PREDATION AND SPECIES REPLACEMENT IN AMERICAN SOUTHWESTERN FISHES: A CASE STUDY

Declines in distribution and abundance of native fishes of the American Southwest have been noted for decades, but mechanisms leading to these losses are unclear. The Sonoran topminnow (Poeciliopsis occidentalis) was studied to determine mechanisms leading to local extirpation, as a model of southwestern fish extinctions. Although habitat destruction is the cause of several population losses, interactions with introduced mosquitofish (Gambusia affinis) are responsible for reduction of topminnows through much of their native range. A series of laboratory and field experiments indicated that predation of juvenile topminnows by mosquitofish is a major factor in decline of the species. For lack of evidence, introduced parasites or diseases, hybridization, physiological stress, and resource competition are all dismissed as probable mechanisms of extirpation. Local extinction of native species is a common result of introduction of exotic organisms, and man-induced range extensions are a major threat to the integrity of natural communities (Elton, 1958). Despite potentially disastrous effects of introduced fishes on native communities (e.g., Miller, 1961; Zaret and Paine, 1973), little is known of mechanisms by which intruders replace natives. Although hypotheses such as competition, predation, and habitat alteration abound, few have been tested. Here, I report an experimental analysis of mechanisms involved in replacement of a native Sonoran Desert fish by a non-native species, and discuss losses of other southwestern fishes via exotics.