Amphibian Mortality Linked To Global Climate Change

Scientists have been trying to determine why amphibian populations worldwide have been declining at alarming rates since the late 1970s, but their research has pointed to a confusing variety of causes. Now Joseph Kiesecker, professor of biology at Penn State, reports the research team he leads has shown that global warming causes changes in rainfall patterns, causing stress in moisture-sensitive amphibians, leaving them susceptible to a variety of pathogens. The specific stresses and specific resulting causes of death depend on the specific conditions in the animals' local habitat.

Kiesecker's 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 or snow in Oregon's Cascade Mountains. Other members of the team include Andrew R. Blaustein and Lisa K. Belden, of Oregon State University.

"For over 10 years, we have been collecting data at a number of sites in the Cascades, 4,000 to 7,000 feet above sea level, where there are large breeding populations of western toads," says Kiesecker, whose team backpacks for 10 miles to reach some of the remote sites. The researchers work in tents for weeks to build their experimental devices, which include boxes they designed to anchor the toad eggs at different distances below the surface of the lake in order to learn whether a thicker blanket of water better protects the eggs from the damaging effect of ultraviolet radiation. "We have found that water levels are shallower during years when there is less snow, which exposes the eggs to more ultraviolet light and makes them susceptible to disease outbreaks," says Kiesecker.

In early spring every year, the toads lay their eggs in one big communal mass that may contain a million or more eggs. They breed at the same spot each year, typically a bedroom-sized area near the edge of the pond, where the ice melts earliest. "We have to be ready before the toads explode into the pond from the surrounding woodlands for their annual three to five days of breeding and egg laying," says Kiesecker. His team determines how many pairs are breeding each year by wading into the pond and picking up each breeding pair in order to identify the toads. The researchers give each toad an identification mark the first year it breeds in the pond. "We can estimate how many eggs are laid in the pond during a season by counting the breeding pairs," he says. An average female lays about 12 thousand eggs.

"Around the early 1990s, we started to see 80 to 100 percent mortality," Kiesecker says. The toad eggs laid at shallower depths, which are stressed by overexposure to ultraviolet light, are killed by a water-mold pathogen, Saprolegnia ferax, which generally attacks only organisms that are injured or under stress in some way.

Kiesecker says ultraviolet light is the cause of lethal toad stress in the Cascade mountains, but it may not be a factor at other sites of amphibian decline, where embryos laid under a heavy vegetation canopy are not exposed to ultraviolet light. Other pathogens have been identified as a cause of death 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 says.

"This study shows that if we want to understand the complex ecology of the world around us, we must start looking at the big picture, and there may not be simple or easy answers," Blaustein says.

Kiesecker explains: "There is a very strong link between water depth and embryonic mortality. Then there is a link between the Southern Oscillation Index and rainfall patterns in the Cascades. Next there is a link between rainfall patterns and depth at which embryos develop. And finally there is a link between egg-laying depth and mortality associated with Saprolegnia ferax."

Kiesecker says his team's results reveal the amazing complexity associated with understanding biological systems, while at the same time demonstrating that there may be simple rules for understanding this complexity. "Our results indicate that interpreting the impacts of global climate change will require a detailed mechanistic understanding of local natural history coupled with long-term data that will permit us to gauge responses associated with climate change. We may be able to use simple indicators of global climate fluctuations to make predictions regarding ecological interactions on local scales," Kiesecker says. The Kiesecker team plans to use the Southern Oscillation Index as a predictor, four to six months in advance, of amphibian-disease outbreaks at particular sites in the Cascade mountains.

"Our research sets the stage for other research teams studying amphibian declines to look at their sites in a different way. They need to know their local conditions really well, then they can incorporate measures of long-term climatic fluctuations into their analyses of their population's mortality rates," Kiesecker says.

"Amphibians could be an important bioindicator species because they are particularly sensitive to climate change," Kiesecker adds. "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."

This research was supported by the NIH/NSF Panel on the Ecology of Infectious Diseases and by Penn State.