Print PageDisturbance and resource supply can increase or decrease species diversity by altering the relative roles of community assembly mechanisms. Examining these effects is important to understand patterns of diversity, as well as how anthropogenic impacts on disturbance regimes and nutrient cycling will affect communities and thus ecosystem functioning. Species diversity may not fully capture community diversity, however, because species are not independent entities but a repository of evolutionary information and relationships, which likely relate to ecological similarity among closely related species. Community assembly mechanisms likely act on species traits, thus incorporating evolutionary history into studies of biodiversity may increase our power to predict community responses to changing processes, such as disturbance and resource supply. We examined the effects of local-scale factorial manipulations of disturbance and nitrogen supply on community diversity and variation among communities in the invaded California grasslands. We used both traditional and phylogenetic diversity metrics of richness, evenness, and community similarity to determine how these processes impacted diversity and elucidate the potential roles of various community assembly mechanisms.
Results/Conclusions
Disturbance increased species richness and evenness. Communities were phylogenetically clustered in general relative to a null model of community assembly where all species had equal probability of colonizing the community, however disturbed communities were less likely to be significantly clustered. Species composition differed between disturbed and undisturbed communities, and disturbed communities were also more variable in their phylogenetic composition. Nitrogen supply had little effect on community diversity, but phylogenetic composition was more similar within than among nitrogen treatments. Disturbance and nitrogen interacted in their effects on phylogenetic composition and a combination of disturbance and nitrogen addition decreased species richness. Overall, our results suggest that these grassland communities are structured largely through environmental filtering. Disturbance appeared to increase opportunities for colonization by decreasing competitive effects of resident communities, likely mediated by build-up of exotic grass litter. Further evaluation of the role of dispersal in structuring these communities is necessary, as well as work to relate functional traits to species' phylogenetic relationships.
