Amphibian Population Declines

An unparalleled diminishment in populations is occurring worldwide in many species of amphibians (frogs, toads, and salamanders). Although there are various causes for declining amphibian populations, the most obvious is habitat destruction. However, introduced exotic species, pathogens , pollution, and global environmental changes all contribute. Moreover, various factors can act together to produce adverse effects on amphibians.

Because amphibians are important predators and prey in many ecosystems , declines in their populations may affect many other species that live within the same ecological community. For example, populations of aquatic insects and amphibian predators such as snakes, birds, mammals, and fish may be especially affected by a loss in amphibians. Moreover, the populations of animals that amphibians eat, such as mosquitoes, may increase as amphibians disappear.

Water Quality Factors

Amphibians have permeable, exposed skin and eggs that may readily absorb toxic substances from the environment. Their eggs are laid in water or in moist areas, and their larvae (tadpoles) are aquatic. Because amphibians are intimately tied to an aquatic environment, the quality of the water in which they live can affect their growth, development, and survival. Because pollutants, waterborne pathogens, and global environmental changes can all affect water quality, these factors can in turn affect amphibians. Conversely, amphibians are important indicators of water quality, and are considered a sentinel species, meaning that what affects amphibians presently may affect other animal species in the future.

Acidification.

A number of studies have shown that acidification of fresh water (that is, a reduction in pH to acidic levels) via acid rain, acid snowmelt, or other modes of pollution are harmful to amphibian growth and development. Some species are more tolerant of acid conditions than others. Thus, depending on the species, the amount of acidity, and other environmental variables, amphibians may experience developmental deformities and increased mortality due to acidification.

Acidification can potentially affect amphibian populations and the communities in which amphibians live. For example, some populations of toads in Britain have probably been reduced by water acidification. Salamander populations in Colorado seem to have declined because of increased acidification during snowmelt. Several studies have shown that acidification of the water can affect competition and predation between amphibians. Thus, the larvae of some frog species may have increased survival rates under acid conditions because their salamander predators show reduced predation at low pH.

Nitrates and Nitrites.

Many chemical products used in agriculture and industry pollute aquatic habitats, causing potentially severe damage to ecosystems. For example, the increase in concentration of nitrate in surface water on agricultural land due to numerous sources may be hazardous to many species of fish, wildlife, and even humans. Data suggest that nitrogen-based fertilizers may be contributing to amphibian population declines in agricultural areas. However, some species appear to be more sensitive than others to nitrate and nitrite pollution.

In one experimental study in Oregon, it was shown that some species reduced their feeding activity, swam less vigorously, and showed disequilibrium when nitrate or nitrite ions were added to the water. Importantly, all species tested in this study showed high mortality at nitrite levels deemed safe for warm-water fishes by the U.S. Environmental Protection Agency. Furthermore, significant larval mortality occurred at the recommended limits of nitrite concentration for drinking water.

Toxic Substances.

Just as amphibian species display variation in sensitivity to nitrate-related compounds, they also show variation in tolerance to other toxic substances that may be found in water. Insecticides such as organophosphates, carbamates, and synthetic pyrethroids, which are used mainly in crop production, have a wide array of effects on amphibians. Depending on the concentrations used and the species involved, some of these substances may be lethal, may affect growth and development, or may affect metamorphosis .

Effects of Ultraviolet Radiation

Global environmental changes may also affect amphibians. For example, ambient (natural) but increasing levels of ultraviolet (UV) radiation owing to ozone depletion are harmful to many amphibian species. Recent experimental field studies conducted in the United States, Spain, and Australia have shown that when amphibian eggs of certain species are shielded from UV, they have a greater hatching success than if they are exposed to UV. Several studies have shown that UV may not kill developing amphibian embryos but may cause developmental abnormalities and changes in behavior. The quality of the water can greatly affect how much UV penetration occurs and therefore how exposed amphibians may be to UV. Thus, water that is laden with organic material may effectively block out much of the potentially harmful UV radiation. Furthermore, certain pollutants in the water may interact with UV radiation in a way that increases their toxicity to amphibians.

The adverse effects of UV radiation can be enhanced in the presence of toxic substances and pathogens. For example, different species of amphibians show variation in sensitivity to aquatic pollutants known as polycyclic aromatic hydrocarbons (PAHs), which are found in locations contaminated with petroleum products or urban runoff. PAHs are extremely toxic to amphibians when they are simultaneously exposed to UV radiation. For example, one PAH, known as fluoranthene, causes increased mortality in salamanders and frogs as the amount of UV radiation increases.

UV radiation also increases amphibian mortality when a pathogenic fungus known as Saprolegnia is present. One major source of Saprolegnia is introduced stocked fish that become infected while being reared in hatcheries. It has recently been shown that when infected fish are released into natural lakes and ponds, Saprolegnia can be transmitted to amphibians. Other studies have shown that the adverse effects of UV on amphibians are enhanced when the water is acidic.

Malformations and Deformities

Water quality degradation has been linked to severe physical malformations (including missing, malformed, and extra limbs) reported in dozens of amphibian species from diverse aquatic habitats across North America. Possible causes for these limb deformities include UV radiation, pesticides in water, and parasitic infection.

One likely scenario for increased malformations is that trematode parasites that cause limb deformities in developing tadpoles have increased with their intermediate snail hosts. Snail populations may have increased with increased algal growth, their main food. In certain regions, lush algal growth may be occurring because of eutrophication of water from nitrogen-based fertilizer use on nearby lands.

Obviously, amphibians are being subjected to a variety of human-induced insults that are related to water quality. Special attention must be given to the presence of pollutants, pathogens, and global environmental changes that may affect amphibian growth and development, increase mortality, and eventually lead to unnatural and accelerated population declines.

see also Acid Rain; Chemicals from Agriculture; Ecology, Fresh-Water; Forest Hydrology; Fresh Water, Natural Composition of; Global Warming: And the Hydrologic Cycle; Hydrologic Cycle; Lakes: Biological Processes; Lakes: Chemical Processes; Pollution of Lakes and Streams; Pollution Sources: Point and Nonpoint; Stream Health, Assessing.

Andrew R. Blaustein

Bibliography

Blaustein, Andrew R. et al. "Effects of Ultraviolet Radiation on Amphibians: FieldExperiments." American Zoologist 38 (1998):799–812.

Blaustein, Andrew R., and David B. Wake. "The Puzzle of Declining Amphibian Populations." Scientific American 272 (1998):52–57.

Boyer, Robin, and Christian E. Grue. "The Need for Water Quality Criteria forFrogs." Environmental Health Perspectives 103 (1995):352–357.

Johnson, Pieter T. et al. "Parasite (Ribeiroia ondatrae ) Infection Linked to AmphibianMalformations in the Western United States." Ecological Monographs 72 (2002): 151–168.

Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton, NJ: Princeton University Press, 1995.

Internet Resources

Amphibian Declines and Deformities. U.S. Geological Survey. <http://www.usgs.gov/amphibians.html>.

THE CASE OF THE CASCADES FROG

The Cascades frog (Rana cascadae ) is a species that is threatened throughout its range in the western United States. Populations are disappearing, and eggs are dying as they are laid in lakes and ponds.

Cascades frogs are sensitive to a number of agents associated with water quality. For example, an experimental laboratory study at Oregon State University showed that survival and activity levels of tadpoles of the Cascades frog are greatly affected by ultraviolet radiation, acid water conditions, and nitrate pollution. These stressors, acting together, reduce survival and activity levels in Cascades frog tadpoles.