59 Intraspecific Competition and its Effects

Key Idea: Members of a population compete for the same limited resources. Territories and hierarchies help to ensure that at least some individuals obtain sufficient resources. Intraspecific competition is an interaction in which individuals of the same species compete for a resource. As population numbers increase, the resources available to each individual become fewer and intraspecific competition increases. When the demand for a resource (e.g. food, water, nest sites) exceeds supply, that resource becomes a limiting factor to further population growth. Populations respond to resource limitation by reducing population growth rate (e.g. through lower birth rates or higher mortality).

The outcomes of intraspecific competition

Scramble competition in caterpillars

In scramble competition, all individuals in the population have equal access to a limited resource and many individuals may starve. This limits population growth and can tightly constrain the life cycle events of some species.

Contest competition in wolves

In social groups, access to resources may be determined by the social hierarchy. The most dominant individuals have priority access to food, but low ranked individuals must contest the remainder and may miss out or may be forced to leave the group.

Blue duck (whio)

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In some vertebrates, defended areas, called territories, enable individuals or groups to command sole access to all the resources within a defined area. Blue duck pairs occupy and defend exclusive territories in the same stretch of river year after year.

A hierarchy is a ranked social order within a group. Hierarchies reduce direct aggression by creating orderly access to resources within a group. Example, African Hunting Dogs. Competes with members of the same species.

Limited resources: Food Shelter Mates

Competes with members of the same species.

Threaten from a distance using ritualised signals (such as calls and bellows) and displays.

'Limited aggression' with threat displays at close range, sometimes with physical contact.

Escalation of threat by fighting. May involve pushing or combat using horns and antlers, biting, or kicking. Risk of injury is high. Fights may even be fatal. Escalation of aggression Increasing risk of injury or death

Territories reduce direct aggression by dividing resources

between groups

or individuals.

Hierarchies Territories

f Determines an order of precedence for access to food, mates and breeding sites. Dominant animals have priority access to resources. f May be linear (with pecking order) or may be complex and involve coalitions. f Hierarchies enable individuals to concentrate on more productive uses for their energy (e.g. raising offspring).

Winner gains possession of resources

f May be held by a single animal, a breeding pair, or a group. f May be large multi-purpose territories for feeding, mating, and rearing young. f May be used for a single purpose (e.g. mating grounds called leks). f Boundaries are patrolled and marked using signals (calls, scent). f Territories are effective in reducing intra-specific competition, but territory defence is costly (time and energy).

(b) Why does intraspecific competition occur?

2. Explain how intraspecific competition acts to limit population size:

3. Describe a type of aggressive behaviour that could serve each of the following purposes:

(a) Territorial defence from a distance:

(b) Maintenance of position in dominance hierarchy:

(c) Competition for the right to mate:

4. Describe two ways in which animals can reduce the intensity of intraspecific competition:

(a)

(b)

5. (a) Explain how a territory can reduce aggression between groups:

(b) What are some of the costs associated with defending a territory?

6. (a) How can hierarchies reduce aggression between individuals in a social group?

(b) Discuss the benefits of a hierarchical system for the group (and species) and for individuals: