Interactions Between Populations (OCR A Level Biology)

• 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 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 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

• 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

• 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

Predation

• 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

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