Biological competition is a fundamental interaction in ecology, acting as a primary driving force that shapes communities and influences the evolutionary path of species. It occurs when multiple organisms seek to acquire the same limited resource in an environment. This struggle for resources affects the survival, growth, and reproduction of the organisms involved, making it a powerful natural selective pressure.
The Core Concept and Necessary Resources
Biological competition is formally defined as an interaction between organisms or species where both require one or more resources that are not available in sufficient supply to meet the needs of all individuals. For this interaction to qualify as true competition, the resource must be finite, and the presence of one organism must negatively impact the fitness or population growth rate of the other.
Organisms compete for resources necessary for life and reproduction. For plants, this often involves light, water, and soil nutrients like nitrogen and phosphorus. Animals typically compete for food, water, territory, and nesting or denning sites. Mating partners are also frequently the subject of intense competition within a species.
Competition Among Members of the Same Species
Competition that takes place between individuals of the same species is known as intraspecific competition. The intensity of this interaction is often high because all members of the population share nearly identical needs and therefore have perfectly overlapping resource requirements. Increased population density directly intensifies this competition, as more individuals are vying for the same limited pool of resources.
This intraspecific struggle manifests in two main ways: scramble and interference competition. Scramble competition, also called exploitative competition, occurs indirectly as organisms deplete a shared resource. An example is a large group of caterpillars rapidly consuming all the leaves on a single plant.
Interference competition, or contest competition, involves direct, aggressive interactions where one organism actively prevents another from accessing a resource. Examples include male red deer locking antlers during the rut to secure mating rights or large aphids ejecting smaller ones from better feeding sites on a cottonwood leaf.
Competition Between Different Species
Competition occurring between individuals of two or more different species is termed interspecific competition. This interaction is driven by the concept of the ecological niche, which encompasses the range of environmental conditions and resources a species requires to survive and reproduce. Interspecific competition arises when the ecological niches of two species overlap, meaning they both rely on the same limited resources.
The degree of niche overlap determines the severity of the competitive interaction. If two species rely on the same food source, such as a lion and a hyena competing for the same zebra carcass, the interaction is strong because the availability of prey directly limits both populations. Similarly, two plant species in a forest floor community may compete intensely for available soil nitrogen, with the more efficient species absorbing a greater share.
Long-Term Ecological Consequences
Interspecific competition can lead to changes in ecological communities and the evolution of species. One possible outcome is the Competitive Exclusion Principle, also known as Gause’s Law, which states that two species cannot coexist indefinitely if they utilize the exact same limiting resource in the same habitat. The species with even a slight advantage in resource acquisition will eventually outcompete the other, driving the less successful species to local extinction or emigration.
More commonly, competition leads to resource partitioning, which is a mechanism that allows species with similar niches to coexist. This involves the species differentiating their resource use, such as foraging at different times of the day (temporal partitioning) or in different physical locations (spatial partitioning). An example is the way multiple warbler species in a forest may feed on insects in the same tree but confine their foraging to different branches or heights.
Over evolutionary time, this pressure to reduce niche overlap can result in character displacement, where competing species evolve physical or behavioral differences in areas where they co-occur. A classic example is the change in beak size observed in finches on the Galápagos Islands, where coexisting species developed distinct beak dimensions to specialize on different sizes of seeds, thereby minimizing direct competition.