Competition is a fundamental process in ecology that acts as a major limiting factor on how large a population can grow. Organisms need resources that are finite in any given environment, which explains why no population on Earth increases indefinitely. Competition is a density-dependent factor, meaning its effects intensify as the number of individuals within a given area increases, creating a natural braking mechanism on population expansion. The struggle for limited resources ultimately dictates the health and size of a population, forcing growth rates to slow as the population becomes denser.
The Two Faces of Competition
Competition is categorized based on the relationship between the organisms involved in the struggle for resources. The two primary forms are classified by whether the competing individuals belong to the same species or different species.
Intraspecific Competition
This form of competition occurs among individuals of the same species, such as a group of deer vying for the same patch of grazing land. Because members of the same species have virtually identical resource requirements, intraspecific competition is often particularly intense. This rivalry acts as a powerful self-regulating mechanism, directly linking population density to the availability of resources.
Interspecific Competition
Interspecific competition involves individuals from different species competing for a shared, limited resource. An example is a lion and a cheetah both hunting the same type of antelope in a savanna ecosystem. While their exact needs may differ slightly, the overlap in resource use means that the presence of one species negatively impacts the population growth of the other.
Essential Resources That Become Limiting
The struggle for survival revolves around a suite of resources necessary for growth, maintenance, and reproduction, which become scarce as population size increases. The availability of food and nutrients is a primary constraint, as energy intake directly influences an organism’s ability to live, grow, and reproduce. When a population exhausts its food supply, growth is immediately restricted.
Physical space is another limiting factor, particularly for territorial animals or plants that require a fixed area for survival. For instance, trees compete intensely for both above-ground space to access sunlight and below-ground space to absorb water and soil nutrients. Likewise, animals need territory for nesting, breeding, and establishing hunting grounds, and a lack of this space can halt population expansion.
Water and sunlight are also common limiting resources, depending on the specific ecosystem. In arid environments, water availability dictates population size, while in dense forests, sunlight becomes the most contested resource for plants.
Direct Mechanisms of Growth Limitation
Competition limits population growth by directly altering the demographic rates of a species—specifically, the birth rate (fecundity) and the death rate (mortality). As resources become scarcer due to competition, individuals experience higher levels of stress and poor nutrition. This stress often leads to reduced fecundity, where females produce fewer offspring, or offspring that are less viable.
Competition also drives increased mortality by contributing to starvation, physical injury, and vulnerability to disease. Overcrowded conditions, a direct result of increased density, facilitate the rapid spread of pathogens, causing death rates to climb. Furthermore, malnourished individuals are often weaker and more susceptible to predation, reducing overall survival rates.
Ecologists distinguish between two interaction mechanisms that cause these demographic shifts. Exploitation competition occurs indirectly when organisms consume a shared resource, depleting the supply for others without direct confrontation. Interference competition, conversely, involves direct confrontation, like when two male deer fight for mating rights or an animal actively defends a specific territory from rivals.
Defining the Carrying Capacity
The culmination of these limiting factors and competitive pressures is the concept of carrying capacity, denoted ecologically by the letter K. Carrying capacity is defined as the maximum population size of a species that an environment can sustain indefinitely, given the available resources. It represents a long-term, stable population limit where the birth rate equals the death rate, and population growth ceases.
The relationship between density-dependent competition and K is one of constant feedback. As a population grows closer to its carrying capacity, competition intensifies, causing the population’s per capita growth rate to slow down.
If a population temporarily exceeds this maximum sustainable limit, an event known as population overshoot occurs. Since the environment cannot support the inflated numbers, the population will quickly suffer a decline called a die-off or population crash. This rapid decrease in numbers is caused by the sudden intensification of resource scarcity, starvation, and disease, forcing the population to fall back toward or below the carrying capacity.