Consumer trophic diversity as a fundamental mechanism linking predation and ecosystem functioning

1. Primary production and decomposition, two fundamental processes determining the functioning of ecosystems, may be sensitive to changes in biodiversity and food web interactions. 2. The impacts of food web interactions on ecosystem functioning are generally quantified by experimentally decoupling these linked processes and examining either primary production-based (green) or decomposition-based (brown) food webs in isolation. This decoupling may strongly limit our ability to assess the importance of food web interactions on ecosystem processes. 3. To evaluate how consumer trophic diversity mediates predator effects on ecosystem functioning, we conducted a mesocosm experiment and a field study using an assemblage of invertebrates that naturally co-occur on North Atlantic coastal saltmarshes. We measured the indirect impact of predation on primary production and leaf decomposition as a result of prey communities composed of herbivores alone, detritivores alone or both prey in combination. 4. We find that primary consumers can influence ecosystem process rates not only within, but also across green and brown sub-webs. Moreover, by feeding on a functionally diverse consumer assemblage comprised of both herbivores and detritivores, generalist predators can diffuse consumer effects on decomposition, primary production and feedbacks between the two processes. 5. These results indicate that maintaining functional diversity among primary consumers can alter the consequences of traditional trophic cascades, and they emphasize the role of the detritus-based sub-web when seeking key biotic drivers of plant production. Clearly, traditional compartmentalization of empirical food webs can limit our ability to predict the influence of food web interactions on ecosystem functioning.