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Sensitive indicators of environmental quality

Head of the two-lined salamander. A common species in the watershed.Throughout the Chesapeake Bay watershed, you can find salamanders along the watershed's rivers and streams. The Maryland Biological Stream Survey, however, has found that streamside salamanders are not doing as well as they used to. Statewide, the number of salamander species decreased as urban and agricultural land use increased.

One reason to look at streamside salamanders is that these amphibians are actually sensitive bioindicators of degraded stream habitats. These salamanders are extremely sensitive to changes in their environment and their health is often directly linked to the health of their habitat.

What makes salamanders so susceptible to changing environmental conditions? A close look at their skin and eggs can give us an answer.

Because salamanders, like all amphibians, lay their eggs in the water, their eggs don't have a protective shell like, for example, chicken eggs. This makes the salamander eggs vulnerable to chemical pollutants, ultraviolet radiation, and other factors that disrupt cell division in the early stages of the embryo. As a result, the embryo may not be able to develop properly, and it will die.

Picture: Eggs of the northern two-lined salamander (Eurycea bislineata). Salamander eggs don't have a hard shell to protect them from chemical pollutants or ultraviolet radiation. Picture courtesy of Robin Jung. Photo of more than 50 small, slimy, black and white salamander eggs.

Even when the eggs hatch, the salamanders stay sensitive to their environment. Salamanders, like all amphibians, have a permeable skin; gases and water simply enter or leave the body through this skin. This permeable skin, however, provides little or no protection from toxins in the soil or in the water. With the water and gases that go through the skin, the salamander also takes in toxins. This way, amphibians are environmental sponges, soaking up the chemicals and toxins from the water or air around them.

Photo of a salamander larva, complete with four small feet and a long tail. Picture: Larva of the northern two-lined salamander (Eurycea bislineata). Through their permeable skin, salamanders soak up chemicals that are in the water. Picture courtesy of Robin Jung.

Threats to salamanders

Amphibians have been around for 350 million years, but recent developments are causing a worldwide decline. Habitat loss, acid rain and environmental contaminants, ultraviolet radiation, and invasive species directly affect salamander and other amphibian populations. And what is most mysterious to researchers, even in pristine, protected areas like nature preserves and national parks, salamander populations are decreasing.

Habitat loss
The loss of wetlands and forests equals the loss of amphibians. An alarming 54 percent of wetlands in the United States have been lost in the past 100 years. Temporary habitats, such as the vernal pools that form only in the spring, are also very important to many amphibians. Many amphibian species use these pools to come together and mate, so when these habitats disappear, it directly affects the amphibian populations.

Acid rain and environmental contaminants
Acid rain can lower the pH and thus increase the acidity of a stream to a level that can kill amphibian embryos. Acidic water can also increase the toxicity of other contaminants, such as metals, pesticides, and petroleum products. Even what you put on your lawn can affect salamanders; some widely used, commercially available herbicides contain a chemical that is directly toxic to amphibians. Because wetlands, the prime habitat of salamanders, are low-lying areas where contaminants in stormwater runoff can accumulate, the salamander is not even safe in its own habitat. The Natural History of Amphibians describes environmental contaminants as "closest to being a "single cause" behind widespread amphibian declines."

Ultraviolet radiation
Ultraviolet radiation (UV) disrupts the development of salamander eggs, and a low pH as a result of acid rain can even increase this effect. Vegetation along a stream or pool can provide shade and block radiation from entering the water where the salamander laid its eggs; when the vegetation is gone, the eggs may be exposed to lethal amounts of UV.

Invasive species
Last but not least, salamanders can fall prey to various introduced species, such as game fish and predacious bullfrogs. Because these species are not native to the place they live, the salamanders don't have a natural defense against them. Also, these non-native species can introduce disease-causing pathogens. Viruses, bacteria, and fungi appear to affect salamanders more than other kinds of animals.

Picture: Northern two-lined salamander (Eurycea bislineata). This is the most common streamside salamander in the Chesapeake Bay watershed. The two-lined salamander appears to be more tolerant of degraded habitats and stream conditions than other salamander species. Picture courtesy of Robin Jung. Photo of the two-lined salamander; the salamander has a brown line from head to tail along either side of its body.

Why should we care?

After having been around for 350 million years, amphibians now face worldwide decline. What does that tell us about the world we live in? Ron Heyer, a scientist at the Smithsonian Institution and chair of the Declining Amphibian Populations Task Force states, "All amphibian biologists are now convinced that something unusual and catastrophic is happening to amphibians." Dr. Heyer speculates that amphibian populations are telling us something about the habitat we share with them.

References and further reading

There are many sites providing detailed information on salamanders, amphibians in general, and their global decline.

The Maryland Biological Stream Surveyoutside link attempts to define the problem of acid rain and deposition in Maryland. Bioindicators, such as salamander populations, are used to assess stream quality. You can volunteer to participate in the stream-sampling program.

The Patuxent Wildlife Research Centeroutside link and the North American Amphibian Monitoring Programoutside link of the U.S. Geological Survey also conduct amphibian monitoring. Both have extensive links to other related pages.

The Declining Amphibian Populations Task Forceoutside link seeks to "determine the nature, extent and causes of declines of amphibians throughout the world, and to promote means by which declines can be halted or reversed."