Frogs in (for example) North America often grow up all deformed - without legs, or with extra ones. Since 1995, tens of thousands of deformed frogs have been found. There are probably several reasons for this. One cause appears to be certain pesticides (closely related to nerve gases which are intended to kill people in wars) which deliberately interfere with a nerve chemical (neurotransmitter) which sends nerve impulses called acteylcholine [1].
Even small amounts of nitrate fertilisers - used by farmers and gardeners - can cause frogs to show symptoms of poisoning too. The amounts needed to make the frogs sick are much less than the amounts farmers put on their fields [2].
The same is true with a fungicide (a chemical which kills fungus which attacks plants) called triphenyl tin. This fungicide causes deformity and death in various types of frog. It is highly poisonous to a range of water-living creatures, from dragonfly larvae and snails to fish.
Meanwhile, in Costa Rica, the disappearance of some frogs from the cloud forest looks to be because of global warming, another thing people are causing [4]. Lizards and birds are in trouble too, despite Costa Rica being a haven for wildlife.
1. New Scientist, 3/4/99, 26.