A University of Pittsburgh researcher has discovered that a popular chemical designed to kill weeds may also be highly deadly to baby frogs. Pitt biologist Rick Relyea found that the herbicide Roundup -- the second most commonly used herbicide in the United States -- caused major declines in tadpole population size and diversity when applied at manufacturer-recommended concentrations in outdoor tanks that simulated real ponds.

"This is really the first discovery that something made to kill plants really can be extremely lethal to amphibians, which of course, comes as a complete surprise to a lot of people that apply Roundup," Relyea said. "We can't assume anymore that herbicides aren't going to have any affect on animals." Roundup, made by Missouri-based Monsanto Co., caused a 70 percent decline in the variety of frog species in the 300-gallon tanks and an 86 percent decline in the total mass of tadpoles.

Leopard frog and gray tree frog tadpoles were completely wiped out, while wood frog and toad tadpoles were nearly eliminated. The experiments were conducted at Pitt's Pymantuning Laboratory of Ecology, a field research station in Crawford County. Relyea's research, published Friday in the journal Ecological Applications, builds on his earlier studies that point to the lethal effects of the widely used pesticide carbaryl on tadpoles. They also could suggest one possible cause for the massive decline in amphibians being documented by scientists worldwide, which could have implications for human health, he said.

"If these frogs are declining from a pesticide effect we never new existed, then what are the other effects that pesticides are having that we don't know about because we haven't done the tests yet?" Relyea asked. Chemical manufacturers are required to measure the toxicity of all bug and weed killers they make, and the results must be approved by the U.S. Environmental Protection Agency. Most toxicology data, however, are collected under artificial conditions: Researchers slowly add a chemical to a tank of critters and find out how much is needed to kill half of them over several days. Relyea's experiments are among the first to study the toxicological impact of herbicides in a more natural setting that accounts for the real-world effects of predators and natural stress factors, said Yale University biology professor David Skelly, who also studies amphibians. "Here we have an example of how commonly used chemicals can affect entire systems," Skelly said. "This research isn't opening a floodgate, but it's adding to the steadily growing flow of information that is going to make our status quo approach for regulating pesticides less and less tenable." Jennifer Bails can be reached at jbails@tribweb.com or (412) 320-7991.