Competition is a fundamental process in biology that drives the structure and dynamics of virtually every ecosystem. It occurs when two or more organisms seek to obtain the same limited resource, such as food, water, light, space, or mates. This struggle for finite resources acts as a selective pressure, influencing which individuals survive and reproduce. Competition shapes population size, determines the distribution of species across habitats, and fuels long-term evolutionary change.
Categorizing Competition Based on Participants
The most basic way ecologists categorize competition is by identifying the organisms involved. This distinction separates the struggle occurring within a species from the struggle happening between different species.
Competition among members of the same species is termed intraspecific competition. Because individuals within a species share nearly identical needs, they compete intensely for the exact same resources. For example, two male red deer may fight to secure mating rights, or a dense stand of pine saplings will compete vigorously for limited sunlight and soil nutrients.
When the rivalry occurs between individuals of different species, it is known as interspecific competition. This interaction happens when two distinct species rely on a shared resource that is not plentiful enough for both populations to thrive. For instance, lions and cheetahs hunt similar prey on the African savanna, and the presence of one predator reduces the food supply available to the other.
How Organisms Compete for Resources
Regardless of whether the competition is intra- or interspecific, the method organisms use to vie for resources falls into two distinct mechanisms: exploitation and interference.
Exploitation competition is an indirect rivalry where one organism consumes a shared resource, making it unavailable to another, without direct physical confrontation. The species that consumes the resource faster effectively exploits the supply. For instance, two species of zooplankton competing for the same algae demonstrate exploitation when the faster-feeding species leaves fewer resources for the slower one.
Interference competition involves direct interaction where an organism physically prevents its rival from accessing a resource. This can include aggressive behavior, chemical warfare, or blocking access. A common example is a dominant bird species driving a subordinate species away from a preferred nesting cavity. In plants, some species exhibit interference by releasing chemical compounds (allelopathy) that inhibit the growth of neighboring plants.
Long-Term Ecological Outcomes of Competition
Sustained competition between species leads to profound ecological and evolutionary consequences, determining how communities are structured. One possible outcome is the Competitive Exclusion Principle, which posits that two species cannot occupy the exact same ecological niche indefinitely if resources are limited. If one species is slightly better at obtaining shared resources, it will eventually outcompete the other, leading to the exclusion of the weaker species.
This struggle often leads to the local extinction of the less efficient competitor. This result was demonstrated in laboratory experiments where one species of Paramecium eliminated another when both fed on the same limited resource. While complete exclusion is rarely observed in complex natural environments, the competitive pressure often forces the weaker competitor to adapt or relocate to a less desirable habitat.
A more common long-term outcome, particularly in stable ecosystems, is resource partitioning, which allows competing species to coexist. This occurs when species evolve or behaviorally adapt to use different subsets of the shared resource or use it at different times. For example, several species of anole lizards in the Caribbean coexist by occupying distinct vertical zones on trees, such such as the trunk, canopy, or ground.
Resource partitioning can drive an evolutionary phenomenon known as character displacement. This describes the evolutionary change in physical or behavioral traits of competing species resulting from the pressure to reduce niche overlap. A classic instance is seen in Galápagos finches, where two similar species living on the same island develop noticeably different beak sizes compared to those living alone. This trait divergence allows one species to specialize in larger seeds and the other in smaller seeds, enabling their coexistence.