Amphibian declines linked to climate change
It has been one of the biggest biological mysteries of the past 30 years.
Beginning in the late 1980s, scientists began to notice a rapid drop in amphibian populations throughout broad regions around the world, including Central America, North America and Australia.
Over the past decade there have been documented declines in more than 200 species, and about 20 species are presumed to have gone extinct. While no single cause has been identified to explain this large-scale phenomenon, a number of factors have been implicated, including habitat loss, disease, invasive species and chemical exposure.
Now a piece of the puzzle has been discovered that might tie together all of these declines. For the first time, scientists have made a direct link between global warming trends and amphibian declines. The findings are published in the April 5 issue of the journal Nature.
Following 10 years of study, the research team found a direct link between the Southern Oscillation Index, which tracks temperature fluctuations including the El Niño warming cycles in the South Pacific, and the amount of rain and snow in Oregon's Cascade Mountains. Altered precipitation patterns resulted in lower levels of water in ponds and lakes, where amphibians lay their eggs. "Around the early 1990s, we started to see 80 to 100 percent mortality," said lead author of the study Joseph Kiesecker.
For the declining population of Western toads, shallow ponds created a more stressful environment for the young embryos, which in turn, made them more susceptible to disease, the researchers found.
Roughly 80 percent of the embryos that were placed in less than 8 inches of water developed infections and died. Yet in eggs that were allowed to develop in water deeper than about 22 inches, the mortality rate was only 12 percent.
Increased exposure to ultraviolet radiation caused most of the eggs that were laid in shallow depths of water to contract the water-mold pathogen, Saprolegnia ferax, which usually only attacks organisms that are injured or under stress.
A number of other pathogens have been identified as a cause of amphibian declines in other parts of the world.
"Stress-related disease is the one consistent factor that may link amphibian deaths worldwide, and we have demonstrated that amphibian stress in the Cascades is ultimately linked to recent global climate fluctuations," Kiesecker said.
The research provides further evidence for the connection between climate and epidemics," said J. Alan Pounds, a biologist at the Monteverde Cloud Forest Preserve and Tropical Science Center in Costa Rica, in a commentary that accompanied the Nature article. A number of recent reports that have linked climate change with population declines in birds and butterflies.
"Amphibians could be an important bioindicator species because they are particularly sensitive to climate change," Kiesecker added. "Many researchers who started trying to solve the puzzle of amphibian declines during the past decade now have become even more motivated by the feeling that amphibians may be telling us something important about the threats to biodiversity on our planet."
In the future, Kiesecker plans to use the Southern Oscillation Index to predict, four to six months in advance, of outbreaks of amphibian disease at specific locations.
"Our research sets the stage for other research teams studying amphibian declines to look at their sites in a different way," Kiesecker said. "This study shows the amazing complexity of biological systems that we will need to grasp if we translate global climate change into local species loss."
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