Print PageThe importance of species diversity for the stability of populations, communities and ecosystem functions is a central question in ecology. Biodiversity experiments have shown that diversity can impact both the average and variability of stocks and rates at these levels of ecological organization in single trophic level ecosystems. Whether these impacts hold in food webs and across trophic levels is still unclear. We asked whether resource species diversity, community composition and consumer feeding selectivity in food webs impact the stability of resource or consumer populations, community biomass and ecosystem functions. We tested the relative importance of resource diversity and community composition. We also tested a number of mechanisms which have been proposed to stabilize aggregate community functions. We used microcosm experiments with phytoplankton species as resources and zooplankton as consumers. We manipulated the composition and species diversity of the phytoplankton community, as well as the feeding selectivity of the consumer. We monitored population, community and ecosystem dynamics over twelve weeks. In particular, we measured zooplankton density, phytoplankton population biovolume, resource community biovolume, chlorophyll-a, particulate carbon, nitrogen and phosphorus, and δ13C (as a proxy for photosynthesis).
Results/Conclusions
We found that resource diversity negatively affected resource population, but not consumer population stability, despite an increase in the generalist’s average population density. Resource diversity also had positive effects on resource community biomass, most ecosystem functions and their stability. Specifically, there were positive effects of diversity on the mean and stability of resource community biomass and particulate carbon, nitrogen and phosphorus. These effects were positively related to trends in chlorophyll-a. Community composition, however, generally explained more variance than resource species diversity per se. Among the stabilizing mechanisms tested, increased average resource community biomass had the greatest positive impact on stability. The specialist consumer’s density was related to the abundance of edible resource biomass, whereas the generalist’s biomass was related to total resource biomass. The lack of a stabilizing effect on both consumers likely resulted from contrasting stabilizing and destabilizing influences of resource species diversity, particularly for the generalist consumer. Our results suggest that resource diversity and composition are generally important for determining the functioning and stability of the aggregate properties of whole food webs, but not necessarily consumer populations.
