Worldwide, amphibians are dying. And University of Pittsburgh ecologist Rick Relyea said he knows one way to kill them: Spray them with a little Roundup, the best-selling weed killer from St. Louis-based Monsanto.
In a new study from Relyea, published in this month's issue of the journal Ecological Applications, Roundup killed 98 percent of tadpoles during a three-week test in simulated shallow ponds. In a separate dry experiment, Roundup killed 79 percent of young frogs and toads after just one day. "It's much deadlier than we thought," Relyea said.
Monsanto says that Roundup isn't meant to be used near water and that its directions clearly say so. But many amphibians live in shallow puddles, Relyea said. He said he worries that wetlands within fields and forests are accidentally being sprayed.
Something clearly is killing amphibians. They have declined drastically since the 1970s, biologists say. Nearly a third of the world's amphibians are threatened, according to a global survey last year by the International Union for the Conservation of Nature and Natural Resources.
By contrast, 12 percent of bird species and 20 percent of mammals are threatened, according to the union's Web site, www.redlist.org.
"The debate is whether amphibians are the canaries in the coal mine," Washington University biologist Jonathan Chase said. There are reasons to suspect they are."
Amphibians' permeable skins make them vulnerable to toxins. Global warming, acid rain and increased ultraviolet light all seem to harm them. So even if Roundup has a toxic effect, Chase said, it's unlikely to cause the global declines on its own. Rather, there are likely many causes with the biggest being loss of habitat, he said.
"The No. 1 cause is that we're building parking lots and malls and expanding our footprint on the world," he said.
Relyea said he agrees that habitat loss is the most important factor. There isn't evidence yet that Roundup is contributing to the worldwide decline, he said. But his experiments show its striking lethality.
Relyea added one tablespoon of Roundup Grass and Weed Killer to 250 gallons of water in cattle-watering tanks where tadpoles were growing with soil and food. That amount mimicked a worst-case accidental spraying of a small wetland, Relyea said.
But Monsanto spokeswoman Mica DeLong said Relyea's concentrations were too high and unlikely to be found in nature. She also criticized the artificial setting of Relyea's dry experiment, in which he sprayed frogs and toads who sat in plastic tubs lined with moist paper towels.
"We believe this needs to be studied in a natural setting where other factors come into play," she said, citing a field study last year by Canadian scientists, published in the journal Environmental Toxicology and Chemistry. It shows that even when small wetlands are accidentally sprayed, Roundup concentrations never come close to the levels Relyea applied.
Roundup is a product name for a herbicide, one of many in a general class that use the chemical glyphosate, which Monsanto pioneered. Glyphosate is now the top agricultural pesticide in the U.S., according to the Environmental Protection Agency.
In 1993, the EPA renewed its permit for Roundup. It noted that glyphosate itself is not toxic to aquatic life. The problem was with one of its common surfactants, which is toxic. A surfactant is a soapy additive used so glyphosate can stick to and penetrate plants.
In Australia and Europe, Monsanto sells Roundup Biactive, a version with a different surfactant that doesn't harm amphibians.
"Why don't we have the other surfactant?" Relyea asked. "Either it's less effective at killing weeds or it's more expensive to make."
Monsanto toxicologist Donna Farmer said the surfactant in Roundup Biactive was less effective on North American weeds and also would be subject to a cumbersome EPA approval process.St. Louis Post-Dispatch
