Population Size

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Carrying Capacity

The number of individuals in a population cannot increase continuously because resources will eventually run out. The maximum size a population can maintain is called the carrying capacity.

Exponential growth

Exponential growth of a population is the continuous growth in population size.
Exponential growth is only possible where there is an infinite supply of resources.
In real ecosystems, this is not the case.

Limited resources

There is a limited supply of resources in ecosystems.
Individuals in an ecosystem are competing to use the resources.
Species that have adaptations that allow them to gain resources better than other species are more likely to reproduce.
Reproduction causes the size of a population to increase.

Carrying capacity

Populations cannot increase to an infinite size because there are limited resources.
The maximum size a population can maintain for a long period of time is called the carrying capacity.

Influence of Abiotic Factors

The carrying capacity of a species is influenced by abiotic and biotic factors. Abiotic factors are non-living factors.

Promoting growth

Abiotic conditions can promote the growth of a population.
This is when the abiotic conditions are favourable for the species so more of the population reproduce.
- E.g. during the summer there is more light exposure in a day. There is more light for photosynthesis so plants have more energy for reproducing.

Slowing growth

Abiotic conditions can slow or stop the growth of a population.
This is when the abiotic conditions are unfavourable for the species so less of the population reproduce.
- E.g. cold climate means that mammals use more energy in maintaining their body temperature and less energy is available for reproducing.

Influence of Biotic Factors

The carrying capacity of a species is influenced by abiotic and biotic factors. Biotic factors are living factors.

Interspecific competition

Interspecific competition is competition between individuals in different species.
Interspecific competition can reduce the amount of resources available. This slows the rate of population growth.
- E.g. Lions and cheetahs compete for the same prey. When lions and cheetahs inhabit the same area, both species have a smaller population size than when they inhabit different areas.

Out-competing

When two species compete for the same resource, one species can out-compete the other.
A species is out-competed when one species is more well-adapted to the environment than the other. This species is better at gaining resources and reproducing.
Out-competing allows one species to increase in population size and causes the other species to decrease in population size.

Intraspecific competition

Intraspecific competition is competition between individuals of the same species.
Intraspecific competition causes fluctuations in population size.
- E.g. When a population of lions increases, competition for food, space and mates increases. Fewer individuals survive and reproduce so the population size decreases. If the population size decreases, there is more food available so population size increases.

Predation

Predation is when one species (prey) is killed and eaten by another species (predator).
Predators and prey interact to influence each other's population size.
- If the prey population size is small, resources are limited for predators and predator population size decreases.
- When the predator population size falls, the risk of death is lower for prey so the prey population size increases.
- More prey are available, so predator population size increases.