<figcaption>Scientists propose that global warming might be helping chytrid fungus kill frogs. This immunostained sample shows the fungus (in red) covering a section of frog skin. Credit: © CSIRO / Lee Berger</figcaption>
Scientists propose that global warming might be helping chytrid fungus kill frogs. This immunostained sample shows the fungus (in red) covering a section of frog skin. Credit: © CSIRO / Lee Berger

Scientists became aware of extinctions in various frog species in the 1980s, when J. Alan Pounds and colleagues reported the disappearances of golden toads (Bufo periglenes) and harlequin frogs (Atelopus) in the Monteverde Cloud Forest Reserve of Costa Rica.1 It wasn't until the late 1990s that researchers found a suspect for the mysterious case of global frog declines: an infectious disease, chytridiomycosis, caused by the chytrid fungus, Batrachochytrium dendrobatidis.

Two years ago Pounds, a resident scientist at the Monteverde Cloud Forest Reserve, and his colleagues implicated another factor in frog deaths. In this Hot Paper, they compiled data from multiple sources and found a correlation between unusually warm years and the disappearance...

A climate for chytrids

For 104 of the 113 species of Atelopus, the Pounds group obtained data for the last year each species was observed in its regional habitat. They interpreted the last year observed as the year of extinction and looked for any corresponding patterns in air temperature, sea surface temperature, elevation, and the number of dry days. They found a correlation between frog declines and years when the temperature exceeded historical averages: Populations would dwindle in the warm year and disappear in the following year. "The study is the first to link some recent species extinctions to climate change," says Pounds.

Since Batrachochytrium reportedly grows best at temperatures between 17° and 25° C, with the ideal temperature of 23° C, Pounds argues in the Hot Paper that because temperatures shifted toward this ideal, climate change is creating an environment conducive to the fungus.

"Everybody suspects that global warming is going to be a huge stress on populations, with lots of extinctions," says David Reed, at the University of Mississippi. "In this case, you have a massive extinction over a short time, which allows you to document the causes. That's the beauty of this study." Reed says the Pounds paper supports his general hypothesis that multiple stresses on a population, including pollution, competitors, and diseases, act multiplicatively rather than additively (see "Climate change and the biosphere").

Scientists working in this field have criticized the study. Lee Skerratt at James Cook University, for example, comments that the Pounds group found a correlation between warm years and frog declines, but it gave no evidence to implicate the chytrid fungus as the culprit, especially since they did not have frog corpses to examine. Skerratt argues in a 2007 paper3 that the effects of climate change have not been significant enough to be a primary cause of the chytridomycosis outbreaks. "You don't need climate change for a disease to wipe frogs out," he says.

Resolving the debate

Pounds agrees that other causes, such as additional pathogens, may also be to blame for killing the frogs. Since the 2006 Hot Paper appeared in Nature, researchers have focused on resolving what is involved. Jason Rohr and colleagues at the University of South Florida in Tampa, for example, found that temperature fluctuations (not just temperature increase or decrease alone) can weaken immunologic responses and increase susceptibility to infection in red-spotted newts.4 He's now reexamining the climate data that the Pounds group used, along with additional data gathered since 2006, to see whether temperature fluctuations might also harm frogs' immunity.

While Rohr is reexamining the climate data, Karen Lips at Southern Illinois University and her colleagues are currently reanalyzing the amphibian data. Lips was a coauthor on the paper that provided the original data Pounds used regarding the last year each frog species was observed. Now she and her colleagues are exploring the hypothesis that the disease moves in wave-like spatiotemporal trends in South America.5

There are also hypotheses independent of the Pounds group's research. Steven Whitfield of Florida International University, for example, attributes frog declines to reductions in the quantity of standing leaf litter.6 Still, "we're far from the end of the story of the chytrid, and global warming and its effect," says David Wake at the University of California, Berkeley. Researchers looking into the cause of amphibian extinctions "are attempting to look at these things and say there's some sort of synergistic interaction. It may be too early to tell, but the quest is turning up interesting things."


1. J.A. Pounds, M.L. Crump, "Amphibian declines and climate disturbance: the case of the golden toad and the harlequin frog," Conservation Biol, 8:72-85, 1994. 2. J.A. Pounds et al., "Widespread amphibian extinctions from epidemic disease driven by global warming," Nature, 439:161-7, 2006. (Cited in 90 papers) 3. L. Skerratt et al., "Spread of chytridiomycosis has caused the rapid global decline and extinction of frogs," EcoHealth, 4:125-34, 2007. 4. T.R. Raffel et al., "Negative effects of changing temperature on amphibian immunity under field conditions," Func Ecol, 20:819-28, 2006. 5. K.R. Lips et al., "Emerging infectious disease and the loss of biodiversity in a neotropical amphibian community," Proc Nat Acad Sci, 103:3165-70, 2006. 6. S.M. Whitfield et al., "Amphibian and reptile declines over 35 years at La Selva, Costa Rica," Proc Nat Acad Sci, 104:8352-6, 2007.

Data derived from the Science Watch/Hot Papers database and the Web of Science (Thomson ISI) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age.

J.A. Pounds et al., "Widespread amphibian extinctions from epidemic disease driven by global warming," Nature, 439:161-7, 2006. (Cited in 90 papers)

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