Abstract: Aim: The aim of the present study was to describe the diversity patterns along
elevational gradients that are crucial for conservation management and for
understanding diversification processes in montane environments. This article
analyses the distributional range and elevation patterns of endemic and nonendemic
(NE) vascular plants living on a unique set of Neotropical table
mountains (tepuis). To investigate the potential causes of the high degree of
diversity and endemism we tested the role of elevation, area and the middomain
effect (MDE). We also aimed to discuss the origin of the current tepuian
biota.
Location: The Guayana Highlands (northern South America). Emphasis is
placed on the mountaintops above 1500 m elevation, which form the highly
biodiverse Pantepui biogeographical province.
Methods: We examined the distribution patterns of vascular plant species richness
in relation to elevation, area, and the MDE using generalized additive
models. We used Range Model for the MDE.
Results: We found that regional endemics richness show a hump-shaped curve
in relation to elevation. Single-tepui endemics (STE) increase with elevation,
whereas NE and total species richness decrease. Area and MDE influence this
pattern for Pantepui and STE, but not for NE. We also observed that the spatial
distribution of endemic richness displays a left-skewed distribution pattern
due to the dominance of STE.
Main conclusions: Our results demonstrate that a combination of elevation,
area and MDE provide a basic explanation for the diversity of vascular plants
in Pantepui. In addition, the present study indicates that maxima of STE are
located at the highest altitudes, where the possibility of biotic connection (via
migration) and gene flux has been minimal, even during glacial phases when
most migration pathways amongst the tepui mountains were open. We also
suggest that climatic filtering due to the extreme conditions atop the tepuis
and low dispersal capacity stand out as the main drivers of the decline in NE
species richness with elevation.
Abstract: Earlier palynological studies of lake sediments from Easter Island suggest that the island underwent a
recent and abrupt replacement of palm-dominated forests by grasslands, interpreted as a deforestation
by indigenous people. However, the available evidence is inconclusive due to the existence of extended
hiatuses and ambiguous chronological frameworks in most of the sedimentary sequence studied. This
has given rise to an ongoing debate about the timing and causes of the assumed ecological degradation
and cultural breakdown. Our multiproxy study of a core recovered from Lake Raraku highlights the
vegetation dynamics and environmental shifts in the catchment and its surroundings during the late
Holocene. The sequence contains shorter hiatuses than in previously recovered cores and provides a
more continuous history of environmental changes. The results show a long, gradual and stepped
landscape shift from palm-dominated forests to grasslands. This change started c. 450 BC and lasted
about two thousand years. The presence of Verbena litoralis, a common weed, which is associated with
human activities in the pollen record, the significant correlation between shifts in charcoal influx, and
the dominant pollen types suggest human disturbance of the vegetation. Therefore, human settlement
on the island occurred c. 450 BC, some 1500 years earlier than is assumed. Climate variability also
exerted a major influence on environmental changes. Two sedimentary gaps in the record are interpreted
as periods of droughts that could have prevented peat growth and favoured its erosion during the
Medieval Climate Anomaly and the Little Ice Age, respectively. At c. AD 1200, the water table rose and the
former Raraku mire turned into a shallow lake, suggesting higher precipitation/evaporation rates coeval
with a cooler and wetter Pan-Pacific AD 1300 event. Pollen and diatom records show large vegetation
changes due to human activities c. AD 1200. Other recent vegetation changes also due to human activities
entail the introduction of taxa (e.g. Psidium guajava, Eucalyptus sp.) and the disappearance of
indigenous plants such as Sophora toromiro during the two last centuries. Although the evidence is not
conclusive, the American origin of V. litoralis re-opens the debate about the possible role of Amerindians
in the human colonisation of Easter Island.
Abstract: An important feature of the ongoing debate about the acceptance of the Anthropocene as a formal chronostratigraphic unit with the same rank as the
Holocene (epoch) has been either the existence or the lack of a Global Stratotype Section and Point (GSSP). In addition, the utility of the Anthropocene
as a stratigraphic unit has also been questioned. In this paper, it is proposed that the discovery of the GSSP may not be a major problem and could only
be a matter of time. However, the term Anthropocene itself, defined on the basis of the stratigraphic expression of human activities (e.g. large-scale
agriculture and land clearance, accelerated release of greenhouse gases to the atmosphere) may significantly impact the current stratigraphic framework
guided by the International Commission on Stratigraphy (ICS). Indeed, the formal usage of this term can not only lead to stratigraphic and terminological
inconsistencies but can also influence the future development of the established chronostratigraphic scheme. These points should be considered by the
ICS Anthropocene Working Group before making a final decision. The stratigraphic status of the Anthropocene, however, is a formal issue that should
not affect current and future research on human-induced environmental and sedimentary changes, including their stratigraphic imprint. The message is
twofold: leave the formal chronostratigraphic aspects to the ICS, and keep producing and organizing knowledge independently of the formal debate.
Doing so would require the development of a parallel and likely transitory chronological system without formal stratigraphic value, from which the term
Anthropocene would be, at least temporarily, excluded.
Abstract: Pantepui is a biogeographical province composed of a group of approximately 50 tabular mountain summits,
or tepuis, in the southeast of Venezuela. This region, also known as Guayana Highlands (GH), lies
between 1500 and 3014 m elevation and has an approximate extension of 6000 km2. The summits of
the tepuis are characterized by exceptional vascular plant diversity and high endemism. However, it is
expected that ongoing global warming will produce upward displacements of summit taxa, which may
cause the extinction of certain species due to habitat loss. This study assessed the potential extinction risk
of the >600 Pantepui endemic vascular plant species during the 21st century due to global warming,
revealing that 30â50% of endemic species would lose their habitat by the end of this century. In light
of these results, prioritization of threatened species for conservation purposes became necessary. This
was achieved through the calculation of an Environmental Impact Value (EIV) for each species, and a subsequent
division of these species into priority categories, which should be used in a chronological
sequence to guide decision-making and future research aimed at establishing the most suitable conservation
strategies. A number of in situ and ex situ conservation alternatives were discussed. In situ conservation
by means of designating protected areas does not appear a viable option because of the
upward habitat displacement involved in this case. Conversely, seed banks, living plant collections and
managed relocation were suggested in this chronological order to preserve the species studied here.
Abstract: This paper aims to show the relevance of past ecological records, at centennial to millennial timescales, for community ecological principles and theory, mainly in relation to temporal dynamics and the origin of present-day community patterns. The underlying assumption is that ecological time is a continuum and the ecological understanding of the present biosphere needs inputs from multilevel timescales. In particular, the so-called Q-time, embracing the Quaternary (the last 2.6 million years), is proposed as a key time period to understand present-day patterns and the underlying causal processes, as for example the latitudinal diversity gradient, the relationship between species richness and stability, the equilibrium/non-equilibrium conditions between communities and the environment, the main trends and clues on the origin of present-day species and the
communities they form, the community succession under changing environmental conditions, or the nature (individual vs collective) of such biotic responses, among others. In this temporal context, neoecological studies and modeling, based on short-term evidence and calibration/validation data sets, are viewed as an important source for hypotheses to be tested with long-term ecological (i.e., palaeoecological) and molecular phylogenetic studies. The considerations around these topics provide valuable insights to address the potential future state of modern communities under the predicted global change, which would be useful to propose suitable conservation strategies. It is hoped that this paper will promote constructive discussions leading to a more close collaboration between neoecologists and palaeoecologists, in the way towards the natural convergence of both into one single, time-independent, discipline as is (or should be) ecology. As this paper has been conceived for both neo- and palaeoecologists, the message is twofold: to neoecologists, care about time; to palaeoecologists, care about ecology.
Abstract: Macrofossil analysis of a composite 19 m long sediment core from Rano Raraku Lake (Easter Island)
was related to litho-sedimentary and geochemical features of the sediment. Strong stratigraphical
patterns are shown by indirect gradient analyses of the data. The good correspondence between the
stratigraphical patterns derived from macrofossil (Correspondence Analysis) and sedimentary and
geochemical data (Principal Component Analysis) shows that macrofossil associations provide sound
palaeolimnological information in conjunction with sedimentary data. The main taphonomic factors
influencing the macrofossil assemblages are run-off from the catchment, the littoral plant belt, and the
depositional environment within the basin. Five main stages during the last 34,000 calibrated years BP
(cal yr BP) are characterised from the lithological, geochemical, and macrofossil data. From 34 to
14.6 cal kyr BP (last glacial period) the sediments were largely derived from the catchment, indicating
a high energy lake environment with much erosion and run-off bringing abundant plant trichomes,
lichens, and mosses into the centre of Raraku Lake. During the early Holocene the infilling of the lake
basin and warmer conditions favoured the growth of a littoral plant belt that obstructed terrigenous
input. Cladoceran remains and Solanaceae seeds are indicative of reduced run-off and higher values of N
and organic C indicate increased aquatic and catchment productivity. From 8.7 to 4.5 cal kyr BP a swamp
occupied the entire basin. The increase of Cyperaceae seeds reflects this swamp development and, with
oribatid mites and coleopteran remains, indicates a peaty environment and more anoxic conditions in
Raraku. At around 4.5 cal kyr BP dry conditions prevented peat growth and there is a sedimentary hiatus.
About 800 cal kyr BP, peat deposition resumed. Finally, in the last few centuries, a small lake formed
within the surrounding swamp. Evidence of human activity is recorded in these uppermost sediments.
Abstract: Current global average temperatures are 2 C cooler than during the last interglacial period. The expected 28
increase in temperature during the 21st century will be most stressful for cold-adapted and stenothermic 29
mountain species, forcing them to migrate upwards, and eventually to concentrate into either large areas 30
with favourable climates (refugia) or small stands under locally favourable microclimates (microrefugia). 31
We investigate potential refugia in the neotropical Guayana Highlands mountain biome (Pantepui), con- 32
sisting of 50 isolated table mountains (tepuis), to develop strategies for conserving biodiversity during 33
future global warming. We predict the amount of loss of altitudinal habitats of endemic vascular flora of 34
26 tepuis and evaluate potential threats to these taxa with respect to species extinction, habitat loss, hab- 35
itat connectivity and the degree of isolation. We compare past, present and future Pantepui landscape 36
configurations through fragmentation analysis and identify potential in situ refugia. Spatial analysis fore- 37
casts more species isolation and declining biodiversity at the end of this century relative to current and 38
past levels. Habitats are predicted to experience >80% loss, with the disappearance of 38 habitat patches. 39
One large patch (Chimantá massif) accounts for 46% of the predicted remaining habitat. This patch can be 40
considered a potential refugium for future vascular flora, as it is predicted to contain some present-day 41
resistant species fromlower altitudinal levels and other species eventually persisting in microrefugia. The 42
easternmost Pantepui district, containing the Chimantá massif and other tepuis, seems to be the most 43
suitable for the application of in situ conservation strategies.
Abstract: In a recent paper, Ricklefs (2012) argued that the current emphasis on theoretical models undermines the relevance of natural history in ecology and advocated for a return to the observation of the natural world. This comment notes that part of the natural history observations needed to address keystone ecological patterns and processes are in often ignored past records. As performed by Ricklefs, the origin and maintenance of extant biodiversity are used to illustrate the potential usefulness of past natural history in theoretical ecology and modeling. It is concluded that rather than testing current theories, past natural history calls for their reformulation.
Abstract: This paper summarises the NPP studies developed so far in a wide range of environments from Venezuelan lowlands, midlands and highlands, as an example of a growing discipline in a Neotropical area. The studies discussed include both modern analogues from surface sediments and Late Quaternary sequences combining pollen and spores with NPP analyses. Emphasis is placed on the utility of NPP as palaeoecological indicators both individually and collectively (NPP assemblages), as well as in combination with other proxies. The main advantages of using NPP instead of only pollen and spores are highlighted using case studies as examples. Among them, the occurrence of NPP in samples barren for pollen and spores, the independent indicator nature of NPP, and their capacity to reveal previously unnoticed environmental shifts (notably those related to local conditions), are emphasised. The main results obtained in all these analyses are discussed in the frame of several areas of the NPP study that are considered crucial (methods, taxonomy, indicator value and statistical treatment). Some ideas for future developments are proposed in each of these areas, and the need for closer collaboration among NPP specialists is strengthened, in a way towards unification and standardisation.
Abstract: The karstic Lake Montcortès sedimentary sequence spanning the last 1548 yr constitutes the first continuous, 26
high-resolution, multi-proxy varved record in northern Spain. Sediments consist of biogenic varves com- 27
posed of calcite, organic matter and detrital laminae and turbidite layers. Calcite layer thickness and internal 28
sub-layering indicate changes in water temperature and seasonality whereas the frequency of detrital layers 29
reflects rainfall variability. Higher temperatures occurred in Lake Montcortès in AD 555â738, 825â875, 30
1010â1322 and 1874âpresent. Lower temperatures and prolonged winter conditions were recorded in AD 31
1446â1598, 1663â1711 and 1759â1819. Extreme and multiple precipitation events dominated in AD 32
571â593, 848â922, 987â1086, 1168â1196, 1217â1249, 1444â1457, 1728â1741 and 1840â1875, indicating 33
complex hydrological variability in NE Spain since AD 463. The sedimentary record of Lake Montcortès re- 34
veals a short-term relation between rainfall variability and the detrital influx, pronounced during extended 35
periods of reduced anthropogenic influences. In pre-industrial times, during warm climate episodes, popula- 36
tion and land use increased in area. After the onset of the industrialization, the relationship between climate 37
and human activities decoupled and population dynamics and landscape modifications was therefore mostly 38
determined by socioâeconomic factors.
Abstract: Cycad diversification and tropical biodiversity.- The recent unexpected discovery that living Cycadales are not Jurassic-Cretaceous (200-65 Mya) relicts, as all their extant genera began to diversify during the Late Miocene (12 Mya), has challenged a classical evolutionary myth. This brief note shows how this finding may also provide new clues on the shaping of the high tropical biodiversity.
Abstract: Pantepui is a phytogeographical
province made up of a group of approximately 50 tabular mountain summits or tepuis in southeast
Venezuela. This region lies between 1500 and 3014 m a.s.l and covers an approximate area of 6000
km2. Its pristine state of conservation is remarkable. The summits of the tepuis contain an excep-
tional level of vascular plant diversity, including 2446 known species, of which 771 are endemic to
Pantepui. It is expected that the ongoing effects of global warming will produce upward displace-
ments of summit taxa, which could result in the extinction of certain species due to habitat loss.
