When two species depend on the same limited resource, they compete for access to it. If the competition is strong enough, one species may become less abundant, move to a different resource or habitat, or be driven out of the local area. The other species may increase as competition for that resource decreases.
This is called interspecific competition: competition between different species. It occurs when organisms need the same resource and that resource is not available in unlimited supply. Food, water, light, nutrients, nesting sites, shelter, and living space can all become objects of competition.
The outcome is not always that one species disappears. The species may divide the resource, use it in different ways, or compete strongly only under certain environmental conditions.
Why competition happens
Competition occurs when organisms require the same resource and using that resource by one organism reduces what remains for another.
For example, two plant species growing close together may both need sunlight, water, and nitrogen from the soil. If those resources are plentiful, their overlap may cause little noticeable competition. But when water or nutrients become scarce, each plant can limit the resources available to the other.
Competition can also involve resources that are not consumed directly. Two birds might compete for the same nesting sites, while animals may compete for territory or shelter. In each case, the important feature is that access is limited.
Competition therefore depends on both resource overlap and resource limitation. Two species can use similar resources without strongly competing if those resources are abundant.
What happens when competition is intense?
When two species use the same scarce resource in similar ways, competition can reduce the growth, survival, or reproduction of both species. An individual may get less food, grow more slowly, produce fewer offspring, or have difficulty surviving.
If the species are affected equally, their populations may both decline until competition becomes weaker. But if one species is better at obtaining or using the resource, the outcome can be different.
The stronger competitor may maintain or increase its population while the weaker competitor declines. If the two species remain heavily dependent on exactly the same limiting resource, the weaker species may eventually disappear from that particular location.
This possibility is known as the competitive exclusion principle. It does not mean that one species must always eliminate another whenever they compete. Rather, it describes what can happen when two species have essentially the same ecological requirements and compete for the same limiting resource under stable conditions.
Species can avoid direct competition
Competition does not necessarily end with one species losing. Species that use the same general resource can often divide it in ways that reduce their overlap.
This is called resource partitioning.
For example, two species might feed on different parts of the same plant, search for food in different areas, or use the same food at different times. Two birds could depend on insects but forage at different heights in a tree. The species still use a related resource, but their specific patterns of use differ enough to reduce competition.
Resource partitioning allows species with overlapping needs to coexist.
The differences can involve space, time, or the type of resource being used. A species may occupy a particular part of a habitat, feed during a particular period, or specialize on a particular kind of food. These differences can make competition less intense even when the species initially appear to have similar ecological roles.
Competition can change where species live
Strong competition can affect not only population size but also where a species is found.
A species may be able to survive in several habitats but perform poorly where a stronger competitor is present. In areas where that competitor is absent, the same species may be more abundant or occupy a wider range of conditions.
This means a species’ actual distribution can be narrower than the range of conditions it could tolerate on its own.
Competition can therefore help determine the realized niche of a species—the conditions and resources it actually uses in the presence of other organisms. Its fundamental niche is the broader range of conditions and resources under which it could survive without those biological restrictions.
The better competitor can change with conditions
There is not always one universally superior competitor.
Environmental conditions can change which species has an advantage. One species might compete more effectively when water is scarce, while another might perform better when water is plentiful. Temperature, light, nutrients, predators, disease, and other ecological factors can also alter competitive relationships.
As conditions change, the balance between competing species can shift. A species that is less successful under one set of conditions may become more successful under another.
This is one reason natural communities can contain multiple species with overlapping resource requirements rather than being dominated permanently by a single competitor.
Competition can affect evolution
Persistent competition can also influence evolution.
Individuals within a species vary in their characteristics, and some differences may affect how effectively they obtain resources. If certain traits improve access to food, space, light, or other limiting resources, individuals with those traits may leave more offspring. Over generations, natural selection can increase traits that help organisms use resources more effectively or reduce competition with other species.
Competition between species can therefore contribute to character displacement, in which populations evolve differences that reduce overlap in resource use. Such evolutionary changes can make it easier for closely interacting species to coexist.
Competition is only one interaction among species
Competition is part of a larger network of ecological interactions. A species may compete with another species for food while simultaneously being eaten by a predator, receiving benefits from a mutualistic partner, or altering the habitat used by other organisms.
As a result, the effect of competition cannot always be understood by looking at two species in isolation.
If one competitor declines, for example, the resource it had been using may become more available to another species. That change can affect additional populations and alter relationships throughout the community.
Competition ultimately comes down to a simple ecological problem: two species need something that is limited. What happens next depends on how much their resource use overlaps, how strongly the resource is limited, how effectively each species obtains it, and whether they can divide its use. They may coexist by using different resources or using the same resource differently, or intense competition may cause one species to become much less abundant or disappear from the local community.
