Frogs: The ultimate indicator species

  • The ventral view of a spotted frog

    Sue Bellagamba
 

Note: This article is a sidebar to this issue's feature story: Utah ushers its frogs toward oblivion

Native frog populations are plummeting all over the world. No one knows exactly why, but there are six prominent possibilities. Destruction of wetlands is one, contamination of water supplies by biocides, pollutants, and acid rain another. A third is the introduction of non-native predators such as voracious game fish from hatcheries, as well as bullfrogs and crawfish that eat the eggs of native frogs. A fourth is natural population variation. The fifth factor is a fungal disease called Saprolegnia, which has long plagued hatchery fish and recently spread to the wild, where it is suspected of affecting frogs as well as fish. The sixth is the depletion of the ozone layer: Now that it is known that sunbathing sets the stage for skin cancer, many scientists wonder what the effects are upon frogs, whose bodies are protected from the elements by a thin skin permeable to both water and gas.

Some scientists wonder why the crash of frog numbers in Utah is outpacing decline in most other areas. Does it foreshadow a system collapse? Are the frogs aquatic versions of the canary in the coal mine? Jasper Carlton, director of the Biodiversity Legal Foundation, says the answer to both questions is yes.

"Leopard frogs, boreal toads, spotted frogs and tiger salamanders are experiencing serious declines," said Carlton from his office in Boulder, Colo., where he files numerous lawsuits on behalf of anurans (frogs and toads). "We often attribute species decline to habitat destruction. What is particularly alarming is that many amphibians occupying undisturbed wilderness habitats are also disappearing at a previously unseen rate. These declines appear to be widespread and have been particularly serious for 20 years."

The National Biological Survey and the National Academy of Sciences have gathered unsettling evidence of amphibian and reptile decline. Here are a few examples:

* Leopard frogs (Rana spp.) which are commonly used in teaching and research institutions, were once prolific in most of the United States. Populations in this diverse group have declined, sometimes significantly, in portions of the Midwestern, Rocky Mountain, and Southwestern states including Utah, to the point they may be candidates for listing as endangered.

* Several species of anurans are fading from small forest streams in the Pacific Northwest. Frogs apparently can't tolerate clearcuts because the toppling of trees removes shade from their spawning areas and exposes eggs to sunlight. Because timber is harvested without adequate streamside protection, many populations of the tailed frog (Ascaphus truei) and torrent salamanders (Rhyacotriton spp.) have been severely affected; some populations could soon be considered for listing.

* In the southeastern U.S., numerous salamander and frog populations have been damaged by stream degradation and the conversion of natural pinewood and hardwood forests and associated wetlands to plantation forestry, agriculture and urban development. Among the victims are Rana capito, better known as the gopher frog, and the hellbender (Cryptobranchus alleganiensis).

Since 1980, the number of endemic amphibian species suffering from serious declines more than doubled. More than 130 species of amphibians and reptiles are listed as threatened or endangered worldwide and 10 of those are frogs and salamanders in the United States.

There are dozens of frog subpopulations that conservationists say warrant listing. For example, David Ross' survey of Western boreal toads turned up only three individuals in northern Utah. Boreal toads are also in trouble in the southern Rockies. Now that the 13-month freeze on listing species ended early this month, the U.S. Fish and Wildlife Service must tackle a backlog of hundreds of species that need protection; the first species Interior Secretary Bruce Babbitt is expected to list is the California red-legged frog.

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