Populations

Assessment Statement

5.3.1Outline how population size is affected by natality, immigration, mortality and emigration

5.3.2Draw and label a graph showing a sigmoid (S-shaped) population growth curve

5.3.3Explain the reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases

5.3.4List three factors that set limits to population increase

Another way to analyze what goes on in an ecosystem is to see how separate populations grow and maintain themselves.

There are four factors that influence the size of a population

Population Growth

The growth of the number of individuals in a population usually follows a sigmoid (S shaped) curve. The curve can be divided into four phases:

  1. Lag phase –
  2. Exponential Growth or log phase –
  3. Linear or transitional growth phase –
  4. Plateau phase –

To explain each phase, we will use a population of rabbits. A group of new, young rabbits moves into a meadow in which all other rabbits are excluded. After familiarizing themselves with the new territory, and establishing burrows, they have unrestricted access to food supplies and breeding starts.

The population of rabbits would increase rapidly, but eventually because of the very large number of rabbits, the vegetation would be used up faster than it grew. Increases in population would stop and the population enters a plateau or stationary phase. The supply of food would be a limiting factor in the growth of the rabbit population.


All organisms can undergo exponential growth if controlling factors are not present or limited.

For example, Elephants which are considered to be very slow breeders, can go from one pair to 19 million alive elephants in 750 years.

A bacteria can also go through exponential growth as shown below.


The only limit is how fast the organism can reproduce.

Exponential growth only occurs occasionally. In the transition or linear growth phase, the increase will slow down until it reaches zero. This occurs when the environment suddenly changes and only a few individuals survive. Interspecies competition results, where the strong survive. This is when the habitat has reached its carrying capacity.

Carrying Capacity is the number of individuals in the population has reached the maximum, which can be supported by the environment.

As a population increases, it begins to experience environmental resistance, because space and resources are reduced and competition for space and resources increases.

If the numbers start to increase above carry capacity, shortage of resources reduces the numbers of offspring produced and the population regulates itself at the carry capacity. If the population is reduced, by heavy predation, for example, the additional resources that then become available lead to an increase in reproductive rate, and the carrying capacity is again reached. In other words, the populations tend to be self-regulating.

Take for example, humans over the years.


Limiting Factors

Populations fluctuate in the wild. There are times of boom and bust. Why does this occur?

There are several factors that can limit populations.

  1. Density Dependent – affect the proportion of the population as the density increases. Examples are predation, disease or intra-species competition. The effects of these factors increase with increasing population numbers.
  1. Density Independent – affect the proportion of the population regardless or size, and affect the overall size of the population or reducing the birth and death rates. Examples are death due to weather, earthquakes, drought.
  1. Extrinsic Regulatory Mechanisms – originate outside the population and include biotic as well as physical factors. For example food supplies, natural enemies, disease, weather.
  1. Intrinsic Regulatory Mechanisms – originate in the anatomy, physiology and behaviour. Competition is the main regulating mechanism for large groups.