Testing a biological mechanism of the insurance hypothesis in experimental aquatic communities.

1. The insurance hypothesis predicts a stabilizing effect of increasing species richness on community and ecosystem properties. Difference among species' responses to environmental fluctuations provides a general mechanism for the hypothesis. Previous experimental investigations of the insurance hypothesis have not examined this mechanism directly. 2. First, responses to temperature of four protist species were measured in laboratory microcosms. For each species, we measured the response of intrinsic rate of increase (r) and carrying capacity (K) to temperature. 3. Next, communities containing pairs of species were exposed to temperature fluctuations. Community biomass varied less when correlation in K between species (but not r) was more negative, and this resulted from more negative covariances in population sizes, as predicted. Results were contingent on species identity, with findings differing between analyses including or not including communities containing one particular species. 4. These findings provide the clearest support to date for this mechanism of the insurance hypothesis. Biodiversity, in terms of differences in species' responses to environmental fluctuations (i.e. functional response diversity) stabilizes community dynamics.