6.6.1 Carrying Capacity

Question 1

What is carrying capacity?

Answer 1

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 reaching 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 possible exception of humans (as humans have managed to overcome many of the abiotic and biotic factors that could potentially limit the population growth of Homo sapiens)

The combined effects of limiting factors that prevent the further increase of a population is known as environmental resistance

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

Question 2

What are abiotic factors that affect population size?

Answer 2

Abiotic factors involve the non-living parts of an ecosystem

Some examples of abiotic factors that act as limiting factors of population size include:

Light availability

Water supply

Water mineral content

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

Question 3

What are the biotic factors affecting population size?

Answer 3

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 (between different species)

Intraspecific competition (between different members of one species)

Predation

Question 4

What is interspecific competition?

Answer 4

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