Effects of long-term consumer manipulations on invasion in oak savanna communities

Consumer-plant interactions can alter the outcome of biological invasions when native and exotic plants differ systematically in their resistance to and/or tolerance of consumer impacts. Given evidence for indirect interactions and shifts in plant communities from the few existing long-term studies, it is clear that long-term studies are a critical component for understanding the role of consumers in plant invasions. Moreover, studies of the role of consumers in mediating invasions have focused on the effects of exotic consumers, while the effects of native consumers on invasion have received little attention. Here we examine the long-term impact of a largely native vertebrate consumer community on native and exotic understory plants and recruitment of native oaks in a California oak savanna. We sampled plant community composition, oak recruitment, and soils inside and outside of 10 exclosures (mean area = 1000 m2) that had been in place for an average of 32 years. Plots with consumers present had 41% more exotic species, 31% higher cover of exotic species, and 33% lower richness of native herbaceous perennials, suggesting that native consumers may play an important role in mediating invasion in this system. The presence of oak canopies had a strong impact on the plant community independent of consumer effects, with greater recruitment of oaks, higher cover of native shrubs, and lower cover of exotic species cover under oak canopies. The concordant variation of native tree canopy and native woody plants suggests that adult oaks provide a refuge for their seedlings and other native woody plants. Thus, the widespread loss of native oaks has likely increased exotic invasion into an important refuge for native species in the California oak savanna ecosystem.