Abstract: The functional significance of web decorations in orb-web spiders has been an area of intense study for well over a hundred years. Two main hypotheses, (prey attraction and predator avoidance) have had intermittent support and criticism. By varying the decoration pattern, spiders minimize the potential predation costs of constructing a highly visible signal and deter potential prey such as bees from associating decorations with danger. The prey attraction hypothesis implies that as the signal changes, so should the response of the intercepting insects. In this study, I tested the response of bees to varying decoration patterns. I show that stingless bees (Trigona carbonaria) respond to the silk decorations of Argiope keyserlingi Karsch 1878 in similar ways irrespective of the pattern of decorations. I also demonstrate that the likelihood of prey hitting the capture area is greater than that of hitting the hub area in decorated webs. Since stingless bees respond similarly to different levels of signal strength, I conclude that variation in decorations does not affect prey interception.
Abstract: Stationary predators such as spiders can face competition from conspecifics simply by virtue of the spatial positioning of their webs. Shadow competition, wherein a predator 'upstream' restricts access to prey for another individual further 'downstream', can affect the foraging success of stationary predators. However, in spiders that build orb-webs in proximity to each other, insect prey often 'ricochet' off the outer web and land on the inner web. In this study, I asked whether the negative effect of shadow competition could be compensated for by the ricochet effect. I experimentally show that despite a strong spatial advantage to a spider on the outer side in terms of prey interceptions, the likelihood of prey intercepting the inner web is increased through the ricochet effect. I also show that the degree of overlap between the webs significantly influences both the number of prey intercepted as well as the number of ricochets. This study shows experimentally that a spider that builds its web close to a conspecific's web suffers very little cost in terms of lost prey interception.
Abstract: Abstract
In the course of their foraging bouts, bees frequently encounter spider webs among the vegetation. The ability to see and avoid these webs is vital for the success of the individual bee’s foraging bout. In this study, we report on the response of stingless bees (Trigona carbonaria) towards the webs of the St. Andrew’s Cross spider (Argiope keyserlingi). We studied the ability of bees to avoid webs in different contexts: when bees were on their foraging path or when they were returning to the hive as well as when they were flying North or South. We show that the probability of a bee being able to avoid a web depends on the context of the bee’s flight rather than the visual appearance of the web. Furthermore, the presence of the spider seems to alert the bee to the web, resulting in bees being more able to avoid capture. We show, specifically, that the probability of being captured is higher when the bee is returning to the hive compared with when the bee is foraging. The likelihood of avoiding a web is also influenced by the compass direction of the flight, although to a lesser extent. Our results indicate that the context of the predator–prey encounter has a significant influence on a bee’s ability to escape interception by a spider web.
Abstract: A long-running debate in the spider literature concerns the function of the extra silk decorations in some spider webs. These decorations are appended to the web and constitute a highly visible signal, which is inconsistent with the trend towards web invisibility. Despite the sustained attention of researchers, the exact function of these decorations is yet to be understood. While most studies have focussed on testing particular hypotheses, there has been a dearth of natural history data regarding web decorations in field conditions. In this study we present baseline data regarding the influence of seasonality, microhabitat characteristics and ecology on the presence of web decorations in an Australian orb web spider, Argiope keyserlingi. In particular, we show that there is preference among spiders to build their webs between bushes and to face the south-east, but this preference does not influence decoration building.
Abstract: Many animals have conspicuous body colour that may serve physiological, camouflage or communicative functions. This study investigated the influence of bright coloration in orbweb spiders on the response of predator and prey using Argiope keyserlingi, the St Andrew's Cross spider. This species has three conspicuous yellow bands on its dorsal abdominal surface. These bands could act as camouflage devise through disruptive colouration or attract prey to the web by exploiting colour preferences in the insect visual system. In the field, naturally yellow spiders captured more prey than spiders where the yellow bands were coloured over with black marker. Similarly, some prey (Harlequin beetles: Tectocoris diophthalamus) moved towards yellow spiders and away from blackened spiders in Y-choice tests. However, native bees (Trigona carbonaria) did not seem to discriminate naturally coloured spiders at a distance when approaching a spider on a web or an empty web. Similarly, praying mantid predators (Pseudomantis albofimbriata) preferred blackened spiders over yellow spiders in a Y-maze, but they showed no preference when offered an empty web and a web occupied by a naturally coloured spider. Thus our data suggest that the main function of the conspicuous yellow bands is crypsis, perhaps via disruptive colouration that obscures the outline of the spider.
