Predator influences on prey community structure in a long-term experimental rodent-plant system

Background/Question/Methods Habitat fragmentation and dispersal limitation may lead to extinction of predator species within local habitat patches. Recently published models suggest that predator presence or absence within such patches alters species-area relationships (SARs) among sites by increasing the probability of local extinction of prey species. Therefore, sites where predators are present are predicted to have shallower SAR slopes than sites where predators are absent. Experimental manipulations at a long-term research site, the Portal LTREB, provide a natural study system comprised of desert rodent predators and plant prey to study predator-driven structuring processes. Encompassing 20 hectares that contains 24 fenced plots, Portal plots represent three treatment categories: 1) all rodents have access, 2) large-bodied rodents (Dipodomys spp.) removed, and 3) all rodents removed. Winter and summer annual plant community SARs were compared among plots and treatments using biannual plant census data from 1989-2002 to explore the effects of the dominant rodent seed predators (Dipodomys spp.) on plant richness in a natural system. Previous research at the site suggests that large-bodied rodent species have a greater impact on winter annual plant community structure than small-bodied species by preferentially consuming plants that produce large seeds. We plotted SARs for each plot, calculated slopes, and compared frequencies among treatments.

Results/Conclusions While previous research shows that Dipodomys spp. at the Portal LTREB affect plant species composition, SAR slopes among treatments were not significantly different. Linear models of winter and summer annual species richness were best fit using only area and time, without the inclusion of treatment. Rather, other factors may be more important in determining the shape of plant SARs at the Portal site such as interannual variation in the environment and on-site habitat variation. Plant species composition varies significantly by treatment, suggesting that Dipodomys spp. influence the identity of plant species on patches in which they are present, but they do not appear to influence the number of plant species found within a site. This suggests that seed dispersal is high enough that the plant community can respond to changing predation pressure without a significant loss in the total number of species. Since predators are often disproportionately affected by habitat fragmentation, empirical work in open systems is essential to understanding community diversity patterns at a multi-trophic level and addressing conservation goals in rapidly changing landscapes.