Spotlight on Zoo Science
January 10, 2006
Breaking the Fall of Frogs
National Zoo scientists help to create a safety net for frogs, which are declining around the world at alarming rates.
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| Madagascar's blue-legged mantella (Mantella expetata) is one of many endangered frogs. |
Frogs, salamanders, and other amphibians are in free-fall. At least 122 species of amphibians have gone extinct since 1980, and about a third of all known species—1,856 of 5,743—are considered threatened with extinction.
Calling this “one of the worst extinction crises of our time,” a group of scientists, including the National Zoo’s Scott Miller and Budhan Pukazhenthi, and the Museum of Natural History’s W. Ronald Heyer, participated last fall in an Amphibian Conservation Summit convened by Conservation International to devise a global action plan that will offer a safety net for declining amphibians.
The alarming state of amphibians is all the more worrisome because scientists believe that these animals are the “canaries in the global coalmine,” or, more technically, sentinel species. Living both on land and in water, and absorbing oxygen and water through their porous skins, amphibians may be among the first to suffer from air and water pollution. That the canaries appear to be dying does not bode well for the rest of us.
Apart from pollution, amphibians are declining due to other human-induced environmental impacts: habitat loss, excessive harvesting for food and medicine, and climate change, often working in concert. Further, and perhaps related to climate change, a fatal disease is decimating species of amphibians around the world.
Fatal Frog Disease
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| A new fungal disease that is decimating frog populations was identified in several frogs at the National Zoo, including this green-and-black dart-poison frog. |
Called chytridiomycosis, the disease is caused by a microscopic fungus in the class Chytridiomycetes. The fungus and the disease was first described by a team of scientists at the National Zoo in 1999. Donald Nichols and Allan Pessier, formerly of the Zoo’s Department of Pathology, first isolated the fungus from the skin of a blue dart-poison frog (Dendrobates azureus) that had died at the Zoo, and later found the same fungus in three other Zoo frogs: a green-and-black dart-poison frog (Dendrobates auratus), a White’s tree frog (Litoria caeruleus), and a smooth-sided toad (Bufo guttatus).
Earlier, Nichols had seen a similar disease in wild cricket frogs (Acris crepitans) in Illinois, and in captive arroyo toads (Bufo microscaphus californicus) in California, and these cases turned out to have been caused by the same fungus. Their colleague, Joyce Longcore of the University of Maine, identified the fungus as a new species, which they named Batrachochytrium dendrobatidis. At about the same time, a team working in Australia reported that a chytrid fungus was associated with frog deaths and population declines in Australia and Central America.
Since then, chytridiomycosis, which attacks frogs’ sensitive skin, has been linked to declines and extinctions of frog populations in many parts of the globe. Some scientists suspect that this epidemic may be related to climate change increasing the amount of cloud cover in some frogs’ habitats. Exposure to sunlight usually kills or limits the growth of chytrids, while the fungus prospers under cloudy conditions, enabling it to proliferate on frogs’ skin to the point of becoming deadly.
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| The critically endangered Panamanian golden frog was bred at the National Zoo in 2005. Photo by Cesar Jaramillo, Smithsonian Tropical Research Institution. |
A study in this week's Nature underscores the link between climate change and decreased biodiversity. Comparing sea-surface and air temperature shifts with the disappearance of two-thirds of the more than 110 known species of Atelopus frogs in the late 20th century, researchers reinforced the theory that large-scale warming led to the proliferation of deadly chytrids. Alan Pounds, an ecologist at the Monteverde Cloud Forest Preserve and Tropical Science Center in Costa Rica and the report's lead author, said, "Disease is the bullet killing frogs, but climate change is pulling the trigger."
Call to Action
Participants in the Amphibian Conservation Summit developed an Amphibian Conservation Action Plan (ACAP) that outlined four essential interventions:
- Expanded understanding of the causes of declines and extinctions, including further study of emerging amphibian diseases like chytridiomycosis and their link to climate change.
- Ongoing documentation of amphibian diversity, and how it is changing. The status of many amphibian species is unknown, and scientists suspect that there may be as many as 2,500 species that have not even been described.
- Development and implementation of long-term conservation programs that protect amphibian habitat, control harvesting, and include reintroductions of frogs to aid in their recovery.
- Emergency responses to immediate crises focused on developing large-scale “captive survival assurance programs” to buy time for species on the brink of extinction.
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| The blue poison-dart frog of Surinam in South America is considered vulnerable to extinction. |
This last is where expertise at the National Zoo is particularly relevant. Pukazhenthi, a cryobiologist at the Zoo, hopes that zoos can take in the most critically endangered frogs and establish populations for future reintroduction. He is quick to note, though, that adequate capacity in terms of infrastructure and long-term funding must be established for the plan to succeed.
It also is hoped that, apart from rearing these species in captivity, participating zoos will also pursue research to improve husbandry practices and captive breeding, enhance disease monitoring and treatment, and establish genome resource banks to store cyropreserved sperm, eggs, and other biological samples from endangered amphibians.
Update: In 2009, the National Zoo, along with seven other institutions, formed the 
Panama Amphibian Rescue and Conservation Project aimed at saving amphibians from extinction as the result of chytrid.
