Abstract: This book provides a reference guide to 450 major invasive plant species. Each species has an entry providing information covering life form, synonyms and commercial use, geographic distribution, habitats invaded, description of morphology, ecology and control. Key references to each species are also provided. The book will provide essential reading for researchers and practitioners in plant ecology, nature conservation and weed science.
Abstract: Invasive alien plants tend to have a greater specific leaf area and more nutrient-rich tissues than the invaded native vegetation. To test whether these traits differ between introduced and native populations of the same species, we compared 20 European (introduced) and 22 American (native) populations of Solidago gigantea Aiton (Asteraceae) in a common-garden experiment.
Five plants per population were grown for 2 years in pots and for one summer outdoors in nutrient-rich soil. We recorded shoot number and biomass, leaf production and senescence, flowering, leaf morphology and nutrient concentrations of leaves and litter. In laboratory assays, we compared litter decomposition and nutrient mineralization.
Shoot growth and leaf traits varied three- to 10-fold among the 42 populations. European plants produced, on average, more shoots than American plants, but did not differ in shoot size, leaf traits or litter decomposition.
The shoot number and total shoot biomass per plant in the experiment correlated positively with the number of new rhizomes produced by shoots of the same populations at their original field sites.
We conclude that introduced S. gigantea populations tend to produce more shoots through clonal growth than native populations. This may increase their ability to compete against the established vegetation in dense stands or at nutrient-poor sites.
Abstract: We investigated habitat use of the common warthog (Phacochoerus africanus) on a large, recently abandoned ranch in Tanzania. Fifty years of ranching had heavily modified the savanna vegetation, especially close to the paddocks where cattle were kept overnight. We recognized 4 characteristic vegetation zones along transects radiating from these paddocks: paddock center (PC), paddock margin (PM), acacia scrub (AS), and unmodified savanna (SV). Within these zones, we recorded warthog traces such as footprints, dung pellet groups, and diggings, and used these data to calculate presence and activity indices. Warthog traces were more abundant in the PM and AS than in PC and SV zones, and during dry periods the presence and activity indices were highest within and close to paddocks. From our results, we concluded that 1) warthogs are pioneer species, being one of the first native ungulates to recolonize cattle-impacted savanna vegetation; 2) areas close to former paddocks are particularly attractive to this species because of their vegetation structure and nutritional quality; 3) management to restore wildlife populations in former rangeland should initially focus on those areas where wildlife recolonization commences; and 4) indirect observations help to identify habitat-use patterns and point out key wildlife areas important for future protection.
Abstract: Weed problems continue to be an obstacle in plant protection and in natural areas, causing high costs of control and asking for adequate prevention measures. New weed problems in any given area (risk area) may arise from three basic sources: by introduction and naturalization of new weedy plant species, by increasing spread of weedy taxa that are already present in the risk area, and by the evolution of new weedy taxa in the risk area. Introduction and subsequent naturalization of new plant species is taking place worldwide and the number of alien plant species being introduced is steadily increasing. Newly recorded alien plant species prevail in disturbed sites and semi-natural habitats, increasing the risk for new plant invasions into areas of high conservation value. New weed problems in agroecosystems are associated with recently naturalized species, with adaptation and evolution of plant species, but also due to land use change, global warming, and changes in the environmental conditions. These may affect the dynamics of weed populations, triggering sudden range expansion and abundance increase. Evolution of new weedy taxa involves crop–weed hybridization, hybridization between native and alien plants in the risk area, and hybridization between formerly separated taxa that co-occur in the risk area. Evolution of herbicide resistance is another source of new weed taxa. A literature survey revealed more than 40 new weed species of agroecosystems within the last 20 years and about 35 new plant species invading natural and semi-natural habitats during the last ten years. Measures towards mitigating problems associated with new weeds includes regular monitoring and risk assessments, aiming at preventing entry of new weeds and prioritizing weed species for control.
Abstract: Solidago gigantea is a rhizomatous perennial herb native to northern America and introduced in Europe. It is a serious invader of disturbed mesic sites, often forming dense monospecific stands. We review the literature on taxonomy, morphology, population biology, cytology, physiology, chemistry, and ecology of this species.
