Print PageDiverse communities are typically considered stable and resistant to invasion by exotic plant species. This stability is thought to arise as an emergent property from an assembly of species that complimentarily utilize limiting resources. Consequently, new species can only enter the community when disturbance makes an otherwise limiting resource available. While this hypothesis is supported by many studies, the specific aspects of diversity that confer resistance to invasion are poorly understood. We examine support for species diversity, functional group diversity, and functional trait diversity contributing to community stability and contrast their roles with the influence of disturbance on invasion. We selected sagebrush (Artemisia spp.)-steppe communities that are rapidly being replaced by the annual grass Bromus tectorum (cheatgrass) as a model system. We manipulated communities by removing sagebrush which effectively increased diversity, removing grasses and forbs which effectively decreased diversity, or removing both which reduced biomass but left diversity unchanged. Sample plots in two study sites, one high in shrub biomass the other low, in eastern Idaho were manipulated in summer and seeded with cheatgrass in autumn.
Results/Conclusions
Invasion was greater in the study site with lower overall biomass. Cheatgrass establishment was minimal in control plots and plots with considerable understory biomass. Soil surface disturbance was more important in influencing invasion than species, functional, or trait diversity. The presence of some species, such as a sedge (Carex sp.), were especially effective at inhibiting cheatgrass establishment. These results suggest that diversity at large scales, but disturbance at small scales influence invasion by an exotic annual grass.