Abstract: Red lists of taxa are important documents guiding the prioritization of conservation efforts. However the actual process of arriving at red lists has been contentious, because of the paucity of hard ecological data. In this article, we examine the red listing of plant species at two geographical scales: regional and local in the Western Ghats, India. At the regional level, we compared the rarity (or abundance) of the red listed and the co-occurring non-red-listed species in fifteen sites across Western Ghats. For the local level analysis, we compared the distribution and demographic profile of red-listed medicinal plants with co-occurring non-redlisted species at two field sites in the Western Ghats. At both the regional and the local scales, our analyses showed that the red-listed species, as a group are not any more disadvantaged than the non-red-listed species. Our results lay caution on the process of inclusion of species in red-lists and urge the necessity of strong field data to make the red-listing process more robust.
Abstract: Although conservation and management of tropical ecosystems requires that we understand the threats to these areas, there are no standardized methods to quantify threats to ecosystems. We used a geographic information system-based protocol with several physical and socioeconomic attributes to assess the threats to a protected area, a wildlife sanctuary in southern India. Physical attributes included threats from major and minor roads and the accessibility of an area (given as inverse of the slope of the area), and socioeconomic attributes included the number of human settlements and human, cattle, and sheep populations. We divided the sanctuary into 30-ha grids, and for each grid we computed three threat categories: (1) settlement-associated threat from humans, cattle, and sheep; (2) development-associated threat resulting from major and minor roads; and (3) accessibility-related threat caused by the steepness of the terrain. Combining all three threats, we derived a composite threat index for each grid and mapped five levels of threats in the sanctuary. We collected data on human activities, tree species richness, and diversity in the transects laid in areas corresponding to these five threat levels. Although the threat levels of the transects were strongly correlated with the human-related disturbance activities, the composite threat indices of the transects were negatively correlated with tree species richness, indicating that the threat values we derived served as a good surrogate of the actual threat experienced by the sanctuary. With appropriate modifications, the protocol developed here can be applied to other ecosystems as well.
Abstract: Non-timber forest products (NTFPs) constitute the single largest determinant of livelihoods for scores of forest fringe communities and poor people in the tropics. In India over 50 million people are believed to be directly dependent upon NTFPs for their subsistence. However, such human dependence on NTFPs for livelihood gains (win) has most frequently been at a certain ecological cost (lose). If livelihoods are to be maintained, the existing 'win-lose' settings have to be steered to a'win-win'mode, otherwise, there could be severe erosion of the biological resources and loss of livelihoods ('lose-lose'). Examining the dependence of forest fringe communities on NTFPs at three sites in south India with contrasting human and ecological settings, three key factors (extent of dependence on NTFPs, indigenous ecological knowledge and market organization) are likely to constrain reaching the win-win situation. How these factors shape the ecological cost of harvesting NTFPs at the three sites is examined. Within the parameter space of these factors, it is possible to predict outcomes and associations that will conform to win-win or win-lose situations. Empirical data derived from the three study sites demonstrate the causality of the observed associations. The key for long-term livelihood gains lies in reducing the ecological cost. Certain interventions and recommendations that could optimize the balance between livelihood gains and ecological cost are proposed.
Abstract: Cyrtophora cicatrosa weaves a three-dimensional dome web and is commonly found in thorny bushes and cacti. The factors that could be responsible for this selection were studied, both by field observations and laboratory experiments. Structural requirements in terms of space availability were found to be an important factor in the selection of the web site, though other factors could be significant. The relationship between C. cicatrosa and its immediate environment is discussed.
