High community dissimilarity at low productivity causes the productivity-richness relation to vary with observational scale
It is widely reported that the productivity-richness relation (PRR) is highly variable, and several field studies suggest that the PRR varies with observational scale. Here we provide the first experimental study to test whether the PRR is scale-dependent when all replicate ecosystems have similar initial conditions. We also test the relation between productivity and compositional dissimilarity, and whether the PRR varies with ecosystem size. Moderately complex replicated microcosms were assembled consisting of a range of protozoa, algae, and a diverse bacterial flora. We found that the PRR of protozoan and algal communities varied with observational scale, but was unrelated to ecosystem size. Specifically, protozoan and algal richness increased monotonically with productivity at the local scale, but became flattened at the regional scale. This varying PRR at different scales occurred because dissimilarity among replicates decreased with productivity. Thus, in this model system, our experimental approach found a different form of scale dependence than previous field research. We speculate that this difference results from different processes governing extinctions at low levels of productivity.
KeywordsDissimilarity Ecosystem size Microcosms Observational scale Productivity-richness relation
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