Disturbance, spatial turnover, and species coexistence in grassland plant communities

Abstract

Humans have dramatically altered natural disturbance regimes. We thus need to understand how these alterations affect plant communities and whether natural disturbance regimes can be restored. I explored the effect of disturbance on plant community patterns and species coexistence in grasslands of northeastern Kansas. In the first chapter, I examined the impact of disturbance associated with the five most common grassland management practices on plant community patterns. I measured species richness and differences in community composition among habitat patches at three spatial scales, at two levels of ecological resolution, and at three levels of taxonomic resolution. There were extensive changes to plant community structure associated with grassland management practices, which may be due to reduced environmental heterogeneity, increased dominance by perennial grasses, and/or decreased functional diversity. The second chapter investigates the smaller-scale disturbance associated with prairie vole, Microtus ochrogaster, burrows. I found that vole disturbance affected the mean values of nine environmental variables, contributed to environmental heterogeneity, increased local plant species richness, metacommunity evenness, and the presence of fugitive species. Variation in community composition was high among burrows because disturbance shifted the identity of dominant species away from the species dominant in the undisturbed matrix and allowed fugitive species to persist in higher abundances. These patterns are consistent with a successional mosaic and alternative successional trajectories among burrows disturbed at different times. In the third chapter, I used prairie vole burrows as a model system to develop a field experiment testing whether the timing of small-scale disturbances contributes to environmental heterogeneity, and whether the functional complementarity of species in the species pool affects the ability of community composition to reflect heterogeneity through species sorting. Disturbance treatments affected coexistence by creating colonization opportunities and successional niche heterogeneity. The effect of environmental heterogeneity on variation in community composition among habitat patches was the greatest in the presence of a complementary species pool. This interaction between complementarity and heterogeneity demonstrates the importance of trait variation among species for exploiting environmental variation among patches and suggests niche-based coexistence through species sorting.