Why frogs are croaking

In the quest to find out why frog species have been declining so dramatically, various researchers have blamed climate change, disease, pollution, and increases in ultraviolet light from the sun reaching the surface. If two new studies are any indication, the answer increasingly appears to be: all of the above.

Researchers led by the University of South Florida’s Jason Rohr have found that an herbicide commonly used by farmers growing corn and sorghum as well as high phosphate levels from fertilizer appear to be triggering a decline in the northern leopard frog. After fieldwork in 18 wetlands in Minnesota, as well as lab studies, the team says that the combination of atrazine and phosphates in freshwater ecosystems are acting as a one-two punch on the leopard frogs.

Atrazine reduced phytoplankton in the water, making more food and light available for algae. It also appears to speed snail reproduction. The snails, often carrying larvae of parasites harmful to frogs, feed on the increased amounts of algae. Then frogs feast on the snails. In addition, the atrazine deadens the frogs’ immune systems, leaving them less capable of countering the parasites. The results appear in today’s issue of the journal Nature.

Meanwhile, in Yellowstone National Park, climate change appears to be the lead culprit in what another team calls a “precipitous decline” in amphibians during the past 16 years. Climate change has  three broad effects, according scientists from Stanford University and South Dakota State University.

Warming appears to have changed frogs’ reproductive behavior and body condition. Warming also appears seems to be destroying habitat as droughts dry up ponds and reduce the amount of leaf litter on the ground. And, the team says, warming can also increase a frog’s susceptibility to disease. The results appear in this week’s issue of the Proceedings of the National Academy of Sciences.