Abstract: Lonicera henryi is an introduced climbing woody perennial that has become naturalized in scattered localities in Switzerland. Growth-related traits of this vine were measured at one forest site near Zurich to assess its invasion potential. Current-year shoots were 47-150 cm long, indicating fast growth. There were 128-366 shoots or stems and 0-96 rooted nodes per m-2 of forest floor, indicating considerable competition for other species. Species richness was significantly lower in plots covered by L. henryi mats compared to adjacent control plots. The results suggest that L. henryi has a considerable ability to cover forest floors with a carpet of intertwining stems, and to climb rapidly on shrubs and trees. Further spread of this vine may impede forest tree establishment and crowd out native species.
Abstract: A Europe-wide survey was conducted by sending questionnaires to weed scientists in order to evaluate currently troublesome weeds and those which may cause problems in the future. Recipients were asked to list species that are spreading and cause problems in agroecosystems, and to rate these according to three scores (degree of weediness, degree of spread potential, and degree of control success), with three levels for each score (low, medium and high). In total, 281 species were reported from 26 European countries. Most of them were annuals (48%), followed by perennials (34%) and biennials (14%). There were significant differences in weed scores among these life forms. Weed scores were unrelated to each other, implying that they have different meanings with respect to the biology of the species. Weed scores did not correlate with European range size, implying that they indicate the weediness of the species independently of the geographical distribution and can be used to prioritize weed species for management.
Abstract: The mechanisms that allow introduced plants to become invasive are poorly understood. Here, we present a test of the evolution of increased competitive ability hypothesis, which holds that because specialized natural enemies may be absent from the introduced range, exotic plants may evolve to invest less in anti-herbivore defenses and thereby gain a competitive advantage over native plants. We grew Solidago gigantea plants derived from both the native range (North America) and the invasive range (Europe) in a common garden in the native range for 2 years. Half the plants were treated with insecticide to protect them from insect herbivores and the other half were exposed to insects that colonized the garden from nearby fields. Insect herbivore biomass was significantly higher on European plants than US plants in the first year but not the second. European plants were more heavily attacked by pathogens in both years of the study. When exposed to insect herbivores, US plants produced more seed than European plants, but when plants were protected from herbivores, seed production was equivalent between US plants and European plants. The presence of insect herbivores suppressed seed production of European plants much more than that of US plants, even though the level of herbivory experienced by European and US plants was similar in the second year, suggesting that the ability to tolerate herbivory was diminished in European plants. These results partially support the EICA hypothesis: plants from the introduced range were more susceptible to some natural enemies and benefited more from insect removal than plants from the native range. The prediction that European plants would perform better than US plants in the absence of insect herbivores was not supported.
Abstract: Introduced plant species that became successful invaders appear often more vigorous and taller than their conspecifics in the native range. Reasons postulated to explain this better performance in the introduced range include more favourable environmental conditions and release from natural enemies and pathogens. According to the Evolution of Increased Competitive Ability hypothesis (EICA hypothesis) there is a trade-off between investment into defence against herbivores and pathogens, and investment into a stronger competitive ability. In this study, we conducted field surveys to investigate whether populations of the invasive perennial Solidago gigantea Ait (Asteraceae) differ with respect to growth and size in the native and introduced range, respectively. We assessed size and morphological variation of 46 populations in the native North American range and 45 populations in the introduced European range. Despite considerable variation between populations within continents, there were pronounced differences between continents. The average population size, density and total plant biomass were larger in European than in American populations. Climatic differences and latitude explained only a small proportion of the total variation between the two continents. The results show that introduced plants can be very distinct in their growth form and size from conspecifics in the native range. The apparently better performance of this invasive species in Europe may be the result of changed selection pressures, as implied by the EICA hypothesis.
Abstract: The spread of invasive plant species is of growing concern to managers of natural resources and ecologists. Most invasive plant species are exotic where they have been introduced and can have serious impacts on the invaded communities. Invasive plant species form a heterogenous group of plants with regard to their taxonomic position, ecological traits and modes of introduction. Most invaders have been originally introduced as ornamentals and for the purpose of erosion control. Therefore, horticulture plays an important role in exchanging plant species around the world and introducing them to new areas. The spread of potentially invasive plant species has been recognized and attempts to prevent such spread are discussed.
Abstract: A risk assessment system was developed to assess the invasion potential of new environmental weeds in central Europe. A pre-evaluation step excludes species that are officially controlled, widespread, or intended for use in protected cultures only. Species eligible for risk assessment are classified into three categories (high risk, further observation required, low risk) by rating them according to various biogeographical and ecological aspects. The rating system was validated by testing 47 well-known invasive plant species of temperate Europe and 193 exotic plants which have failed to establish themselves in Switzerland. The overall accuracy was 65%. Accuracy of correctly predicting invasive species was 77%, while accuracy of correctly predicting non-invasive species was 62%. The proposed risk assessment protocol should be understood as a first attempt for a European country and needs modifications. These can only be achieved by applying the system in practice.
