This frog is missing two limbs. Scientists say high levels of nitrogen and phosphorus in farm and ranch runoff may be a cause of this frog's deformity. (Photo: Craig Line/AP Images)

The growing number of frogs born with extra or missing legs, eyes, and other body parts has puzzled scientists for at least a decade. Now, they believe that they have found a cause–water runoff from U.S. farms and ranches.

Scientists at the University of Colorado in Boulder conducted experiments using dozens of artificial ponds to find out what was causing the increase in deformed frogs. These deformities have been observed for more than 10 years, ever since a group of schoolchildren made a troubling discovery at a pond in Minnesota. About half of the frogs there were missing limbs.

University of Colorado scientists say the problem is high levels of nitrogen and phosphorus in water runoff from U.S. farms and ranches.

Nitrogen and phosphorus are nutrients often found in large quantities in fertilizers. The two substances can help things grow, but too much of either can throw an ecosystem significantly out of balance.

The scientists say that high levels of nitrogen and phosphorus can lead to the increase of certain parasites in contaminated ponds. These parasites infect tadpoles, or baby frogs, and are believed to cause the frogs' limbs to grow abnormally.

Scientists have long suspected that a parasite infection was hurting tadpoles. However, the cause of the increase in parasites had been a mystery, the university researchers said.

Scientists confirmed their theory through experiments in 36 artificial ponds they built. They added nitrogen and phosphorus to some of the ponds. Scientists found that the contaminated ponds had more parasites than the ponds without the added nutrients.