Interspecific competition is the interaction between organisms of different species that both require the same limited resources within an ecosystem. This can be pictured as two people vying for the last available taxi after an event. The resource—the taxi—is finite, and both individuals need it. In nature, these resources include food, water, territory, or sunlight, and this struggle impacts the health, survival, and reproductive success of the species involved, shaping biological communities.
Types of Interspecific Competition
Interspecific competition manifests in two ways that differ in how organisms interact. The first is exploitative competition, an indirect contest where species affect each other by consuming the same scarce resources. For example, in a forest, different species of trees require sunlight and soil nutrients to grow. A taller, faster-growing tree species might absorb more sunlight, leaving less for shorter, slower-growing species and harming them without any physical confrontation.
The second mechanism is interference competition, which involves direct and often aggressive interactions between species. In this case, one species actively prevents another from accessing a resource, even if that resource is not scarce. A classic example is the relationship between lions and spotted hyenas on the African savanna. These predators consume the same prey and directly confront each other, fighting over carcasses and territories in displays of aggression that can result in injury or death.
Consequences for Species
The long-term results of interspecific competition can influence the evolution and distribution of species. One outcome is the competitive exclusion principle, which states that two species competing for the same limited resources cannot coexist indefinitely in the same place. The more efficient competitor will thrive, driving the other species to local extinction. Russian ecologist Georgy Gause demonstrated this in experiments; when two Paramecium species were grown with a fixed amount of food, P. aurelia consistently outcompeted and eliminated P. caudatum.
However, extinction is not the only possible outcome. Species can also coexist by minimizing competition through a process known as resource partitioning. This occurs when competing species evolve to use different resources, or the same resources in different ways or at different times. For example, several species of warblers can live in the same spruce tree because they partition the habitat, with each species feeding in a different part of the tree.
A related outcome is character displacement, where the physical characteristics of competing species diverge over time. This evolutionary shift reduces competition by making the species more specialized. For instance, if two bird species with similar beak sizes compete for the same seeds, natural selection might favor individuals in each species with beaks that are slightly smaller or larger than the average. Over generations, this can lead to a noticeable difference in beak size between the two species where they coexist, but not where they live apart.
Competition in Natural Ecosystems
A documented case of competitive exclusion involves the native red squirrel and the invasive eastern grey squirrel in the United Kingdom. Since its introduction from North America, the larger grey squirrel has outcompeted the red squirrel for food and habitat. Grey squirrels are better at digesting acorns and can tolerate a broader range of habitats, which has pushed red squirrel populations into specific coniferous forests where they have a competitive edge.
The rocky intertidal zone provides another example of how competition shapes a community. Different species of barnacles, such as Balanus balanoides and Chthamalus stellatus, compete for limited space on the rocks to attach and grow. Balanus, a faster-growing species, can physically undercut or crush Chthamalus, illustrating interference competition. However, Chthamalus can tolerate drying out better, allowing it to survive higher up on the shore where Balanus cannot, creating distinct zones where each species dominates in a clear demonstration of resource partitioning.