AN EXPERIMENTAL MANIPULATION OF VEGETATION STRUCTURE - CONSEQUENCES FOR DESERT SPIDERS

We examined the influence of changes in the structure of the shrub lay er in a desert habitat on the community of web-building spiders inhabi ting this vegetation. Vegetation structure was modified to simulate ch anges predicted with increasing aridity. We predicted that changing th e vegetation structure would affect the diversity and abundance of spi ders by means of (1) changing available web supports and (2) changing the abundance of available prey. Using a randomized block design, we p erformed two manipulations: pruning the shrubs to one-half their heigh t and thinning the plot to one-half its initial shrub density. We surv eyed the distribution, abundance, and species identity of spiders befo re the manipulation, and twice after the manipulation. Potential arthr opod prey were censused during the two post-manipulation spider survey s. We found no influence of the treatments on the potential prey. Six weeks after the manipulation, spider abundance was reduced significant ly in plots of both treatments, and species diversity was significantl y lower in the pruned plots. The reduced species diversity in pruned p lots may be explained by the propensity of spiders of different specie s to construct their webs at different heights in the vegetation. Prun ing selectively eliminates the crown of a shrub, and eliminates the po tential web sites of species which prefer the crown, while thinning re moves entire shrubs, eliminating web sites for all species of spiders equally. Neither spider abundance nor diversity differed among treatme nts in the second survey, ten months after the manipulation. We sugges t that the lack of a treatment effect on spider species diversity is r elated to the fact that the second survey was in spring, when the cool er 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 chang es in species diversity and abundance are consistent with the hypothes is that the physical structure of the vegetation influences the spider community of the shrub layer, independently of any trophic influences .