Compensation masks trophic cascades in complex food webs

Ecological networks, or food webs, describe the feeding relationships between interacting species within an ecosystem. Understanding how the complexity of these networks influences their response to changing top-down control is a central challenge in ecology. Here, we provide a model-based investigation of trophic cascades - an oft-studied ecological phenomenon that occurs when changes in the biomass of top predators indirectly effect changes in the biomass of primary producers - in complex food webs that are representative of the structure of real ecosystems. Our results reveal that strong cascades occur primarily in small and weakly connected food webs, a result very much in agreement with empirical studies. The primary mechanism underlying weak or absent cascades was a strong compensatory response; in most webs predators induced large population level cascades that were masked by changes in the opposite direction by other species in the same trophic guild. Thus, the search for a general theory of trophic cascades in food webs should focus on uncovering the features of real ecosystems that promote or preclude compensation within functional guilds.