Abstract Perturbations to the density of a species can be propagated to distant members of a food web via shifts in the density or the traits (i.e. behavior) of inter- mediary species. Predators with differing foraging modes may have different effects on prey behavior, and these effects may be transmitted differently through food webs. Here we test the hypothesis that shifts in the type of predator present in a food web indirectly affect the prey’s resource independent of changes in the density of prey. We assessed the importance of predator identity in mediating the grazing effects of the freshwater snail Physa integra on its periphyton resources using field and mesocosm studies. Field observations showed that Physa used covered habitats more in ponds containing fish than in ponds containing crayfish or no predators at all. A field experiment confirmed that snail behavior depended on predator identity. Physa placed near caged pumpkin- seed sunfish (Lepomis gibbosus) selected covered habi- tats, but Physa placed near caged crayfish (Orconectes rusticus) moved to the surface of the water. The effects of predator identity on periphtyon were then examined in a mesocosm experiment, using caged predators. Habitat use of Physa was similar to their habitat use in the field experiment. In the presence of caged sunfish, periphyton standing crop in covered habitats was reduced to 34% of the standing crop in the presence of crayfish. In contrast, periphyton in near-surface habitats was 110% higher in the presence of fish than in the presence of crayfish. Thus, the effects of predator identity on Physa behavior cascaded through the food web to affect the abundance and spatial distribution of periphyton. Keywords Predator identity · Trait-mediated indirect effects · Physa · Littoral food webs · Predator avoidance Introduction Changes in the abundance of predators affect not only the population size of foragers, but also their behavioral, mor- phological, life-historical, and physiological traits (re- views in Kerfoot 1987; Lima 1998). Because the per-capi- ta effect of foragers on other species may depend on their phenotypic traits, perturbations to the abundance of top predators can be transmitted through a food web via shifts in the traits of intermediary species, independent of any density effects (Turner and Mittelbach 1990). Interactions in which the per-capita effects of one species on another depend on the abundance of a third species have been called higher-order interactions (Abrams 1983; Werner 1992), interaction modifications (Wooton 1994), and trait- mediated indirect interactions (Abrams 1995; Abrams et al. 1996). Theory suggests that trait-mediated indirect ef- fects may be common and important in structuring food webs (Abrams 1992, 1995; Werner 1992), and a few em- pirical studies demonstrate that the strength of trait-medi- ated indirect effects can rival that of density-mediated in- direct effects (Peacor and Werner 1997; Schmitz et al. 1997; Peacor and Werner 2000; Relyea 2000). To predict the occurrence of trait-mediated indirect effects, community ecologists must first consider the specific features of predators that induce trait shifts in foragers. Most studies on trait-mediated indirect interac- tions have focused on predator-induced shifts in habitat use and foraging activity (Turner and Mittelbach 1990; Peacor and Werner 1997; Turner 1997; Kuhara et al. 1999). Because predators vary in their hunting mode and distribution, predators with differing foraging modes have different effects on prey behavior (Sih et al. 1998; McIntosh and Peckarsky 1999; Turner et al. 1999). Thus, different predators will likely vary in the food web ef- fects they create due to differing effects on prey traits (McIntosh and Townsend 1996). R.J. Bernot ( ✉ ) · A.M. Turner Department of Biology, Clarion University of Pennsylvania, Clarion, PA 16412, USA e-mail: rjb5578@ksu.edu Tel.: +1-785-5326814, Fax: +1-785-5326653 Present address: R.J. Bernot, Division of Biology, Kansas State University, 232 Ackert Hall, Manhattan, KS 66506, USA Oecologia (2001) 129:139–146 DOI 10.1007/s004420100705 Randall J. Bernot · Andrew M. Turner Predator identity and trait-mediated indirect effects in a littoral food web Received: 23 October 2000 / Accepted: 27 February 2001 / Published online: 23 May 2001 © Springer-Verlag 2001