Except for "ecosystem," the word that most pervades anything written about ecology is "niche." For a term that is so basic to the science, this one is remarkably hard to clearly define. Hmm. Where have we heard that before?
Niches belong to populations, not to individual organisms.
They are multidimensional entities, involving the places the population members live, what they eat, who eats them, and any other aspect of their influence on the ecosystem that you can think of.I once heard niche defined as a combination of the address and the profession of the population, but it's really more than that. If you think of an ecosystem metaphorically as a huge, very complicated three dimensional jigsaw puzzle, the niche of a population is its piece of that puzzle, and all the little projections and indentations represent the ways the species interacts with other species in the ecosystem. I like this analogy, but it's still inadequate, since it conveys the "space" part of the niche without supplying an image for the "effect" part.
We talk about two different dimensions of a species' niche — the fundamental and the realized. You can think of these as the "potential" and "realistic" niches (though don't trade those terms for the correct ones).
Consider a species of small birds — for instance, a species of finch. There are a lot of kinds of places the birds of this species could live, and a lot of different things they could eat, like small and medium sized seeds, a variety of insects, leaves and fruits of plants, etc. The fundamental ("potential") niche of this species includes all of the places they could nest, and all of the things they could eat. It may be very broad, since many species are capable of being generalists.
However, very few species have their ecosystem to themselves. They must share the ecosystem with other species, and many, many species have fundamental niches which overlap. The competitive exclusion principle — derived from many hundreds of hours of observation of ecosystems — tells us that in a stable ecosystem, no two species are in direct competition with each other. So what happens when two or more species whose fundamental niches overlap occupy the same ecosystem? They work out an arrangement which we call "resource partitioning." This means that they jostle around until each species has reduced its niche size until there is no competition — in other words, they divide up the goodies so no one is consistently fighting over them. The result is that, in a real ecosystem, a population is almost always utilizing only a part of the niche they could have used if they were the only species in the ecosystem — so they are using only part of their fundamental niche.
This portion of their fundamental niche — the part they use in a real ecosystem — is called their realized niche. It may be much smaller than the fundamental niche.
A significant thing to note about realized niches is that populations of the same species may have very different realized niches. Exactly what the limits of the realized niche in any particular ecosystem may be depends upon things like the precise commodities of the ecosystem and the nature of the other species involved in the resource partitioning.
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