Zoology

Causes of Worldwide Amphibian Declines



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I have been an amphibian biologist for nearly 20 years. Throughout this time, scientists have been observing worldwide declines of amphibian populations. Because of their permeable skin and biphasic (terrestrial and aquatic) life cycle, amphibians are considered indicators of environmental decay. Finding a reason for amphibian declines, therefore, has ramifications for human quality of life.

What are some proposed reasons for amphibian declines? Several have been proposed and have been reported in the scientific literature.

Habitat alteration
Alteration of amphibian habitats is the largest reason for the declines in their population numbers. Agricultural crops stand now stand in the way between breeding ponds. Water tables have been altered, causing some ponds to dry prematurely. Habitat around bodies of water has been mowed and fertilized.

Contaminants
Most amphibians live part of their lives in an aquatic environment and part on land. Because of this, they are exposed to contaminants in two different environments. Aquatic contaminants can enter amphibians through their permeable skin. Additionally because many species occupy two places in the food web (herbivorous tadpoles and carnivorous adults), they have two sources from which to eat contaminated food items.

Disease
One definitive cause of amphibian declines is the pathogenic fungus, Batrachochytrium dendrobatidis (known simply as chytrid fungus). It is found everywhere in nature, but can turn lethal if conditions are right. Reports of ponds littered with frog carcasses have been attributed to this pathogenic fungus. Another fungus, Saprolegnia ferax, is also responsible for injuring amphibians. Other diseases such as ranaviruses and parasitic infections have also spread in recent years and have been directly associated with amphibian mortality.

Introduced species
Many species are now living in habitats that are new to them. As a consequence, native amphibians now have new competitors and predators and are experiencing declines in numbers because of these novel interactions. Bullfrogs, which are native to only a portion of the US, are now widespread. Tadpoles of this species routinely outcompete tadpoles of native species. Trout have been introduced into mountain lakes for sporting purposes, only to fell the entire population of amphibians living there.

Increased UV radiation
Thinning ozone layers have been blamed for increased UV intensities on the earth. Because amphibian skin is unprotected against UV radiation, some suspect that this may be a reason for amphibian population declines.

Overexploitation
Amphibians are often collected from the wild and used in the pet trade, or for food in some countries.

It is likely that no one factor is responsible for having caused all amphibian declines. Researchers have discovered that when many of these factors interact with one another, they can have greater impacts than when these factors are alone. For instance, some contaminants are made more toxic in the presence of UV radiation. Amphibians may be more susceptible to disease because they are exposed to contaminants. The research possibilities are endless, and great strides are being made daily.

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