Competitive interactions among the populations of two species will lead to the exclusion of one of the species when the realized niche of the superior competitor encompasses the fundamental niche of the inferior competitor. This is known as the competitive exclusion principle. (Note that the fundamental niche of a species describes all possible combinations of resources and conditions under which species' populations can grow, survive, and reproduce; the realized niche describes the more limited set of resources and conditions necessary simply for the persistence of species' populations in the presence of competitors and predators.) The fact that so many species in a vast array of ecological communities are able to coexist means that species must differ in their realized niches. Such niche differentiation is critical to avoid competitive exclusion and allow coexistence.
Niche differentiation refers to differences among species in their physiology, morphology, and behavior, and concurrently in their use of resources and tolerance of conditions. Perhaps the simplest idea is that species partition available resources; that is, there is differential utilization of resources among species. Resources are often separated either spatially and/or temporally. For example, native mammals can often coexist in the same area because they forage and seek protection in different microhabitats provided by variation in native vegetation. In the Australian arid zone, for instance, the spinifex hopping-mouse Notomys alexis forages in the open while the sandy inland mouse Pseudomys hermannsburgensis is more commonly associated with hummocks of spinifex grass. The two species differ considerably in their morphology, related to their differential use of spatially segregated resources. The hopping-mouse has large hind legs for bipedal motion, which provides for rapid movement and quick changes in direction which are necessary for predator avoidance in the open habitats in which it forages. On the other hand, the sandy inland mouse is much more 'mouse'-like in its limb structure, using all four limbs for quadrupedal motion, an efficient means of movement in tangled spinifex clumps.
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