The natural world is characterized by constant interactions between different species vying for limited resources such as food, water, light, or space. This struggle is a fundamental driver of community structure in ecology. To understand how communities of organisms are organized, it is necessary to grasp the principle that dictates the outcome of this interspecies rivalry. The competitive exclusion principle provides a powerful framework for predicting which species will persist and which will decline when their needs overlap.
Defining the Competitive Exclusion Principle
The competitive exclusion principle asserts that two species cannot indefinitely coexist in the same habitat if they compete for the exact same limited resource. This concept, often summarized as “complete competitors cannot coexist,” predicts that even a slight advantage in resource acquisition or utilization efficiency will allow one species to dominate the other. The result of this direct competition is the decline or local disappearance of the less successful species.
Understanding this principle requires defining the ecological niche, which represents the specific role and requirements of an organism within its environment. A niche is not merely a species’ physical location, but the sum of all the conditions, resources, and interactions it needs to survive and reproduce. When two species possess completely identical niches, their resource requirements overlap entirely, leading to intense competition. The marginally more efficient species will experience higher fitness, ultimately leading to the exclusion of its rival from that specific habitat.
Classic Experimental Proof
The principle was formalized and demonstrated under controlled conditions by the Russian biologist Georgy Gause in the 1930s, using laboratory cultures of single-celled organisms. Gause conducted experiments with two species of protozoa, Paramecium aurelia and Paramecium caudatum. When each species was grown separately in a culture vessel with a constant supply of food, both populations thrived and reached stable sizes.
Gause then placed both species together in the same vessel, forcing them to compete for the identical, limited food source. The result was a clear demonstration of competitive exclusion: P. aurelia rapidly outcompeted P. caudatum. This led to the eventual decline and extinction of the P. caudatum population within the culture, confirming that the more efficient species dominates when resources are shared.
When Exclusion Occurs in Nature
While laboratory experiments clearly show exclusion, the principle is also evident in real-world ecological systems, often taking place over longer timescales. A classic example involves two groups of barnacles on the rocky shores of the intertidal zone: Balanoid and Chthamaloid barnacles. Balanoid barnacles possess a rapid growth rate that allows them to physically undercut and overgrow the slower-growing Chthamalus species.
This competitive dominance restricts Chthamalus to the very upper fringe of the intertidal zone, where it is exposed to greater periods of desiccation and heat stress. Since Balanoid barnacles cannot survive these harsh upper-zone conditions, this area creates a refuge for the less competitive Chthamalus species. Another modern example involves invasive species, which are often competitively superior to native organisms. For instance, the European starling aggressively outcompetes native cavity-nesting birds for limited nest sites, resulting in the displacement and population reduction of native species. Similarly, the rapid spread of Asian carp in parts of the Illinois River has led to them consuming a large portion of the zooplankton and phytoplankton, driving down the populations of native fish that rely on the same food sources.
Resource Partitioning and Coexistence
The observation that many similar species successfully coexist in nature suggests that complete competitive exclusion is often avoided. This coexistence is achieved through resource partitioning, where competing species evolve or adapt subtle differences in their ecological niches. These differences prevent the complete overlap of resource requirements that would otherwise trigger exclusion.
Resource partitioning can manifest in several ways, categorized by how the resource is divided.
Spatial Partitioning
Spatial partitioning occurs when species use different areas of the same habitat. For example, five different species of warblers coexist in spruce and fir forests by foraging for insects in distinct parts of the same tree. Some species feed near the top, others in the mid-section, and still others at the base.
Temporal Partitioning
Temporal partitioning involves using the same resource but at different times. An example is hawks hunting mice during the day while owls hunt the same prey at night.