Living organisms interact with their environment and other species. These interactions often involve a shared demand for essential resources like food, water, light, or living space. When these resources are in limited supply, different species can find themselves in direct competition. This dynamic interplay shapes the structure and diversity of ecological communities.
Defining the Principle
The competitive exclusion principle, sometimes referred to as Gause’s Law, articulates a fundamental concept in ecology. This principle states that two species cannot indefinitely coexist if they are competing for the exact same limited resources in the same place at the same time. One species will inevitably outcompete the other, leading to the exclusion or local extinction of the less successful competitor. Russian ecologist G.F. Gause first proposed this idea based on his experimental work in the 1930s.
For this principle to operate, the competing species must be “complete competitors,” meaning they have virtually identical requirements for the same finite resources. If their ecological niches—their roles and positions in the environment—overlap entirely, their direct competition becomes unavoidable. The superior competitor will more efficiently utilize the shared resource, preventing the other species from obtaining enough to sustain its population.
Conditions for Its Operation
The competitive exclusion principle is most evident under specific ecological conditions. A stable environment, where resource availability and environmental factors remain constant, provides the setting for direct and sustained competition. Fluctuating or unpredictable environments can sometimes allow less competitive species to persist by offering temporary windows of opportunity.
The principle also requires truly limited resources; if resources are abundant, competition may not be intense enough to drive extinction. Species must also occupy virtually identical ecological niches, meaning they exploit the same resources using the same methods and occupy the same physical space. Any slight difference in resource use or habitat preference can reduce the intensity of competition, potentially allowing for coexistence.
Ecological Outcomes
The most direct outcome of competitive exclusion is the local elimination or displacement of one competing species. This can occur through population decline until it vanishes, or by one species moving to a different, less desirable habitat.
However, competitive pressure can also lead to evolutionary adjustments that allow for coexistence. One common response is niche partitioning, where species evolve to use different aspects of a shared resource or habitat. For example, species might feed at different times of day, consume different parts of a plant, or forage in different microhabitats.
Another adaptive outcome is character displacement, where competing species evolve physical or behavioral differences that reduce their niche overlap. These shifts help minimize direct competition, enabling multiple species to persist.
Real-World Illustrations
G.F. Gause’s pioneering experiments with Paramecium species provide a classic illustration of the competitive exclusion principle. He grew Paramecium aurelia and Paramecium caudatum separately with a limited food source; both thrived. When grown together, P. aurelia outcompeted P. caudatum for food, leading to its decline and extinction in the mixed culture.
The introduction of invasive species into new ecosystems provides another example. Non-native species with a similar niche can outcompete native species for limited resources.
For instance, the introduction of the gray squirrel (Sciurus carolinensis) to Great Britain led to the decline of the native red squirrel (Sciurus vulgaris) in many areas. The gray squirrel is larger, more adaptable, and more efficient at foraging, giving it a competitive advantage.