Are the missing legs from predation or look like birth defects?

Amphibians are known as the "canary in the coalmine" of aquatic systems. They absorb water through their skin so they are way more susceptible to all sorts of pollution. A huge percentage of amphibians are endangered, critically endangered, or extinct for this reason, more than any other class of animals (class Amphibia). A major factor is birth control building in water. Modern waste management systems can't take it out so it builds in public waters where waste water is disposed. It actually turns the male amphibians into females so they go extinct.

This is probably not the case for your water, as far as birth control is concerned. But the line about missing limbs makes me wonder if you some have some kind imbalance or pollution in your pond. I know phosphate's can affect amphibians so alot of farm run off could be an issue.