Plant invasion in Pacific grasslands: Is exotic richness a red herring?

Naturalization of exotic species can reduce native abundance and homogenize spatial structure in native communities. These impacts are associated with three critical stages in the naturalization process: initial establishment in the new region, local population increases, and dispersal across the invasive range. To date, most studies have focused on the first of these stages, the establishment of novel species, by examining factors that control exotic species richness. However, many exotic species have little if any impact on the native flora, and it remains unclear whether the exotic species richness has any relationship to more direct measures of impacts such as the local dominance by exotic species and spatial homogenization of communities. Here we examine all three impacts across a 2000 km transect of grasslands along the Pacific Coast of North America. We found that native and exotic richness were positively correlated at  spatial scales spanning seven orders of magnitude (1-10⁷ m²).  Exotic dominance was not positively correlated with exotic richness, but was instead driven by the presence of a small subset of particularly abundant exotic species.  At local scales, increasing native richness was correlated with decreasing exotic dominance indicating that local process (e.g., resource competition) may limit exotic dominance at high native diversity, but regional scale processes (e.g., propagule pressure, climate) act similarly on establishment of native and exotic species, leading to a positive correlation in richness at higher spatial scales.  Exotic species had lower species turnover (beta diversity) than did natives at all spatial scales.  Thus increasing dominance by exotic species will homogenize plant communities. Species identity rather than species number were most important in determining exotic dominance and biotic homogenization, implying that the current debate over the drivers of exotic richness may be a red herring.