Abstract: The homoclime approach has been used to estimate the potential distributions of the exotic goldenrods Solidago altissima, S. gigantea, and S. graminifolia in Europe. These three rhizomatous perennial plants were introduced as ornamentals about 250 years ago. Whereas S. altissima and S. gigantea have become widespread and serious invaders of abandoned fields, forest edges, and river banks, S. graminifolia is still confined to a few sites in Europe. Climatic profiles of their native ranges have been established for each species based on nine parameters from stations throughout their American ranges. I determined potential distributions for each species by mapping European stations that match the climatic profiles. The potential distributions of all three species are substantially larger than their current distributions in Europe. Areas not yet colonized but predicted to be climatically suitable included parts of southern and southeastern Europe, Scandinavia, and the Middle East. Among the three species, the potential range of S. altissima was largest in its latitudinal extent. The discrepancy between current and potential range was highest in S. graminifolia because of its restricted distribution. My results strongly suggest that the spread of S. altissima and S. gigantea has not yet reached its limits and that their range expansion will continue. Large parts of Europe were estimated to be climatically suitable to S. graminifolia, but invasion has not yet occurred. This species is either in lag phase or its spread is limited by factors other than climate.
Abstract: The increasing number of naturalized non-native plant species with a negative ecological impact on the communities where they grow (invasive species) is viewed as a major component of global change and is an important topic of current ecological research. In most regions of the world, the number of alien species is increasing as a result of trade, tourism, and disturbance, thus increasing the likelihood of plant invasions. Several international organizations have incorporated the invasive plant species issue in their main activities and have formulated guidelines for the management and eradication of invasive species. Switzerland as a central European country does not have as many invasive species as for example countries of other continents; however, some species are regarded as being invasive and are of special concern due to the highly fragmented and intensively used landscape. With the exceptions of the Alps, wildlife and areas of high conservation value are restricted to usually small areas, surrounded by heavily disturbed habitats or urban areas. In such places, invasive plant species may pose additional threats to the native diversity. Species of high concern are for example the north American Robinia pseudoacacia, Solidago altissima, S. gigantea, and the Asian species Impatiens glandulifera and Reynoutria japonica. In this article, the invasive species issue is highlighted with regard to the Swiss flora, and the needs for actions are discussed.
Abstract: The increasing number of invasive exotic plant species in many regions and the continuing alteration of natural ecosystems by humans promote hybridization between previously allopatric species; among both native as well as between native and introduced species. We review the ecological factors and mechanisms that promote such hybridization events and their negative consequences on biological diversity. Plant invasions through hybridization may occur in four different ways: hybridization between native species, hybridization between an exotic species and a native congener, hybridization between two exotics and by the introduction and subsequent spread of hybrids. The main harmful genetic effect of such hybrids on native species is the loss of both genetic diversity and of locally adapted populations, such as rare and threatened species. The spread of aggressive hybrid taxa can reduce the growth of, or replace, native species. The main factor promoting the formation of hybrids is species dispersal promoted by humans. However, the success and spread of hybrids is increased by disturbance and fragmentation of habitats, thus overcoming natural crossing barriers, and range expansions due to human activity. There are differences in flowering, pollination and seed dispersal patterns between parental species and hybrids. Hybrid resistance to pathogens and herbivores may also enhance the success of hybrids. To predict the mechanisms and consequences of invasions mediated by hybridization, extensive data on hybrid ecology and biology are needed, as well as carefully designed field experiments focused on the comparative ecology of parental populations and hybrids.
Abstract: There is an urgent need to list regional data on alien species diversity in order to explore worldwide patterns of species invasion. In this sense, developing countries have received little attention. We have characterized the alien flora of northern Africa with regard to the taxonomic composition, life history characteristics, geographic origin and habitats invaded. A total of 343 vascular plant species from 69 families non-native to the regions were found in the literature. Alien species richness ranged from 143 (Algeria) to 60 (Tunisia). Most of these were of Mediterranean and North American origin. Over 40% of the alien species were therophytes. Crop fields, dumps and wet areas such as fresh water streams were the habitats with the higest number of aliens. About 10 % of species in the alien flora are considered to be serious plant invaders in other places of the world. Among these species there are many shrubs and trees such as Acaciaand Eucalyptus. The impact of these species must be surveyed in the field.
