In ecology, the smallest unit of a habitat that is occupied by an organism. Habitat niche refers to the physical space occupied by the organism; ecological niche refers to the role it plays in the community of organisms found in the habitat (sometimes referred to as an animal's profession). Niche is also the functional role of an organism within a community; not only where that organism lives, but also what it does and eats, how it responds to the environment, and how it is constrained by other species and external forces.