Biodiversity, Density Compensation, and the Dynamics of Populations and Functional Groups

Species richness can affect both the size and stability of populations. Many simple theories predict that complex speciose communities should be less stable than simple ones. In contrast, stability of collective community attributes, such as functional groups, may be greater in more complex communities. We used aquatic microcosms containing microbial food webs to generate an experimental gradient of species richness. Biweekly counts of population samples permitted estimation and comparison of density, persistence time, and temporal variation in density for populations and trophic functional groups embedded in communities of varying richness. Densities of nearly half of the species declined as species richness increased, demonstrating broad community-wide density compensation that resulted from more abundant competitors and predators in species-rich communities. Persistence time, a measure of population stability, also declined with increasing species richness. Another measure of population stability, temporal variation in density, indicated that abundances of most species embedded in diverse communities varied no more than in less diverse communities. In contrast to results for individual species, temporal variation of entire functional groups composed of multiple species decreased as species richness increased. The stability of individual populations was not predicted by that of functional groups. Interspecific interactions and statistical averaging may both contribute to the reduced temporal variability of functional groups created by aggregating multiple species.