While much effort has been expended on examining the total and relative effects of vertebrate and invertebrate herbivores on terrestrial plant communities, the competitive relationship between these consumer groups has received substantially less attention. Strong competition between distantly related species has been extensively documented in the marine intertidal where these interactions can drive resource availability, recruitment, and community composition; however most terrestrial studies considering distantly-related consumers, especially those with extreme body size differences, examine their predator-prey relationships rather than their non-consumptive or indirect interactions. Interference between herbivore groups may have ecosystem-level ramifications, including reducing the top-down control of plant biomass and composition by herbivores, and affecting ecosystem services such as pollination and nutrient cycling. In spite of this, few long-term studies have explored the indirect interactions among herbivore groups. Here we studied the long-term impact on the arthropod community of removing vertebrate herbivores from a California oak savannah. We used paired plots to sample arthropods and plant communities inside and outside 11 vertebrate exclosures (mean area = 1000m²) that had been in place for 14-41 years.  

Results/Conclusions

Arthropod community composition was most strongly correlated with sampling plot and specific geographic coordinates, as well as biotic factors including exotic and native plant cover, exotic plant richness, and percent canopy cover. Arthropod richness, diversity, and community composition did not change predictably with exclusion of herbivores, even though large herbivore exclusion affected native and exotic plant species richness and cover. Thus, landscape- and local-level variation in the plant community, not the presence or absence of vertebrate herbivores per se, provided the best model for predicting arthropod community composition. While compensation among herbivore guilds may limit the strength of top-down ecosystem effects, the results of this study, which demonstrate no consistent effect of vertebrate herbivores on arthropod community composition, do not support this idea; however, the link between plant composition and arthropod community, consistent with past studies, highlights the potential for plant extinctions or invasions to alter the composition and function of the arthropod community.