LETTER Predator diversity and the functioning of ecosystems: the role of intraguild predation in dampening trophic cascades Deborah L. Finke* and Robert F. Denno Department of Entomology, University of Maryland, College Park, MD 20742, USA *Correspondence: E-mail: dfinke@wsu.edu Abstract Single trophic-level studies of the relationship between biodiversity and ecosystem functioning highlight the importance of mechanisms such as resource partitioning, facilitation, and sampling effect. In a multi-trophic context, trophic interactions such as intraguild predation may also be an important mediator of this relationship. Using a salt- marsh food web, we investigated the interactive effects of predator species richness (one to three species) and trophic composition (strict predators, intraguild predators, or a mixture of the two) on ecosystem functions such as prey suppression and primary production via trophic cascades. We found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of trophic cascades. In addition, increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity. Therefore, trophic composition of the predator assemblage can play an important role in determining the nature of the relationship between predator diversity and ecosystem function. Keywords Biocontrol, biodiversity, food webs, herbivore suppression, indirect interactions, omnivory, predator–predator interactions, predator–prey interactions, salt marsh, trophic dynamics. Ecology Letters (2005) 8: 1299–1306 INTRODUCTION Studies investigating the importance of biodiversity for ecosystem functioning have become widespread because of concern over the currently unprecedented rate of biodiver- sity loss (Loreau et al. 2001). Many studies, in this area, have focused specifically on the role of producer diversity (Tilman et al. 2002), whereas the consequences of biodiver- sity loss at higher trophic levels have been often overlooked (Duffy 2003; but see Naeem & Li 1998; Norberg 2000; Paine 2002; Duffy et al. 2003; Hillebrand & Cardinale 2004; Downing 2005). As a result, the commonly implicated mechanisms by which changes in biodiversity can influence ecosystem functioning, such as resource use partitioning, facilitation, and sampling effect, have emerged primarily from single trophic-level investigations of plant assemblages (Tilman et al. 1997; Hooper 1998; Fridley 2001). Little is known about the impact of trophic interactions, particularly predator–predator and predator–prey interactions, on the relationship between biodiversity and ecosystem functioning in natural systems (Morin 1995; Wilby & Thomas 2002; Cardinale et al. 2003; Montoya et al. 2003; Finke & Denno 2004; Snyder et al. 2005). Incorporating such a multi-trophic perspective will necessitate the consideration of additional mechanisms by which changes in biodiversity might influence ecosystem functioning, including the occurrence of intraguild predation (Ives et al. 2005). Impacts of the entire predator trophic level on critical ecosystem functions such as prey suppression and primary production via the occurrence of trophic cascades have been documented. The predator trophic level as a whole is capable of reducing herbivore populations and these top- down effects can propagate through the food web to positively influence primary producers, resulting in a trophic cascade (Carpenter et al. 1985; Schmitz et al. 2000; Halaj & Wise 2001; Shurin et al. 2002). However, diversity within the predator trophic level is likely to have significant conse- quences for the occurrence of trophic cascades as well, Ecology Letters, (2005) 8: 1299–1306 doi: 10.1111/j.1461-0248.2005.00832.x Ó2005 Blackwell Publishing Ltd/CNRS