A NASA-funded study to search for links between local climatic variation and the beginning of specific amphibian declines that have occurred in three areas of the world in the past several decades has turned up no significant correlation between the two.
But University of Colorado at Boulder Professor Cynthia Carey, who undertook the study with National Oceanic and Atmospheric Administration climatologist Michael Alexander, said they will continue to look for links between climate change and amphibian mortality. A professor in CU's environmental, population and organismic biology department, Carey said it is possible that the direct cause of death in many amphibians recently, disease, may be indirectly influenced by climate and environmental factors.
The results of the study were presented at the annual American Association for the Advancement of Science Meeting held Feb. 17 to Feb. 22 in Washington, D.C.
For the study, they selected three geographical areas -- Queensland, Australia, Costa Rica/Panama, and the Colorado mountains -- places where years and locations of significant amphibian die-offs due to disease were well documented. They looked at climate data sets of each area using meteorological information gathered from ships, satellites and weather stations to look for unusual temperature or moisture patterns coinciding with known amphibian die-offs from the 1970s to the 1990s.
A lack of correlation between unusual weather and amphibian die-offs in the three regions made them conclude that temperature and moisture effects on the outbreaks of amphibian disease, including fungal infection, were not apparent. In 1998, the fungus Batrachochytrium dendrobatidis was identified as a major cause of global amphibian deaths.
"We don't know anything about what sorts of environmental factors could tip the balance between pathogens and amphibian immune defenses," she said. The fungus appears to sicken amphibians by infecting their skin, which the animals use for respiration, ion and water transport and defense against water-borne pathogens. But researchers do not yet know enough about fungal interactions with amphibian immune systems to understand how the fungus kills the creatures, Carey said.
The chytrid fungus group to which B. dendrobatitis belongs is a parasite that feeds primarily on algae, higher plants and small invertebrates. Key questions still unanswered are when it began infecting amphibians, whether it has spread around the world in recent decades, and if its virulence fluctuates, causing corresponding fatalities in amphibian species. Carey was one of the first U.S. researchers to tie amphibian die-offs to disease.
"It's clear this fungus is the pathogen whacking amphibians in Australia, Costa Rica, Panama and the Western United States," Carey said. "It infects and kills populations very rapidly, taking out all the breeding adults in a very short time."
Of the three largest populations of endangered boreal toads in Colorado, two are now infected with the fungus, said Carey. The largest boreal toad populations in the state are found in Rocky Mountain National Park, the Collegiate Peaks in the central Colorado mountains and in Clear Creek County on the Eastern Slope.
Mass die-offs of the boreal toad and leopard frog in the Colorado mountains, which began in 1973 and continued until about 1982, probably were caused by fungal infections, she said. Major mortality in the few remaining Colorado boreal toad populations began again in 1997 and almost certainly is tied to the fungus, Carey said.
"We used to see total die-offs of isolated boreal toad populations take place in two years, but they do not seem to be decimated so quickly in recent years," she said. "It may be that an innate resistance to the infection allowed these few populations to survive the first onslaught of the fungus in the 1970s, which decimated 80 percent to 90 percent of the populations at the time. It also may be the fungus is not as virulent this time around."
Carey emphasized the worldwide amphibian decline is a multi-pronged problem. "There has been a lot of deforestation, lake and pond draining, pesticide use and predator introductions in areas of amphibian die-offs. But one of the bizarre things about the declines is that many amphibians are dying in relatively undisturbed areas where man-made environmental degradation is not obvious."
The researchers hope to determine if the same chytrid fungus is travelling around the world, if specific local environmental factors cause local populations of the fungus to become virulent, and how specific climatic conditions may affect the susceptibility of amphibians to such infections, she said.
"We need to move on this problem quickly," said Carey. "There is not much time left to identify and fix the problem before Earth loses most of its amphibians."
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