Intraspecific Competition in Ecology

Northern Parula male singing in breeding territory.
Danita Delimont / Getty Images

In ecology, competition is a type of negative interaction happening when resources are in short supply. Intraspecific competition occurs when it is individuals of the same species that are faced with a situation when resources for survival and reproduction are limited. A key element of this definition is that the competition occurs within the ranks of a species. Intraspecific competition is not just an ecological curiosity, but an important driver of population dynamics.

Examples of intraspecific competition include:

  • Larger, dominant grizzly bears occupying the best fishing spots on a river during the salmon spawning season.
  • Songbirds like Eastern Towhees defending territories from which they exclude their neighbors in an effort to secure resources.
  • Barnacles competing for space on rocks, from which they filter water to obtain their food.
  • Plants using chemical compounds to discourage competitors, even those from the same species, and preventing them from growing too close.

Types of Intraspecific Competition

Scramble competition occurs when individuals obtain a declining fraction of the available resources as the number of competitors increases. Every individual suffers from limited food, water, or space, with consequences for survival and reproduction. This type of competition is indirect: for example, deer feed on woody browse all winter long, putting individuals in indirect competition with each other for a resource they cannot defend from others and keep for themselves.

Contest (or interference) competition is a direct form of interaction when resources are actively defended from other competitors. Examples include a song sparrow defending a territory, or an oak spreading its crown to gather as much light as possible, elbowing a spot within a forest canopy.

Consequences of Intraspecific Competition

Intraspecific completion can suppress growth. For example, tadpoles take longer to mature when they are crowded, and foresters know that thinned-out tree plantations will lead to bigger trees than those left alone to grow at high density (density is the number of individuals per unit of area). Similarly, it is quite common for animals to experience a decrease in the number of young they can produce at a high population density.

To avoid high-density situations, many juvenile animals will have a dispersal phase when they move away from the areas where they were born. By striking off on their own, they increase their chances of finding more abundant resources with less competition. It comes at a cost though as there is no guarantee their new digs will have sufficient resources to raise a family of their own. Dispersing young animals are also at increased risk of predation as they travel through unfamiliar territory.

Some individual animals are able to exert social dominance over other ones to ensure better access to resources. That dominance can be applied directly by having better fighting abilities. It can also be demonstrated through signals, like coloration or structures, or behaviors like vocalizations and displays. Subordinate individuals will still be able to access resources but will be relegated to less abundant food sources, for example, or to areas with inferior shelter.

Dominance can also be expressed as a spacing mechanism, including by establishing a pecking order. Instead of competing directly over resources with other individuals of the same species, some animals protect a space from other ones, claiming property over all the resources within. Fighting can be used to establish territory boundaries, but given the risks of injuries, many animals use ritualistic, safer alternatives like displays, vocalizations, mock fighting, or scent marking.

Territoriality has evolved in several animal groups. In songbirds, territories are defended to secure food resources, a nesting site, and young-rearing sites. Most of the springtime birds singing we hear is evidence of male birds advertising their territory. Their vocal displays serve to attract females and to announce the location of their territorial boundaries.

In contrast, male bluegills will only defend a nesting site, where they will encourage a female to lay eggs which he then fertilizes.

Significance of Intraspecific Competition

For many species, intraspecific competition has strong effects on how population size varies over time. At high density, growth is reduced, fecundity is suppressed, and survival is affected. As a result, the size of the population increases more slowly stabilizes, and then eventually starts declining. Once the population size reaches lower numbers again, fecundity picks back up and survival is increased, putting the population back in a growth pattern. These fluctuations keep the population from getting too high or too low, and this regulating effect is a well-demonstrated consequence of intraspecific competition.