REVIEW AND SYNTHESIS The functional role of biodiversity in ecosystems: incorporating trophic complexity J. Emmett Duffy, 1 * Bradley J. Cardinale, 2 Kristin E. France, 1 Peter B. McIntyre, 3 Elisa The ´ bault 4 and Michel Loreau 5 1 School of Marine Science and Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, VA 23062-1346, USA 2 Department of Ecology, Evolution and Marine Biology, University of California-Santa Barbara, Santa Barbara, CA 93106, USA 3 Department of Biological Sciences, Wright State University, Dayton, OH 45345, USA 4 NERC Centre for Population Biology, Division of Biology, Imperial College London, Silwood Park Campus, Ascot, Berkshire SL5 7PY, UK 5 Department of Biology, McGill University, Montreal, Que ´ bec, Canada H3A 1B1 *Correspondence: E-mail: jeduffy@vims.edu Abstract Understanding how biodiversity affects functioning of ecosystems requires integrating diversity within trophic levels (horizontal diversity) and across trophic levels (vertical diversity, including food chain length and omnivory). We review theoretical and experimental progress toward this goal. Generally, experiments show that biomass and resource use increase similarly with horizontal diversity of either producers or consumers. Among prey, higher diversity often increases resistance to predation, due to increased probability of including inedible species and reduced efficiency of specialist predators confronted with diverse prey. Among predators, changing diversity can cascade to affect plant biomass, but the strength and sign of this effect depend on the degree of omnivory and prey behaviour. Horizontal and vertical diversity also interact: adding a trophic level can qualitatively change diversity effects at adjacent levels. Multitrophic interactions produce a richer variety of diversity-functioning relationships than the monotonic changes predicted for single trophic levels. This complexity depends on the degree of consumer dietary generalism, trade-offs between competitive ability and resistance to predation, intraguild predation and openness to migration. Although complementarity and selection effects occur in both animals and plants, few studies have conclusively documented the mechanisms mediating diversity effects. Understanding how biodiversity affects functioning of complex ecosystems will benefit from integrating theory and experiments with simulations and network-based approaches. Keywords Ecosystem functioning, grazing, horizontal diversity, niche breadth, top-down control, trophic cascade, vertical diversity. Ecology Letters (2007) 10: 522–538 INTRODUCTION Global biodiversity is increasingly threatened by human domination of natural ecosystems and concomitant impacts that accelerate rates of population and species extinction, and homogenization through invasion (Vitousek et al. 1997; Sala et al. 2000). These changes raise fundamental questions, such as: What are the community and ecosystem-level consequences of biodiversity loss? Will extinctions alter basic ecosystem processes, including those that produce food, purify air and water, and decompose harmful wastes? To address such questions, the relationship between biodiversity and ecosystem functioning has emerged during the last decade as a vigorous new research area linking community and ecosystem ecology (see general syntheses in Loreau et al. 2001, 2002; Hooper et al. 2005). Well before the recent surge of interest in the functional significance of biodiversity, ecologists recog- nized that community structure can strongly affect the functioning of ecosystems. In particular, a large body of research had shown that loss of predator species can have impacts that cascade down a food chain to plants, altering basic ecosystem processes. One classic example is the kelp – sea urchin – sea otter food chain in the northeast Pacific. Hunting of sea otters by fur traders in the late 19th century caused a population explosion of their sea urchin prey, and consequent overgrazing of kelp forests (Estes & Palmisano 1974). Loss of kelp led to local extirpation of numerous other species that depend on kelp for habitat, as well as increased coastal erosion and storm damage since kelp was a primary buffer from wave action. Similar cascading effects of predator removal have Ecology Letters, (2007) 10: 522–538 doi: 10.1111/j.1461-0248.2007.01037.x Ó 2007 Blackwell Publishing Ltd/CNRS