Interspecific Competition and Resource Partitioning

Interspecific competition is competition between individuals of different species for the same limited resource, and it can lead to competitive exclusion, in which one species eliminates the other, or to resource partitioning, in which the species use different parts of the resource and can coexist. In brief: the competitive exclusion principle (Gause’s principle) states that two species that need exactly the same limiting resource cannot coexist indefinitely in a stable environment. In practice, species coexist by differences in niche, such as feeding on different prey, in different places or at different times.

Competition shapes which species live together and how many individuals each has. This guide explains the types of competition, the competitive exclusion principle and Gause’s experiments, resource partitioning and character displacement, compares the outcomes in a table, works through an example based on the Lotka-Volterra idea of competition and corrects common mistakes.

What is competition?

Competition (a negative interaction, −/−) occurs when individuals use a resource that is limited, such as food, water, light, nutrients, nesting sites or space, so that each reduces the amount available to others. It can be:

• Intraspecific (within a species), which regulates the population and favors the better-adapted individuals.
• Interspecific (between species), which can affect the distribution and abundance of both species.
• Exploitation (indirect) competition, in which species use up a shared resource, or interference (direct) competition, in which one species prevents access to the resource, as when animals defend territories or when plants release chemicals that inhibit others (allelopathy), as in black walnut trees.

Both species are affected, but usually not equally, and often one is more strongly affected than the other (asymmetric competition).

What is competitive exclusion?

The competitive exclusion principle is named after the Russian biologist Georgii Gause, who tested it with the single-celled organisms Paramecium aurelia and Paramecium caudatum in the 1930s. Each species grew well alone in a test tube with bacteria as food. When grown together, P. aurelia multiplied faster, and P. caudatum was eliminated. In contrast, P. caudatum and Paramecium bursaria could coexist, because P. bursaria feeds in a different part of the tube on different organisms and also uses algae within its cells.

Field examples include the barnacles studied by Joseph Connell on the Scottish coast in the 1960s: Chthamalus lives in the upper intertidal zone, and Balanus in the lower. When Connell removed Balanus, Chthamalus could live in the lower zone as well. So Balanus excludes Chthamalus from the lower zone, which is its competition, and Chthamalus survives in the upper zone because it tolerates drying that Balanus cannot. This showed the difference between the fundamental niche (where a species could live without competitors) and the realized niche (where it actually lives in the presence of competitors).

What is resource partitioning?

Resource partitioning is the division of resources among species with similar needs, by use of different parts of the resource, so competition is reduced and they can coexist. It can be by:

• Food: in Robert MacArthur’s classic study (1958) of five species of warblers in spruce trees, each species feeds mainly in a different part of the tree: some at the top, others near the trunk or at the outer branch tips.
• Space or habitat: different species of Anolis lizards use different perches, such as the ground, trunks or twigs.
• Time: some species are active by day and others at night, or feed at different seasons. Bats and swifts both eat flying insects, but at different times.
• Size of food: birds with beaks of different sizes eat seeds of different sizes.
• Depth of roots: shallow-rooted and deep-rooted plants use water from different soil layers.

The “overlap” between two niches can be thought of as the extent of competition. A smaller overlap means more stable coexistence.

What is character displacement?

When two similar species live in the same area, natural selection can favor individuals that differ more from the competitor. Over generations, a trait such as beak size or body size diverges in the area where the species overlap (sympatry), but is more similar where each lives alone (allopatry). A well-known example is Darwin’s finches: on islands where Geospiza fortis and G. fuliginosa live together, the beaks differ more in size than on islands where each lives alone. The difference reduces competition for seeds of the same size.

Outcomes of competition compared

Worked example: two species, one resource

Two species of seed-eating birds, A and B, share an island with a seed supply that can support at most 1 000 birds of species A alone (K_A = 1 000) or 800 of species B alone (K_B = 800). Lotka-Volterra competition models use a coefficient α, the effect of one individual of the competitor on the growth of the other, in units of the equivalent of an individual. Suppose α (effect of B on A) = 0.8, and β (effect of A on B) = 1.5, meaning that a bird of species A uses the resource 1.5 times as much as a bird of species B.

1. Equilibrium condition for A to stop growing: N_A + α N_B = K_A, that is N_A + 0.8 N_B = 1 000.
2. Equilibrium condition for B to stop growing: N_B + β N_A = K_B, that is N_B + 1.5 N_A = 800.
3. Check when A is alone: N_A = 1 000. For B to grow: 0 + 1.5 × 1 000 = 1 500, which is greater than 800, so B cannot increase when A is at its carrying capacity.
4. Check when B is alone: N_B = 800. For A to grow: 0.8 × 800 = 640, which is less than 1 000, so A can increase when B is at its carrying capacity.
5. Result: A can invade B’s territory, but B cannot invade A’s. Species A eventually excludes species B.

In a rule of thumb, species coexist if each limits itself more than it limits the competitor. Here, species A has a stronger effect on B (β = 1.5) than B has on A (α = 0.8), so competition is unequal and A wins. If niche differences made both coefficients smaller, for example α = 0.5 and β = 0.5, both species could coexist. The equilibrium follows from solving the two equations N_A + 0.5 N_B = 1 000 and N_B + 0.5 N_A = 800. Multiplying the second equation by 2 gives 2 N_B + N_A = 1 600. Subtracting the first equation leaves 1.5 N_B = 600, so N_B = 400, and then N_A = 1 000 − 200 = 800. Both species persist, at 800 and 400 birds. The models are simplified, but they show how niche differences allow coexistence.

How does competition affect invasive species?

An introduced species that is a better competitor may exclude native species. The gray squirrel from North America has replaced the native red squirrel in much of Britain and Ireland, partly because it is better at using acorns and because it carries a virus (squirrelpox) to which it is immune but which kills red squirrels. This is an example of apparent competition mediated by a disease. Understanding competition helps to predict and manage such invasions.

How was competition studied?

Alfred Lotka (1925) and Vito Volterra (1926) developed the mathematical models of competition. Gause carried out the experiments in the 1930s. G. Evelyn Hutchinson defined the niche in terms of the range of conditions a species uses (1957). Joseph Connell’s field experiments in the 1960s and Robert MacArthur’s work on warblers brought experimental methods to ecology.

Which mistakes are common?

Mistake 1: Saying that competition always leads to extinction

Often it leads to coexistence through partitioning or character displacement.

Mistake 2: Confusing interspecific and intraspecific competition

Interspecific is between species. Intraspecific is within a species.

Mistake 3: Mixing up fundamental and realized niche

The fundamental niche is the potential. The realized niche is the part actually used in the presence of competitors.

Mistake 4: Thinking that the principle applies to species with different needs

Competitive exclusion applies to species that compete for the same limiting resource in a stable environment.