Environmental heterogeneity can promote species diversity by allowing a greater variety of coexistence mechanisms to operate than in a homogeneous environment. This implies that heterogeneous habitats may be more invasible by exotic species, but also that native species in such habitats may be more likely to coexist with invaders. Quantifying community patterns in time and space provides a first step to understanding how invasion and coexistence processes are operating in an invaded ecosystem, and thus to predicting whether native species may persist without management action. We examined plant community patterns for four years at three reserves spanning 700 km in the invaded California grasslands to compare native and exotic plant richness and abundance over time and space. We calculated the temporal and spatial variability in species richness and abundance and the correlation between richness and abundance of natives and exotics. Our goal was to determine if the two groups of plants responded differentially to the environment, and thus if native plants utilized conditions unfavorable to exotics to bolster their population growth.
Results/Conclusions

Exotic species richness and abundance were consistently greater than those of native plants but showed similar trends. Species richness was highest in the wettest year of the study, and abundance was lowest in the driest year. Species richness and abundance were consistently lower at one site and some locations were consistently more heavily invaded. Abundance of native species varied more from year to year than that of exotics, but richness was similarly variable for both groups. Abundance of native species was more variable in space than that of exotics, and spatial variability of native plant abundance differed by site. Species richness of natives and exotics within plots was positively correlated in all years, but abundance was not correlated. Our results suggest that native and exotic species respond similarly to the environment, with climatic conditions potentially contributing more to richness and abundance in each year than biotic interactions, such as competition. We observed temporally-variable site effects, suggesting that future analyses must consider the interactive effects of spatial and temporal environmental factors on plant community patterns to better understand the potential mechanisms for coexistence of native species with exotics.