Amphibians as biological indicator

Plethodontidae, a large family of lungless salamanders that lives under leaf litter, may number 700 to 900 individuals per acre, exceeding the biomass of all other vertebrates in woodlands of Eastern North America. They feed on insects and spiders, and become food for raccoons, skunks, shrews, snakes, chipmunks and various birds.

One of the most notorious problems in amphibian populations is the dramatic rise in deformed specimens. Extra limbs have been observed in a small percentage of the frogs for hundreds of years, but recently the incidence has increased to anywhere from 15 to 90 per cent in some locations, often affecting more than one species of frog. The formation of extra limbs is known to be caused by a naturally occurring parasite, Ribeiroia, which encysts in developing limb buds, however healthy populations of frogs can thrive in water infected with the virus. A wide range of environmental factors appears to affect the rate of infection and deformity, as described in an article in the March-April 2004 issue of American Scientist (Amphibian decline and emerging disease). Clearly, many different causes contribute to varying degrees in different locations: climate changes, increased UV radiation, loss of biodiversity, introduction of exotic species and diseases, disturbance and development of wetland habitats, to name a few.

These strange deformities are just the beginning. This global decline was not noticed by the scientific community until 1989, but since then 125 species of amphibians have become severely threatened or gone extinct. Never in over 300 million years have they suffered so many extinctions in such a short period.

The article in American Scientist makes a dramatic connection between the plight of amphibians and the increased incidence of emerging diseases in humans. Since the 1980s, the human population has experienced an increase in new diseases, such as Ebola, AIDS and SARS, resurgences of old diseases such as cholera that were previously coming under control, new strains of diseases, and diseases such as West Nile virus spreading into areas where they were formerly unknown. The point of this connection is that the same conditions making amphibians susceptible to disease may also affect humans.

Until now, scientists have debated whether amphibians can be regarded as a biological indicator of global climate change. Perhaps now the debate must come closer to home, as we consider whether the sickness of frogs is part of a wider environmental phenomenon already affecting us. The severity of their decline may serve as a warning of our own susceptibility, and a picture of our own future.

Bibliography

Kiesecker, J. M., L. K. Belden, K. Shea and M. J. Rubbo. 2004. Amphibian decline and emerging disease.

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