What is the Term "Niche" Mean in Ecological Biology?

The term niche, when used in the science of ecological biology, is used to define an organism's role in an ecosystem. Not only does its niche include the environment that a given organism lives in, but it also includes the organism's "job" in that environment. A niche may also encompass what the organism eats, how it interacts with other living (biotic) elements, and also how it interacts with the nonliving (abiotic) aspects of the environment, as well.

Fundamental Niche vs. Realized Niche

All living organisms have what is called a fundamental niche. The fundamental niche includes all possibilities open to the organism within that environment: all possible sources of food, all open behavioral roles in the environment, and all suitable habitats available to it. For example, a black bear (Ursa americanus) is a broadly distributed, omnivorous species that has very large fundamental niche, since it can eat meat as well as a broad range of vegetation, and can thrive in low woodlands as well as grassy mountain regions. It thrives in deep wilderness, but is also highly adaptable to areas near human settlement.

In reality, though, an organism cannot use all suitable resources in an environment at the same time. Instead, the organism will have a narrower range of foods, roles, and habitats that it actually makes use of. This more specific role is called the organism's realized niche. For example, circumstances or competition may reduce a black bear's realized niche into one where foods consist exclusively of berries and carrion meats, and shelter is limited to earthen burrows. Rather than a hunter, its niche may become that of a browser. 

Relationships With Other Organisms

Symbiotic relationships also come into play to determine an organism's niche. Predators that are in the area can limit an organism's niche and particularly where it can find safety and shelter. Competitors will also limit food sources and other nutrients, so they can also have an effect on where an organism makes its home. For example, the black bear and brown bear (Ursus arctos) overlap over much of their ranges, and where this occurs, the more powerful brown bear will generally have its pick of shelter and game, limiting the niche available to the black bear. 

Not all relationships are competitive. An organism may also seek out other species to have positive interactions with in order to define its niche. Commensalism and mutualism with other species in the area can make an organism's life easier. Commensalism is a relationship in which one species benefits while the other is unaffected; mutualism is a relationship in which both species benefit. A black bear that learns to feed on an abundance of raccoons killed along a highway is practicing commensalism; a bear that devours large quantities of blackberries. then "plants" new berries by distributing them through its scat deposits is practicing mutualism.

Relationships with Non-Living (Abiotic) Factors

Abiotic factors, such as water availability, climate, weather—and in the case of plants, soil types, and amount of sunlight—can also narrow an organism's fundamental niche to its realized niche. Faced with a prolonged forest drought, for example, our black bear may find its realized niche redefined as favored plants dwindle and game species become more scarce, and as water shortages force it to seek shelter in other locations.

To some degree, an organism can adapt to its environment, but its basic needs must first be met in order in order for it to establish a niche.