Competition is a fundamental force in nature, acting as a primary driver of structure within biological communities. Community ecology focuses on how interactions between different species shape the distribution and abundance of organisms. Competition is one of the most powerful biotic interactions, determining which species survive and thrive in a given habitat. This interaction occurs whenever two or more organisms seek to acquire the same limited resource, such as food, nesting sites, light, or water. The result is a reduction in the fitness, growth, or reproductive success of all involved parties.
Defining Competitive Interactions
Ecologists classify competition based on whether the organisms belong to the same species or different species.
Intraspecific Competition
Intraspecific competition happens among individuals of the same species, such as two male deer fighting over a mate or a territory. Because individuals within the same species have nearly identical needs, this competition is often intense and acts as a density-dependent regulator on population size.
Interspecific Competition
Interspecific competition occurs between individuals of different species who share a common resource requirement. For instance, a lion and a cheetah may both hunt the same limited population of gazelles. Although their needs are not completely identical, the overlap in their ecological niches limits the population growth of both species.
Competition is also categorized by the mechanism of the interaction, which can be direct or indirect.
Exploitation Competition
Exploitation competition is an indirect interaction where one organism consumes a resource, thereby depleting the amount available for others. A plant taking up nitrogen from the soil reduces the nutrient supply for its neighbor without direct confrontation.
Interference Competition
Interference competition involves a direct physical or chemical interaction that prevents a competitor from accessing a resource. Examples include a male bird aggressively defending a nesting site, or a plant species releasing toxins (allelopathy) to inhibit the growth of nearby rivals. Both intra- and interspecific competition can occur through exploitation or interference mechanisms.
Direct Impact on Species Abundance
When interspecific competition is intense, the immediate consequences can drastically alter the local population sizes of the species involved. The most significant outcome is the Competitive Exclusion Principle, also known as Gause’s Law. This principle states that two species cannot indefinitely coexist if they occupy the exact same ecological niche and compete for identical limited resources.
If one species possesses even a slight advantage in resource acquisition, it will ultimately outcompete the other. The less competitive species will face a population decline, leading to local extinction or migration. This principle was famously demonstrated by ecologist Georgy Gause, who grew two species of Paramecium (P. aurelia and P. caudatum) in the same culture.
In Gause’s experiments, P. aurelia consistently drove P. caudatum to extinction when competing for the same bacterial food source. The superior efficiency of P. aurelia meant it won the exploitative competition under stable laboratory conditions. This outcome shows that when ecological conditions are uniform and niches overlap completely, competitive exclusion is the inevitable consequence.
However, many natural communities exhibit high species diversity, suggesting that mechanisms for coexistence frequently operate. Environmental factors that fluctuate over time or space can prevent the superior competitor from dominating completely.
Environmental Heterogeneity
Environmental heterogeneity, such as variations in soil moisture or light levels, allows different species to specialize in different microhabitats. For example, two closely related fiddler crab species coexist in estuarine environments by specializing on microhabitats that differ in shading, linked to their thermal stress tolerance.
Periodic Disturbance
Periodic disturbance, such as moderate fires or floods, can interrupt competitive exclusion. This concept is formalized in the Intermediate Disturbance Hypothesis, which proposes that species diversity is maximized at intermediate levels of disturbance. Disturbances temporarily reduce the population of the dominant competitor, creating open space and resources that allow weaker competitors or fast-colonizing species to persist. This dynamic prevents any single species from achieving complete dominance.
Long-Term Community Structure
Over evolutionary timescales, the pressure of interspecific competition drives changes in species traits and behavior, fundamentally shaping community structure.
Resource Partitioning
One primary long-term result is resource partitioning, where species evolve to use different facets of a shared resource, minimizing niche overlap. This allows multiple species to coexist in the same habitat by specializing on different parts of the resource spectrum.
A classic illustration involves five species of warblers inhabiting North American spruce forests, studied by Robert MacArthur. Although they appeared to occupy the same niche, MacArthur found they reduced competition by foraging in different zones of the same tree. For example, the Cape May warbler focused on the outer upper branches, while the Bay-breasted warbler foraged on the middle interior sections.
This specialization allows the birds to divide the available insect prey by location and foraging technique, effectively partitioning the food resource. Resource partitioning leads to higher overall species richness by packing more specialized niches into a single community. This evolutionary response transforms intense competition into stable coexistence.
Character Displacement
Another profound evolutionary consequence is character displacement, which describes the divergence of a morphological or behavioral trait in two species where their geographic ranges overlap. This divergence results from selection pressure to reduce interspecific competition in the area of sympatry. The most famous example involves the beak sizes of Darwin’s finches on the Galapagos Islands.
On islands where two finch species live separately, their beak sizes are similar, allowing them to eat a broad range of seeds. However, on islands where the two species coexist, their beak sizes diverge dramatically. One species evolves a smaller beak for smaller seeds, and the other a larger beak for larger seeds. This evolutionary change reduces direct competition for food, enabling stable coexistence in the shared environment.