Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions Andrew M. Turner Turner, A. M. 2004. Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions. / Oikos 104: 561  /569. A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non-lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non-lethal effects of predators on preygrowth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis . Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rateswere reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non-lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system-level properties. A. M. Turner, Dept of Biology, Clarion Univ., Clarion, PA 16214, USA (aturner@clarion.edu). Animals often respond to increased predation risk by altering their behavior, morphology, physiology, or life history (Kats and Dill 1998, Lima 1998a, Tollrain and Harvell 1999). These trait shifts usually engender a number of costs, including a reduction in the effort devo- ted to foraging (Lima and Dill 1990, Werner and Anholt 1996, Lima 1998a). If resource availability remains unchanged, reduced foraging effort will result in prey acquiring fewer resources, and ultimately depressed growth rates, reproductive effort, and population size (Werner et al. 1983, Tonn et al. 1992, Walters and Juanes 1993, Diehl and Eklo ¨v 1995). In this way predators may have substantial non-lethal effects on prey population dynamics (Lima 1998b, Werner and Peacor 2003). A large number of studies show, however, that resource availability is not static, but instead is a function of consumer foraging rate (among other things). Consider, for example, a simple three-level food chain of predators, consumers, and resources. Increased predation can reduce the number of consu- mers foraging on resources, which in turn can result in higher resource standing crops (e.g. a trophic cascade, Leibold et al. 1997, Pace et al. 1999) and higher resource availability for the remaining consumers (Paine 1980, Morin 1986, Wilbur 1987). Similarly, a reduction in the foraging effort of consumers, induced by increased predation risk, can also result in higher resource avail- ability (a trait mediated trophic cascade, Turner and Accepted 11 August 2003 Copyright # OIKOS 2004 ISSN 0030-1299 OIKOS 104: 561  /569, 2004 OIKOS 104:3 (2004) 561