OCR A Level Biology

Revision Notes

6.6.2 Interactions Between Populations

Interactions Between Populations

  • Populations always tend towards a natural, stable size
  • The stable population size is limited by a number of factors
    • Interactions between one population and another (of the same species) will limit the size of those populations through intraspecific interactions
    • Interactions between one species and another can also limit the size of populations through interspecific interactions such as competition and predation

Intraspecific competition

  • Intraspecific competition occurs when individuals from the same species compete for the same resources (“intra” means within)
  • For example:
    • When resources are plentiful, the population of grey squirrels increases
    • As the population increases, however, there are more individuals competing for these resources (e.g. food and shelter)
    • At some point, the resources become limiting and the population can no longer grow in size – the carrying capacity has been reached

Intraspecific competition (grey squirrels), downloadable IGCSE & GCSE Biology revision notes

Intraspecific competition between two grey squirrels (same species) for a limited resource

Interspecific competition

  • Competition between different species for the same resource is described as interspecific competition (“inter” means between)
  • A well-known example of this occurs between the grey squirrel (Sciurus carolinensis) and the red squirrel (Sciurus vulgaris)
  • The introduction of the grey squirrel into the southern UK caused the native red squirrel to be outcompeted for food and shelter
  • The grey squirrel also carries a disease, parapoxvirus, that is harmless to itself but can be fatal to the red squirrel
  • Conservation efforts in northern England and Scotland have slowed the spread of the grey squirrel by
    • Protecting the red squirrels’ habitats and food
    • Giving the red squirrel legal protection
    • Reintroducing the pine marten, a natural predator of the grey squirrel

Interspecific competition (grey and red squirrels), downloadable IGCSE & GCSE Biology revision notes

Interspecific competition between a grey squirrel and a red squirrel (different species) for a limited resource

The effects of competition

Competitive exclusion

  • If two organisms occupy very similar niches, then competition can exist between them for resources
  • One species may be slightly better adapted to compete than the other, so the other species could starve or not find a habitat
  • The second species would then move to find a different niche, and possibly evolve over generations to adapt to its new niche
  • This is called competitive exclusion

Resource partitioning

  • A particular resource eg. a fruit tree is divided up (partitioned) to satisfy the needs of different feeders
  • The fruit may be grazed by one species, the leaves by another
  • The two species are not competing directly for the fruit tree but coexisting on different parts of it

Character displacement

  • Closely related species that inhabit the same geographical region and occupy similar niches differentiate in order to minimise niche overlap and avoid competitive exclusion
  • An example is beak size and shape in Galapagos finches, as observed by Charles Darwin
  • Each species differs in beak dimensions
  • Food, in the form of seeds, comes in different shapes and sizes from a variety of sources
  • The finches with the stronger beaks eat large, tough seeds, while the finches with smaller beaks eat the smaller, softer seeds
  • Natural selection therefore favours those species that compete less with other species


  • Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers who are themselves eaten by tertiary consumers
  • Consumers that kill and eat other animals are known as predators, and those eaten are known as prey
  • In a stable community, the numbers of predators and prey rise and fall in cycles, limiting the population sizes of both predators and prey
  • The graph below demonstrates some of the key patterns of predator-prey cycles:
    • Initially, the number of predators increases as there is more prey available
    • The number of prey then decreases as there are now more predators
    • Then the number of predators decreases as there is now less prey available
    • As a result, the number of prey increases as there are now fewer predators
    • The cycle now repeats

Carrying Capacity, downloadable AS & A Level Biology revision notes

An example of a graph used to model a predator-prey cycle between the Canadian lynx and the snowshoe hare


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