Human Populations (AQA A-Level Biology): Revision Notes
Human Populations
Population growth and size
The human population has more than doubled in less than 50 years, now exceeding 7 billion people worldwide. Understanding how human populations change over time involves examining several key factors that influence growth patterns.
The primary factors determining whether a population increases, decreases, or stays the same are birth rate and death rate. The balance between these two rates determines the natural change in population size.
Migration also affects population size within specific regions. This movement of individuals occurs in two forms:
- Immigration - when people join a population from outside areas
- Emigration - when people leave a population to settle elsewhere
Migration patterns can significantly alter local population dynamics even when national birth and death rates remain stable. Urban areas often experience rapid population growth through immigration, while rural areas may see population decline through emigration.
Population growth calculations
Population change follows a straightforward mathematical relationship:
To express this as a percentage growth rate:
These calculations help demographers predict future population trends and plan resource allocation accordingly.
Worked Example: Calculating Population Growth Rate
A city has a population of 50,000 at the start of the year. During the year:
- Births: 800
- Deaths: 600
- Immigration: 1,200
- Emigration: 400
Step 1: Calculate population change
Population change = (800 + 1,200) - (600 + 400) = 2,000 - 1,000 = 1,000
Step 2: Calculate percentage growth rate
Percentage growth rate = (1,000 ÷ 50,000) × 100 = 2%
Age structure and population pyramids
The age and gender composition of a population provides valuable insights into future growth trends. Population pyramids are graphical representations showing the percentage of males and females in different age groups, displayed as horizontal bars.
Three distinct types of population pyramids exist:
Stable population
Shows balanced birth and death rates with no overall population increase or decrease. The pyramid has relatively straight sides with a slight taper towards older age groups, indicating consistent replacement of the population across generations.
Stable populations are relatively rare in today's world, as most countries are experiencing either growth or decline due to economic and social changes.
Increasing population
Characterised by high birth rates creating a wide base in the population pyramid compared to a stable population. This triangular shape with fewer older people relative to younger generations typically occurs in economically less developed countries where families tend to be larger.
Decreasing population
Features lower birth rates resulting in a narrower base of the population pyramid and higher proportions of elderly people, creating a more top-heavy structure. This pattern appears in certain economically developed countries like Japan, where birth rates have fallen below replacement levels.
The shape of a population pyramid can predict future population trends: wide bases indicate future growth even if current birth rates decline, while narrow bases suggest future population decline as smaller generations reach reproductive age.
Demographic transition model
As countries develop economically, their populations typically follow a pattern called demographic transition. This process can be divided into four distinct stages based on birth rates, death rates, and total population size.
Stage 1: Small and stable population
- Birth rate: High
- Death rate: High
- Population size: Small with little change over time
High death rates result from factors like famine, disease outbreaks, and poor living conditions. High birth rates compensate for high infant mortality, but overall population remains stable.
Very few countries remain in Stage 1 today. Most isolated or undeveloped regions that might fit this pattern lack reliable demographic data.
Stage 2: Early expansion
- Birth rate: High (remains elevated)
- Death rate: Decreasing
- Population size: Beginning to grow
Improvements in nutrition, sanitation, and medical care reduce death rates while birth rates remain high. This creates rapid population growth as more children survive to adulthood.
Stage 3: Late expansion
- Birth rate: Decreasing
- Death rate: Low
- Population size: Continuing to grow but at a slower rate
As living standards improve further, families choose to have fewer children. Education, urbanisation, and changing social attitudes contribute to declining birth rates, though population continues growing due to previous high birth rates.
Stage 4: Large and stable population
- Birth rate: Low
- Death rate: Low
- Population size: Large but stable
Both birth and death rates stabilise at low levels. The population reaches a new equilibrium at a much larger size than Stage 1, with slow or zero population growth.
Example: Demographic Transition in Practice
The United Kingdom demonstrates classic demographic transition:
- Stage 1 (pre-1750): High birth and death rates, stable population around 6 million
- Stage 2 (1750-1880): Death rates fell due to improved sanitation, population grew rapidly
- Stage 3 (1880-1940): Birth rates declined with urbanisation and education, slower growth
- Stage 4 (1940-present): Low birth and death rates, population stable around 67 million
Links to other topics
Human population studies connect to ecosystem dynamics, resource management, and environmental impact assessment. Understanding population trends helps predict future demands on food systems, energy resources, and habitat conservation needs.
Population growth in developing countries often puts pressure on natural resources, while ageing populations in developed countries face challenges with healthcare costs and workforce sustainability. These demographic patterns influence everything from climate change to economic policy.
Key Points to Remember:
- Population change depends on the balance between births, deaths, immigration, and emigration
- Population pyramids reveal whether populations are growing, stable, or declining based on their age structure
- Demographic transition follows predictable stages as countries develop economically
- Wide-based pyramids indicate growing populations; narrow bases suggest declining populations
- The demographic transition model helps explain how birth and death rates change with economic development