Many exotic plant species have been introduced or escaped into grasslands where they combine to form low diversity ‘novel ecosystems’ of species with no evolutionary history of interaction. Ecologists have long argued over whether low diversity of exotic communities results from species interactions (driver model) or the greater nutrient availability or frequency and intensity of disturbances that often is associated with exotic invasions (passenger model). We sampled 15 sites in an observational study, and established and monitored 208 experimental plots to compare species diversity and temporal niche measures between native and exotic grasslands. We used a paired species approach to experimentally determine species dynamics. We used monocultures and 9-species mixtures to test whether yielding behavior and niche overlap explained diversity declines.

Results/Conclusions

Species diversity and richness increased linearly with the proportion of native species across sites in the observational study. Sites fell into two clusters, a group of diverse native-dominated fields and a group of low diversity exotic-dominated fields. The experiment was conducted to better understand what underlies these differences. Species diversity was 41% higher and richness was 22% higher in native than exotic mixtures after two growing seasons. Reduced diversity in exotic communities resulted from lower complementarity and higher temporal niche overlap, occurred in plots that were both irrigated during summer and not irrigated, and was manifest in differences in phenology. These results indicate that exotic grasslands differ fundamentally from native systems. These differences should be taken into account in management and restoration plans.