Multiple drivers of apparent competition reduce re-establishment of a native plant in invaded habitats

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. Using consumer-specific experimental exclosures and seed additions in the invaded grasslands of California, we demonstrate that multiple mechanisms of apparent competition are capable of limiting the re-establishment of the native grass Nassella pulchra in the absence of direct competition with exotic plants. The effect of small mammalian consumers (mice and voles) and larger consumers (e.g. rabbits, squirrels, deer) decreased with distance to the exotic forb Brassica nigra, which varied from 0 33 meters from focal N. pulchra. The effect of larger consumers also depended upon characteristics of the plant community directly adjacent (i.e. approx. 1 m) from focal N. pulchra. The effect of large consumers 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 which consumers have access, 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.