Consumer trophic diversity as a fundamental mechanism linking predation and ecosystem functioning Jes Hines 1,2 * and Mark O. Gessner 3,4 1 Smithsonian Environmental Research Center, Edgewater, MD, USA; 2 Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, Du ¨bendorf, Switzerland; 3 Department of Experimental Limnology, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Stechlin, Germany; and 4 Department of Ecology, Berlin Institute of Technology (TU Berlin), Berlin, Germany Summary 1. Primary production and decomposition, two fundamental processes determining the function- ing 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 function- ing, 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 com- posed 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 con- sumer 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 compartmentali- zation of empirical food webs can limit our ability to predict the influence of food web interactions on ecosystem functioning. Key-words: above-ground, below-ground, food web complexity, nutrient cycling, trophic cascade Introduction One of the central challenges in ecology is to identify factors that affect essential ecosystem processes such as primary pro- duction and decomposition (Borer et al. 2005; Reiss et al. 2009; Gessner et al. 2010; Isbell et al. 2011). Because of the concern that rapid population declines and species extinc- tions will compromise the functioning of ecosystems, an increasing number of studies have examined the sensitivity of particular ecosystem processes to changes in biodiversity and trophic structure (Schmitz, Ha¨ mback & Beckerman 2000; Reiss et al. 2009). Most studies have focused on the role of species diversity within a particular trophic level such as primary producers (Isbell et al. 2011), consumers (Srivastava et al. 2009) or predators (Schmitz 2007). However, the topology of real food webs is far more complex than simpli- fied trophic levels suggest, but it currently remains unclear how much of the complexity is needed to understand the effects of species interactions on ecosystem processes (Duffy et al. 2007; Hillebrand & Matthiessen 2009; O’Gorman et al. 2009). Trophic cascades, the indirect effects of predators on non- adjacent trophic levels, have been found to affect ecosystem processes in both green sub-webs consisting of predators– herbivores–plants (Schmitz 2007) and brown sub-webs com- posed of predators–detritivores–detritus (Santos, Phillips & Whiford 1981; Rosemond et al. 2001) (Fig. 1a). Despite evidence that generalist predators often feed on both herbivores and detritivores, the effects of predator–prey *Correspondence author. Department of Experimental Limnology, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Alte Fisherhu¨ tte 2, 16775 Stechlin, Germany. E-mail: jessica.hines@ eawag.ch Journal of Animal Ecology 2012, 81, 1146–1153 doi: 10.1111/j.1365-2656.2012.02003.x Ó 2012 The Authors. Journal of Animal Ecology Ó 2012 British Ecological Society