The competitive exclusion principle states that two species cannot occupy the same niche. When two species compete for the same habitat and resources, the end result is that one species eliminates the other. This concept was demonstrated by a series of experiments by the biologist G.F. Gause.
G.F. Gause studied the growth rates of two paramecium species when they were grown separately versus grown in the same medium. When cultured separately, both species grew and flourished under the same conditions. When grown together in the same medium, one strain eventually outgrew the other and eliminated its competitor.
The niche of an organism can be described on several levels. The fundamental niche is the range of environments in which an organism can thrive and multiply without competition from other species. The realized niche is a habitat where the organism can live without competing against other species for resources. The realized niche is generally smaller than the fundamental niche.
Most organisms experience competition in their niches at some point in time. A species can occupy several realized niches, and different factors play roles in which niches they occupy. The presence of predators, for example, may lead a species to occupy an alternative niche instead.