Introduced Species

(September 23, 2003)

Humans have spread non-native animals and plants around the world and they have done so at an unprecedented rate in the last century. The establishment and spread of exotic species are a major threat to worldwide biodiversity (Drake and ICSU 1989, Cox 1999). Amphibians have been greatly affected by introduced species through direct predation and competition. In addition, recent studies have found that other factors of decline, such as habitat modification (Adams 2000), chemical contaminants (Relyea Rick and Mills 2001), UV-B (Kats et al. 2000), and disease (Kiesecker et al. 2001) work synergistically to exacerbate the negative affects of introduced species on native amphibians. Below we describe a few well studied examples.

Introduced trout and the decline of the mountain yellow-legged frog

Prior to the mid-1800s, more than 99% of the lakes and ponds in the Sierra Nevada above 7,000 ft were fishless (Knapp 1996). Documented historical accounts from Joseph Grinnell and Tracy Storer (published in 1924) from the University of California's Museum of Vertebrate Zoology claim that the mountain yellow-legged frog, Rana muscosa, was once the most common vertebrate in these high elevation ponds and lakes. Starting in the mid-1800’s, trout have been introduced throughout the Sierra Nevada for sport fishing. Now, more than 80% of these naturally-fishless lakes contain non-native trout and R. muscosa has declined from greater than 80% its historic range (Bradford et al. 1994). The most widely introduced non-native trout species include rainbow trout (Oncorhynchus mykiss), golden trout (Oncorhynchus mykiss aguabonita), rainbow trout x golden trout hybrids, brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta) (Knapp 1996). Introduced trout prey on R. muscosa (Needham and Vestal 1938; Mullally and Cunningham 1956) and several studies have implicated introduced trout as one of the main sources of their decline (Bradford 1989; Bradford et al. 1993; Jennings 1996; Knapp 1996; Drost and Fellers 1996).

R. muscosa and trout photos © Vance Vredenburg

Despite the extensive evidence that trout are capable of eliminating R. muscosa populations, they are still being stocked in wilderness areas of the US forest service. However, fish stocking halted in many but not all national parks in North America between 1970 and 1980 (Knapp et al. 2001). A recent study compared the occurrence of frogs and fish in Kings Canyon National Park to those in the John Muir Wilderness and found that frogs were more abundant in the National Park whereas fish where more abundant in the wilderness area (Knapp and Matthews 2000). Field experiments have shown that when non-native trout are removed from lakes, mountain yellow-legged quickly re-colonize (Vredenburg, Knapp, Milliron unpublished data from three separate studies). These re-colonization events give hope that this frog can return to being the most common vertebrate in the high elevation Sierra Nevada if they can also withstand disease, contaminants and UV radiation. If you would like to learn more about R. muscosa visit the species account on AmphibiaWeb Rana muscosa or visit www.mylfrog.info.

Other Fish and Other Places

In addition to non-native trout in the high elevation Sierra Nevada, brown trout and rainbow trout have also been introduced throughout the range of the spotted tree frog (Litoria spenceri) in South-Eastern Australia (Gillespie 2001). L. speceri has suffered severe population declines (it is listed nationally as critically endangered, see our AmphibiaWeb watch list) and the geographic patterns of decline correspond to the pattern of trout introductions, suggesting that trout are a significant factor in their declines. In addition, Gillespie (2001) conducted field and laboratory experiments to examine the effect of introduce trout on larval L. spenceri and found that brown trout significantly reduce survivorship.
world map of amphibian declines
Litoria spenceri photo ©2002 Jean-Marc Hero

Besides trout, other fish species have been introduced accidentally and intentionally into freshwater habitat worldwide. Several Centrarchid fish species, such as bluegill sunfish (Lepomis sp.) and smallmouth bass (Micropterus dolomieui), have been widely introduced throughout Western North America (Moyle and Light 1996), where they pose serious threats to amphibian populations. For example, Adams (2001) found that, in an enclosure experiment, bluegill sunfish (Lepomis gibbosus) decrease the survival of the northern red-legged frog (Rana aurora) and pacific treefrogs (Hyla regilla).
Red-legged frog and bluegill
Rana aurora photo ©2003 William Flaxington and Lepomis macrochirus photo ©2002 John White

