Climate Change Linked to Decline in Amphibians
A series of environmental dominoes beginning with global climate change may explain one of the decade’s most perplexing biological mysteries--the sharp decline in amphibians--according to a new study of toads in Oregon.
A team of researchers studying western toads in the Cascade Range has tied together a series of seemingly unrelated events: warm weather patterns over the South Pacific, decreased rainfall in the Pacific Northwest, ultraviolet radiation and a fungus-like pathogen.
Simply put, the research team found that unusually dry winters caused by El Nino meant ponds in which toad embryos mature contained less water, making them more vulnerable to both ultraviolet radiation and a destructive fungus.
“We don’t think it’s one single answer,” said Andrew R. Blaustein, biology professor at Oregon State University. The research team’s conclusions are published today in the journal Nature.
The Oregon scientists “have identified, for the first time, a complete chain of events whereby large-scale climate change leads to high mortality in a declining population,” said ecologist J. Alan Pounds at the Golden Toad Laboratory for Conservation in Costa Rica.
The study has implications for other species, by demonstrating the “growing recognition of the connection between climate and epidemics,” Pounds said in a commentary accompanying the report.
“Climate only loads the dice for disease outbreaks.”
The research team led by Joseph K. Kiesecker, a professor of biology at Pennsylvania State University, used data from as far back as 1990 to monitor toad breeding activity at sites 4,000 to 7,000 feet above sea level in lakes and ponds in Oregon’s northern Cascades.
The findings suggest how a global phenomenon such as climate change may be interacting with local factors to destroy amphibians that have existed on Earth since before the age of dinosaurs. Since 1980, at least 20 amphibian species have become extinct, and many others are shrinking dramatically in regions as distant and different as Oregon and Costa Rica.
“What’s coming home is that these environmental fluctuations appear to be sufficiently severe that, when coupled with other factors, they’re having a devastating effect on wildlife,” said David Wake, a UC Berkeley professor of integrative biology and a leader in the study of decreasing amphibian populations.
“This is a new kind of environmental biology,” Wake said, “and we’re really getting introduced to it in a grim way.”
To study the toad’s breeding season, the team backpacked to remote mountain ponds, often in snow and rain, with the temperature hovering around freezing.
Toads breed just as the mountain snows begin to melt, and females then move into ponds to lay their eggs, Kiesecker said.
The water levels in the ponds are governed by the amount of snow and spring rain. The El Nino weather phenomenon generally produces less rainfall in the Pacific Northwest, meaning the ponds are shallower.
The researchers measured the number of embryos deposited during toad breeding season and noted what percentage succumbed to the destructive pathogen known as Saprolegnia ferax.
In the ponds the embryos mature, looking like jet-black dots encased in membranes that act as conduits for wastes and gases. The membranes are wrapped in a jellylike coating that slips through the fingers like egg white.
Although these delicate cases have protected toad embryos for millenniums, scientists found that an unusually high number of embryos were failing in the mountain ponds. More than 50% of the embryos that grew in shallow water developed fungal infections. But the mortality rate held at 19% or less for embryos in water deeper than 45 centimeters (about 18 inches), the Nature article reports.
Scientists also found that embryos raised in deeper water received nearly 44% less ultraviolet radiation than those in shallow water.
“Our findings support the hypothesis that climate-induced fluctuations in water depth have caused unusually high mortality of embryos by influencing their exposure to UV-B radiation and consequently their vulnerability to [fungal] infections,” Kiesecker, Blaustein and coauthor Lisa K. Belken wrote.
Scientists are still wondering why a fungus that has long coexisted with the western toad may be wreaking havoc today. Kiesecker and fellow researchers have been trying to determine if the fungus becomes more common when hatchery-raised fish are introduced to natural bodies of water. They plan to present more information in an upcoming paper, Kiesecker said.
The chain of events apparent in the Oregon mountains may not be replicated in other areas that are experiencing amphibian declines, but scientists say the study shows how global climate change and local factors can combine to harm wildlife.
In a piece accompanying the Nature article, Pounds writes, “Today, there is little doubt that both phenomena--amphibian declines and global warming--are real. If there is indeed a link between the two, as the work of Kiesecker et al suggests, there is clearly a need for a rapid transition to cleaner energy sources if we are to avoid staggering losses of biodiversity.”