This study is an assessment of the potential extinction risk during the 21st century due to global
warming. In order to determine those species that will potentially become extinct during each time
frame (2011-2030, 2046-2065 and 2080-2099), we used Altitudinal Range Displacement (ARD)
analysis and different scenarios to forecast expected temperatures, as predicted by the Intergov-
ernmental Panel on Climate Change (IPCC). The ARD results revealed that at least 169, and up to
321, endemic species would lose their habitat by the end of this century based on the more con-
servative and pessimistic scenarios, respectively. In light of these results, a number of in situ and
ex situ conservation alternatives are discussed. In situ conservation by means of protected areas
does not appear be a viable option because of the upward habitat displacement that would occur.
Conversely, ex situ conservation techniques (seed or pollen banks, in vitro culture, collections un-
der cultivation, etc.) have great potential to preserve the species discussed herein.
Abstract: The occurrence of the Younger Dryas cold reversal in northern South America midlands and lowlands remains
controversial. We present a palaeoecological analysis of a Late Glacial lacustrine section from a midland lake (Lake Chonita,
4.6501 8N, 61.0157 8W, 884m elevation) located in the Venezuelan Gran Sabana, based on physical and biological proxies. The
sediments were mostly barren from 15.3 to 12.7 k cal a BP, probably due to poor preservation. A ligneous community with no clear
modern analogues was dominant from 12.7 to 11.7 k cal a BP (Younger Dryas chronozone). At present, similar shrublands are situated
around 200m elevation above the lake, suggesting a cooling-driven downward shift in vegetation during that period. The interval
from 11.7 to 10.6 k cal a BP is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire
incidence. The intensification of local and regional fires at this interval could have played a role in the vegetation shift. A change to
wetter, and probably warmer, conditions is deduced after 11.7 k cal a BP, coinciding with the early Holocene warming. These results
support the hypothesis of a mixed origin (climate and fire) of the Gran Sabana savannas, and highlight the climatic instability of the
Neotropics during the Late Glacial.
Abstract: During the Mid-Late Holocene, the Iberian Peninsula underwent large climatic and hydrologic
changes, but the temporal resolution and regional distribution of available records is still insufficient
for a comprehensive assessment of the regional variability. The high sedimentation rate of the karstic,
meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a high-resolution palaeoreconstruction
of its ecology for the last 5,340 years based on diatom analysis and supported by aquatic pollen,
sedimentological data, and historic documentary records. The results show marked fluctuations in
diatom species assemblage composition, mainly between dominant Cyclotella taxa and small
Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta
reflects changes in the trophic state, while the succession of centric and pennate species most likely
evidences significant changes in the hydrologic balance of the lake. The diatom assemblages identify
six main phases: increased bioproductivity and likely lower lake levels prior to 2,350 BC; lower lake
levels and strong arid phase between 2,350-1,850 BC; lake level recovery between 1,850-850 BC;
relative lake level raise with fluctuating conditions during the Iberian and Roman Epochs; lower lake
levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by
increased human activities in the watershed during Medieval times; relatively higher lake levels during
the LIA and afterwards. The combined study of diatoms, algae and pollen provides a detailed
reconstruction of climatic events, which refines understanding of regional variability and intertwined
climatic and socio-economical events in the Pyrenees.
Abstract: Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km2; z max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS 14C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000â3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690â1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770â1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.
Abstract: We report vegetation changes of the last millennium inferred from palynological analysis of a sediment core from Lake Montcortès, situated at ~1,000 m elevation in the southern pre-Pyrenean flank. The record begins in the Middle Ages (~AD 800) and ends around AD1920, with an average resolution of ~30 years. The reconstructed vegetation sequence is complex and shows the influence of both climate and humans in shaping the landscape. Pre-feudal times were characterized by the presence of well-developed conifer forests, which were intensely burned at the beginning of feudal times (AD 1000) and were replaced by cereal (rye) and hemp cultivation, as well as meadows and pastures. In the thirteenth century, a relatively short period of warming, likely corresponding to the Medieval Warm Period, was inferred from the presence of a low Mediterranean scrub community that is today restricted to <800 m elevation. This community disappeared during Little Ice Age cooling in the fifteenth century, coinciding with a decline in human activities around the lake. Forest recovery began around AD 1500, at the beginning of the Modern period, coinciding with wetter climate. Forests, however, declined again during the seventeenth century, coinciding with maximum olive and hemp cultivation. This situation was reversed in post-Modern times (nineteenth century), characterized by an intense agricultural crisis and a significant decline in population that favored forest re-expansion. Correlations with nearby Estanya Lake, situated about 350 m below, provide a regional picture of environmental change. Besides some climate forcing evident in both sequences, human activities seem to have been the main drivers of landscape and vegetation change in the southern Pyrenean flank, in agreement with conclusions from other studies in high-mountain environments.
Abstract: This paper reports the preliminary palynological results, at a millennial scale, of a Holocene peat bog sequence, since around 8.0 cal kyr BP to the present, obtained in the summit of the Apakará-tepui (2170 m elevation), in the Chimantá massif, located in the neotropical Venezuelan Guayana. The early Holocene was characterized by a vegetation different to the present, in which trees and shrubs dominated and Myrica was the main element of the gallery forests around a permanent water body, as indicated by the continuous presence of Isoëtes in high percentages. Around the middle Holocene (5.3 cal kyr BP), a shift towards more herbaceous and non-flooded communities occurred, and the present day vegetation established. This has been interpreted as a shift from warmer and wetter climates to cooler and drier conditions. The first phase, from 8.0 to 5.3 cal kyr BP, falls within a warming phase widely documented worldwide, known as the Holocene Thermal Maximum. The millennial trends shown here will be refined with further studies at centennial to decadal time scales. These results support the hypothesis that the best sites to detect paleoenvironmental changes in the summits of the tabular Guayana mountains are close to altitudinal ecotones.
Abstract: Palaeoecology, as an ecological discipline, is able to provide relevant inputs for conservation science and ecosystem management, especially for issues involving long-term processes, such as ecological succession, migration, adaptation, microevolution, and extinction. This use of palaeoecology has been noted for several decades, and it has become widely accepted, especially in the frame of ongoing and near-future global warming and its potential biotic consequences. Selected palaeoecological insights of interest for conservation include the following: 1) species respond in an individualistic manner to environmental changes that lead to changes in community composition, suggesting that future ecosystems would have no modern analogues; 2) in the short-term, acclimation is more likely a response of species that are expected to persist in the face of global warming, but the possibility of evolutionary change linked to the existence of pre-adapted genomes cannot be dismissed; 3) species unable to acclimate or adapt to new conditions should migrate or become extinct, which has been observed in past records; 4) current extinction estimates for the near-future should be revised in light of palaeoecological information, which shows that spatial reorganisations and persistence in suitable microrefugia have been more common than extinction during the Quaternary; 5) biotic responses to environmental changes do not necessarily follow the rules of equilibrium dynamics but depend on complex and non-linear processes that lead to unexpected âsurprisesâ, which are favoured by the occurrence of thresholds and amplifying positive feedbacks; 6) threshold responses can cause the movement of ecosystems among several potentially stable states depending on their resilience, or the persistence of transient states; 7) species and their communities have responded to environmental changes in a heterogeneous fashion according to the local and regional features, which is crucial for present and future management policies; 8) the global warming that occurred at the end of the Younger Drays cold reversal (ca. 13.0 to 11.5 cal kyr BP) took place at similar rates and magnitudes compared to the global warming projected for the 21st century, thus becoming a powerful past analogue for prediction modelling; 9) environmental changes have acted upon ecosystems in an indirect way by modifying human behaviour and activities that, in turn, have had the potential of changing the environment and enhancing the disturbance effects by synergistic processes involving positive feedbacks; 10) the collapse of past civilisations under climate stress has been chiefly the result of inadequate management procedures and weaknesses in social organisation, which would be a warning for the present uncontrolled growth of human population, the consequent overexploitation of natural resources, and the continuous increase of greenhouse-gas emissions; 11) the impact of fire as a decisive ecological agent has increased since the rise of humans, especially during the last millennia, but anthropic fires were not dominant over natural fires until the 19th century; 12) fire has been an essential element in the development and ecological dynamics of many ecosystems, and it has significantly affected the worldwide biome distribution; 13) climate-fire-human synergies that amplify the effects of climate, or fire alone, have been important in the shaping of modern landscapes. These general paleoecological observations and others that have emerged from case studies of particular problems can improve the preservation of biodiversity and ecosystem functions. Nature conservation requires the full consideration of palaeoecological knowledge in an ecological context, along with the synergistic cooperation of palaeoecologists with neoecologists, anthropologists, and conservation scientists.
Abstract: This paper shows a Holocene paleoecological reconstruction based on a peat bog sequence (El PaujÃ, 4º28âN - 61º35âW, 865 m elevation) located in the transition zone between the Gran Sabana (SE Venezuela) savannas and the Amazon rainforests. Paleoecological trends are based on the analysis of pollen and pteridophyte spores, algal and animal remains, fungal spores, and charcoal particles. The whole record embraces the last ca. 8000 cal years BP, and was subdivided into five pollen zones, representing the following vegetation succession: savanna/rainforest mosaic (8250-7715 yr BP), dense rainforests (7715-5040 yr BP), savanna/rainforest mosaic (5040-2690 yr BP), secondary dry forests (2690-1440 yr BP), and peat bog in an open savanna landscape (1440 yr BP-present). These vegetation changes have been attributed to the action of climate and/or land use changes, as well as the corresponding synergies between them. Fire has been determinant in the landscape evolution. Based on the reconstructed fire and vegetation shifts, a changing land use pattern could have been recognized. Between the early and the mid Holocene (ca. 8.3-5.0 kyr BP), land use practices seem to have been more linked to shifting agriculture in a rainforest landscape -as is usual in Amazon cultures- with medium fire incidence affecting only local forest spots or surrounding savannas. More extensive forest burning was recorded between ca. 5.0 and 2.7 kyr BP, followed by land abandonment and the dominance of drier climates between 2.7 and 1.4 y BP. The modern indigenous culture, which prefers open environments and makes extensive use of fire thus preventing forest re-expansion, seem to have established during the last 1500 years. Therefore, a significant cultural replacement has been proposed for the region, leading to the present-day situation. Changing human activities have been instrumental for ecological evolution in this savanna-rainforest transitional region, as well as for the shaping of modern landscapes.
Abstract: Fires are among the most important risks for tropical ecosystems in a future climatic change scenario. Recently, paleoecological research has been addressed to discern the role played by fire in neotropical landscapes. However, given the magnitude of the Neotropics, many studies are relegated to infer just local trends. Here we present the compilation of the paleo-fire records developed until now in the southern Gran Sabana (SE Venezuela) with the aim to describe the fire history as well as to infer the possible forcing factors implied. In this sense, southern Gran Sabana has been under fire perturbation since the Late Glacial, with the concomitant effects upon vegetation, and persisted during the Holocene. Around 2000 cal yr BP onwards, the fire activity highly increased promoting the expansion of pre-existing savannas, the decrease of forests and the appearance and establishment of Mauritia palm swamps. The continuous fire incidence registered for several thousands of years has likely promoted the supremacy of treeless savannas upon other vegetation types and the degradation to secondary landscapes. Based on the available evidence, the anthropogenic nature of this high fire activity has been postulated. If so, it could be hypothesized that the timing arrival of Pemón, the present-day indigenous culture in the Gran Sabana, would be ca. 2000 cal yr BP onwards, rather than the last centuries, as it has been formerly assumed. The implications of these ancient practices in the area are also discussed for present Gran Sabana landscapes sustainability and future conservation strategies.
Abstract: The southern Gran Sabana (SE Venezuela) holds a particular type of neotropical savanna characterized by the local occurrence of morichales (Mauritia palm swamps), in a climate apparently more suitable for rain forests. We present a paleoecological analysis of the last millennia of Lake Chonita (4º39âN - 61º0âW, 884 m elevation), based on biological and physico-chemical proxies. Savannas dominated the region during the last millennia, but a significant vegetation replacement occurred in recent times. The site was covered by a treeless savanna with nearby rainforests from 3640 to 2180 cal yr BP. Water levels were higher than today until about 2800 cal yr BP. Forests retreated since about 2180 cal yr BP onwards, likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales. The simultaneous appearance of charcoal particles and Mauritia pollen around 2000 cal yr BP supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion. The whole picture suggests human settlements similar to today -in which fire is an essential element- since around 2000 years ago. Therefore, present-day southern Gran Sabana landscapes seem to have been the result of the synergy between biogeographical, climatic and anthropogenic factors, mostly fire.
Abstract: The origin of the extant neotropical biodiversity has been a controversial topic since the times of Darwin. Here, the timing and the potential driving factors associated with diversification are discussed using the more recent evidence from molecular phylogenetics. These studies provide new insights on the subject but they are sensitive to dating approaches and targets, and can eventually lead to biased conclusions. A careful analysis suggests that the origin of extant neotropical biodiversity cannot be attributed to the action of one or few events during key time intervals. Rather, it is the result of complex ecological and evolutionary trends initiated by the Neogene tectonic events and palaeogeographic reorganisations, and lasted under the action of Pleistocene climatic changes.
Abstract: The "Great American Biotic Interchange" (GABI) is regarded as a defining event in the biogeography of the Americas. It is hypothesized to have occurred when the Isthmus of Panama closed ca three million years ago (Ma), ending the isolation of South America and permitting the mixing of its biota with that of North America. This view of the GABI is based largely upon the animal fossil record, but recent molecular biogeographic studies of plants that show repeated instances of long-distance dispersal over major oceanic barriers suggest that perhaps the land bridge provided by the isthmus may have been less necessary for plant migration. Here we show that plants have significantly earlier divergence time estimates than animals for historical migration events across the Isthmus of Panama region. This difference in timing indicates that plants had a greater propensity for dispersal over the isthmus before its closure compared with animals. The GABI was therefore asynchronous for plants and animals, which has fundamental implications for the historical assembly of tropical biomes in the most species-rich forests on the planet.
