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