The Amphibian Conservation Challenge
Amphibian populations are declining worldwide, and amphibians are experiencing high extinction rates due to habitat loss, chytrid fungus, pollutants, pesticides, invasive species, and climate change. Amphibians are the most threatened class of vertebrates and merit attention when planning projects such as highways, power lines, power plans, water lines, reservoirs, and other infrastructure projects as well as plans for natural resource lands, such as national forests and parks.
Amphibian Population Crash Events
Many amphibians have disappeared. James Collins and Martha Crump, in their book Extinction in Our Times, Global Amphibian Decline, describe several events where amphibian populations crashed:
- Yosemite toad populations decreased by a factor of nine between 1974 and 1982 in the high Sierra Nevada mountains of California.
- California red-legged frogs disappeared from more than 70 percent of their historic pre-1975 range by 2001.
- Boreal toads disappeared from the West Elk Mountains of Colorado.
- Northern leopard frogs disappeared from the Red Feather Lakes region of Colorado and 33 other areas in Colorado.
- Tiger salamanders disappeared from numerous high elevation lakes in Colorado.
- Blanchard’s cricket frogs disappeared from parts of Illinois, Indiana, Iowa, Michigan, Minnesota, and Wisconsin.
- Green salamanders populations decreased in the Blue Ridge Escarpment of southern North Carolina by 98 percent from 1970 to the 1990s.
- Lungless salamanders (Plethodontidae) experienced decreases in 180 populations, disappeared in 32 locations, but increased in 25 locations in the 1990s.
- Western chorus frogs in Quebec went from being abundant to one of the rarest frogs in Quebec.
- Dink or robber frog populations had substantial population declines in Puerto Rico, including Karls’ robber frog, golden coqui, and Villalba robber frog extinctions.
Amphibian extinctions have occurred around the world according to Crump and Collins. Golden toads and harlequin frogs disappeared from Monteverde, Costa Rica, in the 1980s. Twenty-four species of frogs experienced population declines or became extinct in the Andes of Ecuador in the late 1980s. Kihansi spray toads became extinct in the Kihansi River Gorge of Tanzania. Gastric brooding frogs disappeared from southeastern Queensland, Australia, in 1981. Female gastric brooding frogs brooded their young in their stomachs, a distinctive life process that no longer exists.
Amphibian Extinction Rates
Malcolm McCallum, of Texas A&M University, calculated extinction rates in a 2007 Journal of Herpetology article. The current amphibian extinction rate could be hundreds or thousands times the background extinction rate based on the fossil record. Thus, human activities are increasing the amphibian extinction rate.
Causes of Amphibian Population Declines
- Habitat Loss – Wetland and forest habitat loss impacts amphibians. Forest harvesting, infrastructure development, and water diversions impact frog and salamander species. Tropical forest loss is of special concern since tropical forests support the highest amphibian species diversity. Preventing habitat loss is a top priority for Amphibian Refuge, especially on federal lands.
- Habitat Fragmentation – Separating habitat areas can make it difficult for amphibians to access suitable habitat. Amphibians lives are lost when they have to cross non-habitat areas, such as roadways and railroads. Amphibian Refuge seeks to prevent and mitigate habitat fragmentation.
- Climate Change – As with other animal species, climate change impacts amphibians. Changing rainfall patterns, reduced rainfall, and rising temperatures affect the suitability of amphibian habitats. Amphibian Refuge promotes policies to reduce greenhouse gas emissions to limit the impacts of climate change.
- Pollutants and Pesticides – Since amphibians breathe through their skin, they are especially sensitive to pesticides and pollutants. Pesticide use near wetlands affects water quality. Deformed frogs with missing or twisted limbs have been observed near wetlands. Pollutants, parasites (such as flatworms), and predators are suspected causes of deformities, but more research is needed.
- Invasive Species – Non-native frog species can crowd out native species. Bullfrog ranges have expanded west in the United States and have impacted native western frog species. The coqui frog of Puerto Rico has disrupted Hawaiian ecosystems. Cuban tree frogs, introduced to the Everglades, are known to eat five frog species native to the Everglades.
Amphibian decline data is limited, and there are likely more population declines occurring that have not been identified by biologists. Long-term amphibian population monitoring is needed throughout the world for biologists and environmental managers to gain a better understanding of amphibian population trends and causes of amphibian population declines. Chytrid fungal infections are a current research priority.
Chytrid Fungus
The chytrid fungus (Batrachochytrium dendrobatidis or Bd) is a major cause of amphibian declines. The chytrid fungal infection begins with a zoospore entering an amphibian and forming a cyst under the skin. Rhizoids grow out of the skin and a zoosporangium develops. This zoosporangium releases zoospores that infect other amphibians. Some amphibians are able to resist the chytrid fungal infections. Red-backed salamanders, four-toed salamanders, and mountain yellow-legged frogs have bacteria on their skin that inhibit chytrid fungal growth. Antimicrobial peptides that inhibit chytrid fungal growth are produced by many amphibians. Other amphibians develop behavioral or immunological resistance to the chytrid fungus. And as if the chytrid fungus was not enough of a problem, scientists found another fungus in Europe (Batrschochytrium salamandrivorons), which has infected species such as the fire salamander.