Carrying Capacity (AQA A Level Biology)

• The maximum stable population size of a species that an ecosystem can support is known as the carrying capacity
• Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth
  • In reality, however, there are many abiotic and biotic factors that prevent every individual in a population from making it to adulthood and reproducing

• This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)
  • This is why no single species has a population size that dominates all other species populations on Earth, with the exception of humans perhaps (as we have managed to overcome many of the abiotic and biotic factors that could potentially limit the population growth of our species)

• 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

An example graph showing the population growth of a population of lions and the point at which the carrying capacity of this population is reached

Abiotic factors affecting population size

• 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

• For a population of mammals, for example, if the temperature of their surroundings is significantly lower or higher than the optimum body temperature for that species, individuals will have to use up a significant amount of energy to maintain their optimum body temperature (via homeostasis)
  • As each individual now has less energy available for growth and reproduction, fewer individuals reach reproductive age and successfully reproduce, meaning that population growth is limited

Biotic factors affecting population size

• 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

• This is competition for the same resources between individuals from different species
  • For example, red squirrels and grey squirrels in the UK compete for the same habitat, nesting sites and food resources

• In some cases, competition between species means that the sizes of both populations are limited
  • This can occur as each species has access to fewer resources and therefore less chance of survival and reproduction
  • This usually occurs if the two species are similarly well-adapted to the habitat

• In other cases, competition between species can lead to a decrease in the population size of one of the species and an increase in the population size of the other
  • This can occur if one species is able to out-compete the other for resources and therefore has a greater chance of surviving and reproducing
  • This usually occurs if one of the species is better adapted to the habitat

• For example, the population of grey squirrels in the UK has increased, whilst the red squirrel population is now greatly reduced
  • This has occurred because grey squirrels are better adapted: they eat a wider range of foods, are larger and are able to store greater amounts of fat over the winter months
  • This increases their chances of surviving the winter and reproducing successfully, compared to the red squirrels

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

Intraspecific competition

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

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:
  • The number of predators increases as there is more prey available
  • The number of prey then decreases as there are now more predators
  • The number of predators decreases as there is now less prey available
  • 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