(hii l.ivuv^ Resources — Ncii-nanvc Species 



453 



snakes in ihe wild (Fitch l%5). All of the larg- 

 er, older Mexican gailer snakes have damaged 

 tails from repeated bullfrog bites, and the 

 largest and oldest one was found dying in 1993 

 with gross intlammation of the tail. It appears 

 that without successful reproduction by some of 

 these old snakes, the study population will 

 shortly disappear. 



Bullfrog Removal Experiments 



Before 1993 intensive bullfrog removals 

 were conducted two to three times per year at 

 SBNWR. At one study pond, 854 large (80+ 

 mm body length! bullfrogs had been removed 

 from about 0.2 ha (0.5 acre) of habitat. After the 

 3 to 4 active-season months between removals, 

 we saw a 50^-80% rebound toward preremoval 

 numbers, and we observed weak evidence of 

 positive effects on native leopard frogs and 

 garter snakes (Schwalbe and Rosen 1988). 

 Because a bullfrog can have as many as 20,000 

 eggs per clutch and has multiple clutches each 

 year, the bullfrog was clearly uncontrollable at 

 our initial level of effort. 



Starting in 1993, we increased our efforts to 

 remove bullfrogs from SBNWR by eliminating 

 adult bullfrogs and catching juveniles as they 

 matured. 



Discussion 



If adult-free bullfrog populations are attained 

 at SBNWR during 1994, we predict that this will 

 result in successful recruitment of juvenile 

 Mexican garter snakes. We propose to translo- 

 cate leopard frogs from nearby areas into fenced, 

 newly created, bullfrog-free ponds. A primary 

 objective is to have at least one natural area to 

 save genetic stock of the local leopard frogs. 



The SBNWR, with its numerous highly pro- 

 ductive water sources, was probably a historical 

 regional metapopulation (a set of populations 

 connected by immigration and emigration) cen- 

 ter (Gilpin and Hanski 1991) for leopard frogs. 

 During times of drought, it was likely the main- 

 stay of the species in the San Bernardino Valley 

 system. Some of the unexplained frog declines 

 in western North America (Gary 1993) may ulti- 

 mately be traceable to catastrophic, localized 

 extinctions in such refugia (Sjogren 1991; 

 Bradford et al. 1993). An observation of proba- 

 ble rapid migratory spread by an introduced 

 leopard frog species in Arizona ( 1 2 km/yr: Platz 

 et al. 1990) suggests that individuals do disperse 

 enough to consider metapopulation models. 

 Information related to metapopulation phenom- 

 ena could markedly enhance management for 

 leopard frogs. 



It is notable that the checkered garter snake, 

 with an evolutionary background of geographi- 



cal overlap with bullfrogs, succeeds with intro- 

 duced bullfrogs in the West. Similarly, the acci- 

 dentally introduced and rapidly spreading Rio 

 Grande leopard frog {Rana herUindieri) in 

 Arizona (Platz et al. 1990) also evolved with 

 bullfrogs. In fact, this leopard frog is spreading 

 into areas where the endemic Yavapai leopard 

 frog (/?. yavupciiensis) has been extirpated, 

 probably by introduced predators as well as 

 habitat alteration (Vitt and Ohmart 1978; 

 Jennings and Hayes 1994). 



Conclusion 



Introduced predators such as the bullfrog 

 can have devastating effects on faunas that 

 evolved without equivalent predatory types. The 

 bullfrog, as an exotic in the absence of key orig- 

 inal enemies (the basses, pikes, snapping tur- 

 tles, and water snakes of the eastern United 

 States), attains tremendous population densi- 

 ties. Such non-native predators, in core popula- 

 tion areas of native species, can lead to regional 

 extinctions, and may account for some unex- 

 plained amphibian declines. 



We now have abundant documentation that 

 introduced predators, especially fish, crayfish, 

 and bullfrogs, have caused major declines of 

 frogs and other species in western North 

 America. In Arizona, cuirent trends suggest that 

 inaction could lead to disappearance of three of 

 five native leopard frog species within a decade. 

 We urge, in addition to simply monitoring 

 declines, active management where appropriate, 

 within a controlled and documented frainework. 

 There is a pressing need for a practical, suc- 

 cessful, and vigorously supported management 

 strategy to preserve genetic stocks and restore 

 habitats of native ranid frogs. 



References 



Bradforcl, D. 1989. Allotopic distribution of native frogs and 

 introduced fisties in higfi Sierra Nevada lakes of 

 California: implication of the negative effect of fish 

 nitroduction. Copeia 989:775-778. " 



Bradford, D.F., F Tabatabai, and D.M. Graber. 199.1. 

 Isolation of remaining populations of the native frog, 

 Rana miiscosa. by introduced fishes in Sequoia and 

 Kings Canyon national parks. California. Conservation 

 Biology 7:882-888. 



Bury. R.B., and J. A. Whelan. 1984. Ecology and manage- 

 ment of the bullfrog. U.S. Fish and Wildlife Service 

 Resour. Publ. 155. 23 pp. 



Cary. C. 199.1. Hypothesis concerning the causes of the dis- 

 appearance of boreal toads from the mountains of 

 Colorado. Conservation Biology 7:355--162. 



Fellers. G.M.. and C.A. Drost. 1993. Disappearance of the 

 Cascades frog Rana cascadae at the southern end of its 

 range, California, USA. Biological Conservation 65:177- 

 181. 



Fitch. H.S. 1965. An ecological study of the garter snake, 

 Thanmophis sirtalis. University of Kansas Publications 

 of the Museum of Natural History 15:493-564. 



Gilpin, M.E., and I. Hanski. 1991. Metapopulation dynam- 

 ics. Academic Press, London. 336 pp. 



Canelo Hills Region 

 25 



20- 



!15- 



LiL 



1 50 250 350 450 550 650 750 850 950 



Body length (mm) 



San Bernardino National Wildlife Refuge 

 10 



1 50 250 350 450 550 650 750 850 950 



Body lengtfi (mm) 



Fig. 2. Population structure of the 

 Mexican garter snake. Numerous 

 young snakes (200-700 mm, 1-3 

 years old) show successful repro- 

 duction in apparently intact popu- 

 lations (top), whereas bullfrog- 

 affected populations (bottom) are 

 composed mainly of older (700- 

 1 ,000 mm. 3+ years old) snakes. 



