Effect of Prey Vulnerability on Population Size Structure of a Gape‐Limited Predator

Abstract

It is becoming apparent that population size structure can significantly affect ecological interactions (Ebenman and Persson 1988). Theoretical and empirical studies, for example, show how population size structure can affect population regulation (Lomnicki 1988) and community structure (Neill 1988, Persson 1988). To understand fully how size structure affects ecological interactions, we must understand the factors that contribute to size structure of populations. Size variation within a population can arise from age differences among individuals (e.g., different year classes or asynchrony in breeding) or growth differences among individuals of the same age. Increased size variation within even-aged populations is generally considered to result from unequal food intake among individuals (Lomnicki 1988), which can be affected by the intensity of competition (Begon 1984, Maret and Collins 1994), the relative strengths of exploitative versus interference competition (Lomnicki 1988, Smith 1990), and differences among individuals in foraging behavior under risk of predation (Turner and Mittelbach 1990). We hypothesized that prey vulnerability could be another factor influencing population size structure. If, in a population of gape-limited predators, some types of prey are not equally vulnerable to all individuals, unequal food intake may occur (Smith and Petranka 1987), resulting in increased size variation among individuals. In a gape-limited predator, prey vulnerability is a function of prey size relative to predator size, and this disparity in size, not abundance of prey alone, determines whether or not prey can be caught or eaten by a predator (Smith and Petranka 1987, Dong and Polis 1992). Factors influencing relative size of predator and prey thus determine a predator's success. In instances of cannibalism, population size structure (i.e., relative distribution of small to large conspecifics) can have a major influence on the ratio of potential predators to prey within a population (Crowley et al. 1987, Wissinger 1989, Dong and Polis 1992). Understanding the factors affecting size structure can be fundamental to understanding what controls frequency of cannibals in a population.