Indirect effects of herbivory by deer reduce abundance and species richness of web spiders

Abstract

:We examined the abundance and species richness of web spiders in forests with and without deer to test the hypothesis that herbivory by deer simplifies habitat structure, thereby reducing the number of web spiders. The number of individuals, the number of species, and the availability of potential web sites were all lower where deer were present. The decrease in the abundance of spiders in the presence of deer was more prominent in large species. The species richness standardized by rarefaction was still lower where deer were present in one of two seasons, suggesting that the reduced number of species in the presence of deer was not simply a by-product of the decreased number of individuals. Web site availability was positively correlated with the number of individuals as well as with the number of species when both forest types were combined. However, prey availability, estimated by the number of insects captured with sticky traps, did not differ significantly between sites with and without deer. Thus, the decrease in spiders was most likely caused by indirect non-trophic effects of herbivory that were mediated by the simplification of habitat structure, not by a decrease in prey abundance.