Mosquitofish (Gambusia affinis) have been introduced throughout the world to control mosquito populations and these introductions have had negative affects on amphibians. In experimental studies, mosquitofish decreased the survival of larval pacific treefrogs (Hyla regilla) (Goodsell and Kats 1999) and California newts (Taricha torosa) (Gamradt and Kats 1996) and inflicted tail injury, reduced metamorph size and altered activity patterns of larval California red-legged frogs (R. draytonii ) (Lawler et al. 1999).

The American Bullfrog
Rana catesbeiana

The American bullfrog, Rana catesbeiana, has been accidentally and intentionally introduced throughout the world. Their large size, high mobility, generalized eating habits, and huge reproductive capabilities, have made bullfrogs extremely successful invaders and a threat to biodiversity. They have successfully spread through much of the Western United States and British Columbia and are currently, spreading through Europe, Asia and South America. In their native range, because of their large size and aggressive behavior, they are important competitors and predators that influence the presence and abundance of other frog species (Hecnar and M'Closkey 1997). In their introduced range, bullfrogs pose a serious threat to several sensitive aquatic species (Rosen and Schwalbe 1995).

In California, the abundance of the federally threatened California red-legged frog (Rana draytonii) is negatively correlated with the presence of introduced bullfrogs (Moyle 1973, Fisher and Shaffer 1996). Lawler et al. (1999) found that in the presence of bullfrog tadpoles, the survivorship of California red-legged tadpoles was reduced to 5% from 34% in artificial ponds. In field enclosures, bullfrog adults and larvae significantly affected the growth, development and survivorship of tadpoles of the northern red-legged frog (Rana aurora), and adult bullfrogs decreased metamorph survival by 33% (Kiesecker and Blaustein 1998).

In Arizona, R. catesbeiana appears to be responsible for the declines in native leopard frog populations, Rana yavapaiensis and R. chiricahuensis. Schwalbe and Rosen (1988) found only one site out of 80 where R. yavapaiensis and R. chiricahuensis coexist with R. catesbeiana. Out of 252 stomach contents examined, 14.6% contained vertebrates and the dominant vertebrate found was other anurans, suggesting that predation may be a significant factor.

Literature Cited

Adams, M. J. 2000. Pond permanence and the effects of exotic vertebrates on anurans. Ecological applications 10:559-568.

Bradford David, F., M. Graber David, and F. Tabatabai. 1994. Population declines of the native frog, Rana muscosa, in Sequoia and Kings Canyon National Parks, California. Southwestern Naturalist 39:323-327.

Bradford, D. F., F. Tabatabai, and D. M. Graber. (1993). "Isolation of remaining populations of the native frog, Rana muscosa, by introduced fishes in Sequoia and Kings Canyon National Parks, California." Conservation Biology, 7, 882-888.

Bradford, D. F. 1989. "Allotopic distribution of native frogs and introduced fishes in high Sierra Nevada lakes of California: implication of the negative effect of fish introductions." Copeia, 1989, 775-778.

Bury, B. R., and A. J. Whelan. 1984. Ecology and Management of the Bullfrog. United States Department of the Interior, Fish and Wildlife Service, Resource Publication 155, Washington D.C.

Cox, G. W. 1999. Alien species in North America and Hawaii : impacts on natural ecosystems. Island Press, Washington, D.C.

Drake, J. A., and International Council of Scientific Unions. Scientific Committee on Problems of the Environment. 1989. Biological invasions : a global perspective. Published on behalf of the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU) by Wiley, Chichester ; New York.

Fisher, R. N., and H. B. Shaffer. 1996. The decline of amphibians in California's Great Central Valley. Conservation Biology 10:1387-1397. Gamradt, S., C., and L. Kats, B. 1996. Effect of introduced crayfish and mosquitofish on California newts. Conservation Biology 10:1155-1162.

