Answer:
Predators eat prey. When they do so, they are keeping the population or number of prey in check. Now, a lot of prey are either herbivores or omnivore, meaning plants are in their diet. If there were to be no predators, an absurd overpopulation of prey would arise. This would cripple the growth of young trees by the prey feeding on them, and overall would over-run their habitat, making the food sources scarce. When the food source gets so low they will die out or continue to move on to other surrounding areas and create the same problems.
hope this is a decent answer :)