Amphibian Population Declines

In the early 1970s, biologist Cynthia Carey was studying western toads in remote areas of Colorado. The first year she began her work, the toads were so numerous she had to be careful not to run over them with her car. Two years later, hardly any toads were left. Those still alive were barely able to walk and had puffy, red legs.

In the remote Sierra Nevada Mountains of Yosemite National Park, Gary Fellers admired frog choruses as a young boy and college student in the 1960s. By the 1990s, he found that many of the once resounding lakes were silent. Historical records dating from 1915 (such historical records are unusual for amphibians) showed that of seven species present shortly after the turn of the century, five had suffered serious declines by the 1990s. Only one species, the bullfrog, which is not native to the Sierras, was doing well.

In the early 1990s, similar reports of species declines and extinctions began pouring in from other corners of the globe. Costa Rican golden toads and harlequin frogs, plentiful in the early 1980s, disappeared suddenly. In Australia, the gastric brooding frog, the female of which swallowed its eggs and incubated them in her stomach, became extinct in 1980. These and other reports of extinctions prompted herpetologist Ronald Heyer to comment, "The frogs seem to be telling us something. We'd better find out what it is, and soon."

The story of frog declines has much in common with the story of amphibian malformations. Similar to the situation with malformations, controversy brewed as to whether the declines and extinctions were a new phenomenon or simply normal population fluctuations. A 1991 study of amphibians in South Carolina ponds showed that huge population fluctuations are indeed normal. The study also found that over many years, local extinctions (e.g., in a single pond) are also normal. However, many scientist believe that the number of recent extinctions and declines, and the fact that they are occurring on several continents, is alarming.

Concern over the recent population declines motivated scientists to look for a cause. Scientists have proposed a number of hypotheses to explain these declines, including the following:

  • Introduced Species – To meet demands for recreational fishing, millions of fish have been stocked in lakes and rivers. Bullfrogs have also been introduced in many places in the western United States, where they are not native. These fish and bullfrogs may eat native amphibians or take over their habitat.

  • Human Consumption – The Indian bullfrog may be experiencing declines due to the export of 3,000-4,000 metric tons of frog legs exported annually to France for human consumption.

  • Ultraviolet Radiation – Field experiments in mountain lakes show that eggs of some frogs and toads are damaged by ultraviolet radiation. Scientists also found that a pathogenic fungus acts together with ultraviolet radiation to kill frog and toad eggs.

  • Acid Rain and Acid Soils – Water and soil with a pH below 4 can kill amphibian eggs and tadpoles. Field studies have shown that amphibians may disappear from regions that suffer from acid rain.

  • Pesticides – In Canada, scientists found only five species of frogs in lakes that had been sprayed with DDT and 12 species of frogs in lakes where no pesticides had been sprayed. DDT residues in frogs from sprayed lakes averaged 5,000-47,000 micrograms/kg, whereas frogs from unsprayed lakes had 6 micrograms/kg of DDT. In the Midwest, frog populations increased on a farm where pesticide use was stopped.

  • Diseases – Scientists in Australia, Costa Rica, and the United States recently have discovered a new species of fungus in dead frogs. Evidence of trematode, bacterial, and viral infections also has been found.

  • Malformations – The observed high rates of malformations in some areas could play a role in population declines. Currently, none of the frog species with high rates of malformations is in danger of extinction, although malformations are suspected to be linked to amphibian declines.

  • Habitat Destruction – Because amphibians spend time in both aquatic and terrestrial habitats, they suffer from loss of wetlands and forests. Some amphibians depend on temporary ponds that are present only in the spring (vernal ponds); these ponds are often drained for mosquito control, farming, or construction. Habitat loss probably is the single most important factor in the decline of amphibians and other animals worldwide.

Much of the concern over amphibian declines is based on the premise that amphibians may be especially sensitive to human disturbance and pollution in the following ways:

  1. Amphibian eggs do not have protective shells and are extremely vulnerable to drying out.

  2. Many amphibians lay their eggs in the water, where the eggs may be exposed to chemical pollution and ultraviolet radiation.

  3. Introduced fish and other predators can easily prey on the aquatic eggs and young.

  4. Many amphibians have highly permeable skin, which makes them susceptible to contaminants, both on land and in water.

  5. Many amphibians depend on the preservation of both aquatic and terrestrial habitats to survive at different times in their lives. It is not enough to protect just one of these habitats.

People have a number of reasons for being concerned about malformations and population declines in amphibians. Some people feel that amphibians have value of their own, and that it is ethically wrong to harm them by contaminating the environment. Others may be more concerned with the possible meaning of amphibian malformations for humans. If amphibians are becoming unhealthy and dying, will humans soon follow? Are amphibians the "canary in the coal mine"?

Use this background information with the lesson plan Why Care About Amphibian Population Decline and Malformation?

Excerpted from Hands-On Herpetology.


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