Predation is the killing of one living organism, plants or animal, by another for use as food. Predation as a means of maintaining the health and vigor of a prey population is well known.
i feel strange in the latter sentence. My knowledge of "a prey population" is supposed to be "something being eaten", so I would feel more confortable in it uses "predator".
please help me clearify it
You are misunderstanding the passage. The existence of predators (like lions) thins out the sick and weak members of the prey population (like gnus), so that those gnus who are healthier have more grass to eat and more chances to mate and raise young.