Over the last few centuries we have shot, trapped and skinned the predators that formerly thrived at the top of the food chain in the wild. Wild bears, wolves and big cats are all in retreat, and a growing number of scientists are discovering that by eliminating predators, we have changed the environment. Removing predators from the wild has thrown ecosystems off-kilter, triggering domino effects that scientists are just beginning to understand. In "Wild Predator Invasion," NOVA follows scientists who are trying out a simple but controversial solution: returning apex predators --like wolves, bears, and panthers -- to their natural environments. Can these newly reintroduced predators restore the natural balance of their ecosystems without threatening the humans who live among them?