Intraspecific competition is a fundamental ecological interaction where individuals of the same species compete for the same limited resources in their shared environment. Since all members of a species have nearly identical needs for survival and reproduction, this competition is a powerful force in nature. This struggle results in a reduction in the overall fitness for the competing individuals. The outcome of this contest determines which individuals are successful and ultimately shapes the genetics and size of the entire population.
The Driving Force: Limited Resources
The basis for intraspecific competition is the imbalance between finite resources and the potential for infinite population growth. When a population grows, the demand for life-sustaining resources increases, but the supply often remains static. For animals, these resources include food, water, shelter, and mates, while plants compete for light, soil nutrients, and water. Competition becomes unavoidable once the population density reaches a certain point.
Mechanisms of Competition
Intraspecific competition manifests through two main mechanisms: scramble competition and contest competition. The distinction lies in whether the interaction is indirect and shared, or direct and confrontational.
Scramble Competition
Scramble competition is an indirect form where a shared resource is depleted by all individuals without direct physical interaction. All members of the population have equal access to the resource, but as the population grows, the resource is divided into smaller fractions. For example, insect larvae consuming a finite supply of foliage will indirectly compete, leading to scarcity. This equal sharing of an inadequate resource can cause a sudden population crash if too few individuals obtain enough to survive.
Contest Competition
Contest competition, also known as interference competition, involves a direct, aggressive interaction where individuals actively fight or defend a resource. This mechanism results in clear winners and losers, with successful individuals monopolizing the resource. A classic example is male red deer locking antlers to fight for access to females or a prime territory, excluding subordinate individuals or relegating them to less desirable areas.
Impact on Population Size
The long-term consequence of intraspecific competition is the regulation of population size, acting as a density-dependent factor. As the number of individuals increases, the intensity of competition also increases, leading to higher rates of mortality and reduced birth rates. This mechanism slows the population’s growth rate as it becomes more crowded.
This regulation prevents the population from growing indefinitely and overusing its environment. The maximum population size that an environment can sustainably support over a long period is known as the carrying capacity. Intraspecific competition is the primary force that pushes the population growth rate toward zero as the population size approaches this limit.
The pressure of constant competition also acts as a powerful driver of natural selection and evolutionary change. Individuals better adapted at acquiring limited resources—whether through aggression or efficiency—are more likely to survive and pass on their traits. This ongoing struggle can even promote diversification within the species, as some individuals shift their behavior to use alternative, less-contested resources.