Individuals in most natural populations of predators vary in size, and size differences among individuals often result in cannibalism. Cannibalism is an extremely common phenomenon in the animal kingdom, particularly among generalist predators that engage in intraguild predation (IGP). However, few studies have specifically addressed the effects of cannibalism on IGP. The aim of my study was to investigate how trophic and behavioral interactions between 2 size classes of an intraguild (IG) predator influenced the survival and behavior of IG prey and a shared prey resource. I tested for these effects with larval odonates by exposing a shared prey resource (Ischnura verticalis) to the presence or absence of IG prey (Pachydiplax longipennis) and 2 size classes of IG predators (small or large Anax junius) in a 2 × 2 × 2 factorial design. Mortality rates of the shared resource in all single-predator treatments were significantly greater than in nonpredator controls, and risk reduction was observed when the shared resource was exposed to combinations of predators. The significant negative effect of large A. junius on P. longipennis survival and activity level was greater than that of small A. junius. Cannibalism occurred between large and small A. junius in size-structured IG predator treatments, and the effects of the size classes were not additive for the survival of IG prey. Cannibalism was not solely responsible for risk reduction in IG prey, and reduced activity level of small A. junius in the presence of larger conspecifics probably had a positive influence on P. longipennis survival. My results demonstrate that cannibalism among IG predators can influence the survival of IG prey and might contribute to coexistence among predators in systems with strong IGP.
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