Abstract: Despite what their names might suggest, ecology and palaeoecology have progressed historically as well separated disciplines. This unfortunate disjunction is analyzed here from a theoretical point of view. Among the factors that have facilitated the separation are: 1) the past-present dissociation characteristic of the human mind, 2) the diversity of fields of provenance of palaeoecologists, 3) the contrasting nature of the evidence and associated methodological differences, and 4) misunderstandings caused by the use of prefix palaeo-. The principle of uniformitarianism emphasizes that past, present and future are not discrete units but a time continuum through which species and communities flow, change and evolve; and that ecology and palaeoecology are only different approaches with a common objective, which is the ecological understanding of the biosphere. Therefore, a terminological clarification is needed. Ecology in a broad sense, includes inferences about the past (palaeoecology), present studies (neoecology or contemporary ecology) and future projections (predictive ecology). Palaeoecology is thus a means by which ecology studies the past using proxies. Other disciplines beginning with the prefix palaeo- (notably palaeoclimatology and palaeoenvironmental reconstruction) are not necessarily ecological. It is recommended that ecologists and palaeoecologists develop joint projects, and that palaeoecologists increase their participation in ecological journals, books and meetings. These collaborations will demonstrate that a palaeoecologist is not simply a palaeoscientist whose data may be of interest for ecology but is primarily an ecologist working on another time scale, with different methods.
Abstract: The existence of palm-dominated forests covering the island since the last glaciation and the recent
deforestation by humans are paradigmatic in Easter Island's paleoecological reconstructions. The timing and
mode of the deforestation are controversial, but there is general agreement that it actually occurred, and it is
often given as an example of a human-induced environmental catastrophe with philosophical implications
for the future of the whole planet. To evaluate whether this is the only well-supported hypothesis or if there
might be other scenarios compatible with the paleoecological data, this paper reviews all the available
evidence on past vegetation changes on Easter Island. The discussion is centered on three main points: 1) the
alleged nature and extension of the former forests, 2) the taxonomic identity of the dominant palms, and 3)
the nature of the recent ecological changes leading to a treeless island. The potential causes of the assumed
deforestation are beyond the scope of this study. Concerning the first point, palynological and
anthracological results obtained so far are not only compatible with a forested island, but also with other
scenarios, for example a mosaic vegetation pattern with forests restricted to sites with a high freshwater
table (gallery forests), which are mostly around the permanent lakes and along the coasts. With regard to
palm identity, some extant species have been proposed as potential candidates, but the palms that
dominated these forests seem to have become extinct and their identity remains unknown. The existence of
a sedimentary hiatus around the dates of forest decline complicates the picture and reinforces the possibility
of climatic changes. It is concluded that the hypothesis of a previously forested island has yet to be
demonstrated. Therefore, the recent ecological disaster, human-induced or not, is still speculative. Several
types of future studies are proposed for a better understanding of Easter Island's ecological history,
including: modern analog studies from similar situations, pollen dispersal modeling, high-resolution multiproxy
studies along the cores obtained so far, more coring campaigns in the search for older sediments, and
DNA and isotopic analyses of plant remains for taxonomic identification purposes.
Abstract: The occurrence, or not, of the Younger Dryas cold reversal in the tropical Andes remains a controversial
topic. This study reports a clear signal for this event in the Venezuelan Andes, employing high-resolution
palynological analysis of a well-dated sediment core from Laguna de Los Anteojos, situated around
3900 m elevation, within grass páramo vegetation. The lake is surrounded by some Polylepis forests
which are close to their upper distribution limit. The section of the core discussed here is 150-cm long
and dated between about 14.68 and 9.35 cal kyr BP, using a polynomial age-depth model based on six
AMS radiocarbon dates. Between 12.86 and 11.65 cal kyr BP, an abrupt shift occurred in the pollen
assemblage, manifested by a decline of Podocarpus, Polylepis and Huperzia, combined with an increase in
Poaceae and Asteraceae. The aquatic pteridophyte Isoëtes also decreased and disappeard, and the algae
remains show their minimum values. Pollen assemblages from the Younger Dryas interval show
maximum dissimilarity values compared with todayâs pollen assemblage, and are more similar to
modern analogs from superpáramo vegetation, growing at elevations 400e500 m higher. A lowering of
vegetation zones of this magnitude corresponds to a temperature decline of between 2.5 and 3.8 C.
During this colder interval lake levels may have been lower, suggesting a decrease in available moisture.
The vegetation shift documented in Anteojos record between 12.86 and 11.65 cal kyr BP is comparable to
the El Abra Stadial in the Colombian Andes but it differs in magnitude. The Anteojos shift is better dated
and coincides with the Younger Dryas chron as recorded in the Cariaco Basin sea surface temperature
reconstructions and records of continental runoff, as well as in the oxygen isotope measurements from
the Greenland ice cores. When compared to other proxies of quasi-immediate response to climate, the
time lag for the response of vegetation to climate is found to be negligible at a centennial scale. The
Polylepis pollen curve is especially noteworthy, as it reproduces the overall pollen trends and matches
well with paleoclimatic reconstructions based on other proxies. Hence, Polylepis might be used as
a reliable paleoclimatic indicator in lake sediments close to its uppermost distribution boundary.
Abstract: This work is a pollen-morphological study of various plant species from Pantepui (Venezuelan Guayana), a region with high biodiversity and endemism, where global warming is expected to have a high impact. The study consists of a series of morphological descriptions of selected taxa from the Maguire pollen reference slide collection of The New York Botanical Garden (NYBG). The collection was initiated under the supervision of Senior Curator Bassett Maguire to advance systematic, palynological, and medical studies; today it has become also useful for other disciplines such as paleoecology, paleoclimatology or forensic studies. The aim of this pollen-morphological study is to enhance the database of pollen descriptions and illustrations for identification purposes, to be used in the ongoing paleoecological reconstructions and, eventually, in other types of studies using pollen, particularly from the Guayanan tepui summits.
Abstract: Palynological studies including records of non-pollen palynomorphs (NPP) are uncommon in the Neotropics,
in spite of their demonstrated usefulness in other regions. Modern analog studies to improve
palaeoecological interpretations of NPP are even more scarce. Here, we report the NPP assemblages
recorded in modern surface samples from an altitudinal transect of the Venezuelan Andes, ranging from
about 2300 and 4600 m. We compared the assemblages with the results of previous pollen analyses of the
same samples. The variables considered to explain NPP patterns along the transect are altitude and the local
habitat of the sampling site (âsample typeâ). A total of 65 NPP taxa â classified into algal and zoological
remains, and fungal spores â have been found. Unidentified taxa (23) have been named with a code,
depicted and described for further reference. Fungal spores are well represented along the whole transect,
whereas algal and zoological remains are absent or very scarce in the lower and the uppermost ranges. The
altitudinal zonation of fungal spores matches with that of pollen and the corresponding vegetation belts,
suggesting a close relationship. The known environmental requirements of some of the fungi identified
allowed inferences on particular ecological features, in agreement with previous palynological interpretations.
Both elevation and sample type are needed to explain the observed differences in the assemblages of
the three groups, the elevation being more decissive for fungal spores and the local habitat of the sampling
site for algal and zoological remains. The sample type effect is minimised when the NPP studied are
considered altogether, thus increasing their usefulness as palaeoecological proxies. This study reinforces the
utility of modern analog surveys of NPP with palaeoecological purposes and encourages further research,
particularly in poorly known areas, as for example tropical regions.
Abstract: The existence of microrefugia and/or cryptic refugia has been proposed to explain the colonization patterns of temperate continents after the Last Glacial Maximum, as well as to gain an understanding of the present genetic structure of species and their populations. The concepts 'microrefugia' and 'cryptic refugia' largely overlap but do not fully coincide, which may lead to terminological duplication and eventual semantic misconceptions. Here, after a terminological and conceptual analysis, it is proposed that most situations currently characterized as cryptic refugia can be accommodated within the definition of microrefugia, with one single exception, for which some alternative terms are suggested. Moreover, the concept and definition of cryptic refugia are considered to be more imprecise, perishable and subjective than those of microrefugia, which is the preferred term in the current context.
Abstract: This work aims to estimate the potential effects of the global warming
projected for the twenty-first century on the biodiversity of a remote and pristine
region of the Neotropics called Pantepui. Habitat loss and fragmentation by upward
migration of bioclimatic conditions is analyzed using Species-Area Relationships
(SAR) and Altitudinal Range Displacement (ARD) analysis. The ARD is a tool that
uses the present-day lapse rate to estimate the upward migration of the species based
on the global warming predicted by the IPCC. The results show that around 80% of
the vascular flora, ca. 1,700 species of which up to 400 would be Pantepui endemics,
are threatened of extinction. These estimates should be considered preliminary,
but the danger is real. Therefore, suitable conservation or mitigation strategies are
needed.
Abstract: The concept of microrefugia is widely accepted to explain the post-glacial colonization
of continents, which led to the present-day biotic configuration of the
Earth. However, so far, microrefugia are no more than a âtheoretical necessityâ
without an appropriate biogeographical and ecological characterization. Here, a
tentative definition and a classification of microrefugia, based on the existing
literature, are proposed for discussion. A deeper biogeographical analysis of the
concept is suggested using novel ecological models and methods, in order
to develop new hypotheses to be tested with palaeoecological and molecular
phylogenetic tools.
Abstract: The neotropical Gran Sabana region of Venezuela
is dominated by apparently anomalous vegetation
types, treeless savannas and savanna-forest mosaics, considering
the present-day warm and wet bioclimatic
conditions. Past climatic changes and fire have been proposed
as the more probable causes. Recent palynological
studies show that savanna vegetation has been present
since the beginning of the Holocene, but the earliest fires
recorded so far only go back to 3,800 cal years B.P. This
paper uses pollen and charcoal analyses to show the existence
of early Holocene regional fires in the Gran Sabana,
and to show the intimate connection between the proxies
for fire (charcoal) and savanna vegetation (pollen)
throughout the Holocene. Although the cause of such fires
is not yet known, the possibility of early Holocene human
occupation of the Gran Sabana is suggested.
Abstract: Modern proxy-calibration studies are a powerful tool for paleoecological interpretation. This paper analyzes the relationships among modern pollen rain, vegetation and altitude in the Central Pyrenees, where several paleo-palynological studies have been developed, but a modern analog survey is still unavailable. The work analyzes the pollen content of moss polsters from different vegetation communities along an altitudinal transect, as well as the flora and vegetation using the Braun-Blanquet system. DCCA showed that altitude satisfactorily explains both vegetation (r2 = 0,988) and pollen (r2 = 0.841) gradients. Besides the complexity of pollen-vegetation relationships, some regularities were found to be useful for paleoecological and paleoenvironmental interpretation. In general, altitudinal vegetation and pollen patterns show similarities, but pollen belts and boundaries are less well defined, likely due to the homogenizing effect of upward wind transport. Palynological differentiation of montane from subalpine/alpine belts is straightforward from the trends of the more significant pollen types, mainly the low-altitude deciduous trees and the high-mountain herbs. Palynological differences between subalpine and alpine belts, which boundary coincides with the treeline, are more subtle and need quantitative criteria and complementary proxies. From an individual point of view, four main groups of pollen were distinguished, in relation to their usefulness as vegetation and altitudinal indicators: 1) very good indicators, 2) good indicators, 3) non indicators, and 4) allochthonous pollen types. The first two groups resulted to be useful as indicator taxa for modern analogs for paleovegetational and paleoaltitudinal reconstruction, while the latter two groups should be interpreted with caution in paleoenvironmental studies.
Abstract: The previously recorded vegetation constancy during most of the Holocene, atop some summits of the
Guayana tabular mountains (or tepuis), led to the hypotheses of either environmental stability or site
insensitivity. As high-mountain biomes are considered to be especially well suited for recording past
environmental changes, a palynological study on the uppermost summit of the Chimantá massif was
designed to test its suitability for these purposes. A peat sequence was obtained spanning the last ~13.0 cal
kyr BP, but an acceptable resolution for paleoecological reconstruction is available only for the last
~4000 years. Around 4.3 cal kyr BP, the modern vegetation was established and has remained virtually
unchanged until today; minor paleoenvironmental changes recorded in other sequences around 2.5 cal kyr
BP were not detected here. The main paleoclimatic trends are in good agreement with other neotropical
records, especially from Lake Valencia and the Cariaco Basin. It is concluded that high-altitude tepuian sites
are useful to record paleoenvironmental changes of moderate to high intensity but once a dense vegetation
cover is established, gentle shifts remain hidden due to the capacity of plant communities to absorb the
changes. The best sites for paleoecological research atop the tepuis are those lying on or near altitudinal
ecotones, especially between the meadows and the paramoid shrublands (~2200 m elevation). Sites within
the meadow domain, as most well-studied so far, are relatively insensitive to Holocene paleoenvironmental
changes.
Abstract: The Gran Sabana region (SE Venezuela) is characterized by extensive neotropical savannas, in a warm and wet climate, apparently more suitable for the existence of rain forests. Two main types of hypotheses have been proposed to explain this situation: natural factors, such as climate changes or soil poverty, and burning by humans. This study analyzes the pollen and charcoal content of a 7,500 years old peat bog, at the shore of Laguna Encantada. The savanna landscape has dominated the area during the whole interval, but local fires did not appear until around 1,200 years ago, when the present communities began to establish. An expansion of gallery forests, likely due to a climatic shift, has been recorded around 4,000 years ago, followed by a deforestation triggered by fire, leading to present-day situation. These results, together with former similar studies, suggest that the present Gran Sabana landscape resulted from the interaction of diverse factors, being climate changes more related to its origin and human induced fires more linked to the recent expansion.
Abstract: The evolutionary origin of extant species in the Neotropics, one of the most biodiverse
regions of the world, has been widely debated. One hypothesis is that neotropical species
emerged primarily during the Quaternary (the last ~2 million years), favoured by alternating
glacial/interglacial climates. An opposite view proposes an older Tertiary origin linked
primarily to palaeogeographical changes. Here, a thorough review of the available literature
on DNA molecular dating shows that the TertiaryâQuaternary debate no longer makes sense.
