Human Population Density, Distribution and Structure

Sparse and dense: population distribution and population density

Population distribution is how people are spread over an area, and population density is how many people there are in each square kilometre of it. They are always used together because a place with few people overall can still be crowded where they actually live. Sparsely populated places contain few people: a desert, a polar ice sheet or a high mountain range. Densely populated places contain many people: a river floodplain, a coastal plain or a large city.

Density is a calculation, so learn the formula and the unit together: population density = total population / area in km2, and the answer is in people per km2. A district of 5,600 km2 with 840,000 people has a density of 840,000 / 5,600 = 150 people per km2. Convert before you divide: if the population is given as '4.8 million', write 4,800,000, and if the area is in square miles, convert it to km2 first.

An average density hides the pattern, because it treats every square kilometre as if it were identical. A country with an average of 20 people per km2 might have 20 people in every square kilometre, or it might have almost everyone living on one small strip of cultivated land with empty desert for the rest, which is the pattern along the Nile in Egypt. So when a question asks you to describe distribution, describe the pattern - clustered on a coastal plain, strung out along a river valley, packed into one city - and not just the size of the population.

The same country can also be dense and sparse at once. Brazil is one of the world's larger economies, yet most Brazilians live in the crowded south-east coast while huge areas of the Amazon basin hold almost nobody. The distribution, not the total, is what a government has to manage when it plans roads, clinics and power lines.

• Population density = total population / area in km2, measured in people per km2.
• Population distribution = how people are spread over an area.
• Sparsely populated: few people in a large area, such as a desert or a mountain range.
• Densely populated: many people in a small area, such as a river delta or a coastal city.
• A national average density does not tell you how people are distributed within the country.

Physical factors: why people settle where they do

The land itself rules out most of the planet. Relief matters first: flat lowland is easy to plough and easy to build on, while a steep slope loses soil, is hard to terrace and expensive to reach with a road, so mountain slopes are usually sparse. Altitude matters in the tropics, where the lowlands are hot and malarial and the cooler highland plateaus, such as the Ethiopian highlands and the Altiplano of Bolivia and Peru, carry most of the population.

Climate decides whether farming can feed a dense population. People crowd into temperate and monsoon climates with reliable rain, warmth and a long growing season, and avoid the very cold, the very dry and the permanently wet. Siberia is enormous but holds a small share of Russia's people because the growing season is short and the ground is frozen for much of the year, and the Sahara and the Atacama Desert in Chile are close to empty for the opposite reason.

Water supply and soils are the two natural resources that keep people in one place. Most of the world's dense rural populations sit on deep, fertile alluvium laid down by rivers - the Nile, the Ganges and the Mekong - or on volcanic soils that are rich in minerals and renew quickly, as they do on Java in Indonesia and on the slopes of Mount Merapi. Where the water table is shallow and wells are cheap, villages can spread away from the river; where groundwater is deep or salty, they cannot.

Finally, resources and hazards pull in opposite directions. Coalfields, oilfields and ore bodies attract workers and the towns and railways that serve them, which is how the Ruhr in Germany and the goldfields of South Africa filled up. Fertile floodplains and volcanic slopes are risky but productive, so people stay and accept floods, lahars and ash falls because the land gives back more than it takes - until one event destroys the settlement and the survivors move.

Human factors: work, transport and government decisions

Work is the strongest human pull. People move towards mines, factories, ports, tourist resorts and offices, and a town that loses its main employer shrinks once the jobs go. Because workplaces attract shops, schools and hospitals in turn, an advantage early on keeps growing, which is why the same cities stay at the top of a country's hierarchy for centuries.

Transport and services decide which of two equally good sites fills up. A site on a navigable river, a natural harbour or a railway junction becomes a trading city, while a site with no route to market stays a village. Roads, power lines, piped water, clinics and secondary schools all pull people in, and their absence pushes people out towards the nearest town.

Government decisions can move people against the physical geography. Brazil built its planned capital, Brasilia, deep in the interior and moved the government there in 1960 to draw population and investment away from the crowded south-east coast. The Netherlands, where a quarter of the land is below sea level, kept a dense population on land that would otherwise be marsh by building polders, dykes and pumping stations. Both are management strategies that work, and both carry a cost: Brasilia is an expensive city in a thinly populated region, and the polders depend on constant maintenance of dykes and on pumping that uses energy all year round.

