Decline in frog populations
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Since about 1950, the populations of many species of amphibians (caecilians, frogs, toads, salamanders and newts) throughout the world have declined markedly; some species have become extinct. In many instances, these declines are attributable to adverse human influences acting locally, habitat degradation as a result of deforestation or draining of wetlands, urban encroachment, and pollution.
In 1988, however, herpetologists from many parts of the world reported declines in amphibian populations in protected, apparently pristine habitats, such as national parks and nature reserves, where such local effects could not be implicated. This led to the suggestion that there may be one or more global factors that are adversely affecting amphibians. Possible candidates for such influences are climatic and atmospheric changes (for example, increased UV-B radiation), widespread pollution (e.g. acid rain), infectious disease and parasitic infestation.
Other factors include the introduction of non-indigenous predators and competitors.
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Pollution
Pollutants are causing frog deformities such as multiple limbs, stunted growth or misplaced eyes. Researcher and naturalist Gary Fellers has been tracking the deformation and decline of the frog population in Yosemite National Park in California. He attributed these occurrences directly to "pesticides wafting over the Sierra Nevada mountains from...farms in California's Central Valley". ("What's Killing the Frogs?” Newsweek May 13, 2003 pg 46).
Pollutants have varying effects on frogs. Some alter the central nervous system; others like atrazine cause a disruption in the production and secretion of hormones (UC Berkeley biologist Tyrone Hayes).
Recent evidence points to ozone as a possible contributing factor to the worldwide decline of amphibians (Dohm, M.R., et al. 2005. "Effects of ozone exposure on nonspecific phagocytic capacity of pulmonary macrophages from an amphibian, Bufo marinus. Environmental Toxicology and Chemistry 24 (January):205-210).
Climate change
Large stands of ecosystem such as rainforests are being destroyed at a phenomenal rate decreasing available habitat. Also pollutants indirectly affect frogs by way of ozone depletion causing sun burn damage to the delicate moist skins of frogs, and further affecting their immune systems.
Parasites
Researcher Joseph Kiesecker focused research on a rural area of Pennsylvania where trematodes were present in ponds. His research indicated that frogs sheltered from the infiltration of parasites developed normally, while those that were exposed developed deformities. He also monitored the pesticide levels in the same ponds and found a magnification in the rate of parasitism in the polluted ponds suggesting that the presence of the pollutants reduces the frog’s immune response to parasitism. ("Pesticides Mess with Immunity" J. Pickrell Science News. July 13, 2003 p. 20)
Chytrid fungus
A parasitic fungus belonging to a chytrid family has been killing frogs in Australia, South America, Central and North America. The disease is called chytridomycosis. The time from infection to death has been found to be 1-2 weeks in experimental tests.
In 1998, following large-scale frog deaths in Australia and Panama, research teams in both countries came up with identical results— a previously undescribed species of pathogenic fungus, Batrachochytrium dendrobatidis. Subsequent research has established that the fungus has been present in Australia since at least 1978. It now appears that some recent extinctions of Australian frog species were caused by this fungus.
The fungus may have been exported from Africa, perhaps traveling with the African Clawed Frog, Xenopus laevis, which are sold in pet stores and used in laboratories around the world.
Exotic species
Non-native predators and competitors are also affecting the viability of frogs in their habitats. The mountain yellow-legged frog which typically inhabits the Sierra Nevada lakes have seen a decline in numbers due to stocking of non-native fish (trout) for recreational fishing. The developing tadpoles and froglets fall prey to the fish in large numbers. This interference in the frog’s three year metamorphoses is causing a decline that is manifest throughout their ecosystem ("On the Rebound." Elizabeth Daerr, National Parks Magazine, January-February 2005, p. 49 (http://www.npca.org/magazine/january_february/Rare_Endangered.asp)).
External links
- FrogWeb: Amphibian Declines & Malformations (http://www.frogweb.gov/)