Intraspecific competition is an ecological interaction where individuals from the same species vie for the same finite resources, leading to a decrease in fitness for the involved organisms. This stands in contrast to interspecific competition, which involves members of different species. Because individuals of the same species have nearly identical resource needs, intraspecific competition is often a more intense force in nature. The core of this interaction is resource limitation; if resources were infinite, populations could grow exponentially.
Causes of Competition Within a Species
As a population grows, the demand for these resources increases, leading to heightened competition among its members. The main causes of this competition stem from the scarcity of food, water, physical space or territory, and opportunities for mating.
Food availability is a frequent point of contention. A large herd of deer in a forest during winter, for instance, may exhaust the available browse, forcing individuals to compete for every last bit of vegetation. Similarly, in arid or drought-stricken environments, animals may congregate around a diminishing waterhole, with each individual’s access to water coming at the expense of another’s.
Space is another resource that provokes competition. Many bird species compete for a limited number of suitable nesting sites, which are required for successfully raising young. In the plant kingdom, competition for space is equally intense, as plants in a dense forest must compete for physical room to grow, which directly relates to their access to sunlight and soil nutrients.
Beyond simple survival, many species compete for mates, a process that drives sexual selection. Male birds developing elaborate plumage or engaging in complex courtship displays do so to outcompete rivals for the attention of females.
Mechanisms of Competition
The methods by which individuals compete can be broadly categorized into two distinct mechanisms: exploitation and interference. These mechanisms describe whether the competition occurs indirectly through resource use or through direct confrontation.
Exploitation competition is an indirect process where individuals deplete a shared resource, making it less available for others without ever physically interacting. For example, with a population of caterpillars feeding on a single host plant, the caterpillars that hatch and begin eating first consume leaves that are then no longer available for those that follow. This directly impacts the latecomers’ growth and survival even though the caterpillars may never encounter one another. This form of competition often rewards the individuals who are first to claim the resource.
Interference competition involves direct and often aggressive interactions. This is when one individual actively prevents another from accessing a resource. Classic examples include two grizzly bears fighting over a prime fishing spot on a river or male red deer clashing antlers to control access to females during the mating season. This mechanism is not limited to physical fights; it also includes animals that defend a territory through songs or chemical markers, effectively excluding rivals. Some plants engage in a form of interference competition known as allelopathy, where they release chemicals into the soil that inhibit the growth of nearby competitors.
Population-Level Consequences
Intraspecific competition is a major factor in regulating population size, preventing indefinite growth. The effects of this competition become more pronounced as the number of individuals in a given area increases, a concept known as density-dependent regulation. As population density rises, competition intensifies, leading to reduced survival and birth rates.
This density-dependent process is what slows a population’s growth rate as it expands. Instead of growing exponentially forever, the population follows a logistic growth model, often visualized as an “S-shaped” curve. Growth is rapid when the population is small and resources are plentiful, but it slows as competition increases and eventually levels off when the population reaches the environment’s carrying capacity, symbolized as K. The carrying capacity represents the maximum population size that can be sustained by the available resources.
A clear illustration of density-dependent regulation occurs in plant populations through a process called self-thinning. When seeds germinate in a crowded area, a large number of seedlings begin to compete for limited light, water, and soil nutrients. As they grow, the stronger, more successful seedlings thrive, while the weaker individuals are outcompeted and die. This mortality reduces the population density, allowing the surviving plants to grow to maturity.