Interaction Dimensionality Scales Up to Generate Bimodal Consumer-Resource Size-Ratio Distributions in Ecological Communities

Understanding constraints on consumer-resource body size-ratios is fundamentally important from both ecological and evolutionary perspectives. By analyzing data on 4685 consumer-resource interactions from nine ecological communities, we show that in spatially complex environments—where consumers can forage in both two (2D, e.g., benthic zones) and three (3D, e.g., pelagic zones) spatial dimensions—the resource-to-consumer body size- ratio distributiontends towards bimodality, with different median 2D and 3D peaks. Specifically, we find that median size-ratio in 3D is consistently smaller than in 2D both within and across communities. Furthermore, 2D and 3D size(not size- ratio) distributionswithin any community are generally indistinguishable statistically, indicating that the bimodalityin size-ratios is not driven simply by a priori size-segregation of species (and therefore, interactions) by dimensionality, due to other factors. We develop theory that correctly predicts the direction and magnitude of these differences between 2D and 3D size- ratio distributions. Our theory suggests that community-level size-ratio bimodalityemerges from the stronger scaling of consumption rate with sizein 3D interactionsthan in 2D, which both, maximizes consumer fitness, and allows coexistence, across a larger range of size-ratios in 3D. We also find that consumer gape-limitation can amplify differences between 2D and 3D size-ratios, and that for either dimensionality, higher carrying capacity allows coexistence of a wider range of size-ratios. Our results reveal new and general insights into the sizestructure of ecological communities, and show that spatial complexity of the environment can have far reaching effects on community structure and dynamics across scales of organization.