Competition is a fundamental ecological interaction that prevents populations from growing indefinitely. Competition is classified as a density-dependent factor, meaning its intensity and impact on the population growth rate increase directly with the population’s size. This process begins when resources become scarce due to a high number of individuals in a given area. It is the primary force that regulates the size of all biological populations within an ecosystem.
The Two Forms of Population Competition
Competition is broadly categorized based on the relationship between the organisms involved, specifically whether they belong to the same species or different species. The first type, intraspecific competition, occurs among members of the same species struggling for the identical set of limited resources. Because all individuals of a species require the exact same food, nesting sites, and mates, this form of competition is the most intense. For example, two male deer fighting over access to a female during the rutting season is a direct expression of intraspecific competition for reproductive resources.
The second type is interspecific competition, which takes place between individuals of different species that share a common, limited resource. This might involve a population of coyotes and a population of bobcats hunting the same small rodents in a shared territory. While both species are vying for the same prey, they often have slightly different hunting strategies or resource preferences, which can reduce the direct intensity compared to intraspecific conflict. Strong interspecific competition can still lead to the competitive exclusion of one species, where the less efficient competitor is driven out of the habitat.
Direct Mechanisms Limiting Population Growth
Competition limits population size by actively reducing the availability of essential resources, which in turn affects the fitness and survival of individuals. As population density increases, the shared pool of resources, such as food, water, or sunlight, is depleted at a faster rate than it can be replenished. This resource scarcity leads to lower energy reserves in individuals, making them less robust and more vulnerable to other environmental pressures.
The struggle for resources manifests in two main ways: scramble and contest competition. Scramble competition involves all individuals exploiting the shared resource pool equally, leading to a general reduction in the amount available to everyone. When food is thinly distributed, every individual gets less, resulting in widespread undernourishment and weaker overall health. Conversely, contest competition involves direct, physical interactions, such as territorial defense or fighting over a water source.
Both forms of competition ultimately result in increased mortality rates within the population. Scramble competition increases mortality indirectly through starvation or weakened immune systems, while contest competition increases mortality directly through injury or the stress of being excluded from the best foraging areas. Furthermore, intense competition causes a significant reduction in reproductive success, which is a major limiter of population growth. Organisms must divert energy away from mating and raising offspring simply to survive, leading to delayed maturity, fewer successful pregnancies, or smaller litter sizes.
An additional mechanism is the physiological stress response caused by constant competitive pressure in dense populations. Elevated stress weakens the immune system, increasing susceptibility to diseases. Since dense populations facilitate the rapid transmission of pathogens, this combination dramatically increases the death rate, further limiting population size.
The Ecological Outcome: Reaching Carrying Capacity
The long-term result of these competitive mechanisms is the stabilization of a population at a specific size known as the carrying capacity, or K. Carrying capacity is defined as the maximum number of individuals of a particular species that an environment can sustainably support. This limit is not static, as changes in the environment, such as a drought or a new food source, can shift the value of K.
A population’s growth curve will slow down as it approaches the carrying capacity, transitioning from rapid increase to a near-zero growth rate. At this point, the limiting effect of competition means that the number of births is roughly balanced by the number of deaths. Any temporary increase in population size above K intensifies competition so severely that the death rate quickly surpasses the birth rate, forcing the population size back down.