Indeed, the > 1400 neotropical species whose origin has been dated have appeared in a
continual fashion since the late Eocene/early Oligocene (~39 million years before present)
to the Quaternary. Palaeogeographical mechanisms of speciation are relatively well accepted,
but diversification processes linked to climate are still controversial. These results are
important to unravel both the origin of present-day biodiversity patterns at both local and
global scales and the genetic and environmental mechanisms involved, which are two
crucial aspects for suitable biodiversity conservation strategies.
Abstract: This paper evaluates the potential usefulness
of non-pollen palynomorphs or NPPs (microfossils
other than pollen and spores present in palynological
preparations) contained in lake sediments in the paleolimnological
reconstruction of high altitude environ
ments ([4,000 m) from the Venezuelan Andes. A
synthetic, quantitative approach is employed, instead of
the classical analytical and mostly qualitative approach
commonly used so far for NPPs. The main sources of
variation are the PediastrumâBotryococcus alternation
and the relationship between these two algae and animal
remains such asAcari legs, postabdomina of Cladocera,
mandibles of other invertebrates, and an unknown type
called LVA-1. Other significant microfossils are
remains of Rivularia-type and turbellarian oocytes,
including Gyratrix. The sequence initiates around
15,000 calibrated years before present (cal BP) with
the deglaciation of the lake catchment, high water levels
and still cold climates. A phase of lower lake levels was
recorded between about 12,000 and 6,000 cal BP.
Temperatures increased by around 9,000 and 7,000 cal
BP, and then decreased until 6,000 cal BP. Since that
time, both lake levels and temperature increased again
and stabilized at about 4,000 cal BP, when they reach
modern-like values. These results show a good agreement
with previous studies based on pollen, diatom and
oxygen isotope analyses, and provide additional paleoecological
information, as for example the possibility
of a previously unrecorded Younger Dryas signal. The
inclusion of quantitative NPP analysis in routine
paleolimnological studies using synthetic methods is
thus recommended. Possible future improvements are
suggested, mainly those related with the development
and use of NPP modern analogs.
Abstract: Aim The assumedly anomalous occurrence of savannas and forestâsavanna
mosaics in the Gran Sabana â a neotropical region under a climate more suitable
for tropical rain forests â has been attributed to a variety of historical, climatic,
and anthropogenic factors. This paper describes a previously undocumented shift
in vegetation and climate that occurred during the early Holocene, and evaluates
its significance for the understanding of the origin of the Gran Sabana vegetation.
Location A treeless savanna locality of the Gran Sabana (430¢â645¢ N and
6034¢â6250¢ W), in the Venezuelan Guayana of northern South America, at the
headwaters of the Caronı´ river, one of the major tributaries of the Orinoco river.
Methods Pollen and charcoal analysis of a previously dated peat section
spanning from about the Pleistocene/Holocene boundary until the present.
Results Mesothermic cloud forests dominated by Catostemma (Bombacaceae)
occupied the site around the Pleistocene/Holocene boundary. During the early
Holocene, a progressive but relatively rapid trend towards savanna vegetation
occurred, and eventually the former cloud forests were replaced by a treeless
savanna. Some time after the establishment of savannas, a marked increase in
charcoal particles indicates the occurrence of the first local fires.
Main conclusions The occurrence of cloud forests at the Pleistocene/Holocene
boundary contradicts the historical hypothesis according to which the Gran
Sabana is a relict of the hypothetical widespread savannas that have been assumed
to have dominated the region during the last glaciation. The first local fires
recorded in the Holocene were on savanna vegetation, which is against the
hypothesis of fire as the triggering factor for the establishment of these savannas.
Climate change, in the form of global warming and a persistently drier climate,
emerges as the most probable cause for the forestâsavanna turnover.
Abstract: Aim To reconstruct in detail the potential migration routes and barriers for
vascular plants from the summits of the Guyana mountains during the
Quaternary, in order to test the possibility of migration among them during
the glaciations. These changes in connectivity are predicted based on the
altitudinal migration of plant communities associated with glacial cooling. To
examine the effects of these cycles, the extent of the potential biotic interchange
and its influence on patterns of endemism was modelled.
Location The summits of the tepuis or table mountains of the Neotropical
Guyana Highlands, which constitute the peculiar and discontinuous Pantepui
phytogeographical province (total surface 5000 km2, altitudinal range 1500â
3014 m a.s.l.), and is characterized by a unique and diverse flora with a high
degree of endemism.
Methods GIS-based palaeotopographical reconstruction using a high-precision
digital elevation model, combined with phytogeographical analysis by means of a
data base built up from the Flora of the Venezuelan Guyana, which includes the
geographical and altitudinal ranges for each Pantepui species.
Results During the Last Glacial Maximum, which serves as a representative of a
standard Quaternary glaciation, most migration pathways among tepuis were
open for species with lower altitudinal levels (LAL) £ 1500 m (1678 species or c.
69% of the total Pantepui species), and closed for species with LAL â¡ 2300 m
altitude (c. 3%). The species in between these altitudes have intermediate
migratory possibilities, depending on the district and the tepui considered.
If these local factors are considered, the number of species with no possibility of
glacial interchange increases to 202 or c. 8% of the total. The strongest
topographical barrier separated eastern locations above 1600â1700 m elevation
from all others. The highest possibility of interchange was among the tepuis of the
eastern sector, in which internal topographical barriers were only effective for
species with LALs at or above 1900 m.
Main conclusions The Quaternary evolution of the vascular flora from the
Guyana Highlands took place in a predominantly migration-prone, glacial-era
landscape, in which more than 70% of the flora (maximum estimate) was able
to move from one tepuian district to another, thanks to the downward
bioclimatic shift caused by cooling. Interglacials were too short to drive
significant evolutionary diversification. A number of present high-altitude local
endemics are species that were unable to migrate, even during glaciations.
However, some endemic species do appear to have been able to migrate
among regions, suggesting that topographical isolation alone is not enough
to explain patterns of endemism. Other factors such as tepui summit
area, habitat heterogeneity or pre-Quaternary evolution are considered.
Abstract: The Quaternary paleoclimatic trends documented for the Northern
Hemisphere also affected the neotropical region, and contributed
to the shaping of its present-day biodiversity patterns.
DNA molecular-clock studies on extant neotropical species
have recorded a significant acceleration of speciation rates in
the last 5 million years (my), coinciding with a marked Plio-Pleistocene
global cooling. Furthermore, around half of the species
studied originated during the last 2.6 my, in the frame of Pleistocene
glaciations. The refuge hypothesis is considered inadequate
for the Neotropics, and alternative diversification mechanisms
linked to climate change are discussed herein. Among
them, recurrent vertical migrations controlled by alternating
glacial/interglacial climates, and the resulting connection and
disconnection of lowland and highland biotas, have been considered
important speciation factors. A significant number of
endemic taxa from the neotropical highlands is potentially
threatened of extinction by habitat loss/fragmentation, due to
the global warming predicted by the end of the present century.
Abstract: The evolutionary origin of extant species has been largely debated. The controversy is especially vivid in the Neotropics, one of the more biodiverse regions of the world. One hypothesis is that Neotropical species emerged primarily during the Quaternary (the last ~2 million years), favored by the alternating glacial/interglacial climates. An opposite view proposes an older Tertiary origin linked primarily to paleogeographic changes. Here, a thorough review of the available literature on DNA molecular dating shows that the Tertiary-Quaternary debate no longer makes sense. Indeed, the &amp;gt;1400 Neotropical species dated so far appeared in a continual fashion since the late Eocene/early Oligocene (~39 million years before present) to the Quaternary. Speciation rates maximized during the Plio-Pleistocene (the last ~5 million years), coinciding with a global climatic cooling. Paleogeographic mechanisms of speciation are relatively well known, but diversification processes linked to climate are still poorly understood. These results are important to understand the origin of present-day biodiversity patterns at both local and global scales, as well as the genetic and environmental mechanisms involved, two crucial aspects for suitable biodiversity conservation strategies.
Abstract: The numerical relationship between modern pollen assemblages and altitude in high mountain
environments from the northern Andes is analysed, in order to found inference models that allow
estimating palaeoaltitudes and palaeotemperatures from past pollen records. The calibration set (DM)
consists of a 50-sample altitudinal transect between /2300 and /4600 m altitude. The overall and
individual pollen responses to altitude were tested by correspondence analysis (CA), generalized linear
regression (HOF) and weighted averaging (WA). Transfer functions were derived by weighted averagingpartial
least squares (WA-PLS) regression. Overall, altitude is the main controlling factor for the
composition of pollen assemblages, as shown by the high correlation between altitude and the first CA
component (r//0.88). Individually, around 35% of the 82 pollen taxa show a significant response to
altitude through monotonic or unimodal functions. The best transfer function obtained has a good
statistical performance, as shown by the determination coefficient (r2
jack/0.78). The prediction power, as
measured by the root mean square error of prediction (RMSEP), is of 256 m (12% of the total altitudinal
gradient), which is equivalent to /1.58C. These parameters fall within the performance range of the
inference models developed elsewhere using pollen and other biological proxies. It is concluded that the
DM training set is useful to reconstruct Pleistocene and major Holocene palaeoclimatic trends. This study
demonstrates the suitability of establishing reliable transfer functions for palaeoclimatic estimation in the
highest altitudes of the tropical Andes, and encourages their continued improvement.
Abstract: The fully vegetated summits of the table mountains that form the Guayana Highlands
(GH), in northern South America, hold amazing biodiversity and endemism levels, and
unique vegetation types. In spite of their present-day healthy appearance, their biota is
seriously threatened of habitat loss by upward displacement, because of the projected
warming for the end of this century. Available data are still insufficient for a definite
assessment, but preliminary estimations based on representative endemic vascular plant
species show that roughly one-tenth to one-third of them would loss their habitat with
the 2â4 1C temperature increase predicted for the region by AD 2100. Given the underlying
endemism, the eventual loss of biodiversity will be of global nature. Other
mountain ranges around the world with similar characteristics of the GH, namely
topographical isolation, high endemism and absence of nival stage because of the lower
altitude, would be under similar unexpected risk, and should be urgently considered for
conservation purposes.
Abstract: The underlying causes of late-Holocene climate variability in the
tropics are incompletely understood. Here we report a 1,500-year
reconstruction of climate history and glaciation in the Venezuelan
Andes using lake sediments. Four glacial advances occurred between
anno Domini (A.D.) 1250 and 1810, coincident with solaractivity
minima. Temperature declines of 3.2 1.4°C and precipitation
increases of 20% are required to produce the observed
glacial responses. These results highlight the sensitivity of highaltitude
tropical regions to relatively small changes in radiative
forcing, implying even greater probable responses to future anthropogenic
forcing.
Abstract: Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9-N) Andean lake was conducted in order to reconstruct local and
regional vegetation dynamics since deglaciation. Although deglaciation commenced ¨15,500 cal yr B.P., the area around the Laguna Verde
Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no
modern analog colonized the superpa´ramo. Although this community persisted until ¨6000 cal yr B.P., it began to decline somewhat earlier, in
synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was
replaced by a low-diversity superpa´ramo community that became established ¨9000 cal yr B.P. This replacement coincides with regional
declines in temperature and/or available moisture. Modern, more diverse superpa´ramo assemblages were not established until ¨4600 cal yr
B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen
influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between
14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds
transported lower elevation forest pollen to the superpa´ramo, causing the apparent increase in tree pollen at high altitude. The association
between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this
relationship.
Abstract: This article reports the occurrence of a Middle Wisconsin interstadial, Pedregal, in the northern Venezuelan Andes, as demonstrated by
high-resolution pollen analysis of a previously dated peat layer. Paleoclimatic trends are deduced mainly from changes in the abundance of
tree pollen from the uppermost Andean forests. Previous calibration of this pollen with modern analogs has enabled reconstructions of prior
altitudinal displacements of montane ecological belts and temperature changes. Paleotemperatures were deduced from the current lapse rate
(K0.6 8C/100 m). After a glacier advance, represented by an underlying till, average temperatures increased to 3 8C lower than modern
temperatures during the Pedregal interstadial and then dropped again to approximately 7 8C lower than modern. It is the oldest Quaternary
paleoclimatic event reported thus far in the area.
Abstract: Radiocarbon-dated sediment cores from seven lakes and two bogs spanning the
Cordillera de Me´rida in the Venezuelan Andes were used to identify and date the regional history
of late Pleistocene and Holocene glacial activity. Coring sites were selected at different elevations
across a pronounced rain shadow from southeast (wet) to northwest (dry). Sediment lithostratigraphy
and magnetic susceptibility, in conjunction with AMS radiocarbon dates on macrofossils and charcoal,
were used to constrain deglaciation. The local expression of the Last Glacial Maximum
occurred between 22 750 and 19 960 cal. yr BP. On the wetter southeastern side of the Cordillera
de Me´rida, glaciers had significantly retreated by 15 700 cal. yr BP, followed by several minor glacial
advances and retreats between 14 850 and 13 830 cal. yr BP. At least one major glacial readvance
occurred between 13 830 and 10 000 cal. yr BP in the wetter southeastern sector of the region. The
drier northwest side of the Cordillera de Me´rida records initial glacial retreat by 14 240 cal. yr BP.
Multiple sites on both sides of the Me´rida Andes record a further phase of extensive deglaciation
approximately 10 000 cal. yr BP. However, the north-northwest facing Mucubajı´ catchment
remained partially glaciated until ca. 6000 cal. yr BP. Deglacial ages from the Venezuelan Andes
are consistently younger than those reported from the Southern Hemisphere Andes, suggesting an
inter-hemispheric deglacial lag in the northern tropics of the order of two thousand years.
Abstract: Among the potential consequences of the projected global warming on high-mountain environments
is biodiversity loss by upward migration of environmental conditions and the
consequent habitat vanishing for upland species. So far, such effect has been considered only
for temperate and boreal mountain ranges. Here, the same situation is analysed on the neotropical
Guayana mountains, in northern South America. A preliminary appraisal based on
83 species of vascular plants shows that 8-33% of them are threatened of extinction by habitat
loss. Most are endemic, hence their disappearance would affect global biodiversity.
