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Outline how population size is affected by
natality, immigration, mortality and
• Natality – offspring are produced and
added to the population
• Mortality – individuals die and are lost
from the population
• Immigration – individuals move into the
area from somewhere else and add to the
• Emigration – individuals move out of the
area and are lost from the population
Draw a graph showing
the sigmoid (Sshaped) population
Explain reasons for the exponential growth
phase, the plateau phase and the
transitional phase between these two
Population increases exponentially because
the natality rate is higher than the mortality
rate. This is because there is an
abundance of food, and disease and
predators are rare.
Difference between natality and mortality
rates are not as great, but natality is still
higher so population continues to grow,
but at a slower rate.
Food is no longer as abundant due to the
increase in the population size. May also
be increase predation and disease.
Natality and mortality are equal so the population
size stays constant.
• shortage of food or other resources
• increase in predators
• more diseases or parasites
If a population is limited, then it has reached its
The maximum population size that can be
supported by the environment
List three factors which set limits to
1. shortage of food or other resources
2. Increase in predators
3. More diseases or parasites
The IBO assessment statements:
5.3.1 Outline how population size is affected by natality,
immigration, mortality and emigration.
•Aim 7: Simulation exercises can be performed.
5.3.2 Draw and label a graph showing a sigmoid (Sshaped) population growth curve.
5.3.3 Explain the reasons for the exponential growth phase,
the plateau phase and the transitional phase between
these two phases.
5.3.4 List three factors that set limits to population increase.