Niche

In ecology, a niche (pronounced "nich", "nish" or "neesh") is the place in an ecosystem that is fillable by a species. The idea is best explained through examples, so here we go.

Virtually everywhere on earth, the sun provides a swath of niches for any organism that can convert UV energy into a biologically usable form (plants and algae). This in turn forms the basis of nearly all ecosystems, opening up niches for anything that can eat these organisms (or parts of them) for the stored energy/chemicals. These animals then open up niches for organisms that prey on those organisms, which can open up niches for parasites to feed on those animals in turn, and so on.

In most parts of the world, the niche of small, skittish, ground-dwelling vegetarian is played by a placental mammal, like a chipmunk or mouse. However, in Australia, the niche is taken up by a small, ground dwelling marsupial, while in New Zealand, the equivalent niche is a flightless, ground-adapted omnivorous bird, the kiwi. In fact, most of the niches normally occupied by mammals in the rest of the world are filled by birds in New Zealand. The only native mammals in New Zealand are bats (which conveniently take up the night-hunting flying insectivore niche).

One thing to note is that no two species can fill the same niche simultaneously. This is one of the reasons invasive species are so damaging to ecosystems.