Conflict, land tenure and housing costs complete the picture. War and insecurity empty whole regions, as they have done in Syria and Sudan, while a shortage of affordable housing pushes poorer households to the edge of a city or onto land that nobody else wanted - a steep slope, a floodplain or a rubbish tip. Human factors also work in combination: a fertile delta with a port, a railway and a government investment programme is far denser than the same delta with none of them.

• Work: mines, factories, ports, tourism and offices concentrate people where the jobs are.
• Transport: rivers, natural harbours, railways and motorway junctions decide which sites grow into cities.
• Services and infrastructure: schools, clinics, piped water and electricity attract people, and their absence pushes them away.
• Government policy: planned capitals, resettlement schemes, land reform and land reclamation (the Dutch polders) can shift population.
• Conflict and insecurity: war and unrest drive people out of a region, often permanently.
• Housing and land cost: high prices push poorer households onto steep slopes, floodplains and other marginal land.

How a population changes: births, deaths and migration

A population changes in only two ways: people are born and die, and people arrive and leave. The birth rate is the number of live births per 1,000 people per year and the death rate is the number of deaths per 1,000 people per year. Natural increase is the birth rate minus the death rate, and it is positive when there are more births than deaths. In most low-income countries the birth rate is high and the death rate has fallen with better food, clean water and vaccination, so natural increase is large; in most high-income countries both rates are low, so natural increase is small and sometimes negative.

Natural increase can also be written as a percentage of the population: (births - deaths) / total population x 100. A country of 50.0 million people with 1,100,000 births and 400,000 deaths in a year has a natural increase of 700,000 people, which is 700,000 / 50,000,000 x 100 = 1.4 per cent that year. Keep the top and bottom of the fraction in the same units and the calculation is straightforward.

Migration is the movement of people into and out of a country. Immigration is arrival, emigration is departure, and net migration is immigration minus emigration. Push factors drive people away - unemployment, low wages, drought, war, poor services - and pull factors attract them - jobs, higher wages, safety, family, schools and hospitals. A country with more arrivals than departures has positive net migration; one that loses more people than it gains, as several Pacific island states do when their young adults take work overseas, has negative net migration.

Total population change is natural increase plus net migration, and the two can pull in opposite directions. Japan has a low birth rate and a shrinking population, while several Gulf states, including the United Arab Emirates, have a low birth rate among their own citizens but a fast-growing population because temporary workers arrive in large numbers. When you explain a change in population size, always ask which of the two causes is doing most of the work.

Reading a population pyramid

A population pyramid is a bar chart turned on its side. The vertical axis is age, usually in five-year bands from 0-4 at the bottom to 85 and over at the top. The horizontal axis is the size of the population in each band, either as a number or as a percentage of the total so that countries of very different sizes can be compared. Men are conventionally shown on the left and women on the right, so the length of the bars on each side tells you the balance between the male and female population.

Read the pyramid in three bands, because that is how the exam asks about it. The bottom band, roughly 0-14, is the young dependent population. The middle band, roughly 15-64, is the working population. The top band, 65 and over, is the elderly dependent population. Different countries draw the boundaries in slightly different places - the United Kingdom uses 16 and pension age - so state the age bands you are using when you answer.

Every bar records two things at once: how many people were born in those five years, and how many of them are still alive. That is why a wide base means a high birth rate and a narrow top means that most people do not reach old age. It also explains the notches: a short bar in the middle of a pyramid usually marks a group thinned by war, famine or disease, or a group that was never born because births fell during a crisis, and that notch climbs slowly up the pyramid as the survivors age.

The sides of the pyramid usually slope inwards towards the top for two reasons. Fewer people are born now than in the past, and older people die, so there are fewer of them left. Women also live longer than men on average, so the bars on the right are often longer than the bars on the left in the oldest age groups. If you are asked to draw a pyramid, label both axes, keep the male and female sides symmetric about the middle line, and remember that the whole pyramid adds up to 100 per cent when it is drawn as percentages.

