This post rewritten because it was wrong in a couple of ways. However, the frogs still have an extra limb.
A study published in the Proceedings of the National Accounting of Scientists says that agricultural run off is indirectly causing leopard frogs in Colorado to grow with either missing or extra limbs. The real culprit is a really complicated parasite. The fact that this little bastard even evolved is amazing. First it grows inside fresh water snails. The snails then eject the parasite when it gets big enough to become bothersome. Then the parasite infects the tadpoles of the frogs, causing the deformities. Meanwhile birds eat the frogs and then excrete the parasites back into the system where they infect the snails again.
Where do the nutrients kick in? Well, the runoff causes an algal bloom which the snails eat. More snails means more parasite, whihc means more three legged froggies.
The kicker to this whole story is that he first deformed snails were found ten years ago by some school children in Minnesota. Yick.