The answer is competition.
Competition is an interaction between organisms. If resources, such as food, water, sunlight, territory, are limited and several species depend on these resources, then, they must compete with other species to gain the resources. So, if predators are present, this means that they will maintain the size of prey population on some optimal level. With fewer individuals in the population, there will be less competition for resources.
Therefore, In natural ecosystems, the presence of predators lessens the effects of competition among the prey.
