Many species find themselves isolated from predators with which they evolved. This can be natural, as on islands, or unnatural, as in zoos. In response to this isolation, many species lose the ability to respond appropriately to their predators. However, some species, despite many years of isolation, retain anti-predator behavior.
UCLA professor, Dan Blumstein, reviewed the literature and suggests that the key factor responsible for persistence, despite the loss of some predators, is that anti-predator behavior does not evolve independently. Most species have more than a single predator and we should expect co-adapted suites of anti-predator behavior. Thus, the loss of a single predator should have a limited effect on overall anti-predator abilities as long as other predators remain.
Evidence in support of this 'multi-predator hypothesis' comes from studies of wallabies, which retained anti-predator behavior for thousands of years following isolation from some predators, but rapidly lost anti-predator behavior when isolated from all predators.
The multi-predator hypothesis has important implications for conservation biology: taking animals from a predator-free location and putting them back into a predator rich area is more likely to fail than taking animals from an area with a sub-set of historically-important predators.