The Global Amphibian Decline:
Trends, Causes, and Challenges

Mountain yellow-legged frog
(Rana muscosa)
photo by Vance Vredenburg

Global Trends
North America
Central and South America
Australia
Europe

Causes
Habitat Modification
Exotic Species
Acidification and Toxic Contiminants
Ultraviolet Radiation
Climate Change
Pathogens

Challenges

Bibliography

Links

Actual Cause: Exotic Species

Exotic species introduced by humans into natural systems can play a significant role in determining amphibian distribution and population dynamics. Larval amphibians are extremely vulnerable to vertebrate and invertebrate predators, and introducing predatory fish into habitats can markedly reduce the aquatic amphibian diversity. Amphibians breeding in permanent water bodies with predatory fish likely have evolved protective mechanisms such as unpalatability of eggs and larvae or specific escapement behavior. However, local amphibians remain vulnerable to exotic predators because they have no antipredator strategies to protect against introduced species. The negative effects of introduced predators on amphibians may be magnified in high-elevation waters because mountain lakes have lower productivity, increasing the time amphibians spend in the vulnerable larval period, and less habitat structure, which provides less protection against predators. Introduction of several trout species, typically for sport fishing, into high elevation watersheds in California Sierra Nevada has been implicated in the decline of the mountain yellow-legged frog (Rana muscosa, above), since this amphibian frequently restricts larvae to the same deep water bodies that the predators prefer. For similar reasons, introduced trout are also likely responsible for declines of long-toed salamanders (Ambystoma macrodactylum) and Columbia spotted frogs (Rana luteiventris) in Idaho, as well as the spotted tree frogs (Litoria spenceri) in south-eastern Australia. Bullfrogs (Rana catesbeiana), which compete with and prey upon other frog species, introduced to the lower central valley of California are also strongly suspected to be the cause of the decline in the California red-legged frog (Rana aurora draytonii).

Colonization of normally fish-free aquatic systems by predatory fish can result in the rapid extirpation of amphibians. Introduced predators that eat amphibian larvae can also prevent migration among local populations through streams, potentially short-circuiting recolonization of local populations and causing regional extinctions. Moreover, given the metapopulation dynamics of many amphibian species, reduced recruitment due to predation in a source population could result increase the extinction probability of a sink population.

Exotic species are also potential vectors of amphibian diseases responsible for local epidemics and declines. Migratory fish also could transmit diseases between watersheds or introduce new pathogens.