Competition between different species is a fundamental process that shapes the structure of biological communities. When multiple species require the same limited resources, their interactions influence population growth and distribution. The Competitive Exclusion Principle (CEP) is a foundational rule in ecology describing the ultimate result when this competition is intense and sustained. This principle establishes a clear boundary for species coexistence, dictating that a single resource can only support a single dominant species over time.
Defining the Principle
The Competitive Exclusion Principle, often referred to as Gause’s Law, proposes that two species competing for the exact same limited resource cannot stably coexist at constant population levels. If one species possesses even a slight advantage in acquiring or utilizing that resource, it will inevitably out-compete the other. Over time, the superior competitor will increase, while the population of the weaker competitor will decline.
This proposition was formulated by the Russian biologist Georgy Frantsevich Gause in the 1930s, based on his laboratory experiments. Gause showed that when two competing species were introduced into a controlled environment with a single limiting food source, one population consistently drove the other to local extinction. The underlying logic is that if resources are truly identical, the species with the highest efficiency at resource uptake or survival will always prevail. The principle predicts that competition for an identical niche will result in the exclusion of the less-fit species.
The Critical Role of Identical Niches
The concept of an ecological niche is central to understanding the Competitive Exclusion Principle, as it represents a species’ total functional role within its environment. This role encompasses the full range of physical conditions, resources, and interactions a species needs to survive and reproduce. The CEP holds true only when the niches of two species are perfectly identical, meaning they rely on the same food, shelter, and environmental conditions.
Any difference in resource use, such as feeding at a different time of day or consuming slightly different components of a food source, means the species occupy distinct niches. Because perfect competitors are rarely found in nature, the principle highlights the stringent conditions necessary for one species to completely eliminate another. Subtle variations in habitat preference or resource accessibility can prevent total niche overlap and avert competitive exclusion.
Outcomes of Interspecific Competition
When two species share overlapping niches, the interaction leads to one of two major outcomes: competitive exclusion or coexistence. Competitive exclusion occurs when one species successfully monopolizes the shared limiting resource, leading to the local disappearance of the other. This outcome is often observed in controlled laboratory settings or in environments where resources are extremely scarce.
More frequently in natural ecosystems, competition results in coexistence, achieved primarily through resource partitioning. Resource partitioning involves species evolving or adapting to utilize slightly different aspects of the shared resource, effectively differentiating their niches. For instance, one species might feed on the largest seeds, while the other focuses on the smallest, reducing direct competition and allowing both to persist.
An evolutionary consequence of this partitioning is character displacement, where physical or behavioral traits diverge more noticeably in areas where the species co-occur. This divergence accentuates the difference in their resource use, making the niche overlap minimal. By shifting traits like beak size or feeding time, species minimize the detrimental effects of interspecific competition.
Experimental Evidence and Real-World Examples
Gause’s classic laboratory experiments involving two species of single-celled organisms, Paramecium aurelia and Paramecium caudatum, provide the strongest direct evidence for the principle. When grown separately with a constant food supply, both species thrived and maintained stable populations. However, when placed together in the same culture, P. aurelia consistently outcompeted P. caudatum for the limited food source.
The smaller P. aurelia was more efficient at gathering the bacterial food, resulting in its continued growth while the population of P. caudatum declined to zero. This demonstrated that under stable conditions with a single limiting resource, one species’ superior competitive ability led to the extinction of the other. The principle also helps explain the success of invasive species that exploit resources more effectively than native organisms.
A clear real-world example involves the introduction of the Eastern Grey Squirrel (Sciurus carolinensis) to Great Britain, where it largely replaced the native Red Squirrel (Sciurus vulgaris). The Grey Squirrel possesses a competitive advantage, partly due to its ability to digest acorns earlier in the season and its greater resistance to the squirrel poxvirus. This superior competitive edge, combined with niche overlap, has driven the Red Squirrel to local exclusion across much of the mainland.