We examined the influence of changes in the structure of the shrub layer in a desert habitat on the community of web-building spiders inhabiting this vegetation. Vegetation structure was modified to simulate changes predicted with increasing aridity. We predicted that changing the vegetation structure would affect the diversity and abundance of spiders by means of (1) changing available web supports and (2) changing the abundance of available prey. Using a randomized block design, we performed two manipulations: pruning the shrubs to one-half their height and thinning the plot to one-half its initial shrub density. We surveyed the distribution, abundance, and species identity of spiders before the manipulation, and twice after the manipulation. Potential arthropod prey were censused during the two post-manipulation spider surveys. We found no influence of the treatments on the potential prey. Six weeks after the manipulation, spider abundance was reduced significantly in plots of both treatments, and species diversity was significantly lower in the pruned plots. The reduced species diversity in pruned plots may be explained by the propensity of spiders of different species to construct their webs at different heights in the vegetation. Pruning selectively eliminates the crown of a shrub, and eliminates the potential web sites of species which prefer the crown, while thinning removes entire shrubs, eliminating web sites for all species of spiders equally. Neither spider abundance nor diversity differed among treatments in the second survey, ten months after the manipulation. We suggest that the lack of a treatment effect on spider species diversity is related to the fact that the second survey was in spring, when the cooler microclimate found in tall shrubs was less important than in summer. The lack of treatment effect on spider abundance may be a result of low spider densities, such that web sites were not limiting. The changes in species diversity and abundance are consistent with the hypothesis that the physical structure of the vegetation influences the spider community of the shrub layer, independently of any trophic influences.
Colonial spiders construct individual capture webs within a matrix of shared supporting frame threads. Cyrtophora citricola is a colonial orb-weaving spider with a complex three-dimensional web. Colonies may contain a few to several hundred individuals, but individuals may also occur solitarily. Local conditions such as food supply and substrate availability are likely to influence colony formation. In this study we explored the influence of local conditions and dispersal behavior on colony establishment in a desert population of C. citricola in the hyper-arid Arava Valley in Israel. Colonies in the Arava occur primarily on scattered Acacia trees and less commonly on shrubs. The spatial distribution of colonies was clustered and was not influenced by the condition of the Acacia trees (leaf flush, flowering, or fruiting). In a controlled experiment, we showed that dispersing spiders remained longer and built webs faster in trees that contained conspecific webs than in trees without webs. We propose that spiders benefit from establishing webs in the proximity of other spiders, while dispersal to another tree may not result in arrival at an improved habitat. These two factors may promote colony living even in prey-poor environments such as the extreme desert.
We investigated the habitats and interspecific associations of eight species of zodariid spiders in the Negev Desert of Israel. The spiders were collected for 3 years using pitfall traps at 10 sites in the northern and central Negev Desert as part of a large-scale pitfall-trapping project. Zodariidae were the second most abundant family of spiders (after Gnaphosidae) collected in the pitfall traps. One species, Zodarion nitidum, was the most abundant of all trapped spiders (17.5%). Lachesana blackwalli showed a preference for sand; Trygetus sexoculatus, Lachesana rufiventris, Z. nitidum, and Ranops expers, for rock; and Zodarion judaeorum, for loess. Further, T. sexoculatus showed an affinity to wadi habitats, and Z. judaeorum, Zodarion cyrenaicum, and L. blackwalli, to plateau habitats, whereas Z. nitidum and R. expers occurred mainly on slopes. T. sexoculatus and Z. nitidum tended to occur at northern (low-altitude) sites with higher rainfall. Palaestina eremica, L. blackwalli, and R. expers were more abundant in southern (high-altitude) sites, but with low rainfall. Analysis of the interspecific associations provides a list of spiders of other families that are found in association with zodariid spiders.
Current models indicate that intraguild predation is most likely to occur in communities with intermediate levels of productivity. Desert communities fit this criterion and also contain a disproportional amount of generalist predacious arthropods, in particular spiders, suggesting a high degree of intraguild predation in these communities. In this study we looked at intraguild predation in the Negev highlands among five species of spiders, two of which (Poecilochroa senilis, Gnaphosidae and Thyene imperialis, Salticidae) are predators of one or more of the other three (Latrodectus revivensis, Theridiidae; Stegodyphus lineatus, Eresidae; and Mogrus sp., Salticidae). However, unlike the simple interactions frequently modeled, we found complex interactions among the species which enhance coexistence. For example, evidence of higher-order interactions between Mogrus and Stegodyphus apparently enhanced the survival of Stegodyphus: during an annual cycle Mogrus, the preferred prey of Poecilochroa, was able to escape predation as juveniles by becoming scarce in the preferred habitat of Poecilochroa (which then mainly attacked Stegodyphus); when Mogrus returned to the habitat as adults, it relieved Stegodyphus from predation pressure at a time when the Stegodyphus population was most vulnerable (juveniles still in their dead mother's nest). Mogrus did not gain relief from predation by changing habitats as Thyene readily attacked the juveniles in the second habitat. The relationship between Poecilochroa and Latrodectus is also complex. Poecilochroa readily preyed on Latrodectus spiderlings and were often found overwintering in Latrodectus eggsacs. However, adult Latrodectus may themselves prey on Poecilochroa during summer. Thus the presence of a potential predator (Latrodectus) may enhance the survival of Poecilochroa during the winter by being a source of food.
Dispersal and site selection determine the distribution of organisms in relation to habitat features. Natal philopatry and specific habitat requirements may result in a clumped distribution that may have important consequences for population structure. We investigated dispersal of young of the web-building spider Stegodyplzus lineatus (Eresidae), which has a highly clumped distribution in dry watercourses (wadis) in the Negev Desert. The young dispersed from maternal nests in late summer to a median distance of only 2 m, and the dispersal directions differed among families and sites. More than 80% of the young settled in four species of perennial shrubs, a distribution that differed significantly from expected from the relative abundance of these shrubs in the habitat. Occupied shrubs were taller and had larger surface areas than unoccupied ones nearby. We suggest that the limited movement of young during dispersal and their preference for certain types of shrubs results in a clumped distribution.