Interactions between the green and brown food web determine ecosystem functioning

The concepts of top-down and bottom-up controls are central to our understanding of cascading trophic effects on ecosystem functioning. Classical food web theory has focused either on food webs based on primary production (green food webs) or on food webs based on detritus (brown food webs) and generally ignored nutrient cycling. We argue that nutrient cycling connects the two food webs, which questions the traditional concept of top-down and bottom-up controls. By integrating these two food webs and nutrient cycling into simple models, we investigate the cascading effects from one food web to the other one. Both analytical calculations and simulations show that these two cascading effects depend on simple but distinct mechanisms that are derived from different ecological processes. Predators of decomposers can affect primary production in the green food chain. The signs of these effects are determined by relative proportions of nutrient cycling within the brown food chain. Cascading effects within the green food chain can affect decomposer production in a bottom-up way. The carbon/nutrient limitation of decomposers determines the way the green food chain affects decomposer production. These theoretical findings are applicable to explore real interactions and cascading effects between the green and the brown food webs, such as pelagic-benthic interactions or above-ground-below-ground interactions.