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Indicator Species: Using Frogs and Salamanders to Gauge Ecosystem Health

Use amphibians, like salamanders, toads and tree frogs, and other indicator species to show when pollutants threaten local ecosystem health.

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iStockphoto.com/Benjamin Loo
Three bull frogs resting on rocks in a pond.

Ecologists and environmental scientists use a wide range of plants and animals as “indicator species” to get an idea about ecosystem health of streams and forests. Some of these species are tolerant of pollution — such as certain bacteria or algae — so their presence indicates the presence of pollutants like sewage. Other species are intolerant of pollution or environmental disturbance — such as mayflies or many fish species — so their presence is an indicator of a healthy ecosystem, while their absence can indicate problems. One group of organisms most sensitive to environmental change appears to be amphibians, which includes frogs, toads and salamanders.

For many years, scientists have been noticing a severe decline in amphibians around the world. Many species have completely disappeared, while others have become extremely rare or have started developing deformities — like extra legs. As a result, amphibians have come to the forefront as indicator species. But you don’t have to be a professional ecologist or environmental scientist to help monitor this decline. You can do something as simple as keep an eye on your garden.

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Image by Allen Blake Sheldon

The southern red-backed salamander (Plethodon serratus) captured in Cobb County, Georgia.

Why is the amphibian population so sensitive?

One thing that makes frogs, toads and salamanders such sensitive environmental indicators is their skin. Unlike reptiles, birds and mammals, amphibians are still partially tied to the water, spending at least part of their life cycle as eggs and juveniles in an aquatic environment. The eggs don’t have a hard shell around them like the eggs of reptiles and birds, and must remain in the water, or they will dry out. The juvenile forms, or tadpoles, breathe through gills like fish, extracting oxygen from the water. Not until they develop into adults do amphibians possess lungs (well, most of them anyway), allowing them to leave the water.

Even as adults, most amphibians have to keep their skin moist by staying near water or damp areas, since they breathe partially or entirely through their skin. (There are a few species of salamanders called “lungless salamanders” because they have no lungs and depend completely on their skin to breathe.) Because their skin is so porous and absorbs gases, like oxygen, and liquids, like water, amphibians at all stages of life are sensitive to environmental changes, especially many types of pollution that may be in the atmosphere, water or soil.

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Image by Allen Blake Sheldon

A spotted salamander (Ambystoma maculatum) photographed in Monroe County, Tennessee.

What does all this have to do with the health of your garden? Whatever affects amphibians also may affect people. Because frogs, toads and salamanders are so sensitive to pollutants, and because they are so much smaller than humans, they will likely show signs of problems in your garden before it affects you. They can serve the same function that canaries used to serve for coal miners years ago: Being more sensitive to poisonous gases than the humans, when the canary became sick or died, the miners knew something was wrong.

If you provide abundant habitat for amphibians and have a healthy population of them in and around your garden, that is a good indicator that you have a healthy environment. If they start disappearing or showing deformities, you might have some problems, especially with your water or soil.

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Image by Allen Blake Sheldon

The colorful marbled salamander (Ambystoma opacum).

Watch for the warning signs

The pH of your pond (as well as your soil) may affect the development of both eggs and tadpoles of salamanders, frogs and toads. Low-pH water and soil is acidic. As such, it can cause toxic substances, such as heavy metals (like lead), to dissolve more easily and be absorbed more readily through amphibian skin. For those amphibians that stay in or near water even as adults (most frogs and some salamanders), these toxins can be harmful. A wide variety of inexpensive and easy-to-use water and soil pH test kits are readily available.

Environmental pollutants may be in low enough concentrations as to not kill amphibians, yet they may still cause deformities like extra limbs or limbs growing from strange places on the body. In the 1990s, biologists started noticing unusually high numbers of frogs with these malformations and began to look into possible causes. While one culprit seemed to be a naturally occurring parasite called a trematode, research indicated that some environmental factors might have made the frogs more susceptible to these trematodes by weakening their immune systems. Studies on frogs in areas subjected to lots of insecticides and herbicides showed a higher occurrence of deformities than those in areas where these pesticides were not used.

Reducing or eliminating the use of pesticides and other chemicals would benefit your local amphibian populations. Carefully observing the amphibians that live around your yard and garden for signs of deformities is a good way to monitor the natural health of your soil. Also, paying close attention to amphibian numbers is a good way to monitor the health of your immediate environment.

