Distinguish between fundamental and realized niches.
The niche that a species could occupy is often smaller than the niche that the species actually occupies. These potential and actual niches are called the fundamental niche and the realized niche of the species. Differences between the fundamental and realized niches are due to competition.
Other species prevent a species from occupying part of its fundamental niche by out-competing or by excluding it in some other way.
The fundamental niche of a species is its potential mode of existence, given the adaptations of the species.
The realized niche of a species is its actual mode of existence, which resluts from its adaptations and competition from other species.
Competitive exclusion happens when a species is unable to occupy any part of its fundamental niche in an area, so it has no realized niche in that area.