COMPETITION BETWEEN PREDATOR AND PREY - RESOURCE-BASED MECHANISMS ANDIMPLICATIONS FOR STAGE-STRUCTURED DYNAMICS

In predator-prey interactions between size-structured populations, sma ll (young) predators may compete with species that ultimately become t heir prey. We tested experimentally whether such competition occurs be tween young-of-year (YOY) largemouth bass and their eventual prey, blu egill. In a divided experimental pond, target densities of YOY bass an d juvenile bluegill were subjected to different densities of bass and bluegill neighbors to examine the dependence of juvenile growth on fis h density and species composition. After 7 wk, clear differences in gr owth rates of both species existed across densities, Bass growth was r educed in the presence of both bass and bluegill neighbors, whereas bl uegill growth was primarily affected intraspecifically. Bluegill had s trong competitive effects on bass despite substantial resource partiti oning between the two species. These effects were mediated through cha nges in the size-structure of important invertebrate prey in both open water and vegetation habitats. Data collected from a set of seven lak es in Michigan indicate that interspecific competition between juvenil e 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 f unctionally distinct stages that respond differently across a gradient of bluegill density. Because of the competitive stage between predato r and prey, dynamics of the interaction differ from predictions based on classical predator-prey or competition theory.