Abstract: The summits of the table mountains (tepuis) from the Neotropical Guayana region are
remote environments suitable for palaeoecological studies with evolutionary, biogeographical and
palaeoclimatic implications. Here, using palynological analyses of two radiocarbon-dated peat bogs
from a tepui summit, the Holocene palaeovegetational trends are reconstructed, and related to possible
forcing factors. Because of the pristine character of the Guaiquinima summit, the recorded
palaeoenvironmental changes are probably due to natural causes, which makes them valuable
archives of the natural component of climatic change at a millennial time scale. The sequence begins
with pioneer communities or meadows similar to present-day ones, between about 8.4 and 4.5 ky BP.
After this date, and until about 2 kyr BP the expansion of gallery forests suggests an increase in precipitation,
documented also at regional (Neotropical) level. Between ca. 2 kyr BP and the last century,
gallery forests are replaced by forests characteristic of the upper Guaiquinima altitudes, coinciding
with a regional phase of reduced moisture. The present-day meadows, established relatively quickly
during the last century, substituted the former upland forests. In the locality studied, the main controlling
factor of the vegetation during the Holocene seems to have been the moisture balance. In
contrast to other tepui summits, there is no clear evidence for changes linked to temperature oscillations.
This could be due to the elevation of the site, far from any characteristic ecological boundary,
that makes it insensitive to this parameter.
Abstract: Until recently, the high degree of diversity and endemism of the Guayana
Highlands was explained within the frame of the refuge theory. Although this
hypothesis is unsupported by recent palaeoecological evidence, no new diversification
model has been proposed. This paper is a proposal based on the latest
palynological findings that indicate a downward biotic migration of c. 1100 m
altitude during glacials, and the subsequent interglacial upward shift, in response
to colder and warmer climates, respectively. Therefore, during glacials, biotic
mixing is expected in the lowlands, thus promoting sympatric speciation,
hybridization and polyploidy. At the mountaintops, unknown cold-adapted taxa
and pa´ramo-like(?) communities are expected to have occurred, and vicariance
prevailed. In the interglacials, many taxa have had the opportunity for ascending
to the mountains again, allowing genetic interchange among their slopes and
summits, while others would have been adapted to lowlands. The interglacial
highland communities, where vicariance still predominated, experienced some
extinction owing to habitat loss by upland displacement. According to this model,
the successive alternation of glacials and interglacials resulted in a net increase of
diversity and endemism, favoured by the complex topography and habitat heterogeneity,
which allowed high niche diversification. This model has some
similarities with the Andean and Amazon modes of diversification, but the special
topographical characteristics of the Guayana region made it different in other
fundamental aspects. The Guayana Highlands would have acted as a âbiodiversity
pumpâ for the surrounding inner and coastal lowlands, due to the repeated
speciation and further spreading events, as a response to climate. Several working
hypotheses are suggested in relation to the proposed model. The use of coordinated
international multiproxy projects combining palaeoecology and genetic
analysis of modern taxa is strongly encouraged for exploring these ideas.
Abstract: The aim of this paper is to document the Holocene (6000 years BP to the present) vegetation trends on the summit of three
tepuis (table mountains), from one of the largest and highest tepuian massifs of the Neotropical Guayana region, the Chimanta´,
situated in Venezuela. The tepui summits are almost pristine and unique sites to record natural forcings and ecosystem
responses. Here, pollen analysis and radiocarbon dating of four peat sequences obtained with a manual Hiller borer are
presented, and compared with modern analogues from surface samples, for interpretation. Highland tepuian meadows have been
the dominant vegetation type throughout the time interval studied. The sequences studied exhibited different minor vegetation
patterns in time, recording primarily local vegetation dynamics, such as lateral variations in the forestâmeadow ecotones, and
quantitative shifts in the dominant meadow taxa. Moderate climatic shifts formerly reported for other localities were not
recorded here, probably because the intermediate altitude and the geomorphological characteristics of the sites studied made
them insensitive to subtle regional changes in temperature and moisture, which are hidden by local vegetation shifts. The results
of the present study allowed estimation of the magnitude of formerly reported vegetation shifts in the same massif. In the
studied sites, the constancy in the vegetation through time cannot be considered only as the result of a high degree of climatic
stability, but also the consequence of site insensitivity.
Abstract: Aim
To document the occurrence of vertical displacements of vegetation in the
high plateaus of the Venezuelan Guayana (
tepuis
) over the last
c
. 6000 years, and to
discuss their significance for the origin of their flora, especially the endemism
patterns observed in their flat summits. Two hypotheses have been proposed for the
origin of the summit flora. One (the Lost World hypothesis) proposes a long history
of evolution in isolation from the surrounding plains, while the other (the Vertical
Displacement hypothesis) suggests that vertical movements of vegetation during the
Pleistocene glacial-interglacial cycles would have resulted in floristic mixing within
the lowlands, and genetic interchange among plateau summits.
Location
This work has been conducted on the flat summit of the ChurÃ-tepui, in
the Chimantá massif, at 5
°
15
â²
Lat. N and 62
°
01
â²
Long. W, around 2250 m altitude.
Methods
Pollen analysis and radiocarbon dating of two peat outcrops, using modern
analogue technique and numerical methods for palaeoecological interpretation
were used.
Results
The replacement of a high-altitude plant community (a paramoid
Chimantaea
shrubland) by a lower elevation (< 2300 m)
Stegolepis
meadow, occurred about 2500
years before present (yr
ï¢ï°
). This vegetation change is inferred to have resulted from
a regional climatic shift to higher temperature and moisture. A subsequent decrease
in temperature and moisture led to the establishment of present conditions after
about 1450 yr
ï¢ï°
.
Main conclusions
The highland vegetation of the
tepuis
responded to climate
shifts with vertical displacements, supporting the hypothesis of vertical mixing.
However, a physiographical analysis shows that around half of the
tepuis
would
never have been connected by lowlands. Therefore, both hypotheses are needed to
explain the origins of the summit flora in the
tepuis
.
Abstract: Aim
To document the occurrence of vertical displacements of vegetation in the
high plateaus of the Venezuelan Guayana (
tepuis
) over the last
c
. 6000 years, and to
discuss their significance for the origin of their flora, especially the endemism
patterns observed in their flat summits. Two hypotheses have been proposed for the
origin of the summit flora. One (the Lost World hypothesis) proposes a long history
of evolution in isolation from the surrounding plains, while the other (the Vertical
Displacement hypothesis) suggests that vertical movements of vegetation during the
Pleistocene glacial-interglacial cycles would have resulted in floristic mixing within
the lowlands, and genetic interchange among plateau summits.
Location
This work has been conducted on the flat summit of the ChurÃ-tepui, in
the Chimantá massif, at 5
°
15
â²
Lat. N and 62
°
01
â²
Long. W, around 2250 m altitude.
Methods
Pollen analysis and radiocarbon dating of two peat outcrops, using modern
analogue technique and numerical methods for palaeoecological interpretation
were used.
Results
The replacement of a high-altitude plant community (a paramoid
Chimantaea
shrubland) by a lower elevation (< 2300 m)
Stegolepis
meadow, occurred about 2500
years before present (yr
ï¢ï°
). This vegetation change is inferred to have resulted from
a regional climatic shift to higher temperature and moisture. A subsequent decrease
in temperature and moisture led to the establishment of present conditions after
about 1450 yr
ï¢ï°
.
Main conclusions
The highland vegetation of the
tepuis
responded to climate
shifts with vertical displacements, supporting the hypothesis of vertical mixing.
However, a physiographical analysis shows that around half of the
tepuis
would
never have been connected by lowlands. Therefore, both hypotheses are needed to
explain the origins of the summit flora in the
tepuis
.
Abstract: The peculiar biogeography of the so-called
âLost Worldâ, i.e. the summits of sandstone tableaux
(tepuis) in the Neotropical Guyana region of Venezuela,
has generated a debate regarding the factors that are
thought to account for modern vegetation patterns in the
region. Some argue that plant communities on these highelevation
summits reflect a long history of evolution in
isolation, while others surmise that there has been
substantial biotic interchange with the surrounding lowlands
during glacial times. Until now, these apparently
competing hypotheses have not been tested using palaeoecological
methods. I used pollen analysis of Quaternary
sediments and documented past vertical migrations of
vegetation in response to climate changes, which supports
the second hypothesis. Physiographical analysis, however,
shows that about half the tableaux summits are too
high for their flora to have reached the lowlands during
the last glaciation, suggesting that a portion of the
tableaux vegetation has always experienced some degree
of biotic isolation. Thus, a component of the summit
vegetation evolved in isolation, whereas other taxa
experienced interchange, as reflected in endemism patterns
among the tableaux summits. Biogeographical
patterns on the summits are the result of complex
evolutionary processes. The two hypotheses invoked to
explain the vegetation patterns are not mutually exclusive,
but instead complement one another.
Abstract: The uniqueness of the flora from the remote tableaux summits of the Guayana region has been explained either as the result
of a long history of evolution in isolation (Lost World hypothesis or LW) or by alternating upward and downward displacements
during the glacialâinterglacial Quaternary cycles (Vertical Displacement hypothesis or VD). So far, the problem has been
addressed solely on the basis of present-day floristic observations. This paper faces the problem from a Quaternary
palaeoecology perspective using recent palynological findings in the area, comparisons with palaeoecological records from
Neotropical mountains and lowlands of similar latitude, isotopic glacialâinterglacial records from marine and ice cores, and
different points of view about the response of organisms to Quaternary climatic changes, with emphasis on the LGM and the
debate on the existence or not of Neotropical refugia. It is concluded that both LW and VD hypotheses, together with
autoecological and synecological considerations, are needed to explain the present-day specialisation and endemism of the flora
from the tableaux summits. The case of a highly endemic genus (Chimantaea, Asteraceae) is analysed as an example, to
illustrate the usefulness and limitations of the different arguments to account for its biogeographical pattern. Some ideas are
provided for future research, including a more extensive sampling strategy, the use of molecular phylogenetics, the evaluation of
the individualistic versus the community approach, and the use of island biogeography and metapopulation methods on presentday
floristic data.
Abstract: This work consists of a key, descriptions and illustrations, to
identify the most significant angiosperm pollen types (85 genera/
species, belonging to 36 families), from a paleoecological point of
view, of the Gran Sabana and the âtepuiâ (table mountains)
summits from eastern Venezuelan Guayana. The pollen key and
plates contain the most frequent and abundant pollen types found
so far by the author in the Holocene sediments, as well as the
significant medium and minor elements. This is enough to carry
out a standard, successful palynological analysis in the area with
a small percentage of unknown taxa, and can be considered a first
step to be enhanced with future studies, especially from the âtepuiâ
summits of the western Venezuelan Guayana.
Abstract: This paper deals with an Oligocene section that is stratigraphically
homogeneous from both a lithological and palynological point of
view. It has been impossible to subdivide it into discrete units,
using either taxon-range analysis or assemblage-zone approach
based on the relative abundance of palynomorphs. Furthermore,
the common multivariate numerical methods used so far with
success in the region (Cluster Analysis, Principal Components
Analysis, etc.) gave no useful results in this case. The search for
cyclicity using palynocycles and ecologs has been also unsuccessful.
Instead of considering the section of low interest, an alternative,
high-resolution ecological approach was attempted to extract
the information contained in these sediments. Paleoecological
trends were deduced from statistical methods commonly used
in modern and Quaternary ecology, mainly TWINSPAN and
gradient analysis, combined with diversity analysis. As a result,
the fine-scale stratigraphic variability of the data could be successfully
explained in terms of paleoecological succession taking
place in upper delta environments, characterised by a complex
mosaic vegetation including morichales, herbaceous fern swamps,
and gallery forests. The succession could be reconstructed in
detail, and would be of indirect stratigraphic value for highresolution
correlation. This is an example of how the search for
narrow or biased objectives can hidden significant information. It
is more fruitful to have a wider perspective, and to be open to any
information that sediments can provide us, without a priori
limitations.
Abstract: This paper reviews ecological and paleoecological information for a small isolated mangrove
communtiy in NE Venezuela to explain its past and present features. Paleoecological data was provided by
radiocarbon dating plus pollen and fossil wood analysis of samples from an 11 m deep boring section.
Current data were obtained from vegetation characterization plus soil and hydrochemistry analysis. The
oldest dated sediments are 6960 ± 70 14C years old. They were deposited on an extensive mangrove fringe
dominated by Rhizophora sp. and Avicennia sp., and the sea-level was about 9 m below the present one.
Relative sea level has experienced an average rise of about 13 cm/100 years, which disconnected this mangrove
from adjacent communities. Avicennia disappeared and Rhizophora became the only dominant mangrove.
Today, the mangrove community lies about 500 m inland, surficial hydrological connections to the sea
are lacking, and the intermediate area is occupied by coconut plantations. Partial isolation from the sea has
favored the existence of a seasonal low-salinity marsh, maintained by runoff and a marine groundwater
wedge, which produce eutrophic water conditions and favor the establishment of continental and halophytic
terrestrial vegetation. This mangrove community may disappear in a few decades if isolation and human
disturbance continue.
Abstract: This article documents the application of high-impact palynology
(HIP) in the Maracaibo Basin of Venezuela and its influence on such
exploration and production aspects as regional planning and strategies,
risk reduction, optimal drilling decisions and investment, petroleum-
system modeling, new discoveries, and secondary recovery
by fluid injection, among others. High-impact palynology has been
defined as the coupling of high-resolution sequence biostratigraphy,
multidisciplinary work, and the alignment of palynology with the
attainment of business goals. The first part of this article explains
the high-resolution ecostratigraphic methods used and the concept
of integrated work applied. The second part of the article shows
the results obtained in selected case studies, which illustrate the
advantages of HIP. Among the most relevant studies are high-resolution
ecostratigraphic frames at a basin level, timing between
structural trap formation and oil migration, differentiation of petroleum
systems in adjacent reservoirs, the concept of palynoblocks
in structurally complex areas to estimate missing sections, stratigraphical
models for exploratory wells with better predictions of
target horizons, fine-scale reservoir correlations, and discovery of
new reservoirs. The use of HIP in other areas is recommended, with
palynology as a common in-house practice within multidisciplinary
teams formed especially for each specific task.
