The field of ecology seeks to understand the complex relationships that govern the distribution and abundance of organisms within natural environments. These relationships include species interactions, which can be mutually beneficial, predatory, or competitive. Competition between different species is one of the most powerful forces shaping biological communities. This interaction dictates which species thrive, adapt, or ultimately disappear from a given area.
Defining Interspecific Competition
Interspecific competition is a biological interaction occurring when individuals of two or more different species vie for the same limited resource in a shared environment. The term “interspecific” means “between species,” distinguishing it from competition among members of the same species. The presence of each species negatively affects the population growth and survival of the other, resulting in a negative-negative interaction for both parties.
The concept of a limited resource is central to this definition, as competition only arises when the supply of a necessary item is insufficient to meet the demands of all organisms. Common limiting resources include food, water, light, and essential soil nutrients for plants. For animals, limited resources often include physical space, such as nesting sites, den locations, or territorial boundaries.
Intraspecific competition occurs among individuals of the same species, such as two male deer fighting over a mate. While intraspecific competition regulates the size of a single population, interspecific competition influences the structure and diversity of the entire ecological community. This competition often occurs because the ecological niches of the two species—their specific roles and resource needs—overlap, forcing them to use the same finite supply of materials.
Mechanisms of Interspecific Competition
Competition between species does not always involve direct confrontation; it manifests through two distinct mechanisms: exploitation and interference. These mechanisms describe how one species negatively impacts another’s access to resources. Exploitation competition is an indirect interaction where one species consumes or uses a shared resource faster or more efficiently than the other.
Exploitation competition is characterized by resource depletion rather than direct contact between the competitors. For example, two different species of herbivorous insects feeding on the same plant compete exploitatively; the one that eats the foliage more rapidly leaves less food available for the other. Similarly, a plant species with a dense root system may absorb soil water and nutrients so quickly that a neighboring species is stunted.
Interference competition is a direct interaction where one species actively prevents another from accessing the resource. This mechanism often involves aggressive behavior, territorial defense, or the release of toxic substances. For example, two species of birds may fight over a limited number of tree cavities suitable for nesting.
In the plant world, a specific form of interference known as allelopathy occurs when one plant releases biochemicals into the soil that inhibit the growth, germination, or survival of competing species. The black walnut tree is a classic example, producing the chemical juglone, which is toxic to many other plant species and prevents them from growing near its roots. Both exploitation and interference can occur simultaneously, intensifying pressure on the less dominant species.
Ecological Outcomes of Competition
The long-term effects of interspecific competition determine the makeup of a biological community, leading to outcomes ranging from the extinction of one species to stable coexistence. The Competitive Exclusion Principle, also known as Gause’s Law, states that if two species require the exact same limited resource to survive—occupying the identical ecological niche—they cannot coexist indefinitely.
In a laboratory experiment, Georgyi Gause showed that when two species of Paramecium protozoa were grown together with a fixed amount of food, one species consistently outcompeted the other. The superior competitor eventually drove the weaker species to local extinction due to more efficient resource uptake. In natural systems, competitive exclusion means one species will either be eliminated from the area or forced into a less favorable habitat.
However, many species that appear to compete closely manage to coexist through resource partitioning. This involves the evolution of slight differences in resource use that reduce the intensity of direct competition. Species may begin to use a different part of the habitat, feed at different times, or consume different sizes or types of the same general resource.
For example, multiple species of Anolis lizards on Caribbean islands avoid competitive exclusion by occupying different vertical zones within the same forest. Some species prefer the leaf litter while others live high in the tree canopy. This spatial partitioning allows them to share the same general environment while utilizing distinct micro-habitats and food sources, enabling stable populations and allowing diverse communities to persist.