Carrying Capacity (AQA A Level Biology): Revision Note

Carrying capacity

• The maximum stable population size of a species that an ecosystem can support is known as the carrying capacity

• Although all individuals can potentially reproduce, abiotic and biotic factors limit survival and reproduction

• This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)

• Most species remain below or at carrying capacity due to these limiting factors

  • Humans are a possible exception, as we've overcome many natural limitations

• The graph below shows the population growth of a population of lions

  • The point at which the graph started to flatten out (plateau) is the carrying capacity of this population

  • At this point, the environmental (abiotic and biotic) factors that stop all individuals from surviving and reproducing result in the population no longer being able to grow in size

Factors affecting population size

Abiotic factors

• Abiotic factors involve the non-living parts of an ecosystem

• Some examples of abiotic factors that can limit population size include:

  • light availability

  • water supply

  • temperature

  • amount of space available

  • soil pH

• If environmental temperature is far from a mammal’s optimum, individuals use more energy to maintain their optimum body temperature (via homeostasis), leaving less energy for growth and reproduction

  • Fewer reach reproductive age, so population growth is limited

Biotic factors

• Biotic factors involve the living parts of an ecosystem

  • Biotic factors are the interactions between organisms

  • This includes things like competition for resources and predation

• Biotic factors can be split into three main types:

  • Interspecific competition

  • Intraspecific competition

  • Predation

Interspecific competition

• Occurs when different species compete for the same resources (e.g. food, space, nesting sites)

• Can limit both populations if species are similarly adapted, as resources are shared

• If one species is better adapted, it may outcompete the other:

  • The weaker competitor declines in number or is excluded

• For example, in the UK, grey squirrels outcompete red squirrels because:

  • Grey squirrels eat a wider range of food

  • They are also larger and have better fat storage in winter

    • This leads to greater survival and reproduction

Intraspecific competition

• This is competition for the same resources between individuals from the same species

• For example: grey squirrels compete with grey squirrels

• When resources are plentiful, grey squirrel numbers increase

  • More individuals compete for food and shelter

  • Resources become limiting, so the population stabilises at the carrying capacity

Predation

• Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers who are themselves eaten by tertiary consumers

• Predators are consumers that kill and eat prey

• In a stable community, predator and prey populations cycle as follows:

  1. Prey numbers rise → more food for predators

  2. Predator numbers rise → more prey eaten

  3. Prey numbers fall → less food for predators

  4. Predator numbers fall → less pressure on prey

  5. Prey numbers rise again → cycle repeats