Abstract: The quantitative reconstruction presented in this paper documents
paleoecological trends of the northern Maracaibo basin (western
Venezuela), during the Early Miocene, through pollen analysis of
drill core samples from the La Rosa (shallow marine) and Lagunillas
(coastal plain) formations. Palynological assemblages were
grouped statistically into three assemblages representing mangroves,
herbaceous back-mangrove swamps, and inland palm/
fern swamps. Mangroves were the local vegetation during the
deposition of La Rosa Formation and, after a regressive event,
were replaced by herbaceous back-mangroves during the sedimentation
of Lagunillas Formation. The palynological assemblage
representing palm/fern swamps dominates the entire sequence
and is interpreted as a large background signal reflecting
transport by rivers. Mangrove communities had few species, and
represented a transitional phase in the mangrove community
evolution, after the terminal Eocene biotic crisis.
Abstract: Palaeo-biogeographical knowledge of the monosulcate Tertiary form-genus Mauritiidites can be useful
to study important palaeoecological and evolutionary questions in the Neotropical lowlands. However,
the lack of taxonomic diVerentiation within this fossil pollen genus is a handicap, and a broad
morphological and morphometric study, both in space and time, is needed to establish a consistent
Mauritiidites taxonomy.This note shows a morphometric characterisationof early MioceneMauritiidites
from western Venezuela, as a ® rst step. Morphologically, the specimens studied are consistent with the
description of Mauritiidites franciscoi (van der Hammen) van der Hammen & GarcÃõ a de Mutis 1966.
Light microscope measurements show little variability in most of the parameters, except for the aperture
width. However, this is considered part of the natural variability within a single pollen type, which is
commonly found in the pollen of several modern palm genera and species. As a result, a single
morphological and morphometric pollen type is proposed for the specimens studied. Future studies
should characterise more Mauritiidites specimens from other regions and time slices, to found
taxonomically-meaning diVerences.
Abstract: This note presents a preliminary reconstruction of the relative sea-level rise which ocurred on the Caribbean coasts of Venezuela during the last ca. 12,000 years (12 kcal BP), based on existing radiocarbon dates from buried coals and nearshore peats. The curve obtained is considered to be useful for several purposes related to Global Change studies and coastal ecosystem management.
Abstract: A quantitative palynological study of the Paleocene/Eocene
transition in western Venezuela was undertaken to detect
and analyze possible cyclic patterns. Two different methodologies
were used, palynocycles and ecologs, and their results
are compared. A total of 237 outcrop samples from
three formations deposited in continental to coastal environments
were analyzed for pollen and fern spores.
Several palynological cycles are recorded and correlated
with third-order global eustatic cycles. A high-frequency cyclicity
of ca. 220,000-year period also was found. Both
methodologies recorded the same cyclic patterns and can be
considered complementary. Ecologs are easier to use, but
have less interpretative potential. Palynocycles are more
complex, but also more descriptive and help detect small hiatuses.
Diversity values reach maxima at cycle boundaries
and their minima in the middle of cycles. This distribution
has been interpreted in terms of different palynomorph
sources under conditions of high and low sea level. During
the Paleocene/Eocene transition, diversity shows a constant
ascending trend, probably due to a long-term, worldwide
climatic warming.
Abstract: A chrysophyte cyst assemblage from surface sediments of a tropical, brackish-water environment in the southern
Caribbean coast is described and depicted. All the cysts are unornamented and relatively large. The taxonomic diversity
is low, due probably to human disturbance or salinity stress. It is assumed that the chrysophyte taxa involved
are salinity-tolerant, freshwater algae. This is the first record of chrysophyte cysts in mangrove environments, and
extends the range of habitats in which they are commonly found.
Abstract: The purpose of the present report is to document the occurrence of a Holocene mangrove community on the southern Caribbean margin, and to build up a preliminary sea-level curve for Venezuela. This is accomplished through pollen analysis of a buried coastal peat layer at 9.4 m depth, as well as anatomical analysis and radiocarbon dating of an overlying wood fragment. The pollen assemblage from the peat layer is analogous to those found in present-day Caribbean mangroves. The wood comes from a 6,960±70 year old Rhizophora tree. This is used as evidence of a in situ mangrove community and, consequently, of the position of sea level at that time. A tentative curve for sea-level rise is presented, by compilation of radiocarbon dates on mangrove peats and corals. This study is a contribution to the evaluation of recent sea-level increase connected to the global warming, its main value being the possibility of recognizing the natural trend in sea-level rise, and separate it from the human-induced effect.
Abstract: Fire has been considered one of the most important factors in the expansion of
savannas in the Gran Sabana region. In Urue´, an important fire event that occurred before
1.6 kyr BP led to the replacement of âprimaryâ forests by savannas and morichales (monospecific
communities of the palm Mauritia). In the present work, the secondary succession after fire is
reconstructed by palynological analysis of a previously dated clay core, and the results are
compared with those from studies based on present-day ecosystems. Charcoal analysis is used
to infer fire incidence and surface samples are used as modern analogues. The secondary
succession is subdivided into seven seral stages: open secondary forests, helechal or dense fern
community, transitional savanna, wet savanna with morichales, treeless savanna, wet savanna
with morichales, and treeless savanna. Fires were common at the beginning, but climate
constituted the main successional control from the transitional savanna stage onwards. The
process is characterised by a continuous impoverishment of taxa, but there was a steady increase
in Mauritia, due to its ability to colonise new habitats created by disturbances. The conclusions
of this palynological reconstruction show good correspondence with present-day studies on fire
ecology.
Abstract: A floral change occurring in northern South America at the Paleocene=Eocene boundary is analysed using palynological
data. The sequence studied is an outcrop from the Venezuelan Maracaibo basin, deposited in shallow marine to coastal
environments without apparent stratigraphic breaks. Significant pollen and spore counts from 237 samples were studied
stratigraphically and statistically in order to compare Paleocene and Eocene palaeofloras and palaeoecological trends.
The Late Paleocene=Early Eocene transition is the boundary between two floras which differ both qualitatively and
quantitatively. However, the change is not sudden but stepped and gradual. Paleocene taxa seem to be of pantropical
distribution, whereas Eocene assemblages are more restricted to the Neotropics. The global warming well documented
elsewhere is proposed as the major cause for these changes. Trends in sporomorph diversity in this record appear to track
changes in temperature documented in isotopic records from temperate regions. The extinct parent plant of Echitriporites
trianguliformis is tentatively proposed to be intolerant to high temperatures, because of its absence during the Early
Eocene warm phase. Palaeoecologically, although marsh and back-mangrove swamps dominated both Late Paleocene and
Early Eocene assemblages, their taxonomic compositions were different, especially in the inland marsh forests. Mangrove
components are scarce or absent through the whole sequence studied, suggesting the absence of these communities during
the time-interval analysed. A palaeoecological subdivision into assemblage zones was not possible; instead, a recurrent
pattern suggesting palynological cycles was observed. However, palynocycles could not be studied in detail due to the lack
of knowledge of botanical affinities for many of the taxa involved and the apparent absence of mangrove assemblages.
Abstract: A summary is presented of palaeoclimatic
and sea level data from Venezuela
since the Last Glacial Maximum, as
background information for PAGES activities
in this country. In addition, the
possibility of future projects on the studied
sites and in new localities is evaluated,
within the framework of PAGES
guidelines. The review has four foci: discussion
of modern analogs for proxy
data, description of palaeoclimates of the
last glacial cycle (PAGES Stream II) and
of the last centuries (PAGES Stream I),
and new evidence for the postglacial sealevel
rise.
Several modern-analogue studies exist,
but they were focused to relate the main
environmental features and vegetation
types with associated sediments, and not
to calibrate proxies for high-resolution
records. Concerning the last glacial
cycle, most of the continental sites studied
are from high elevations. They have
provided a tentative palaeotemperature
curve, but the resolution is still low.
Lowland data are scarce, but they seem
to record mostly humidity changes. High-resolution
marine records are available
near the Venezuelan coast, showing the
principal world-wide late glacial climate
oscillations, as for example the Younger
Dryas. However, a detailed Holocene
palaeoclimatic record is still lacking.
Correlation between continental and marine
records available so far are satisfactory.
Studies on the last millennium
have shown global oscillations such as
the Little Ice Age, and the influence of
man on ecosystems, but the main constraint
is again the low resolution of the
records. Finally, despite the few studies
available, a preliminary sea level curve
for the last 7 14C ka is presented.
As a general conclusion, the available
palaeoclimatic records are encouraging,
but not enough for predictive purposes.
New research projects are needed, with
emphasis on high-resolution analysis
(sampling and dating), as well as modern-
analogue calibration of proxies. Interesting
areas to be cored are high-altitude
Andean lakes and bogs, and Lake
Valencia sediments. Due to the lack of
laboratories and institutions devoted to
the study of Quaternary palaeoclimates
in Venezuela, international co-operation
is essential, and the role of official research
and technology agencies, such as
CONICIT, becomes fundamental.
Abstract: The palynomorph content of surface samples from a sea-land transect in a small coastal basin was analysed, in order to characterise palynologically the sedimentary subenvironments and vegetation types. These results should provide the basis for the interpretation of further palaeoecological analyses on stratigraphical samples from the same site. The main vegetation types are mangroves, swamps, Cocos plantations and abandoned crops, arranged in a zonal pattern. All the palynomorph types found were recorded and counted, including pollen, fern and allied spores, fungi spores and unknown remains (probably algal material and animal parts). Both general and individual trends of these palynomorphs followed the main vegetation patterns, and both zonal sea-land arrangement of plant communities and their disturbance patterns were reproduced by them. Furthermore, particular types were found to be direct or indirect indicators of given plant zones and local disturbance. Detrended correspondence analysis allowed definition of the sedimentary attraction domains for the involved communities and ordination of both samples and palynomorphs with respect to them. The results are encouraging for palaeoecological studies, since the main key environmental and vegetational patterns are reflected by the palynomorphs of surface sediments. Limitations are also discussed.
Abstract: The available palynological evidence allows reconstruction of the principal trends in the historical biogeography and evolution of the neotropical palm genus Mauritia. Its pollen is known from the Palaeocene, and has been widespread throughout the entire neotropical region, during the Tertiary. From the Late Miocene until the Pleistocene, populations have been progressively fragmented by the Andean orogeny, thus promoting local extinction in some smaller basins and allopatric speciation in others. During the Pleistocene, Mauritia survived the repeated dry and cool periods characteristic for glaciations either by restricting its distribution to assumed Amazon forest ârefugiaâ or by forming large-scale, heterogeneous forest communities. After the last glaciation, palynological studies document a broad dispersal process of Mauritia, leading to its present range. However, Holocene climatic shifts, together with human disturbance, created a heterogeneous distribution pattern with spatial and temporal differences within this process. Therefore, biogeographical inferences based on present-day observations can only account for the most recent changes and are not suitable for generalizations. The history of Mauritia derived from the fossil record suggests opportunities for both vicariance and allopatric speciation, and reveals dispersal patterns through time. The presence of Mauritia in sediments can also be used as a reliable indicator of warm tropical lowland environments flooded by fresh (occasionally oligohalyne) waters. Several hypothesis about its biogeographical history and its palaeoecological implications are offered that can be tested with future studies.
Abstract: Palynological analyses of fluvio-glacial sediments from Mesa del Caballo (3500 m a.s.l.), in the Venezuelan Andes,
allowed reconstruction of palaeoenvironments of high altitudes close to the Last Glacial Maximum. A stadial, called
E1 Caballo, is reported at 16.5 ka B.P. The estimated average temperature was around 7°C lower than today, and
coincides with the Colombian F~quene stadial and the global oxygen-isotopic Stage 2. The rate of sedimentation of
the whole Mesa del Caballo sequence has been up to 50 times higher than others in the Venezuelan Andes. This rate
declines exponentially with altitude for a range of sites, and is probably related to the amount of erosion and the size
of catchment area. Also, a direct relationship exists between sedimentation rates and the age of sediments. To explain
this it is necessary to take into account the decreasing age of sediments with increasing altitude, because of the
removing of older sediments by glaciers during the LGM, combined with the sequential filling of rock cavities upwards
during the deglaciation. The average velocity of altitudinal glacier retreat since El Caballo stadial until the present
day has been 7.3 cm/yr. It has not been constant through time. The maximum occurred in the last centuries, after the
Little Ice Age (Piedras Blancas cold phase), and the minimum after the Miranda warm phase, around 2.5 ka B.P.
Negative retreat values or readvance coincide with the La Culata cold phase. Taxonomic and autoecological studies
of unidentified palynomorphs, as well as palaeolimnological analyses (diatoms and sediment chemistry) will be
reported elsewhere.
Abstract: As a part of a general project that aims to reconstruct the
paleosuccession of Paleogene mangroves of the Maracaibo
Basin, this paper deals with the quantitative reconstruction
of Middle Eocene mangrove communities, and their relation
to potential forcing factors. Four palynological assemblages
were found. These represent, respectively, inland forests
(A1), back-mangrove herbaceous swamps (B1), mangroves
(B2), and an unknown plant community dominated
by the extinct Echitriporites trianguliformis. Mangroves
were dominated by Pelliciera and Nypa; Brevitricolpites
variabilis, which has been considered the dominant taxon
of the early and middle Eocene mangroves in nearby areas,
has not been found in this study. The succession of coastal
vegetation, linked to sea-level changes, could be reconstructed
from these assemblages. The trends constitute a palynocycle
which began and ended with a low sea-level plant
community dominated by unknown stands represented by
E. trianguliformis and interpreted low paleosalinities; intermediate
high sea-level vegetation is represented by mangroves
and interpreted high paleosalinities. This cycle is
correlated chronologically with the global eustatic cycle TEJAS
A 3.4, extending from44 to 42.5 Ma (Lutetian). The floristic
composition of middle Eocene mangroves was very
different from those of the Oligocene to Recent. An important,
probably worldwide, evolutionary change occurred
during the late middle Eocene and the late Eocene in these
communities. Pollen taxa botanically related to known and
extant mangrove elements seem scarce for this time span.
