Researchers looked for chemical pollutants or hormonal changes in the frogs as culprits. But recent evidence linked the deformities missing, extra, or deformed limbs to the presence of Ribeiroia ondatrae, a frog parasite that has been noted in the scientific literature for a century and a half. But no one could explain why the incidence of deformities has increased to upwards of 20 to 30 percent of some frog populations in the 21st century compared with probably less than one percent historically.
Now a collaboration involving ecologists at Washington University in St. Louis and the University of Wisconsin strongly points to farming practices and development, two factors that create a condition called eutrophication in ponds and wetlands.
Eutrophication is caused by higher phosphorus and nitrogen (prime components of agricultural fertilizer) levels in wet ecosystems. Higher levels of these nutrients cause a profound impact on the food web that imperils the frogs' existence.
A warm gun
"What we have is a warm gun, not yet a smoking one," said Jonathan Chase, Ph.D., assistant professor of biology at Washington University in St. Louis. "We have evidence that eutrophication creates a favorable situation for a common snail that thrives on high phosphate and nitrogen levels. This particular kind of snail, the ramshorn snail, found in pet stores, is the snail needed by a different life stage of the same parasite that causes the deformed frogs. So the snail, frog, and parasite are entangled with each other in a complicated food web."
Chase had previously studied the ecology of food webs in small ponds and the important role of the ramshorn sna
Contact: Tony Fitzpatrick
Washington University in St. Louis