Energy input and species diversity patterns in microcosms

Numerous studies document some form of relationship between the energy input to a community (or some surrogate for energy input), and the species richness of all, or part, of the community. Although not consistently so, the relationship is commonly either unimodal or positive. However covariation of energy with other environmental factors, in both field and experimental studies, and the timescale of expected population responses to variation in energy, means that testing the patterns is difficult. Here we use laboratory microcosm systems, of protists and bacteria, to examine the response of artificially constructed communities to an energy gradient (6 levels), on a long timescale (up to 421 days), while in parallel examining the responses of each species individually to the environmental conditions across the gradient. The species richness of individual communities (α‐diversity) showed positive responses initially, but after longer periods either no relationship, or a modestly unimodal one. When all replicate communities were considered together at each energy level (γ‐diversity), there was a more consistently positive relationship, an effect generated by the fact that at low energy the species composition of replicate communities was almost identical, while at high energy there was considerable variation among replicates. Although when each species used in the multispecies systems was exposed to the same energy gradient individually, there were distinct differences in the their responses to the gradient, the patterns of community composition and diversity seen in the multispecies systems could not be explained simply by the individual species’ responses to the environmental conditions along the gradient.