Abstract: The aims of the present study are to document the palynomorphs other than pollen and pteridophyte spores encountered in the Last Glacial alluvial and lacustrine sediments from the Venezuelan Andes, and to test their potential paleoecological and paleolimnological values using taonomic and statistical approaches. A total of 28 fungal spore types, 7 forms of remains of aquatic organisms, and 30 unidentified morphological classes are reported. Seven funagl spores forms follow the same stratigraphical trends as the pollen from páramo vegetation, suggesting that they are controlled by similar environmental (climatic?) factors, or that the parent fungi depend on these vegetation to develop. Five other forms show an inverse behaviour, their maximum coinciding with the minimum estimated temperatures, during the El Caballo stadial. It is proposed tentatively that these two fungal assemblages are inversely related to temperature, and could be used as paleoeclimatic indicators. Remains of aquatic organisms allowed following the local trends in water balance. A glacial paleolake with well-developed littoral communities existed in front of the terminal moraines before the stadial maximum. The subsequent incoming of a colder climate determined the impoverishment of shore communities and finally, their disappearance, so that during the stadial only cold-water tolerant plankton could develop in the lake. The lake dried out immediately after the stadial maximum. Seven unidentified forms follow the same trends as the littoral indicator remains, suggesting that they come from organisms of this habitat. Two other of these forms fit well with pelagic conditions, and two very similar to bryophyte spores are statistically associated to bog representatives. Proposed studies for the future are 1) a characterisation of modern sediments from different environments by their total fossil content, 2) increased documentation and identification of unknown forms by comparison with living material, and 3) macrofossil and geochemical analysis of both sediments and living material.
Abstract: The present paper reviews the palynological studies developed on modern and Quaternary sediments from northern South America, in order to plan future paleoecological work. The present coastal flora and vegetation as related to sedimentary environments and human disturbance are also reviewed, because they are needed to interpret the spatial and temporal patterns of palynological assemblages. Most of the modern analog studies developed so far, are oriented to find a general relationship between palynomorphs in sediments, potential vegetation sources and coastal environments. Detailed studies are scarce, and quatitative calibrations of proxies are lacking. Concerning Quaternary studies, most records embrace the last glacial cycle, and the emphasis is placed on the last glaciation and the climate of the Amazon lowlands during the Last Glacial Maximum. In the coasts, a characteristic, symmetric and recurrent vegetation succession has been recorded during the glacial regressive and transgressive phases. Holocene pollen records are interpreted mainly in terms of sea level rise after the last glaciation. However, detailed studies on the last two millennia, useful for prediction purposes in a Global Change perspective, are lacking. Future work suggested include: 1) detailed modern-analog local studies, preferably near the coring sites, 2) calibration of proxies, especially for parameters such as temperature, salinity and distance to the coast and 3) high-resolution studies on shallow cores, accurate dating and isotopic analysis to provide temperature, sea-level and human-disturbance record during the last centuries.
Abstract: The pollen of 17 genera (19 species) of Moraceae, Ulmaceae and Urticacece from the Venezuelan Andes are described, in order to facilitate their identification in palaeoecological studies. A subdivision into four main morphological groups based on the shape and aperture number is proposed. Furthermore, a morphometric study of the most significant parameters (polar axis, equatorial diameter, pore diameter and exine width) is carried out to separate species within the same morphological type. This study should be extended to the remaining species of Venezuelan urticales.
Abstract: Ecostratigraphy or the stratigraphy opf ecosystems deals with evolution at the community level, in order to apply the ecological advances to geochronology. The present note is an attempt to show the importance of ecological events in chronostratigraphy, despite their environmental component. Ecostratigraphy is a challenge to traditional biostratigraphy, but is a more realistic approach to the community behavior in time.
Abstract: The palynological analysis of an outcrop from the northern Venezuelan Andean flank show that the palynologically-derived Oligo/Miocene chronostratigraphy and paleoenvironmental succession is consistent with previous sedimentological studies for the section. The sequence studied was deposited in alternating coastal and alluvial plain environments, with a variable influence of brackish water. Brackish fern swamps and freshwater alluvial plain forest swamps were dominant, depending on the proximity of the marine shoreline. Paleosalinity was estimated by a ratio between freshwater algae and marine forms. However, freshwater algae Pediastrum and Botryococcus may also tolerate low ranges of salinity, and this factor must be taken into account in future studies. A Late Oligocene phase of increased tectonic activity, possibly related to the Andean uplift, is suggested. In agreement with previous surveys, a striking evolutionary change was observed in Eocene and post-Eocene lower coastal plant communities. The Olio/Miocene palynoflora of the region studied is similar to that of the rest of Venezuela.
Abstract: Palynological evidence currently used to determine the global eustatic cycles of Haq et al (1987) requires the existence of sediments from both continental and marine environments. The present work is a contribution to sequence analysis using sediments of continental nature. The boundaries of palynological zones help to locate and date sequence boundaries (SB) and maximum flooding surfaces (MFS); the SB is characterized by hiatuses or and/or sudden floral changes (type 1 SB) and barren intervals with oxidized kerogen (type 2 SB), while the MFS may have a few brackish or marine water representatives. When marine fossils are absent, only paleovegetational criteria can be used. In this case, multivariate methods are used to find the most reliable pollen associations. Among them, the coastal and the more inshore associations are selected to compute an index that estimates the position of the tidal limit (TLI). The lateral movements of this tidal limit are recorded and correlated with corresponding sea level fluctuations, thus inferring the SB and MFS from paleovegetational data. The application of these methodologies to two wells from the Maracaibo basin (western Venezuela) allowed determination of Maastrichtian to Eocene 3rd order cycles from the suprcycle sets Upper Zuni A and Tejas A, as well as reconstruction of the local paleogeography of the coastal area.
Abstract: Pollen analysis of several peat sections from the summits of Guaiquinima and Chimantá massifs (Venezuelan Guayana) allowed studying the response of plant communities to the Holocene climatic oscillations. No directional trends towards an equilibrated community were observed Modulated succession into the attraction domain (sensu Holling, 1973) of herbazal-type communities was the main process, and no substitutions by other communities were recorded In general, the responses to climatic disturbances have been very slow, except for the case of paramoid shrubland communities, which rapidly reacted to the general humidity increase initiated at about 2,500 BP. The diversity trends were practically always associated to changes in equitability, a typical feature of mature ecosystems. The dynamics of the communities studied suggest a continuous disequilibrium between climate and vegetation, and do not support the existence of a long history of biological constancy and geographical isolation. Furthermore, the origin and maintenance of high ecological diversity is compatible with the hypothesis of niche diversification in heterogeneous environments. Further studies are proposed in order to achieve detailed successional reconstructions of vegetation enabling the identification of the involved ecological processes and the characteristics of transient phases.
Abstract: Sediments of the Tacarigua coastal lagoon in northern Venezuela are rich in organic material. The lagoon and its sediments are therefore important as a modern present-day analog for the interpretation of palynofacies assemblages in clastic sediments that formed in tropical coastal environments. Samples representative of the most important subenvironments of the lagoon were collected and prepared with standard palynological methods. The organic materials were analyzed in terms of biological origin, source area and textural characteristics. The analytical results were evaluated with standard statistical techniques. Based on Principal Components Analysis (PCA) six sample groups are differentiated. Sample groups A and B are vey rich in humic gels and they represent the western part of the lagoon in which sedimentation largely reflects the influence of the River Guapo. sample group C contains both humic gels and degraded plant material. Group C represents the central part of the lagoon where sedimentation is influenced both by the River Guapo and the lagoon mouth. Sample group F contains sub-equal percentages of degraded algal and/or bacterial material, humic gels and degraded plant material. Group F is representative of the eastern part of the lagoon where autochthonous sedimentation is important. Sample groups D and E which have a high percentage of degraded algal and/or bacterial material, are representative of sediments that were laid down in close proximity to mangrove vegetation. I view of the varied character of the organic assemblages in these modern lagoon deposits, the interpretation in environmental terms of similar fossils assemblages is complex and requires a careful consideration of all the evidence.
Abstract: Palynological analysis of the Neogene Urumaco Through sedimentary sequence allowed dating and paleoecological characterization of several fluvio-deltaic sub-environments, a study only approcahed previously from micropaleontology. Sedimentation began in the earlier part of the early Miocene with the deposition of Agua Clara Formation. A subsequent regressive event (middle Early Miocene) started with the sedimentation of Cerro Pelado Formation and culminated at the end of the Early Miocene, with the shales of Querales Formation, which indicates transgressive environments. During the Middle Miocene, a new deltaic progradation occurred, and Socorro Formation was sedimented. Urumaco Formation was deposited during the Late Miocene in coastal, near-shore environment. The sedimentation of Codore Formation began with the El Jebe Member, and ended with the continental sediments of Algodones Member. Fluvial conditions were maintained during the deposition of El Vergel Member of san gregorio Formation (Pliocene). A marine invasion occurred later, during the sedimentation of Cocuizal Member, before the regression represented by the Rio Seco Member (Pliocene).
Abstract: Palaeoecological analysis of lake sediments from carinapay allowed reconstructing the temporal dynamics of a macrophyte littoral hydrosere during the last millennium. The results show that succession has been affected by environmental shifts, namely changes in the water level, in several ways. Nevertheless, the hydrosere persisted due to its resilience. This study is an example of modulated succession, and supports the hypothesis of continuous disequilibrium between the environment and the ecosystem, maintained by constant, slow and continuous oscillations.
Abstract: In order to consider all significant factors from principal components analysis in the interpretation of limnological data, we used the Andrews (1972) plots and the Frontier (1976) method to choose a suitable number of principal components. In this way, we obtained important information that previous analyses on the same data had not provided. The conclusions attained suggest the direction of future studies. We propose the combined use of those methods as a means to increase the information obtainable from principal component analysis in limnology.
Abstract: Eldredge & Gould (1972) interpreted the hiatuses of the fossil record as potential evolutionary gaps. In this way, most of them would indicate phases of geologically instantaneous speciation events, followed by phases characterized by the absence of change or "stasis". This scheme was very different from those of darwinian phyletic gradualists, and challenged the role of natural selection in the evolutionary process. The emergent hypothesis was called Punctuated Equilibrium, and suffered the attacks of the darwinian orthodoxy, expressed as the three typical stages of K. von Baer: first, the new ideas were considered false, later heretic, and finally trivial and well-known. However, they are some fundamental differences between the Punctuated Equilibrium and the Modern Synthetic Theory of Evolution. It is possible that an objective consideration may lead to a more general evolutionary interpretation, but both tendencies must reconsider some points: the neo-Darwinians seem too much dogmatic, and the "punctuationists" offer unsatisfactory explanations for their general views.
Abstract: Maastrichtian to Pliocene/Pleistocene pollen zones were found in a sedimentary sequence from the Maracaibo Basin. Two hitauses account for the lack of Early/Middle Eocene to Oligocene, and Early to Middle Miocene sediments. On the other hand, three pollen assemblages were found in the Misoa Formation (Early and Middle Eocene): alluvial/coastal plain transition (represented by P. medius and M. franciscoi), back-mangrove swamp (D. adriennis, M. annulatus and M. franciscoi) and Nypa mangrove (S. echinatus, D. adriennis and R. amapaensis). The Middle Eocene expansion of the second one suggested a climatic change to more seasonal conditions. Furthermore, a hypothetical second mangrove assemblage, dominated by E. trianguliformis, is proposed as a working hypothesis. Paleovegetation and paleosalinity indices were computed through the scores of the principal components and abundances of aquatic microfossils, in order to compare them and infer eustatic events. The coincidence of inland ecosystems and freswater algae was considered a regressive phase and its peaks were correlated with SBs, whereas the co-occurrence of coastal ecosystems and brackish water algae was interpreted as trangressive indications, and the corresponding peaks were correlated withFSs. In this way, the 3rd order cycles TEJAS A 2.9, 3.1 and 3.3 were found.
Abstract: Three Holocene palynological records from the Gran Sabana region were studied from an ecological perspective, in order to examine and to compare the responses of plant communities to external disturbances, i.e. climatic changes and fire. Attraction domains were defined using modern surface samples to build a multidimensional space, in which successional trends (deduced from palaeoecological data) were plotted. Both dryness and fire produced the substitution of forest by open savanna, but at different time scales. Forests did not recuperate at any time interval of this study, though a humid climate returned. In its place, morichales (swamp communities dominated by Mauritia flexuosa [Palmae]) developed in a general landscape of open savanna. More detailed palaeoecological stidues using fine resolution techniques, with emphasis on transitional phases, are recommended.
Abstract: The statistical relationship between chrysophycean cyst abundances and ecologically known factors, derived from multivariate analyses, is proposed as a useful way to derive palaeoecological information of non-identified cysts. The method is applied to a case-study from the Spanish Pyrenees, and encouraging results are obtained for some of the morphotypes found.
Abstract: The present work consists of the pollen analysis of 15 Holocene sedimentary sequences and 36 surface samples of the "Gran Sabana" and the summit of 3 "tepui" (table mountain) massifs located in the Venezuelan Guayana. The main purpose is to collect direct evidence which could help to reconstruct the climatic history and resolve the biogeographical and ecological questions about this tropical region (origin of the tepui flora, existence or not of a Pleistocene refuge, high ecological diversity, origin of the "Gran Sabana", among others). The results obtained indicate the existence of: 1) climatically arid or semi-arid phases before the Holocene and before 5,000-6,000 years BP, 2) great environmental heterogeneity until some 3,000 years BP, and 3) more or less generally increasing humidity afterward, until some 1,000-1,5000 years BP, when the climate became similar to the present one. These results point towards a mixed origin of the tepui flora (autochthonous and allochthonous), favored by vertical migrations of ecological belts in time. Also, the climatic variability of the Holocene prevents thinking of the Pleistocene in terms of a climatically constant environment and, therefore, of the presence of biotic refuges in this region. The high ecological diversity of the area may be explained by the maintenance of a constant instability between the environmrnt and the ecosystems, processes most likely caused by phenomena of "intermediate perturbations" or continuous change. Climate (aridity) as well as fire caused the irreversible disappearance of the gallery forests of the "Gran Sabana" which are substituted by savannas with palm swamps ("morichales"). The succeeding repetition of events of this type could have increased the size of the savannas in relation to the forests. Direct evidence of human activities has not been found. Finally, some possible lines of investigation for the future, suggested by these results, are proposed.
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Abstract: A review showing the potential contribution of Quaternary palaeoecology to the ecological
theory, focused on the ecosystem evolutionary processes, is presented. By analyzing oceanic
and continental Pleistocene and Holocene records, some reflections about ecological succession,
diversity, rhythms, predictability, stability, and modelling are made, and compared with theoretical
statements derived from neoecology.
