Births and immigration are inputs that increase a human population. Deaths and emigration are outputs that decrease it. This is core SL and HL content in IB ESS Subtopic 8.1, Human populations.
The Population as a System
A human population can be modelled as a system. The number of people in the population is a storage, while births, deaths and migration are flows that change the size of that storage over time.
| Population flow | Effect on population size |
|---|---|
| Births | Input: people are added through reproduction |
| Immigration | Input: people enter the population from another area |
| Deaths | Output: people are removed through mortality |
| Emigration | Output: people leave the population for another area |
The overall change in population size is:
For example, suppose a country experiences 800,000 births, 300,000 immigrants, 500,000 deaths and 200,000 emigrants in one year:
Its population therefore increases by 400,000 people.
The difference between births and deaths is natural increase. The difference between immigration and emigration is net migration. A population can therefore grow even when deaths exceed births if net immigration is sufficiently high. Qatar, for example, has experienced substantial population growth associated with international labour immigration, showing how migration can strongly affect population size and structure.
A common misconception is that population change depends only on birth and death rates. This ignores migration, which may be a major influence and can reflect economic opportunities, conflict, environmental change and government policy. Access to safe migration and resources also raises questions of environmental justice.
Exam Technique
For an IB ESS system diagram, show the population as a rectangular storage, with births and immigration as inward arrows and deaths and emigration as outward arrows. In calculations, use data from the same period and include the correct unit, such as people per year.