FROG FALLOUT: The northern leopard frog is among the many amphibians worldwide struggling with man-made threats, in this case, herbicide pollution.
©Neal Halstead, University of South Florida

The northern leopard frogs that inhabit the boreal U.S. have never recovered from some catastrophic population declines in the 1970s. Some blame it on the acidifying lakes and streams caused by coal-burning, others point to the ongoing loss of wetlands to development, and now new evidence shows that the herbicide atrazine—widely sprayed on crop fields throughout the region—is killing the frogs by helping parasitic worms that feast on them.

"Atrazine provides a double whammy to frogs: It increases both amphibian exposure and susceptibility," says biologist Jason Rohr of the University of South Florida in Tampa, who tested the impact by re-creating field conditions in 300-gallon (1,135-liter) tanks in his lab. "Atrazine is one of the more mobile and persistent pesticides being widely applied. In fact, residues have been found in remote, nonagricultural areas, such as the poles."

That may explain why amphibians are on the decline worldwide. As many as one third of the nearly 6,000 known amphibian species—frogs, toads, salamanders, even wormlike caecilians—are threatened with extinction, according to the International Union for Conservation of Nature (IUCN). And no one knows why.

In the case of the northern leopards, the culprit appears to be the common herbicide acting as a double-edged sword: It suppresses the frogs' immune systems while boosting the population of snails that play host to parasitic worm larvae, the latter of which infect the weakened leopard frogs.

Such herbicides are present in 57 percent of U.S. streams, according to the U.S. Geological Survey, and it is that water pollution—not inbreeding—that is the prime suspect in the high rate of deformity in U.S. amphibian populations, according to new research from Purdue University.

But national parks and other areas protected from pollution and development are providing no refuge. The frogs and salamanders of Yellowstone National Park have been declining since the 1980s, according to a Stanford University study, as global warming dries out seasonal ponds, leaving dried salamander corpses in their wake. Since the 1970s, nearly 75 percent of the frogs and other amphibians of La Selva Biological Station in Braulio Carrillo National Park in the Caribbean lowlands of Costa Rica have died, perhaps due to global warming.

Throughout the tropics, amphibians are also falling prey to a devastating disease, believed to be exacerbated by climate change: chytrid fungus. This pathogen is marching though Central America at present, leaving silent streams—those without the chorus of dozens of frog species—behind. Researchers at the University of Georgia in Athens have surveyed such streams before, during and after such extinctions, and documented the impacts in those waters that have lost all of their amphibians, including muddier waters and a less productive food web.

"There are a whole lot of things that aren't being eaten, mostly insects," says biologist Joseph Mendelson of Zoo Atlanta. "And there are a whole lot of other creatures that don't have prey."

But the really bad news is that amphibians may be just the first sign of other species in trouble. Biologists at the University of California, San Diego, have shown that amphibians are the first to respond to environmental changes, thanks to their sensitivity to both air and water. What goes for amphibians may soon be true of other classes of animal, including mammals.

Whereas the exact causes of the amphibian decline remain a mystery, it is clear that man-made  problems like atrazine pollution and climate change are contributing factors, both of which can be reversed. "The easiest thing to do," Rohr notes, "would be to identify an alternative herbicide that controls the desired pests but is less detrimental to amphibians."