Gillespie, G. R. 2001. The role of introduced trout in the decline of the spotted tree frog (Litoria Spenceri) in south-eastern Australia. Biological Conservation 100:187-198.

Goodsell, J., A., and L. Kats, B. 1999. Effect of introduced mosquitofish on Pacific treefrogs and the role of alternative prey. Conservation Biology 13:921-924.

Hayes, P. M., and R. M. Jennings. 1986. Decline of Ranid Frog Species in Western North America: Are Bullfrogs (Rana catesbiana) Responsible? Journal of Herpetology 20:490-509.

Hecnar, S. J., and R. T. M'Closkey. 1997. Changes in the composition of a ranid frog community following bullfrog extinction. American Midland Naturalist 137:145-150.

Jennings, W.B. (1996). "Status of amphibians." "." Sierra Nevada Ecosystem Project, Final Report to Congress, 2, 921-944.

Jennings, R. M. 1988. Natural History and Decline of Native Ranids in California. Pages 61-72 in H. F. DeLisle, B. K. Brown, and B. M. McGurty, editors. Proceedings of the Conference on California Herpetology, Special Publication.

Kats, L. B., J. M. Kiesecker, D. P. Chivers, and A. R. Blaustein. 2000. Effects of UV-B radiation on anti-predator behavior in three species of amphibians. Ethology 106:921-931.

Kiesecker, J. M., and A. R. Blaustein. 1998. Effects of introduced bullfrogs and smallmouth bass on microhabitat use, growth, and survival of native red-legged frogs (Rana aurora). Conservation Biology 12:776-787.

Kiesecker, J. M., A. R. Blaustein, and C. L. Miller. 2001. Transfer of a pathogen from fish to amphibians. Conservation Biology 15:1064-1070.

Knapp, R., A., P. Corn, Stephen, and D. Schindler, E. 2001. The introduction of nonnative fish into wilderness lakes: Good intentions, conflicting mandates, and unintended consequences. Ecosystems [print] 4:275-278.

Knapp, R. A., and K. R. Matthews. 2000. Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conservation Biology 14:1-12.

Lawler, S. P., D. Dritz, T. Strange, and M. Holyoak. 1999. Effects of introduced mosquitofish and bullfrogs on the threatened California red-legged frog. Conservation Biology 13:613-622.

Moyle, P. B. 1973. Effects of introduced bullfrogs, (Rana catesbeiana), on the native frogs of the San Joaquin Valley, California. Copeia 1:18-22.

Moyle, P. B., and T. Light. 1996. Fish invasions in California: Do abiotic factors determine success? Ecology (Washington D C) 77:1666-1670.

Mullally, D.P., and J.D. Cunningham (1956). "Ecological relations of Rana muscosa at high elevations in the Sierra Nevada." Herpetologica, 12, 189-198.

Needham P.H., and E.H.Vestal. (1938). "Notes on growth of golden trout (Salmo aguabonita) in two High Sierra Lakes." California Fish and Game, 24, 273-279.

Relyea Rick, A., and N. Mills. 2001. Predator-induced stress makes the pesticide carbaryl more deadly to gray treefrog tadpoles (Hyla versicolor). Proceedings of the National Academy of Sciences of the United States of America [print] 98:2491-2496.

Rogers, P. C. 1996. Rana catesbiana (Bullfrog) predation. Herpetological Review 27.

Rosen, P. C., and C. R. Schwalbe. 1995. Bullfrogs: introduced predators in southwestern wetlands. Pages 452-454 in E. T. LaRoe, G. S. Farris, C. E. Puckett, DoranP.D., and M. J. Mac, eds., editors. Our living resources: a report to the nation on the distribution, abundance, and health of U.S. plants, animals, and ecosystems. U.S. Depatment of the Interior, National Biological Service, Washington, DC.

Werner, E. E., G. A. Wellborn, and M. A. McPeek. 1995. Diet composition in postmetamorphic bullfrogs and green frogs: Implications for interspecific predation and competition. Journal of Herpetology 29:600-607.