Conspecific density determines the magnitude and character of predator-induced phenotype

Abstract

The benefits in survival gained from predator-induced phenotypes often come at a cost to other components of fitness. Therefore, the level of expression of an induced phenotype should mirror the level of risk in the environment. When a predator exhibits a saturating functional response the risk of mortality to a given prey decreases as prey density increases. Therefore, for a given predator threat, investment in defense should be lower in prey at high density relative to those at low density. In this study, I test whether the magnitude of predator-induced morphological plasticity decreases with increasing conspecific density by exposing pine woods tree frog (Hyla femoralis) tadpoles at three different densities to predators (present or absent) in a factorial experiment. Tadpole morphology was not affected by changes in density in the absence of predators. However, predators had a significant, density-dependent effect on tadpole morphology. Specifically, the magnitude of morphological response was graded and larger for animals in the low density (high risk) environment. This study demonstrates that tadpoles can modulate phenotypic plasticity in response to mortality risk as a function of both the density of conspecifics and chemical cues from predators, which suggests that they are able to detect and respond to fine-scale changes in the threat environment. In addition, this study highlights the need for analytical approaches that allow morphological plasticity studies to elucidate allometric relationships in addition to simply quantifying size-corrected traits.

Interactive effects of predation risk and conspecific density on the nutrient stoichiometry of prey

“…Such a mechanism was supported previously by Van Buskirk et al. (2011) and McCoy (2007). Van Buskirk et al.…”

“…(2011) reported that Rana temporaria tadpoles respond behaviorally to per capita predation risks imposed by Aeshna cyanea dragonfly larvae and that these responses decreased as prey conspecific density increased. McCoy (2007), in turn, reported that morphological predator‐induced changes in Hyla chrysoscelis were reduced in response to increased conspecific density when subjected to predation risk imposed by the Lethocerus americanus giant water bug.…”

An inducible morphological defence is a passive by-product of behaviour in a marine snail

“…This approach provides an advantage over previous studies of inducible defences in that it allows one to distinguish between morphological differences that are a by-product of behaviour and growth versus a direct response to risk cues. It also aids in the interpretation of studies of phenotypic plasticity where morphological relationships can change with body size, and body size often varies among treatments (McCoy 2007). For example, because of differences in growth, high-food and crab-exposed snails differed in final size, making differences in shell mass and shell deposition between high-food and crab treatments difficult to interpret.…”

Conspecific density affects predator‐induced prey phenotypic plasticity

“…To date, predator-inducible morphological defenses in aquatic organisms have served as excellent model systems for testing predictions about the evolution and maintenance of phenotypic plasticity (Ghalambor et al 2007). However, it has been shown that prey population density (i.e., conspecific density) and its effects on the assessment of predation risk by the prey can be very important for determining the extent to which prey express anti-predator defenses (Roberts 1996, McCoy 2007, Van Buskirk et al 2011. Peacor (2003) has noted that prey need to estimate their own population density to better assess risk, especially if the prey detects the presence of a predator through predation cues.…”

“…Most of the studies that have evaluated the effects of conspecific density on predator-induced responses have aimed to test the role of prey resource competition in limiting phenotypic plasticity (Tejedo andReques 1994, Davenport andChalcraft 2014). Prey should be unable to express predator-induced phenotypes because resources are scarce and impose a physiological limitation on morphological development, a rationale that neglects the principle that conspecific density modifies the prey's perceived risk (McCoy 2007, Van Buskirk et al 2011). In addition, historically, many of the studies addressing the role of predator-induced defenses have examined the effects of conspecific density on a single plastic trait (e.g., activity levels) (Relyea 2001).…”