Predator cannibalism can intensify negative impacts on heterospecific prey

Abstract

Although natural populations consist of individuals with different traits, and the degree of phenotypic variation varies among populations, the impact of phenotypic variation on ecological interactions has received little attention, because traditional approaches to community ecology assume homogeneity of individuals within a population. Stage structure, which is a common way of generating size and developmental variation within predator populations, can drive cannibalistic interactions, which can affect the strength of predatory effects on the predator's heterospecific prey. Studies have shown that predator cannibalism weakens predatory effects on heterospecific prey by reducing the size of the predator population and by inducing less feeding activity of noncannibal predators. We predict, however, that predator cannibalism, by promoting rapid growth of the cannibals, can also intensify predation pressure on heterospecific prey, because large predators have large resource requirements and may utilize a wider variety of prey species. To test this hypothesis, we conducted an experiment in which we created carnivorous salamander (Hynobius retardatus) populations with different stage structures by manipulating the salamander's hatch timing (i.e., populations with large or small variation in the timing of hatching), and explored the resultant impacts on the abundance, behavior, morphology, and life history of the salamander's large heterospecific prey, Rana pirica frog tadpoles. Cannibalism was rare in salamander populations having small hatch-timing variation, but was frequent in those having large hatch-timing variation. Thus, giant salamander cannibals occurred only in the latter. We clearly showed that salamander giants exerted strong predation pressure on frog tadpoles, which induced large behavioral and morphological defenses in the tadpoles and caused them to metamorphose late at large size. Hence, predator cannibalism arising from large variation in the timing of hatching can strengthen predatory effects on heterospecific prey and can have impacts on various, traits of both predator and prey. Because animals commonly broaden their diet as they grow, such negative impacts of predator cannibalism on the heterospecific prey may be common in interactions between predators and prey species of similar size.

Antagonistic indirect interactions between large and small conspecific prey via a heterospecific predator

“…Trait‐mediated indirect effects through predator growth can occur when the population of the predator species is size‐structured, for example, if cannibalism, a size‐ dependent behavior, enhances the growth of the cannibalistic individuals. In the same amphibian predator–prey system, Takatsu and Kishida () found that cannibalism by large salamander hatchlings on small conspecific hatchlings greatly enhances the growth of cannibalistic salamanders, and these bigger salamanders subsequently exert intensive predation pressure on large tadpoles. These results reported by Takasu and Kishida (2015), together with the present results, suggest that trait‐meditated apparent competition between prey items can occur via predator growth: one prey item is vulnerable to predation and causes the predator to grow rapidly, because it is a nutrient‐rich food source, and the other is less vulnerable to predation unless the predator grows bigger.…”

Foraging traits of native predators determine their vulnerability to a toxic alien prey

“…Occasionally, R. pirica frog tadpoles can feed on living animal prey if the prey is small and weak (e.g., amphibian larvae in dying condition). Hynobius retardatus salamander larvae, by contrast, are strictly carnivorous, preying mainly on amphibian larvae including R. pirica tadpoles and conspecifics (Takatsu & Kishida, ).…”

Body size variation in aquatic consumers causes pervasive community effects, independent of mean body size

“…Therefore, the variably sized tadpole populations should lead to an increase in the abundance of zooplankton when newts are present. In some systems, however, the enhanced growth of predators may enhance their capacity to feed upon alternative prey, resulting in greater predation pressure (Takatsu & Kishida, ); a form of apparent competition (Holt, ), although in our study, there are no alternative prey that are larger than the tadpoles. Finally, changes in tadpole abundance and foraging may also indirectly influence zooplankton populations.…”