Unexpected changes in community size structure in a natural warming experiment

Background/Question/Methods: Natural ecosystems typically consist of many small and few large organisms. The scaling of this negative relationship between body mass and abundance has important implications for resource partitioning and energy usage. Global warming over the next century is predicted to favour smaller organisms, producing steeper mass-abundance scaling and a less efficient transfer of biomass through the food web. We tested this theory in a natural warming experiment, involving 13 whole-stream ecosystems within the same catchment that span a temperature gradient of 5-25 °C.

Results/Conclusions: We find the opposite effect to theoretical predictions, with fewer small organisms and larger consumers in the warmest streams. We introduce a mechanistic model that shows how the temperature dependence of basal resource carrying capacity can account for these previously unexpected results. If nutrient supply increases with temperature to offset the rising metabolic demand of primary producers, there will be sufficient resources to sustain larger consumers at higher trophic levels. These new data and the model that explains them highlight important exceptions to some commonly assumed “rules” about responses to warming in natural ecosystems.