Intraspecific competition

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A male Blackbird (Turdus merula) singing. Bird song often functions as a signal in territory defense.

Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem (e.g. food, light, nutrients, space). This can be contrasted with Interspecific competition, in which different species compete.

For example, two trees of the same species growing close together will compete for light, water and nutrients in the soil. Getting less resources, they will perform more poorly than if they grew by themselves (for example lowered growth rates and fewer seed output). Trees have therefore adapted to grow taller or develop larger root systems through natural selection.

Grasshoppers provide an animal example. By eating grass, individual grasshoppers deprive their fellow conspecifics of food. This is an example of exploitation competition, which means that the grasshoppers do not interact directly with each other, but have a negative effect on others' growth and reproduction by their effect on a resource (in this case, grass). In other cases, intraspecific competition may be a case of interference competition, in which the animals interact directly. This is the case, most notably, in territorial animals: some individuals actively prevent others from exploiting a given resource, usually food or space.

Intraspecific competition is a major factor affecting the carrying capacity of a population (maximum population level supported by the environment). The levelling of population growth at high densities (known as density dependent inhibition) can be seen as an effect of intraspecific competition. Indeed, whereas at low densities organisms do not compete for resources, at higher densities resources become limiting, and the population size can no longer increase. In terms of population growth rate, this produces a sigmoidal curve, which is a familiar sight for ecologists.