Causes and consequences of biodiversity in experimental aquatic microcosms

A major challenge facing humans is the need to preserve biological diversity. This challenge is complicated by a fundamental uncertainty about the interplay between biodiversity and various processes operating at the levels of population, community and ecosystem. The studies presented here used a model system of protists and metazoans from aquatic communities to explore questions about the maintenance of biodiversity and consequences of biodiversity loss. One study experimentally assessed whether diversity of temporary aquatic communities is maintained by drying, a non-equilibrium disturbance that could promote richness among rain pool organisms. Laboratory simulations of constantly wet or periodically drying rain pools showed that undisturbed communities had higher diversity than disturbed communities, mainly due to the presence of predatory species which required long periods without disturbance to become active. If disturbance maintains diversity in this system, it occurs under a different drying regime or over more drying cycles.; A second study examined how properties of populations, communities and ecosystems differed along a gradient of species richness, assembled using combinations of 3 to 31 species. Along the gradient of increasing richness, many species declined in abundance, but several showed no difference in variation in abundance over time. In contrast, variation in abundance of most functional groups decreased with increasing richness, implying that the stability of individual populations may not be tightly linked to that of community properties. Variation in abundance of several species was strongly related to variation in ecosystem performance. Species with strong ecosystem associations had high abundance and long persistence time, providing simple criteria for predicting which species may be important in other systems. Higher diversity communities contained more species that had strong ecosystem associations than lower diversity communities. Diverse communities also displayed more predictable carbon dioxide production, an important ecosystem function, than their depauperate counterparts. In moderate to high diversity communities, two other ecosystem measures leveled off, suggesting redundant functional roles of species. Although species composition accounted for much of the variation in ecosystem processes, species richness provided additional information about ecosystem functioning beyond that explained by the densities and identities of species.