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Image iStockphoto.com/craigdamlo

A small frog surveys its surroundings.

Becoming familiar with the species found most often around your place is a good way to monitor environmental health. There are numerous field guides to amphibians and reptiles available at the library or online. Most state wildlife agencies also have some literature available describing the state’s herptofauna (reptiles and amphibians). If familiar species that have been present for a long time suddenly become scarce or disappear completely, that’s Mother Nature’s alarm system telling you something is wrong.

Using amphibians as indicators is not limited to gardeners. The techniques described here could be used by anyone to monitor the health of their favorite natural area, such as duck hunters and fishermen concerned about the environmental health of a marsh, lake or river. Amphibians are ancient and remarkable animals that survived the extinction of the dinosaurs. We still have much to learn from them, not the least of which is their ability to warn us about the quality of the environment we share with them.


Build a habitat, and they will come

The first step in using amphibians as environmental indicators is to have amphibians present. That means providing them with habitat: food, cover, water and, ideally, a breeding site. Most amphibians lay their eggs in temporary or permanent water bodies, although there are a few species of salamanders that lay eggs under rotten logs or moss in damp woodlands. So, the best way to attract amphibians to your garden is to provide them with a pond or pool for reproduction. These don’t have to be large or complex, but it needs clean, chlorine-free water, and, ideally, it needs to be present year-round.

Pools or ponds should have abundant sources of cover both in them and around their edges for adults, eggs and tadpoles. This cover may include a variety of shrubs and trees, ground cover vegetation, leaf piles, or piles of rocks. Small boards of untreated and unpainted lumber can be placed in shady areas. Alternatively, logs or large dead tree branches can be placed in shady areas to provide cover for salamanders. Many amphibians lay their eggs on or under aquatic plants, and these plants also provide protection for the tadpoles from predators, excessive heat, and ultraviolet radiation from the sun. Ultraviolet radiation, specifically, has been shown to disrupt the development of salamander eggs.

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Image iStockphoto.com/Mark Kostich

Salamander eggs are usually seen in the spring.

Snags (dead trees) make excellent habitat for cavity-nesting animals, such as flying squirrels, bats, woodpeckers and tree frogs. If there aren’t any snags on your property, you can provide artificial nest boxes made from untreated lumber. Nest boxes for tree frogs should have small diameter holes to discourage use by birds or squirrels.

Another way to provide refuges for tree frogs is to hang or nail a piece of PVC pipe about 2 feet long and 1 inch in diameter to a tree about 5 or 6 feet above the ground. The bottom should be capped with a small drain hole drilled a few inches from the bottom. This will allow a small amount of water to collect in the bottom but not fill up the entire pipe. Tree frogs have been shown to use these pipes as refuges from the daytime heat. The pipes should be close to a pond or pool so the frogs have easy access to breeding habitat.

Fish always seem to be a natural addition to any pond or pool, but not in this case. Fish and amphibians, especially in a small pond or pool, are not compatible. Fish are voracious predators of most amphibian eggs and even tadpoles. Even some types of amphibians, such as bullfrogs, can be detrimental to maintaining a diverse population of other amphibian species. Bullfrogs can devastate other amphibian species in small ponds, as they will eat just about anything they can swallow. This is especially true in those areas where bullfrogs have been introduced from outside their native range.

Making some of these habitat improvements may be all that is needed to attract frogs, toads and salamanders if your property is near natural areas (streams, lakes, forests) with existing amphibian populations.

If your property is not close to amphibian friendly areas, you may need to introduce egg masses or tadpoles from other areas to “seed” your habitat. Before capturing or transporting any of these animals, check with your state wildlife agency concerning regulations pertaining to handling wildlife. Although all egg masses and tadpoles look pretty much the same at first glance, each species is distinct. You should try to develop at least some familiarity with these differences, especially if there are endangered or threatened species of amphibians in your area. Moving them from their natural habitat, even for good reasons, could get you into a lot of legal trouble.

Read more: Learn all about the American bullfrog: All About the American Bullfrog


John Marshall lives with his family in Benton, Arkansas. He is an instructor of environmental science, biology and botany at Pulaski Technical College.

Published on Jun 6, 2013

Grit Magazine

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