An ecological niche describes the role of an organism within its environment. The niche includes the complete set of biotic, living, and abiotic, non-living, resources that the organism uses. For example, the niche of a hare includes abiotic factors, such as the climate in which it can survive, and the terrain that provides shelter and safety from predators.

Biotic factors might include the other hares it encounters, the plants that it consumes, and predators that feed upon it. A fundamental niche is the full set of resources that an organism could use in its environment, while a realized niche is the narrower set of resources that a species actually uses. Competition with other species is the major factor that limits the extent of the realized niche.

The species is typically more optimally adapted for these conditions, but it may use resources outside of the realized niche if its preferred resources are not readily available, or if competition from other species is removed. While there is often overlap in the resources used by similar species in a given area, multiple species cannot occupy the exact, same niche in an ecosystem, due to competition. One solution that arises over time is resource partitioning, a division of limited resources in an ecosystem that allows species to coexist rather than compete.

Ecologically similar species can either evolve to use different resources, or share the same resources, but use them at different times or places. A classic example of resource partitioning can be found in Robert MacArthur's work on warbler species in the woods of New England. MacArthur found that, although the warblers appear to use the same resources, inhabiting the same trees, and consuming the same insects, they actually occupy different areas of the trees. The spatial partitioning of resources was sufficient to allow five species of warblers to coexist within the same trees.