Effects of Abiotic Disturbance on Coexistence of Predator‐Prey Fish Species

Behavioral responses of animals to large—scale abiotic disturbance are poorly known. Organisms living in habitats with frequent perturbations may evolve behaviors that minimize the impact of, or exploit, the disturbance. Organisms from environments that have fewer or different perturbations should not evolve these behaviors. The Sonoran topminnow (Poeciliopsis occidentalis) a poeciliid fish native to the arid American southwest, and the mosquitofish (Gambusia affinis), a morphologically similar poeciliid introduced from the mesic central and eastern United States, occur sympatrically in a few Arizona localities. Since flash floods of the mountainous Southwest are quantitatively and qualitatively different from the typical spring floods of mesic lowland drainages, I predicted that introduced G. affinis populations would incur greater losses during Southwestern floods than would native P. occidentalis. This was supported by preflood and postflood populations surveys in a natural habitat and by subsequent laboratory simulations of floods. The native species apparently possesses innate behaviors, such as quick response and proper orientation to high discharge, that allow its persistence under flash flood conditions. Mosquitofish show little such behavior and are more easily displaced by floods. Mosquitofish typically extirpate topminnows from native habitats via predation within 1—3 yr after introduction. Replacement is most rapid in localities that rarely or never flood, while long—term coexistence may occur in frequently flooded habitats. Abiotic disturbances may promote coexistence by periodically reducing population sizes of the exotic predator before the native prey species is eliminated. This hypothesis is consistent with data from a variety of other Arizona aquatic systems containing communities of native prey and introduced predatory fishes.