An apex predator is an animal who, as an adult, has no natural predators in its ecosystem. The great white shark is an example of an apex predator.
Apex predators play an important role in keeping ecosystems in check. A study in 2009 showed that on land, or in the ocean, the removal of apex predators (intentionally or accidentally) can have dramatic impacts on an ecosystem, and result in even larger problems.
The study mentioned above found overall that reduction of apex predators results in an increase in 'mesopredators,' or smaller predators, which causes a decline in prey populations. Apex predators can keep these smaller predators in check, keeping important prey populations abundant.
For example, the removal of wolves in the western U.S. in fear of loss of livestock has caused an explosion in the population of coyotes, causing even more damage to livestock than before. In Yellowstone Park, it caused an increase in the elk population, which increased the pressure on the park's vegetation.