in ecology, utilization of the same resources by organisms of the same or of different species living together in a community, when the resources are not sufficient to fill the needs of all the organisms.

Within a species, either all members obtain part of a necessary resource such as food or space, or some individuals obtain enough for their needs while other members, cut off from the resource, die or are forced to inhabit a less suitable or marginal area. Young members of a population are most often adversely affected.

The closer the requirements of two different species, the less likely is it that they can exist in the same area. Species with similar requirements can sometimes exist in the same area if they differ in behavioral ways such as feeding patterns, nesting habits, or activity periods, although they may be forced into direct competition when resources are scarce. Often small populations of two species coexist, but their members have smaller than average bodies or a low reproductive rate.