Competition between Predator and Prey: Resource‐Based Mechanisms and Implications for Stage‐Structured Dynamics

Abstract

In predator—prey interactions between size—structured populations, small (young) predators may compete with species that ultimately become their prey. We tested experimentally whether such competition occurs between young—of—year (YOY) largemouth bass and their eventual prey, bluegill. In a divided experimental pond, target densities of YOY bass and juvenile bluegill were subjected to different densities of bass and bluegill neighbors to examine the dependence of juvenile growth on fish density and species composition. After 7 wk, clear differences in growth rates of both species existed across densities. Bass growth was reduced in the presence of both bass and bluegill neighbors, whereas bluegill growth was primarily affected intraspecifically. Bluegill had strong competitive effects on bass despite substantial resource partitioning between the two species. These effects were mediated through changes in the size—structure of important invertebrate prey in both open water and vegetation habitats. Data collected from a set of seven lakes in Michigan indicate that intraspecific competition between juvenile bluegill and YOY bass is an important process in natural populations. However, once bass become piscivorous, bluegill become a significant resource for bass. As a result, bass populations are split into two functionally distinct stages that respond differently across a gradient of bluegill density. Because of the competitive stage between predator and prey, dynamics of the interaction differ from predictions based on classical predator—prey or competition theory.