An organism's ecological niche refers to its role in its particular environment, including all the ways it affects both its physical habitat and the other organisms within it. For instance, a predator has an effect on its prey populations as well as on any higher predators or parasites that prey on it.
Even organisms that fulfill similar roles in an environment can have somewhat different ecological niches. Two predators can prey on the same species, but they might be more active or reproduce at different times. The way an organism responds to changes in its environment is also an aspect of its niche. In addition, some organisms are capable of altering their environments in ways that affect other species, such as building hives.
Organisms require a niche to survive. If the niches of two organisms overlap, as when two predators hunt the same type of prey, one of the species may be out-competed and must then either focus on different prey or even disappear from the environment. When new organisms enter an environment, they must occupy a niche and compete with overlapping niches of existing organisms. They may also alter the niches of other organisms by providing a new source of food or some other beneficial effect.