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...ecosystem faces severe conservation problems, mostly derived from the extensive urbanization of California and northernmost Baja California. The resulting habitat modification has impacted some of the species of amphibians in southern California so strongly that the arroyo toad (Bufo californicus; sensu Gergus 1998) is officially considered endangered, and the red-legged frog (Rana aurora), threatened. In contrast with their diminished populations in southern California, these species, as well as other frogs and toads, fare much better in Baja California (Grismer 2002).
Besides knowledge of such better conditions, not much is known about the conservation status of amphibians in Baja California. There are a number of pressures that might impair the quality of their habitat, including development, land conversion, dams, sand-mining, gravel mining, roads, water extraction, refuse, chemical contamination, and invasive species (Robert E. Lovich, pers. comm.).
One potentially serious threat is the presence of invasive animals, as such species have impacted native taxa in other aquatic habitats of western North America (Bury and Luckenbach 1976; Dudley and Collins 1995). Alien fishes have been widely introduced to Mexico, including to Baja California (Contreras-B. and Escalante-C. 1984; Follett 1961; Mellink and Ferreira-Bartrina 2000). However, not enough information is available to judge the risks to amphibian conservation derived from invasive aquatic animals in the Mediterranean of Baja California. Here we report on field work carried out in order to obtain a first impression of such risks.
Methods
Between 3 March 1998 and 30 August 2001 we visited 24 sites with standing or running water within the mediterranean region of Baja California, from Arroyo El Rosario to the U.S. boundary (Table 1). Some of the sites were visited once; others, on multiple occasions. At most sites we visually searched for amphibians and captured them by hand or with hand-nets. Specimens were identified on the spot and, if captured, released. At most sites we also captured fishes with a 1 x 10-m beach seine with 3/16" mesh, and with a hand-net. The fishes were brought to the laboratory, in 70% ethyl alcohol, for identification. We examined...
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