The rate of abnormalities like shortened or missing legs was less than 2 percent among frogs and toads on national wildlife refuges, according to a 10-year study.
The findings suggest that the malformations first reported in the mid-1990s were more rare than feared.
However, much higher rates were found in local “hotspots,” suggesting that where these problems occur, they have local causes. The results are published in PLOS ONE.
“We now know what the baseline is and the 2 percent level is relatively good news, but some regions need a deeper look,” says Marcel Holyoak, professor of environmental science and policy at the University of California, Davis, and a co-author on the study.
68,000 frogs and toads
Hotspot regions included the Mississippi River Valley, California, and south-central and eastern Alaska.
Mari Reeves, a graduate student working with Holyoak, led the data analysis and is corresponding author on the paper. Reeves now works at the US Fish and Wildlife Service in Alaska.
The Fish and Wildlife Service carried out fieldwork for the study at 152 refuges across the country between 2000 and 2009. Researchers collected more than 68,000 frogs and toads for the study. The complete dataset is available to researchers and the public online.
The aim of the study was to understand where and when these abnormalities occur—Are they widespread, or localized? Are they persistent, or do they appear and fade away?—rather than to identify specific causes, Holyoak says.
Understanding the patterns of these hotspots in space and time can help researchers home in on likely causes, he says.
The results show that abnormality hotspots occur in specific places, but within these hotspots the rate of malformations can change over time, Holyoak says.
“We see them at an elevated frequency one year or for a few years, and then they recover,” he says.
The most common problems observed were missing or shortened toes or legs, and skin cysts. Only 12 cases of frogs with extra legs were found.
Many different potential causes have been put forward for the abnormalities, including pollution from industry or agriculture, parasites, ultraviolet exposure, and naturally occurring heavy metals leaching into water bodies. The exact cause may vary from place to place, Holyoak notes.
The study comes against a background of a general decline in amphibian populations both in the US and worldwide.
For example, the California red-legged frog is now listed as threatened. Frogs and toads may be especially sensitive to changes in climate and air or water quality.
It’s not clear whether or not hotspots of malformations contribute to this general decline, Holyoak says, but the new dataset will help researchers explore the problem.
The Fish and Wildlife Service funded the study. Other authors contributed from the University of Colorado, Boulder; the US Fish and Wildlife Service, Annapolis; and Indiana University School of Medicine.
Source: UC Davis