Definition

A niche is an organism's job in its ecosystem, including what it eats, what eats it, where it shelters, and when it is active, so a hawk hunting mice by day and an owl hunting similar prey by night in the same forest occupy different niches; two species with exactly the same niche in the same place cannot both last, since one will always get slightly more food and slowly push the other out. More formally, a niche is the full range of environmental conditions and resources a species uses and the role it plays in its community, and because of the competitive exclusion principle, two species cannot occupy the same niche in the same habitat indefinitely, instead dividing resources through resource partitioning or having one eliminated, with adaptations being what fit a species to its niche. Hutchinson formalized the niche as an $n$-dimensional hypervolume of environmental variables within which a species can maintain a population, distinguishing the fundamental niche, the full range it could occupy without competitors or predators, from the narrower realized niche it actually occupies; niche overlap drives competition, character displacement (evolutionary divergence of competing species where they co-occur) is a signature of past competition, and niche breadth separates generalists from specialists, with invasive species often succeeding by exploiting empty niche space.