UNIVERSITY PARK, Pa. -- Even though amphibian populations are declining sharply worldwide, there is no smoking gun to indicate a cause and thus no simple solution to halting or reversing these declines.

That's the conclusion of a national study that was spearheaded by the U.S. Geological Survey and featured important contributions from Penn State researchers.

David Miller, assistant professor of wildlife population ecology in the College of Agricultural Sciences, helped to organize the study and was the lead biometrician in charge of data analysis. Staci Amburgy, a Penn State doctoral candidate in ecology who worked with USGS amphibian researchers as an undergraduate student at Colorado State University, also contributed to the study. She played a key role in organizing and maintaining the database upon which the research relied.

The news about amphibians is grim, noted Evan Grant, a USGS research wildlife biologist who led the study, which was published today in Scientific Reports. The evidence shows that though every region in the United States suffered severe declines, threats differed among regions. These threats include the following:

--Human influence from the Mississippi River east, including the metropolitan areas of the Northeast and the agriculture-dominated landscapes of the Midwest.

--Disease, particularly a chytrid fungus in the Upper Midwest and New England.

--Pesticide applications east of the Colorado River.

--Climate changes across the southern United States and the West Coast.

Amphibian declines are a global phenomenon first documented in the early 1960s. This new research demonstrates that declines are continuing unabated in the United States, even in protected national parks and refuges.

Scientists have broadly linked declines to environmental factors such as climate, human influences such as land-use change, and contaminants and disease. However, they have not been able to use actual scientific data on a large scale to discern causes of the ongoing disappearance of amphibian populations.

The study provides evidence that the average decline in overall amphibian populations is 3.79 percent per year, but the rate of decline is more severe in some regions, such as the West Coast and the Rocky Mountains. If this rate remains unchanged, these species would disappear from half of the habitats they occupy in about 20 years.

"The research involved a truly comprehensive and collaborative effort to bring together data from researchers across the United States," said Penn State's Miller. "We combined nearly half a million actual observations of 84 species across 61 study areas to answer questions about the causes of wide-scale amphibian declines."

The new study, the first to test this linkage at a continental scale, suggests that the presence and intensity of the four main threats -- human influence, disease, pesticide application and climate change -- varies substantially across the United States. The causes of the declines are more variable and more locally driven than had been assumed.

"Losing 3 or 4 percent of amphibian populations might not sound like a big deal, but small losses year in and year out quickly lead to dramatic and consequential declines," said USGS ecologist Michael Adams, a study co-author and the lead for the USGS Amphibian Research and Monitoring Initiative, which studies amphibian trends and causes of declines.