https://amphibiaweb.org/cgi/amphib_query?where-genus=Xenopus&where-species=laevis
Xenopus laevis (Daudin, 1802) African Clawed Frog
John J. Crayon1
Feral clawed frogs have been found in Colorado, Florida, Nevada, New Mexico, North Carolina, Texas, Utah, Virginia, Wisconsin, and Wyoming (Mardht and Knefler, 1973; Bacchus et al., 1993; Tinsley and McCoid, 1996; Blair et al., 1997), but have not established reproducing populations that have persisted over time. Arizona and California are the only states in which apparently permanent populations are known. Other extralimital populations have been documented in Great Britain, the Netherlands, Chile, and Ascension Island (Loveridge, 1959; Veloso and Navarro, 1988; Tinsley and McCoid, 1996).
Most authorities believe that the introduced populations of African clawed frogs found around the world are the subspecies Xenopus l. laevis, descendants of animals originally collected from native populations in South Africa (Carr et al., 1987; Tinsley and McCoid, 1996). There has been speculation that > 1 taxon may be established in California (Stebbins, 1985; M.R. Jennings, 1987a), which has not been confirmed by molecular-level investigations (Carr et al., 1987). All specimens examined from California populations have been morphologically identical to X. l. laevis. In addition, only females of X. l. laevis attain sizes greater than 100 mm in Africa (Kobel et al., 1996), and all long-established populations in California produce individuals larger than this.
Reproduction by feral clawed frogs has been documented in five states: Arizona, California, North Carolina, Virginia, and Wisconsin. The North Carolina and Wisconsin frogs have not persisted, largely due to winter temperature extremes (Tinsley and McCoid, 1996). The Virginia population, first noted in 1982 at the Gulf Branch Nature Center, Arlington, (Zell, 1986), was extirpated by the late 1980s (Tinsley and McCoid, 1996). The Arizona population was introduced in the 1960s to artificial bodies of water on the Arthur Pack Golf Course in Tucson and has remained confined to these sites (Tinsley and McCoid, 1996).
First found as a feral animal in California in 1968 (St. Amant and Hoover, 1969), clawed frogs have since become established in many Southern California drainages, including sites in Los Angeles, Orange, Riverside, San Bernardino, San Diego, Santa Barbara, and Ventura counties (McCoid and Fritts, 1980b; Lafferty and Page, 1997; S. Sweet, personal communication; D. Holland, personal communication). Citations listing clawed frogs in Imperial County (Mardht and Knefler, 1972; Stebbins, 1985) have never been substantiated. The discovery of disjunct populations in California is summarized in Table 8. The details surrounding the collection and reporting of almost all of these populations indicate that frogs were present for some period of time prior to their discovery, in some cases probably as long as several years.
The initial establishment of these widely separated, discontinuous populations was clearly the result of separate introduction events. Today, 25–30 yr after the initial discovery of these populations, clawed frogs have now spread throughout most of the drainages that contained the original release sites. For example, populations in Orange County have spread from the coastal plain in the north to the San Diego Creek and Upper Newport Bay drainages and the foothills of the Santa Ana Mountains as far south as Aliso Creek (B. Goodman, personal communication). In Orange County, clawed frogs have also spread throughout the Santa Ana River drainage into western Riverside and San Bernardino counties (B. Goodman, personal communication).
Clawed frogs originally discovered in tributaries of the Santa Clara River (Zacuto, 1975) have invaded the river and colonized both upstream (J. Dole, personal communication) and downstream to the river's mouth (Lafferty and Page, 1997), now inhabiting the tributaries in Agua Dulce, Soledad, Placerita, and San Francisquito canyons (Tinsley and McCoid, 1996; S. Bautista, personal communication). The clawed frogs first noted in Lake Munz have now colonized neighboring Hughes Lake and Elizabeth Lake and are found throughout the Leona Valley in Amargosa Creek and its tributaries (personal observations). In San Diego County, the Sweetwater, Otay, and Tijuana rivers are now colonized by clawed frogs (R. Fisher, personal communication). These patterns of dispersal mimic patterns of clawed frog dispersal in Africa, where frogs in rivers are carried downstream from breeding habitats, actively move upstream toward headwaters, and utilize human-created bodies of water as "stepping stones" to invade new habitats (van Dijk, 1977).
Some clawed frog populations have stayed confined to relatively discrete locations. The Goleta Slough and Edwards Air Force Base populations have not expanded (the latter primarily due to lack of suitable nearby habitat), and > 20 yr after their discovery, clawed frogs from Vail Lake have not moved downstream to invade the main part of the Santa Margarita River drainage (R. Fisher, personal communication).
Clawed frogs are intolerant of water loss (Hillman, 1980) and not capable of sustained overland travel through the xeric habitats of Southern California. In some areas, the sheer distance between suitable aquatic habitats may thus present an insurmountable barrier to invasion. Most of the spread of the species between drainages has likely been the result of human intervention.
Thirty years after their introduction to California, some patterns of distribution and dispersal of clawed frogs emerge: (1) populations are derived from independent introduction events in five of the counties they now inhabit (San Bernardino and Ventura counties were colonized from neighboring counties); (2) lotic (flowing water) systems are susceptible to complete colonization, including into their brackish interface with tidal waters (e.g., Santa Clara and Sweetwater rivers); (3) some climatic and biological barriers seem to prevent or retard their spread (discussed below); clawed frogs are not present in all apparently suitable habitats (e.g., Santa Barbara and Southern Riverside counties); (4) desert wetlands can sustain clawed frog populations (e.g., Piute Ponds on Edwards Air Force Base); (5) if human-aided introductions continue, there are few freshwater aquatic habitats in California that are not at risk for colonization. Waters that flow either rapidly all year or freeze over completely are among the few systems likely to remain free from invasion.
The introduction of the largemouth bass (Micropterus salmoides) has been implicated in the decline of clawed frogs in Africa, where these frogs are a popular fish bait (Hey, 1949; Mardht and Knefler, 1973). In Southern California, observers believe that the presence of fish has limited the expansion and/or population levels of clawed frogs (Zacuto, 1975; McCoid and Fritts, 1980a).
Introduced animals, including anurans, when released from the limiting effects of predation and interspecific competition, may reach higher densities at colonized sites than where they are indigenous (for anurans see Cohen and Howard, 1958; Lampo and Bayliss, 1996; Lampo and De Leo, 1998). At some sites clawed frogs exist in truly remarkable densities, for example in African fish-free lakes (Tinsley et al., 1996) as aquaculture pests (Schoonbee et al., 1979; Hepher and Pruginin, 1981; Prinsloo et al., 1981; Schramm, 1986), and in California sites free from predatory fish, where their numbers expand to fill the trophic levels fish would normally occupy (Crayon and Hothem, 1998). For example, visual surveys of surfacing frogs, combined with sampling by seining, produced a population estimate of at least 150,000 frogs at Piute Ponds on Edwards Air Force Base, Los Angeles County (unpublished data).
Efforts to eradicate clawed frogs in California are not usually successful (St. Amant, 1975; Zacuto, 1975). There is one known case in which a population in California was permanently eradicated by human efforts—the population on the University of California, Davis campus, Yolo County, cited above. The colonized habitat was a constructed, discrete body of water, which limited frogs to an area where they could be poisoned effectively.
Adult clawed frogs left the water during unsuccessful attempts to poison using rotenone at Vasquez Rock, the site of one of the first populations detected in the upper Santa Clara River drainage (St. Amant, 1975; Zacuto, 1975). Subsequent attempts to develop effective protocols for poisoning in the Santa Clara River also were unsuccessful (J. Dole, personal communication).