As a general conclusion, the necessity of considering palaeoecological findings in ecological
theory is emphasized. In addition, it seems essential to place palaeoecology in a more ecological
framework.
Abstract: In order to determine the sequential prevalence of pollen grains and fungi spores in the city of Caracas by the volumetric method of collection and its effect on the population at risk, the first multidisciplinary team of aerobiological research was structured through a protocol designed for five experimental stages namely: a) Determination of the climatological parameters: temperature, relative humidity, speed and wind direction, precipitation and atmospheric stability, through simultaneous records of meteorological status located in the metropolitan area of Caracas; b) Collection, identification, classification and determination of the local distribution of the most important pollen grains existing in the zone under study; c) Collection, identification and classification of the most important fungi spores in the area; d) Preparation of the first pollinic calendar of Caracas and e) Evaluation of possible implication of the environment and the response through the IgE antibody in the selected patients in the area under study. Our results show: 1) The climatic conditions existing during the sampling period coincide with the analysis of the last 20 years in the Valley of Caracas. 2) The Venezuelan Central University (UCV) station was elected as the most representative point for permanent sampling. 3) An ideal statistical method is obtained in order to determine the spatial arrangement in the sampler rod of a dense type of fungi spores typical of the area under study. 4) The first pollinic calendar of Caracas was structured and 5) A seasonal tendency of the IgE response is shown. These results suggest an evident interrelation between the suggested concept of polyseasonality and antigenic polysensitization, and between individual seasonality with a specific reactivity and, finally, between tropical mixed seasonality with the expression of combined respiratory pathologies in our environment.
Abstract: The pollen analysis of a peat bog in Páramo de Piedras Blancas
altlowed the identification of a lowering of the Andean
vegetational belts, approximately between the 15 th and the
16 th centuries. In this note, we propose that temperature
lowering was the cause of this lowering and corre late this event
with the Little Ice Age, on the basis of radiometric, geomorphologic,
archaeologic and historical evidence. In addition, we
estimate that the mean temperature lowering must have been
around 2º C.
Abstract: The effect of some common palynological preparation methods on the size of the Kochia scoparia pollen was studied. The "swelling" effect of both glycerol and glycerin jelly agreed with previous works, as well as the small size change in silicone oil preparations. The storing in silicone oil preserved size after five years, showing its suitability as reference size procedure.
Abstract: In order to search an uniform criteria for sampling aerobiological particles, three sites were selected along the valley of Caracas. The distribution of vegetation, topographical and meteorological parameters were monitored. Pollen and spores were collected at each site during 24 hr at 6 min-intervals with a Rotorod sampler. Pollen counts were similar in all sites and much less abundant than spores. Cladosporium was the most abundant spore and grass pollen represented more than 50% of the pollen grains. Results showed that the three sites could be selected as suitable sampling area for airborne pollen and spores. Nevertheless, we recommend the selection of the most accessible and central site for a long-term sampling.
Abstract: Venezuelan blackwater rivers show great variability of physico-chemical features, and a classification of them is attempted. Criteria for grouping rivers and river groups were obtained through multivariate statistics; similarity, divergence, and principal components analysis. Mineral content, organic matter, marine influence and weathering of igneous rocks are the most important criteria. Rivers can be classified into five different rgoups, related to geological, climatic, and vegetational features. Our classification can be refined further as more limnological and geochemical information becomes available, but we consider that the general criteria found here will remain valid.
Abstract: Palynological analysis of samples from a flucioglacial terrace at 3920 m elevation, in the periglacial belt of the Paramo Miranda, has shown that, before 11,500 to about 11,120 yr BP, the very few plants growing in the region belonged to the Gramineae, the Compositae (mainly to Coespeletia) and Montia. The climate was colder than at present. During the early Holocene the concentration of pollen and spores in sediments increased and new pollen types from paramo plants were incorporated. Pollen of the caryophyllaceae (mainly Arenaria), of Gentiana, Valeriana, Geranium and a few genera of Compositae reached the site. Long-distance pollen, derived from the cloud forest increased and Podocarpus pollen was more abundant than at present. These results suggest that temperatures increased at the beginning of the Holocene, the vegetation attaining a density similar to that of the present, althoguh with fewer species. From 5470 to 2500 yr BP, pollen and spore concentrations fluctuated and other types, including Rhizocephalum and Isoetes, followed by Draba, Plantago and certain other genera of the Compositae were now present in the sediments.From ca 3000 yr BP onwards the composition of the pollen assemblage was the same as at present. Two cold phases were detected, one from earlier than 11,500 yr BP and another from 6000 to 5250 yr BP. A warm phase from 2690 to 2500 yr BP probably represented a rise of about 1.2 ºC above the modern average temperature. In general terms, the climatic oscillations detected at Miranda are in accordance with those in other parts of South America.
Abstract: Pollen analysis of a peat bog at 4080 m elevation from the Páramo de Piedras Blancas has provided information on the climatic history of the last 13000 years. Before 1230+/-80 C-14 yr BP until ca. 940 yr BP (Phase I) the palynomoprh assemblage indicates a climate similar to the present day but the site was drier than today. From 900 to 740 yr BP (Phase II) temperature and humidity increased. From 700 to 380 yr BP (Phase III) there is a decrease in the abundance of all pollen grains and pteridophyte spores, besides the absence of arboreal pollen, indicating a retreat of the forest to lower elevations, and a very sparse vegetation close to the site. Probably a glacial desert prevailed at the site during this phase, and the climate was very cold. The pollen assemblage from 340 yr BP onwards (Phase IV) suggests an increase of humidity and probably a gradual rise of temperature. Correlation of this section with another peat core at a lower elevation in the same mountains is made. In general terms, the climate oscillations detected at Piedras Blancas find agreement in other parts of the world.
Abstract: The sample size in pollen analysis is discussed on the basis of three fundamental considerations: the percentage and concentration calculations, and the estimation of the diversity. The reliability of percentage and absolute calculations is not acceptable before about 300-400 counts. Furthermore, the necessity of considering not only the number of pollen types but also the evenness of its abundances is emphasized, and a diversity saturation criterion is proposed to determine the adequate counting size.
Abstract: Palynological analysis of two Venezuelan Andes peats provided climatological data on the last thirteen centuries. Both stratigraphic sections show similar vegetational and climatic sequences, starting with a cold phase, followed by a warming and wetter phase, and ending with a colder and drier phase and a marked lowering of the ecological altitudinal belts. This last interval started approximately in the 14th century and extended until at least the 18th century, which correlates with the Little Ice Age. In terms of arcaheological correlations, this event ocurred within the MucuchÃes phase, beginning around the middle IV Period and extending until the V Period.
Abstract: The identity of most of the resistence forms that belong to the Chrysophyceae is unknown. For this reason, their paleolimnological interpretation is limited. More than sixty different forms of these cysts have been found in a carstic depression in the Eastern Pyrenees. The cysts are described and classed. Only in five cases, we could establish a correspondence between the cyst and some living species. In order to explore the indicator value of the resistance forms, the relation between their abundances and the percentages of diatoms was examined. The conclusions of this analysis are still tentative and must be compared with the results of future studies. It would be useless to present the results in a stratigraphic sequence, because the sediment was disturbed by livestock. Nevertheless, we can infer that the characteristics of the temporary pond were constant during the deposition of the sediments. The small variations in the species composition can be interpreted in terms of an annual cylce, with sudden changes.
Abstract: A detailed account is given of the procedure required to estimate the number of grains in one milligarm of pollen. This enables the addition by weight of an accurate number of exotic pollen to sediments for the estaimation of absolute pollen counting, concentration and influx. It avoids the use of pollen tablets and the time-consuming pollen suspensions, and minimizes the error resulting fom the addition of exotic pollen.
Abstract: Secchi disc depths (Ds), and extinction coefficients of total (Kt) and filtered (blue, green and red) visible light were measured in 63 Spanish reservoirs. The best estimate of the constant relating Ds and kt is given by Kt = 1.75/Ds. Attenuation of green light (Kv) is low and regular and is the best predictor for both Ds and Kt. A classification of water types according to their light penetration characteristics was obtained. The statistical relationship between Ds and chlorophyll concentration is low, which is attributed to the effect of suspended particles.
Abstract: Studies about pollen sedimentation and its relationship to vegetation are essential tools for past vegetation reconstruction. This is the aim of this study, developed at Val dâAiguamòg (Val dâAran). The pollen content of moss polsters along an altitudinal transect from around 800 to 2600 m was analyzed, and compared with the local and regional flora and vegetation. Altitude resulted to be the most important factor to explain both vegetation and pollen gradients. Besides the complexity of pollen-vegetation relationships, regularities useful for paleoecological and paleoenvironmental interpretation were found.
Abstract: This paper reports the occurrence of a Middle Würm/Wisconsin interstade, called Pedregal, in the northern Venezuelan Andes. This is accomplished by high-resolution pollen analysis of a peat layer (PED5 VII), previously dated to >60,000 to 58,350 14C yr BP. Paleoclimatic trends are deduced mainly from changes in the abundance of tree pollen from the uppermost Andean forest. Previous calibration of this pollen with modern analogues allowed reconstructing past altitudinal displacements of montane ecological belts and temperature change. Paleotemperatures were deduced considering the present lapse rate (-0.6/100 m). After a glacier advance, represented by the underlying till, average temperatures increased up to 3 ºC below modern during the Pedregal interstade (>60,000 yr BP), and then dropped again to about 7 ºC below modern (58,350 yr BP). These trends agree with previous studies from the Colombian Andes, and fit well with the oxygen isotope curve from Greenland ice cores.
Abstract: The present paper reviews the results of paleoecological and paleolimnological analyses of a sedimentary sequence deposited during the Last Glacial Maximum, in the Venezuelan Andes. The sediment accumulated in a pre-glacial lake which existed until after 16 ka, when it dried. Pollen and algal remains show three intervals of high Andean vegetation (stadials), interrupted by two intervals of superpáramo (interstadials). One of these stadials was called El Caballo and dated to 16.5 14C ka BP. estimated temperatures were around 7 ºC lower than today. After that, the glacier retreat proceeded at an average of about 7.3 cm altitude/y, but with oscillations, due to Postgalcial and Holocene climatic shifts. A pre-glacial lake existed in the site prior to 16.5 ka BP, with well-developed pelagic and littoral communities. The latter disappeared during the colder interval, and only pelagic communities dominated by Botryococcus survived. The lake dried out just after the El Caballo stadial. A total of 28 unidentified fungi spores and 30 other unknown palynomorphs were documented and related to pollen and algae assemblages, in order to test their paleoecological usefulness. Several fungal spores resulted to be associated to the superpáramo vegetation, while others show an inverse trend, suggesting different temperature controls. Among unknown remains, a substantial number seem to be originated from aquatic organisms. The affinities of several of them with littoral, pelagic or bog assemblages could be established. Continued taxonomic work and intensive studies on modern sedimentation patterns of unidentified forms is proposed to improve their ecologocal characterization.
Abstract: Mangroves are forested ecosystems from tropical coasts, dominated by few tree genera resistant to the salintiy of the seawater and with special morphological and physiological adaptations, which determine a characteristic physiognomy. The evolutionary origin of mangroves is commonly associated with the first appearances of fossil pollen and spores from elements of these communitites, and has been placed on the lower Cretaceous. In the present work, the available palynological evidence is analysed, and an Eocene origin is proposed instead. The quantitative pollen analysis of a Paleocene/Eocene section for northern South America is examined as an example of a typicial evolutionary sequence. The first mangrove elements appeared in the Paleocene, but the dominant and characteristic components did not appear until the lower Eocene. However, their relative abundance was too low to account for mangrove ecosystems. The first palynological records of well established mangrove communities were found in the middle Eocene, where the more common occur and show characteristic abundances. The same trend has been observed in other localities from northern South America, suggesting that the first real mangrove communities in the region of Tethys appeared in the Eocene. On the other hand, the evolution of these mangroves has not been a gradual process. Stable phases have alternated with others of more intense speciation, expansions and reductions of the geographical range, and massive extinctions. An outstanding event in the evolution of mangroves took place in the Eocene/Oligocene boundary, coinciding with global mass extinctions, related with climatic changes. The main Eocene mangrove elements disappeared at that time and were replaced by others newly originated, from which the present mangroves derive. From the Oligocene onwards, a progressive taxonomic enrichment without remarkable extinctions has been the rule.
Abstract: Working on the K/T El Kef (Tunisia) type section, Keller (1988) found that some species of the subfamily Heterohelicinae, previously considered to be restricted to the Maastrichtian, enteded their ranges until the Danian. Further work on other classical K/T sections from Netherlands, Spain, Denmark and Mexico produced similar results. This work reports a similar analysis for different Venezuelan basins and biostratigraphic markers. Coinciding with previous obervatiosn elsewhere, several of the species analyzed, as for example Watznaueria barnesae and Micula decussata (calcareous nannoplaknto), Proteacidites dehaani (pollen), and some foraminifera (G. aegyptiaca and A. mayorensis) extend their ranges up to the Early Paleocene. This indicates that a significant group of taxa survived the K/T crisis at both margins of the Tethys sea.
Abstract: A total of 21 Middle Eocene to Late Miocene palynocycles were found in four wells from western Venezuela, using the method of Poumot (1989). The palynocycles are tentatively labeled as PCY (Palynocycle) forllowed by an ordinal, and correlated with the global third order eustatic cycles of Haq et al. (1987). Four palynocycles are presumably lacking, corresponding to the eustatic cycles TA4.3 (Late Eocene), TB1.5 and TB2.1 (Early Miocene), and TB2.6 (Middle Miocene). Future studies and the application of this scheme to practical problems are needed to test its udefulness.
Abstract: The area of the present Lake Maracaibo has been characterized by a succession of compressive and extensive phases since the Paleogene, which have reactivatedpre-existant structures. Here, the potential contribution of palynology to the unraveling of the tectonic history of the area is presented. A detailed palynological analysis of 65 wells from the central lake area allowed defining and mapping a series of "palynoblocks", which helped knowing the paleorelief before the tectono-eustatic peneplanation characteristic of the end of the Eocene.
