Ecology, 91(8), 2010, pp. 2242–2252 Ó 2010 by the Ecological Society of America Cascading diversity effects transmitted exclusively by behavioral interactions SHAWN A. STEFFAN 1 AND WILLIAM E. SNYDER Department of Entomology, Washington State University, Pullman, Washington 99164 USA Abstract. Consumer diversity generally increases resource consumption. Consumers can also impact other species by altering their behavior, but it is unclear how such nonconsumptive effects scale with diversity. We independently manipulated predator species richness and the consumptive and nonconsumptive effects of predator communities to measure the role of each factor in protecting Brassica oleracea plants from caterpillar herbivory. Plant biomass was greatest when diverse predator assemblages induced antipredator behaviors in herbivores, an effect not further strengthened when predators could also kill caterpillars. Predators within diverse communities were more likely to forage on plants and to disrupt herbivore feeding, reflecting greater aversion to foraging among conspecific than heterospecific competitors. Predator diversity, therefore, initiated behavioral changes at the predator and then herbivore trophic levels, both to the benefit of plants. Our results indicate that strong, emergent species- richness effects can be transmitted entirely through behavioral interactions, independent of resource consumption. Key words: biodiversity; Brassica oleracea; ecosystem function; interference competition; intraguild intimidation; Myzus persicae; nontrophic interaction; Plutella xylostella; predator–prey; trait-mediated; trophic cascade. INTRODUCTION The degree of resource extraction by diverse commu- nities generally exceeds that of species-poor communi- ties (Cardinale et al. 2006). In most cases this is attributed to niche partitioning among species, or interspecific facilitation of resource capture (Hooper et al. 2005). In either case, diversity effects are generally envisioned as operating through the consumption of resources. However, multispecies interactions also may be mediated by changes in behavior, which may be reflected in biomass changes that are one or more trophic levels removed from the inciting species (Turner and Mittelbach 1990, Schmitz et al. 1997). For example, predator-avoidance behaviors by herbivores often come at the cost of lost feeding opportunities, leading to reduced plant damage even when herbivore numbers do not change (Lima and Dill 1990, Werner and Peacor 2003, Preisser et al. 2005). Such behaviorally mediated indirect effects can be as strong as those induced by the actual consumption of intermediary species (Schmitz et al. 2004). Analyses confined to predator and prey demographics, therefore, may significantly underesti- mate the effects of predators on community structure (Abrams 1995, Brown et al. 1999, Byrnes et al. 2006, Goudard and Loreau 2008). For predators, there is good reason to expect complex interactions between consumer diversity and the relative strengths of nonconsumptive and consumptive effects (Schmitz 2007). Behavioral interactions are particularly important in predator communities (Sih et al. 1998), perhaps contributing to the widely varying diversity– consumption relationships observed at this trophic level (Ives et al. 2005, Bruno and Cardinale 2008). Prey consumption can decline at higher diversity levels when intraguild predation is common (Finke and Denno 2004) but increase when predators partition prey (Bruno and O’Connor 2005, Wilby et al. 2005, Finke and Snyder 2008) or facilitate one another’s prey capture (Soluk and Collins 1988). Thus interactions between predator diversity and predator consumptive effects themselves often are nonadditive (Ives et al. 2005). Similarly, the consumptive and nonconsumptive effects of single predators can be positively or negatively nonadditive (Peacor and Werner 2001, Griffin and Thaler 2006). It is interesting that in terms of predator–predator intimidation, there is not a clear trend as to whether interspecific interactions tend to differ from intraspecific interactions. Studies isolating inter- and intraspecific competition among predators have shown that behav- ioral interference between heterospecific predators can be more severe than among conspecifics (e.g., Soluk and Collins 1988, Peckarsky 1991, Wissinger and McGrady 1993); however, one study has suggested the opposite Manuscript received 5 May 2009; revised 2 October 2009; accepted 29 October 2009. Corresponding Editor: R. J. Marquis. 1 Present address: Department of Entomology, Washing- ton State University, Tree Fruit Research and Extension Center, 1100 N. Western Ave., Wenatchee, Washington 98801 USA. E-mail: shawn_steffan@wsu.edu 2242