Abstract: The species composition of the flora of any region changes with time as some species decline and new species immigrate. Within the last 150 years, we witnessed a rapid decline of rare species due to habitat destruction and a rapid increase in the number of non-native species introduced and becoming naturalized. The spread of non-native species into natural and semi-natural habitats is an important issue in conservation biology and will increase in its significance as more and more alien species are becoming naturalized. The presence of alien species changes the taxonomic and ecological composition of a flora, making different floras more similar, and leads to the spread of invasive species – fast growing species that become dominant where established and supress native vegetation. Whereas the extent of declining species is rather well documented, data on the extent of the alien flora are lacking for Switzerland. This paper gives an overview on the alien flora of Switzerland and explores trends in the taxonomic composition and geographic origins.
Abstract: Germination, growth, and physiological responses of hybridizing Carpobrotus from coastal California to soil salinity were studied. Hybrids are presumably the result of hybridization and introgression between the exotic Carpobrotus edulis, a succulent perennial invading coastal habitats, and the native or long-naturalized C. chilensis. Germination responses were investigated at 0, 10, 20, and 50% seawater. Seedling growth and physiology were compared by irrigating seedlings with solutions of the same seawater concentrations and in low and high nutrients. Germination was inhibited in the presence of salt, but recovered after transferring the seeds to fresh water. Seeds exposed to salt had higher final germination rates than control. Growth of Carpobrotus was slightly enhanced by low seawater concentrations but reduced at high salinity at both nutrient regimes. Leaf cell sap osmolarity increased with increasing soil salinity, and taxa did not differ significantly in this physiological adjustment. Leaf carbon isotope ratios (13C) ranged from -28 to -22 and became less negative at higher salinities, indicating an improved water use efficiency in the seedlings at high salt concentrations. In addition, 13C values were generally less negative at high than at low nutrients. Differences among taxa were generally small. The results show that salinity affects both establishment and growth of hybridizing Carpobrotus. The overall weak species differences in salt tolerance indicate that the exotic C. edulis can occupy the same sites as C. chilensis in terms of salinity. The similarity of hybrids in their response to salinity suggests that they may contribute to the invasion by Carpobrotus.
Abstract: Carpobrotus edulis (L.) N. E. Br., an exotic perennial that invades the coastal habitats of California, hybridizes with the presumed native Carpobrotus chilensis (Mol.) N. E. Br. To investigate the potential role of hybrids in the invasion process by C. edulis, we compared the growth and plasticity of clones of hybrids and parental species. Ramets of three hybrid classes and both parental species were grown in a common garden with four different environments. Both the parental species and the hybrids exhibited strong plastic responses to the test environments for characters describing clonal growth and physiology. Carpobrotus edulis consistently had the largest biomass, but C. chilensis produced longer branches. Carbon isotope ratios (13C) showed only weak differences among morphotypes. Hybrids were mostly intermediate relative to parental species in both growth and physiological characters. The intermediacy of hybrids in their response to environmental variation indicates that they do not have inferior growth compared with parental species. The high amount of plasticity of the hybrids and C. edulis, as well as their overall similarity, may account for their invasion success in different habitats of coastal California.
Abstract: The patterns of spread of the three exotic species Solidago altissima L., S. gigantea Ait. and S. graminifolia (L.) Salisb. after their introductin to Europe were investigated, based on herbarium specimens and literature records. The spread was analysed by mapping the localities for each decade since 1850. Cumulative numbers of localities as well as numbers of occupied grid squares showed a continuous increase since 1850 for all three species. The slopes, however, were significantly different among the species, Solidago gigantea had the highest colonization rate, followed by S. altissima, and finally S. graminifolia. The latter showed only a slight increase in abundance over time. The increase in diameter of the range in central Europe was logistic for S. altissima and S. gigantea with a rapid increase between 1850 and 1880. The spread of the species in area and time over Europe showed no clear front; new localities at large distances were simultaneously colonized. A large part of the actual range of S. altissima and S. gigantea was already achieved about 1950. It is assumed that the spread of the species followed the hierarchical diffusion model with several independent foci from which the species began to spread. The data suggest that S. altissima and S. gigantea will successfully spread further, leading to an increase in abundance and area, while S. graminifolia seems to spread slowly.
