The ecological niche of a species in the absence of competition is called the fundamental niche.

Every species has a potential niche that can be exploited in its habitat ( a niche refers not only to living space, but to food, temperature, appropriate mating conditions, etc.). However, because of competition from other species, this full potential may not be realized and the species will be restricted. The niche a species actually occupies is known as the realized niche.

For example, a sparrow's fundamental niche may include a huge chunk of forest surrounding its nest, but, because of the group of chicadees located a few trees away, its realized niche is much smaller because of interspecific competition. If these chicadees suddenly disappeared, the realized niche of the sparrows would probably increase.

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