Detecting small environmental differences: risk-response curves for predator-induced behavior and morphology

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 polymorphisms. 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 environmental 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 quantified 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 maximal phenotypic response for each trait, whether the different 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 fine-tuned, graded responses and most of them exhibited a plateauing response with increased predation risk, suggesting either a limit to plasticity or the reflection of high costs of the defensive phenotype. For many traits, a large proportion of the maximum induction occurred at low levels of risk, suggesting that the chemical cues of predation are effective 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 fine-grained changes (i.e., within generation) in their environment, and impacts on ecological communities via trait-mediated indirect effects.

Interactive effects of competition and predator cues on immune responses of leopard frogs at metamorphosis

“…Cages in treatments without predators were similarly disturbed. Previous experimental work shows that the concentration of predator cue used in this experiment is sufficient to induce plastic responses (Schoeppner and Relyea, 2008). The first predator feeding was on 11 May (defined as experiment day 1) and the last feeding was on day 45, just before leopard frog tadpoles began to metamorphose.…”

Top‐down effects on antagonistic inducible defense and offense

“…Adaptive phenotypic plasticity requires reliable signals, and many species appear to use signals that are associated with interaction intensity (Wiackowski and Staronska 1999, Van Buskirk and Arioli 2002, Relyea 2004, Schoeppner and Relyea 2008. For example, if a predator-prey pair is sensitive to the presence of a second predator in the system, then the intensity of their interactions and thus the dosage of cues released (by the primary predator) to induce the prey defense may diminish, thereby reducing the degree of inducible defense expression.…”

Training for Translocation: Predator Conditioning Induces Behavioral Plasticity and Physiological Changes in Captive Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis) (Cryptobranchidae, Amphibia)

“…Conditioned and unconditioned hellbenders reduced their activity by 70.1% and 40.8%, respectively, when presented with kairomones from one largemouth bass, but we did not observe further changes in chamber activity after exposure to high concentrations of kairomones. A threshold response following the addition of one predator is similar among other amphibians [65,66]. Wood frog tadpoles (Rana sylvatica) reduce their activity by 38% when presented with a single predator, but then show no additional differences in activity when two, four, or six predators are presented [65].…”

“…A threshold response following the addition of one predator is similar among other amphibians [65,66]. Wood frog tadpoles (Rana sylvatica) reduce their activity by 38% when presented with a single predator, but then show no additional differences in activity when two, four, or six predators are presented [65]. Oppositely, pool frog tadpoles increase the proportion of inactive individuals by 22% when they are presented with a single caged predator, but have no additional changes when three more caged predators are added to the same holding tank [66].…”