Abstract: Solidago altissima and S. gigantea were introduced from North America to Europe ~250 yr ago and are now considered aggressive weeds in abandoned fields and conservation areas. We studied patterns of genetic differentiation in these two species along their present latitudinal range in Europe (44-61 degrees N). Two generations of clonally propagated ramets from randomly selected genets of 24 populations of each species were grown in a common-garden experiment at latitude 47 degrees N from 1991 to 1992. Both species showed significant variation among populations in morphological and life-history characters: at this southern location, plants from northern populations were smaller and flowered earlier than plants from southern populations. The gradient of clinal variation was more pronounced in the second year of cultivation than in the first and was steeper in S. altissima than in S. gigantea. Within-population variation among genotypes was significant tot most characters in the case of S. altissima. Phenological rate (reciprocal of days to flowering) and size at maturity showed a significant negative correlation among populations bot not among genotypes within populations, indicating that genetic trade-offs may occur at one but not another infraspecific level. We suggest that the pattern of among-population variation reflects rapid adaptive population differentiation after introduction of the species to Europe.
Abstract: Patterns of flowering and pollination visit of Carpobrotus spp. were examined to determine the hybridization potential between the introduced succulent perennial C. edulis and its native congener C. chilensis in coastal California. Both species flowered from March to July and both were visited by the same Thysanoptera and Coleoptera taxa. Flowers from hybrid-appearing individuals were intermediate in corolla diameter. Emasculation and bagging experiments suggest that C. edulis and hybrid morphotypes have facultative agamospermia, whereas C. chilensis has facultative self-fertilization. Emasculated C. chilensis did not set seeds. Pesticide application and flower bagging suggest that 80% of seed set in self-pollinated C. chilensis flowers is mediated by thrips. It is concluded that there is high potential for natural hybridization in Carpobrotus species in California and that Carpobrotus populations can persist without cross-pollination.
Abstract: A geographic and taxonomic overview of the nonindigenous plant species of Europe, based on the 'Flora Europaea' is given. The flora of Europe includes 1568 species which have either expanded their ranges within Europe under human influence (naturalized European species) or are of non-European origin (exotic species). The latter group consists of 580 species (37%) which form a diverse group in terms of their taxonomic composition and geographic origin. The exotics are represented by 113 families, of which the Compositae, Rosaceae and Gramineae are most important. The ratio of species to families is low. Most exotic species in Europe originate from the Americas and Asia. Countries of southern Europe have a higher relative number of exotics in their non-native flora than northern ones. The species-range size distribution differs between naturalized European and exotic species; the latter are on average more widespread than the naturalized.
Abstract: The morphological variation of Solidago gigantea in Europe was assessed by scoring plants from 22 populations along a north-south transect for quantitative and qualitative characters. All size related and floral characters varied significantly among populations. Size related characters exhibited large coefficients of variation and high intraclass correlations. Floral characters varied considerably less and showed rather narrow unimodal distributions. Pubescence of the leaf main vein varied from glabrous to weakly pubescent. Leaf surfaces beneath as well as secondary veins were uniformly glabrous among the shoots. The first three principal components accounted for 56.5% of the variation and formed no separated clusters among the shoots sampled. Chromosome counts revealed only tetraploid cytotypes (2n = 36). The low variation in taxonomic key characters and chromosome numbers suggest that the tetraploid cytotype of S. gigantea has became naturalized in Europe, probably as the result of only a few introductions of this species.
Abstract: Morphological and life-history variation within and between 23 European populations of the introduced and widespread perennial Solidago canadensis L. sensu lato (Asteraceae) was investigated. The populations differed significantly in all 19 characters measured. Coefficients of variation and intraclass-correlations were high for characters related to growth. Floral characters (involucral height, number of disk florets, number of ray florets), and pubescence of stems and leaves varied considerably among sampled shoots, but their frequency distributions were unimodal and the coefficients of variation low. The first three principal components explained 48% of the variation and formed no clusters among the shoots. Several character means correlated with latitude, indicating clinal variation among the populations. Comparison of variation in floral and leaf characters with data from the native range suggests that S. canadensis s.l. naturalized in Europe belongs to a taxon that is close to S. canadensis var. scabra (Muhl.) Torr. & Gray (S. altissima L.), despite the reported diploid chromosome number of this taxon in Europe.
