Oecologia (2008) 154:743–754 DOI 10.1007/s00442-007-0862-4 123 COMMUNITY ECOLOGY - ORIGINAL PAPER Detecting small environmental diVerences: risk-response curves for predator-induced behavior and morphology Nancy M. Schoeppner · Rick A. Relyea Received: 15 November 2006 / Accepted: 7 September 2007 / Published online: 6 October 2007  Springer-Verlag 2007 Abstract Most organisms possess traits that are sensitive to changes in the environment (i.e., plastic traits) which results in the expression of environmentally induced poly- morphisms. While most phenotypically plastic traits have traditionally been treated as threshold switches between induced and uninduced states, there is growing evidence that many traits can respond in a continuous fashion. In this experiment we exposed larval anurans (wood frog tadpoles, Rana sylvatica) to an increasing gradient of predation risk to determine how organisms respond to small environmen- tal changes. We manipulated predation risk in two ways: by altering the amount of prey consumed by a constant number of predators (Dytiscus sp.) and by altering the number of predators that consume a constant amount of prey. We then quantiWed the expression of predator-induced behavior, morphology, and mass to determine the level of risk that induced each trait, the level of risk that induced the maxi- mal phenotypic response for each trait, whether the diVer- ent traits exhibited a plateauing response, and whether increasing risk via increasing predator number or via increasing prey consumption induced similar phenotypic changes. We found that all of the traits exhibited Wne-tuned, graded responses and most of them exhibited a plateauing response with increased predation risk, suggesting either a limit to plasticity or the reXection of high costs of the defensive phenotype. For many traits, a large proportion of the maximum induction occurred at low levels of risk, sug- gesting that the chemical cues of predation are eVective at extremely low concentrations. In contrast to earlier work, we found that behavioral and morphological responses to increased predator number were simply a response to increased total prey consumption. These results have important implications for models of plasticity evolution, models of optimal phenotypic design, expectations for how organisms respond to Wne-grained changes (i.e., within gen- eration) in their environment, and impacts on ecological communities via trait-mediated indirect eVects. Keywords Environmental sensitivity · Threshold response · Graded response · Continuous variation Introduction Phenotypic plasticity is a common response to changing environments. Organisms that possess adaptive plastic traits have the ability to alter their traits in response to envi- ronment cues to produce phenotypes that perform better under the new environmental conditions (Schlichting and Pigliucci 1998; West-Eberhard 2003; DeWitt and Scheiner 2004). The range of phenotypes produced in response to environmental changes depends upon the gradient of the environmental factor that is experienced and the sensitivity of the organism in detecting and responding to environmen- tal change. If sensitivity is low, organisms may only detect Communicated by Steven Kohler. Electronic supplementary material The online version of this article (doi:10.1007/s00442-007-0862-4) contains supplementary material, which is available to authorized users. N. M. Schoeppner · R. A. Relyea Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA N. M. Schoeppner (&) School of Biology, Georgia Institute of Technology, Atlanta, GA, USA e-mail: nschoeppne3@mail.gatech.edu