Introduced Species

May 14, 2008

Humans have spread non-native animals and plants around the world and 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. 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 of California 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-1800ís, 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).
Rana muscosa and non-native rainbow trout photo
Rana muscosa and non-native rainbow trout photos © Vance Vredenburg

With the extensive evidence that trout are capable of eliminating R. muscosa populations, fish stocking has been stopped in National Parks and Forest Service wilderness areas. 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 were more abundant in the wilderness area (Knapp and Matthews 2000). A trout removal study (Vredenburg 2004) at three Sierra Nevada lakes in Kings Canyon showed rapid recovery of Rana muscosa and also identified the vulnerable stage of the frog life cycle as the tadpole with caged fish experiments. Another attempt was made to repeat the results at another series of lakes (Knapp, Boiano and Vredenburg 2007) and the frog populations again quickly increased. In addition, lakes without any frogs were colonized after fish were removed, creating the beginnings of a metapopulation of Rana muscosa. 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. Unfortunately Davidson and Knapp (2007) have suggested that pesticides blown into Sierran aquatic habitats from the California Central Valley are also negatively affecting the populations of R. muscosa and may limit recovery at some locations even after the removal of introduced trout. To learn more, visit the species account on AmphibiaWeb Rana muscosa or visit

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. spenceri has suffered severe population declines (it is listed nationally as critically endangered, see the 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 © 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 © 2003 William Flaxington and Lepomis macrochirus © 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

Rana catesbeiana © 2001 Pierre Fidenci

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). Further, chytrid has been found on Rana catesbeiana farmed for international food trade (Mazzoni et al. 2003).


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.

The Cane Toad Bufo marinus

cane toad
Bufo marinus ©2006 William Flaxington

The cane toad, Bufo marinus, has been accidentally and intentionally introduced throughout the world.They are native to Central and South America, but their large size, high mobility, generalized eating habits, and huge reproductive capabilities, have made cane toads extremely successful invaders and a threat to biodiversity, most notably in Australia, but also in in many areas in the Pacific and Caribbean islands. The adult cane toad has poison glands and the tadpoles are highly toxic to other predators, this combined with their large size and aggressive behavior, impact many native species by competition and predation, but also through other pathways such as poisoning during ingestion attempts (Greenlees et al. 2007).

cane toad
Bufo marinus photo ©2003 John White

In addition, they have a high phenotypic lability, which makes them very successful invaders. In Australia, both their size and relative toxicity has decreased since introduction in 1935. Usually, the cane toads exert their maximum impact on native species when they first arrived in the area; the level of impact then declines over time (Phillips and Shine 2005).

Puerto Rican coqui

© 1979 Alan Resetar

The Puerto Rican coqui, Eleutherodactylus coqui, was accidentally introduced to Hawaii in the late 1980's. It was likely brought in on decorative plants imported from the Caribbean. The tropical climate of the Hawaiian archipelago is ideally suited to the direct development life history of the coqui, and there are no natural predators of the frogs. Therefore the population of the introduced frogs has exploded and they are now found in concentrations as much as five times higher than on their native Caribbean islands. Whereas Puerto Ricans treasure the distinctive call of their native frog, the Hawaiians consider their calls a major source of noise pollution. For more information The University of Hawaii at Manoa's College of Tropical Agriculture and Human Resources has a terrific web page: Control of Coqui frogs in Hawaii.

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