Competition

Learning Outcomes

  • Define the competitive exclusion principle

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown in Figure 1, with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

Graphs a, b, and c all plot number of cells versus time in days. In Graph (a), P. aurelia is grown alone. In graph (b), P. caudatum is grown alone. In graph (c), both species are grown together. When grown together, the two species both exhibit logistic growth and grow to a relatively high cell density. When the two species are grown together, P. aurelia shows logistic growth to nearly the same cell density as it exhibited when grown alone, but P. caudatum hardly grows at all, and eventually its population drops to zero.

Figure 1. Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, the P. aurelia outcompetes the P. caudatum.

Resource Partitioning

Competitive exclusion may be avoided if one or both of the competing species evolves to use a different resource, occupy a different area of the habitat, or feed during a different time of day. The result of this kind of evolution is that two similar species use largely non-overlapping resources and thus have different microniches. This is called resource partitioning, and it helps the species coexist because there is less direct competition between them. These organisms coexist by minimizing direct competition.

The anole lizards found on the island of Puerto Rico are a good example of resource partitioning. In this group, natural selection has led to the evolution of different species that make use of different resources. The figure below shows resource partitioning among 11 species of anole lizards. Each species lives in its own preferred habitat, which is defined by type and height of vegetation (trees, shrubs, cactus, etc.), sunlight, and moisture, among other factors.

Diagram representing resource partitioning among species of the anole lizard. Some live high in the tree, others in the middle of the tree, others on the trunk. Other anole species live in bushes or cactuses. Also, some live in a sunnier drier environment and some in a shadier moister environment. There are eleven species pictured in all, each with a slightly different environment it occupies.

Figure 2. Resource partitioning among anole lizards

Video Review

Watch this video to review competition and how populations share resources in a community:

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