If something is drastically reduced or killed, especially in number, you can say it was decimated. "The oil spill in the Gulf of Mexico decimated the wildlife along the coast."
The verb decimate originally referred to a form of capital punishment for Roman troops. If there was a rebellion, one out of every ten men (thus the dec in decimate) was put to death. So the word's first expanded usage was to mean a ten percent reduction or a ten percent tax. Modern usage gives the word decimate its "drastically reduced" meaning, but the verb can also be used to mean "to wipe out" or "to eliminate."