Human populations — core idea

· Human populations can be studied as open systems with inputs and outputs.
· Main guiding questions: How can population dynamics be measured and compared? and How accurately can future population growth be predicted?
· Exam focus: define population indicators, calculate natural increase and doubling time, interpret age–sex pyramids, explain the DTM, and evaluate population projections/policies.

Population inputs and outputs

· Inputs to a human population = births and immigration.
· Crude birth rate (CBR) = number of live births per 1,000 people per year.
· Immigration rate = number of immigrants per 1,000 people per year.
· Outputs from a human population = deaths and emigration.
· Crude death rate (CDR) = number of deaths per 1,000 people per year.
· Emigration rate = number of emigrants per 1,000 people per year.
· Rates can be compared at different scales: town, country, region, or global population.

This diagram helps students connect birth rates, death rates and population size over time. It is useful for comparing how demographic transitions can happen at different speeds in different countries. Source

Key population measures

· Total fertility rate (TFR) = average number of births per woman of childbearing age.
· Life expectancy = average number of years a person is expected to live, usually from birth, if demographic conditions remain the same.
· Doubling time = time taken for a population to double at its current growth rate.
· Rule of 70: $\text{doubling time} = \frac{70}{\text{growth rate as a percentage}}$.
· Natural increase = birth rate − death rate.
· Natural increase as a percentage = $\frac{\text{CBR} - \text{CDR}}{10}$.
· Positive natural increase = population grows naturally; negative natural increase = population declines naturally, unless migration offsets it.

Human population growth and projection models

· The global human population has followed a rapid growth curve.
· Models are used to project future global human population growth.
· UN projection models use scenarios linked to future fertility rates.
· Future population growth is uncertain because future human fertility rates are uncertain.
· Projections are not predictions; they are modelled scenarios based on assumptions.
· Model limitations: changing fertility, mortality, migration, policy, conflict, healthcare, education, economic development and social change can alter outcomes.

Population and migration policies

· Population policies can directly manage growth rates by influencing birth rates.
· Anti-natalist policies aim to reduce birth rates.
· Pro-natalist policies aim to increase birth rates.
· Migration policies influence immigration and emigration.
· Policies may use cultural, religious, economic, social and political factors.
· Named examples should be used in exam answers, such as a country policy designed to reduce fertility, encourage births, or control migration.
· Strong answers evaluate effectiveness, ethical concerns, equity, human rights, economic impacts and long-term consequences.

Indirect management of population growth

· Population growth can also be managed indirectly through economic, social, health, development and other policies.
· Policies improving gender equality, education, public health and welfare can affect births, deaths and migration.
· Improvements in female education and access to healthcare often reduce fertility by changing opportunities, survival rates and family planning choices.
· Improvements in public health, sanitation and welfare usually reduce death rates and increase life expectancy.
· Use two named examples when explaining indirect population management.

Age–sex pyramids

· Age–sex pyramids model and compare the composition of human populations.
· They show the proportion or number of males and females in each age group.
· They may be measured in absolute numbers or as a percentage of the total population.
· A wide base usually indicates high birth rate and a youthful, growing population.
· A narrower base can indicate falling birth rate and slower future growth.
· A large older population indicates ageing, often linked to high life expectancy and low fertility.
· Use pyramids to infer growth rate, fertility, mortality, life expectancy, dependency and likely future service needs.

Demographic transition model (DTM)

· The demographic transition model (DTM) describes how birth rates and death rates change through stages of development over time.
· The DTM has five stages.
· Stage 1: high birth rate + high death rate = low/stable population growth.
· Stage 2: death rate falls before birth rate = rapid/exponential population increase.
· Stage 3: birth rate begins to fall = growth slows.
· Stage 4: low birth rate + low death rate = stable or slowly growing population.
· Stage 5: birth rate may fall below death rate = possible population decline.
· Link age–sex pyramids to DTM stages: youthful pyramids fit earlier growth stages; rectangular or top-heavy pyramids fit later stages.
· The DTM is a simplified model, so it is useful for comparison but cannot perfectly predict every country’s demographic pathway.

This image summarises the DTM by showing how death rates usually fall before birth rates, causing rapid population growth. It is especially useful for linking model stages to CBR, CDR and population change. Source

Common exam traps

· Do not confuse CBR/CDR with total births/deaths; they are rates per 1,000 people per year.
· Do not forget migration: population change is affected by births, deaths, immigration and emigration.
· Do not describe UN projections as certain predictions; they are scenario-based models.
· Do not assume all countries pass through the DTM in the same way or at the same speed.
· Do not explain environmental stress using population size alone; include resource consumption, biocapacity, equity and planetary boundaries.