4.3 – Populations

4.3 – Populations

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

Population can be affected by:



Or birth. This is because as the birth rate increases, the population increases. This
increase is exponential, so as the population increases, the birth rate increases accordingly.


This is the arrival of organisms to the population from another area, adding to the numbers
of the total population


Or death. The mortality rate also increases as the population increases.


This is when a part of the population migrates to another area. Along with mortality, this helps to stabilise the population growth.

A population is stable when:

Natality + Immigration = Mortality + Emigration

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

At the beginning, the is a short lag phase, as the population adapt to their environment

Exponential Phase

Also called the log phase. This is when there are low or reduced limiting factors, allowing the population to expand exponentially in the habitat. It may be increasing by 2n where n = number of generations. In these circumstances, the rate of natality and immigration is higher than mortality and emigration. However, emigration and mortality will not = 0. During this time, little waste product accumulates, and there is adequate nutrients.

These conditions are typical of a population of germinating annual plants in a new season. Also, a bacterial population during the initial phases of an infection, or of any species occupying a previously unoccupied habitat (succession)

Transition Stage

Also called the linear growth phase. During this time, resources are reduced and the growth of the population becomes limited. This comes as a result of increased competition as the population grows. Some individuals will survive, who obtain the resources, while others will not. This produces a lower rate of population growth than that observed in the exponential phase.

Population Plateau

Also called the stationary phase. This is when the population remains constant over time or generations. The population is determined by the carrying capacity of the habitat at that point in time.

Natality + Immigration = Mortality + Emigration

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

The exponential growth phase comes as a result of the fact that the population has already begun to grow, and rises quickly because there are no limiting factors, and the resources exist in unlimited amounts.

The plateau phase happens once the habitat is supporting the maximum number of organisms at that time.

The transitional phase happens because limiting factors in the environment slow the increase.

4.3.4 – List three factors that set limits to population increase

  • Availability of nutrients
  • Number of predators and parasitism
  • Accumulation of waste materials
  • Disease
  • Shortage of space or territory

In other words, populations tend to be naturally self-regulating.