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In an environment with limited resources, any organisms that utilize the same resources will be in competition with each other.

For example, let's look at two competing species of paramecium, a single-celled organism that feeds on bacteria. If we raise each of these species in isolation, both populations will thrive, at least until their food source runs out.

However, we can raise both species in the same environment and see what happens. Notice that P. aurelia starts out with a slow growth rate, but it eventually out-competes P. caudatum, whose population eventually declines.

Since these two species compete for the same resources, they are said to occupy the same ecological niche.

This experiment, conducted by G.F. Gause, illustrates the competitive exclusion principle, which states that two species in direct competition cannot occupy the same niche.

If two species are found to coexist, then they must be using the resources in their environment in slightly different ways, and therefore occupy different niches.

Looking at a slightly more complex example, here is a marine ecosystem that illustrates how a niche can change depending on the competition of other species.

A species of Chthamalus barnacle will occupy both high and low tide regions, given no competition from other barnacles. Meanwhile, Balanus barnacles will occupy only the low tide region, given no competitors.

You can see tat the Balanus species out-competes Chthamalus in the low tide region, so Chthamalus narrows its niche to the available high tide area.

This example illustrates the distinction between a species' realized niche, which is the role a species actually plays in the ecosystem, and its fundamental niche, which is the role it would play in the ecosystem if competitors were not present.

In many cases, it is impossible to determine a species fundamental niche without removing all of its competitors.

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