Individual and Interactive Effects of Large and Small Mammalian Herbivores on Oak Performance and the Woody Understory Community

Although mammalian herbivores are well known to have substantial effects on their host plants and entire communities, few studies have considered the individual and interactive effects of multiple herbivores in the same system. This is surprising given that most areas have multiple herbivore species that may interact directly or indirectly, thus potentially changing the impacts that each has on the plant community. Using an eight-year factorial exclosure experiment, we studied the influence of black-tailed deer (Odocoileus hemionus columbianus) and various small mammals (primarily meadow voles Microtus califomicus and deer mice Peromyscus maniculatus) on the growth and abundance of oak seedlings and juveniles (Quercus agrifolia, Q. kelloggii and Q. garryana) as well as the characteristics and composition of the understory woody community in northern California. Small mammals significantly reduced the height of oak seedlings and both deer and small mammals significantly reduced the height of juvenile oaks. Neither mammal by themselves affected oak seedling abundance,juvenile abundance or understory oak cover. However, deer had a negative effect on oak abundance and cover when small mammals were present and a positive effect on these\ud variables when small mammals were absent. We found that both small mammals and deer significantly reduced woody species richness. Small mammals significantly reduced the cover of non-oak woody species and deer showed a strong trend in the same direction. Neither deer nor small mammals significantly affected aboveground herbaceous biomass or leaf litter, although there was a trend for small mammals to reduce leaf litter. We hypothesize that the interactive effects of the two herbivores observed in our system occurred because small mammals reduced the overall cover of woody understory species and indirectly increased the intensity of deer browsing on oaks by reducing protective woody cover. Collectively, our data indicate that deer and small mammals have both individual and interactive effects on three dominant oak species and their surrounding community, and that considerable insight will be gained by evaluating the impacts of multiple herbivore species using factorial exclosure experiments.The National Science Foundation; California Native Plant Society (Milo Baker Chapter); Northern California Botanist