Bullfrogs : Introduced Predators in Southwestern Wetlands

Fig.!. The worm has turned! In this unstaged photograph taken at Parker Canyon Lake, Cochise County, Arizona. 1964, an intro­ duced bullfrog is swallowing a Mexican garter snake. normally a frog-eating species. Such preda­ tion appears to be destroying remaining populations of this garter snake in the United States. I n the American Southwest, much of the native fish fauna is facing extinction (Mincldey and Deacon 1991); frogs in California (Fellers and Drost 1993) and frogs and garter snakes in Arizona (Schwalbe and Rosen 1988) are also in critical decline. Habitat destruction and intro­ duced predators appear to be primary causes of native frog declines (Jennings and Hayes 1994), and habitat modification often yields ponds and lakes especially suitable for introduced species. Introduced bullfrogs (Rana catesbeiana) have been blamed for amphibian declines in much of western North America (e.g., Hayes and Jennings 1986; Leonard et aL 1993; Vial and Saylor 1993). Extensive cannibalism by bull­ frogs renders them especially potent predators at the population leveL The tadpoles require only perennial water and grazeable plant mater­ ial; hence, transforming young can sustain a dense adult bullfrog population even if alternate prey are depleted. This may increase the proba­ bility that native species may be extirpated by bullfrog predation. Introduced predatory fishes are apparently an important cause of frog declines (Hayes and Jennings 1986). They have been strongly impli­ cated in one important case of decline of native ranid frog (family Ranidae, the "true" frogs; Bradford 1989). Some introduced crayfish may also be devastating in some areas (Jennings and Hayes 1994). In our study region, however, nei­ ther introduced fishes nor crayfish are dominant We present results that sustain a "bullfrog hypothesis" for some native ranid declines, and we present our study as an example of how evi­ dence accumulates to support such a hypothesis. In 1985 we began documenting historical localities for wetland herpetofaunas (reptiles and amphibians), based on museum records and personal interviews, then revisited these and additional areas to determine current species' status. Results of this process, plus circumstan­ tial evidence, suggested that the bullfrog was a primary cause for declines of leopard frogs and garter snakes in southern Arizona (Schwalbe and Rosen 1988). In 1986-89 and 1992-93 we conducted removal censuses of bullfrogs at Sa Bernardino National Wildlife Refug n (SBNWR), Cochise County, Arizona. We simultaneously monitored native ChiriCahue leopard frogs (R. chiricahuensis) and MexiCa~ garter snakes (Thamnophis eques) at the sites of bullfrog removal. A control site, with no bUll­ frog removal, was established in comparable habitat at Buenos Aires National Wildlife Refuge (BANWR), Pima County, Arizona.