The role of evolutionary processes in determining trophic structure

There are two contrasting hypotheses regarding the drivers of biomass-distribution among trophic levels within ecosystems. Energetic or bottom-up models propose control by supply of energy, either from autotrophy or from the underlying trophic level. Dynamical or top-down models propose control by predators in the overlying trophic level. Although multiple approaches have been used to reconcile these hypotheses, they have rarely considered the evolutionary pressures created by different distributions of biomass. Here, we study the effects of these evolutionary processes using an agent-based, spatially-explicit, eco-evolutionary model. We demonstrate that, when ecosystems are simple and
predator-prey relationships between species are fixed and do not evolve, primary
productivity limits the number of trophic levels. In this case, the abundance in the top trophic level is always limited by productivity. However, productivity-limited trophic levels subject those below them to limitation by predation, and predation-limited trophic levels allow trophic levels below them to grow until limited by productivity. These results match the expectations of the Exploitation Ecosystems Hypothesis (EEH), which predicts the same pattern of alternating top-down and bottom-up control on trophic levels. When species are able to evolve to adaptively adjust their trophic levels, however, selection is liable to lead to collapse of the trophic pyramid through prey switching. Under these conditions, all trophic levels experience bottom-up control. When the evolution of prey switching is restricted, higher trophic levels are evolutionarily stable and the predictions of the EEH are once again met; the stability of long food chains is thus dependent on the difficulty associated with prey switching. These results suggest that, at least under the combinations of model parameters explored in this study, evolutionary limitation is key to maintaining trophic structure.