"It's no secret that, worldwide, amphibian populations are declining," says Donald Sparling, professor of zoology and associate director of SIUC's Cooperative Wildlife Research Laboratory. "Many species have been exterminated in the last 20 years. Habitat destruction has been a major cause, but disease and contaminants are also worldwide problems."

Frogs and toads, with their permeable skin, are especially sensitive to their surroundings and are considered early-warning indicators of environmental problems.

Most of California's native frog and toad species are in trouble. "The hardest-hit areas are from central to southern California, especially east of the Central Valley," Sparling says. These are primarily agriculture-intensive areas, or places downwind of agricultural activity.

Before Sparling came to SIUC, he worked as a biologist with the U.S. Geological Survey. One of his last projects there was to supervise a sort of round-robin experiment in California. Deborah Cowman, a graduate student at Texas A&M University, moved clutches of frog eggs between ponds in Lassen, Sequoia, and Yosemite national parks. She found that, no matter where the eggs originated, many more of the hatched tadpoles died in Sequoia than in Lassen, with Yosemite falling in between.

The translocation experiment showed that genetic differences between populations weren't the cause. Instead, she and Sparling went on to find that pesticide levels in these parks tracked with the mortality rates: Sequoia had the highest levels, followed by Yosemite and then Lassen.

What are pesticides doing in these supposedly pristine places? They are literally floating in from the Central Valley, where they are sprayed from planes. Some of the aerosol droplets drift east, perhaps hitching a ride on dust particles, and end up in snowfall in the Sierra Nevadas. Not coincidentally, Sequoia is the park closest to the Central Valley.

With USGS funding, Sparling and master's student Jill Hunt are finding that even very low pesticide concentrations are problematic. Their lab work focuses on chlorpyrifos, the most widely used pesticide in the state, and on endosulfan, which seems to be the most toxic to amphibians. For several species, they've established an important benchmark figure called the LC50: the concentration of a contaminant that will kill 50 percent of the individuals exposed to it. Take endosulfan, for example: the LC50 is a little over 3 parts per billion (ppb) for the Pacific treefrog and Western toad. But it's less than 1 ppb for the foothills yellow-legged frog. In fact, that level kills 90 percent of yellow-legged hatchlings before they get past the tadpole stage, the researchers found.

Amphibians can tolerate higher levels of chlorpyrifos: 12 ppb for the yellow-legged frog; 20 ppb for the treefrog. (The higher tolerance of the latter may explain why the Pacific treefrog is one of only two frog or toad species not showing a significant decline in California.)

But that's only part of the chlorpyrifos story. Aquatic bacteria and ultraviolet rays both break down chlorpyrifos into a compound called chlorpyrifos-oxon. This breakdown product is 100 times more toxic to amphibians than the original compound, Sparling and Hunt have determined. "Nobody had ever looked at toxicity of the oxon form before," Sparling says.

Not surprisingly, species that spend more of their lives in the water are in greater danger of extinction, Sparling says. For example, it's so cold where the mountain yellow-legged frog lives that it spends two summers as a tadpole. Sparling's studies were included as supporting data when this species was listed as endangered.

In other research, Sparling and master's student Tab Bommarito are investigating what's killing off the extremely rare Barton Springs salamander in Austin, Tex—a totally aquatic species that lives only in one spring-fed wading pond in an Austin park. The likely threat? Particles of coal-tar and asphalt sealants washed from a nearby parking lot. These petroleum-derived substances contain PAHs (polycyclic aromatic hydrocarbons), which can cause genetic damage that leads to cancer or other diseases. Sunlight—specifically, ultraviolet B radiation—increases PAH toxicity about 100-fold.

"We don't yet know how toxic these contaminants can be," Sparling says. "These sealants are used throughout the nation, often close to wetlands, so this research could have implications for other species."

Since the salamander is so rare, Bommarito and Sparling are using two very similar species as research surrogates. They're exposing these individuals to different concentrations of sealant particles in sediment, then looking at DNA damage, survivability, and behavioral responses.

PAHs also kill the little crustacean Hyalella azteca (see In Our Own Back Yards), a key food source for salamanders. The contaminant may be accumulating up the food chain, or the salamanders may be dying because their food source is disappearing. Either case underscores the importance of studying the effects of contaminants on the smallest of the small.

Why care about something as little and scarce as the Barton Springs salamander?

"All species have a role in the ecosystem, whether they serve as predator or prey," Sparling says. "If we see a major group of vertebrates in severe decline, that's a significant portent for the health of the environment in which we live. We hope that what we learn from research on a single species will have applications to many other species that are facing similar exposures.

"We're building [the picture] piece by piece."

—by Marilyn Davis, ed.