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Competitive Exclusion Principle: Definition & Example

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  • 0:01 Definition of…
  • 1:57 Adapt, Migrate, or Die
  • 3:19 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
The ecological concept of competitive exclusion illustrates the fragile and dynamic relationships among organisms competing for the same resources. This lesson examines a definition of this concept, goes through some examples and finishes with a brief quiz.

Definition of Competitive Exclusion

Living things coexisting in the same place at the same time can be like a game of musical chairs. There are often not enough chairs for everyone, and two cannot occupy the same chair at the same time. In nature, a niche is analogous to the chairs in this example; it is a species' role in its habitat. This 'role' includes what it eats, how it eats, where it lives, its impact on nutrient cycles, and how it functions with respect to other living things.

Two different species cannot occupy the same niche at the same time. This is not unlike the roles of different players on a baseball team: at any given time, two players cannot occupy the same position. Each player has a unique job in ensuring the success of the team, and having three people trying to pitch at once would cause catastrophe! In nature, different species with overlapping niches would be in fierce competition for resources. This is where the concept of competitive exclusion comes into play.

In competitive exclusion, one species is displaced by another when their niches overlap and they compete for the same resources. The 'winning' species out-competes the 'losing' species. This can occur during environmental change, when a new species enters an ecosystem and its presence conflicts with native species. Because of competitive exclusion, and other ecological factors, species do not always fill the full niche that they have the potential of filling. This is called the fundamental niche. Instead, they end up occupying a portion of the fundamental niche. This is called their realized niche.

Here is an example of a fundamental versus realized niche, where an organism can withstand a certain range of temperature on the x-axis and humidity on the y-axis. The fundamental niche is shown in yellow and the realized niche in red.

fundamental realized niche

Adapt, Migrate, or Die

Let's use an example of two barnacle species: Balanus barnacles that tend to live in the lower part of tide ranges, and Chthamalus who tend to live in the upper tide range. We will call them B and C, respectively. Even though C larvae tend to settle out in all levels of the tide range - upper, middle, and lower - when only C species are present, there is a clear division of their home zones as adults when B species are also present.

Basically, both species tried to occupy the same niche - the middle and low zones of the intertidal - but species B successfully drove C out of the middle and lower area, and now they each have separate zones, with different levels of sunlight and air exposure and different physiological concerns. The only options for the 'losing' species during competitive exclusion are to adapt to a new niche within the ecosystem, migrate to a different ecosystem, or die off.

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