This much is clear: frogs are dying.
One third of the world's 6,260 amphibian species are globally threatened or extinct. The primary threat to their survival is still habitat destruction, which impacts 61 percent of known amphibian species. But climate change and the deadly chytrid fungus could potentially take the lead over the next century—or at least make things much, much worse for frogs, salamanders and their legless, subterranean cousins known as caecilians.
How much worse? It depends, in part, on whether climate change has the potential to spur—or slow—the growth of the chytrid fungus, which probably originated in South Africa and spread in the 1930s as clawed frogs were exported for frog-based pregnancy tests. Although more than 200 species have been diagnosed with the pathogen, 1,034 have been deemed "susceptible," according to the International Union for Conservation of Nature and Natural Resources (IUCN). And that's where things get tricky.
Some scientists, like Alan Pounds of the Tropical Science Center's Monteverde Cloud Forest Reserve in Costa Rica, believe that warming temperatures have single-handedly triggered the spread of the fungus. Others, like Karen Lips of the University of Maryland (U.M.), College Park, say that climate has played practically no part in its expansion. Still others, like Ross Alford at James Cook University in Australia, contend that hot, dry weather is directly driving most extinctions, and the fungus has had only a tangential role.
In 2008 disease ecologist Jason Rohr of the University of South Florida in Tampa, published a paper saying, in effect, that none of the amphibian extinction theories were quite right—and the parts that were correct were correct for the wrong reasons. To illustrate his point, he showed that frog extinctions were more closely correlated to beer and banana production than to air temperature.
Last week, Rohr traveled to the Ecological Society of America conference in Albuquerque, and presented results from a new analysis and a laboratory study that he hopes will bring the bickering scientists closer to a resolution—his resolution, that is.
First, he took a close look at the timeline of extinctions in Atelopus, the New World frog genus hardest hit by the chytrid: 71 of the 113 species are now presumed extinct. The extinctions have clearly been spreading geographically since 1979, but there's a great deal of year-to-year variation in their numbers.
In the early 1990s, for instance, about 8 percent of Atelopus species were winking out each year. But a closer look shows that just under 3 percent of Atelopus extinctions happened in 1991, whereas 18 percent occur the following year. These peaks and troughs correlate with the El Niño climate cycle, which creates warmer, wetter summers in Central and South America every three to eight years, due to shifts in trade winds and ocean currents. Rohr contends that the frog extinctions occur during cool swings in the midst of warm El Niño years, a hypothesis that goes against Pounds's theory that the fungus grows best at warmer temperatures.
To parse out this complex phenomenon, Rohr and his colleagues decided to bring the frogs and the fungus into the lab. Cuban tree frogs were raised in Styrofoam tanks at either 15 or 25 degrees C for four weeks and then were infected with chytrid fungus. The fungus grew better and killed more frogs at the cooler temperature.
More importantly, Rohr looked at how climate fluctuations would affect the frogs' ability to fight the fungus. If frogs were suddenly moved from the hot temperature to the cooler temperature, they ended up with 25 percent more fungus than if they were kept at the cooler temperature. Rohr suspects that the frogs' ability to secrete fungus-fighting skin secretions is temperature-sensitive.
The bottom line, he explained, is that climate change does matter for the fungus—but probably not as much, or as little, as some would like. Although El Niño events have become more frequent and intense in recent years, scientists are still undecided about their connection to climate change. "Temperature variability has increased in the region," Rohr said, "but I don't think climate is going to account for nearly as much variation as epidemic spread."
Amphibian experts say the new results seem promising. "On a month scale, temperature fluctuations could have strong effects on the immune system," explains immunologist Jacques Robert at the University of Rochester School of Medicine and Dentistry. "When you start to change temperature, going up or down, there is some suppression in the animals."
And chytrid expert Joyce Longcore of the University of Maine, Orono, also agrees that the theory seems reasonable. "I'm primarily on the side of not needing to have climate change to have these infections," she says. "That doesn't mean that in some instances it doesn't slow its spread or make it worse."
Of course, it is still too early to know whether the rest of the amphibian decline community will embrace—or dispute—the new results. Although U.M.'s Karen Lips saw Rohr present the new data, she declined to comment until she sees a paper in print.
This much is clear: frogs are dying.