Conservation and restoration of native plants often requires promoting the establishment of native plants in areas where they have been replaced by exotic plants.  Although rarely examined, apparent competition, whereby exotic plants increase consumer pressure on native plants, could play a significant role in affecting native plant establishment in invaded habitats.  Moreover, although terrestrial consumer communities often contain many consumer species, little is known about which consumers may generate apparent competition, and whether the strength or mechanism of apparent competition differs among different members of the consumer community.  We used consumer-specific experimental exclosures and seed additions in the highly invaded grasslands of California to examine whether the strength of apparent competition varied with consumer guild, and whether apparent competition was capable of limiting the re-establishment of a native perennial grass, Nassella pulchra.

Results/Conclusions

We find that multiple indirect effects via apparent competition are capable of limiting the re-establishment of the N. pulchra in the absence of direct competition with exotic plants.  The effect of small mammalian consumers (mice and voles) decreased with distance to the exotic forb Brassica nigra.  When larger consumers (e.g. rabbits, squirrels, deer) were also allowed access to N. pulchra, consumer effects remained a function of distance to B. nigra, but also increased with the richness of the exotic plant community and the degree to which the exotic plant community was dominated by exotic grasses as opposed to exotic forbs.  Our finding that apparent competition can be driven by different mechanisms, that the importance of each mechanism depends upon the consumer guild, and that each mechanism has a different spatial extent, suggests that the composition of both the consumer community and the exotic plant community may shape the spatial dynamics of reestablishment, the potential for restoration, and the need for conservation.