• Vertical axis: age bands, 0-4 at the bottom upwards in five-year steps.
• Horizontal axis: number of people, or percentage of the total population, in each band.
• Left side: males. Right side: females.
• Bottom band (about 0-14): young dependents. Middle band (about 15-64): working population. Top band (65 and over): elderly dependents.
• A short bar or notch records a war, famine, epidemic or a fall in births, and it moves up the pyramid as that generation ages.

Youthful, ageing and stable populations and the dependency ratio

A youthful population has a pyramid with a very wide base that narrows quickly, giving a large school-age group, a smaller working population and few elderly people. It is produced by a high birth rate together with a death rate that has already fallen. The pressures are all about children: schools, child health care, food and land, and then a race to create enough jobs as those children reach working age. Most low-income countries, including Malawi and Uganda, still have this shape.

An ageing population has the opposite pyramid: a narrow base, because the birth rate is low, and bars in the 60s and above that are almost as long as those in mid-life, because life expectancy is high. Japan, Italy and Germany all have this shape. The pressures move upwards with the population: pensions, social care, hip replacements and long-term medicine, paid for by a workforce that is no longer growing. A stable, or stationary, population sits between the two: the sides of the pyramid are nearly straight from the base to about 50, with a base that is about the same width as the bars above it, showing birth and death rates that have been low and roughly constant for decades.

The dependency ratio measures how many people each working person has to support: dependency ratio = (young dependents + elderly dependents) / working population x 100. The working population is normally taken as 15-64. A country of 60 million with 18 million people under 15 and 6 million aged 65 and over has 60 - 18 - 6 = 36 million of working age, and a dependency ratio of (18 + 6) / 36 x 100 = about 67 dependents per 100 working people.

Two countries can share a dependency ratio of about 67 and need completely different policies. If the dependents are children, as in Malawi, the money has to go on schools, clinics and maternity services, and there is a bulge of future workers to employ. If they are elderly people, as in Sweden, whose base is narrow but whose ratio is still driven upwards by the 65-and-over group, the money goes on pensions, care homes and medicine, and there may be too few workers to fill the jobs. That is why the ratio is always quoted together with what kind of dependents it contains.

The dependency ratio also has limits, and saying so earns marks. It assumes everyone aged 15-64 works and everyone outside that band does not, yet in many low-income countries children work on family farms and many people over 65 keep working, while in high-income countries students, unemployed people and unpaid carers of working age are all dependent in practice. Because these two errors partly cancel out, the ratio is still a useful comparison, as long as you treat it as a rough indicator rather than a measurement.

World Bank income groups and comparing an LIC with an HIC

The World Bank groups the world's economies by income so that countries at similar levels of development can be compared, using gross national income (GNI) per person in US dollars as its main measure. Low-income countries (LICs) have the lowest income per person, middle-income countries (MICs) sit between the other two groups and are split into lower middle and upper middle income, and high-income countries (HICs) have the highest. The exact thresholds are revised every year, but they are roughly a few thousand dollars or less per person for low income and roughly fourteen thousand dollars or more for high income, so Malawi and Niger fall in the low-income group while Germany, Japan and Sweden fall in the high-income group. Middle-income countries such as Brazil, Indonesia and Thailand contain a large share of the world's people and include countries at very different stages of change.

Population structure follows income closely, because income shapes birth rates and life expectancy. An LIC such as Malawi has a pyramid with a broad base and steep sides: most of the population is under 15, the birth rate is high, life expectancy is short and there are relatively few people over 65, so the pyramid tapers quickly to a point. The large young group means the dependency ratio is driven almost entirely by children, and it also means that even if every couple has fewer children from now on, the population will keep growing for decades as those children grow up and have families of their own.

An HIC such as Sweden has a pyramid with a narrow base and sides that fall away slowly. The birth rate is low, life expectancy is long, and the oldest age groups hold a substantial share of the population - men and women in their 80s and 90s are visible as a long, thin top rather than a point. The dependent group is dominated by the elderly, so the money goes on pensions and care rather than schools, and some countries fill labour shortages by encouraging immigration. Japan is the extreme case, with a shrinking workforce and nearly a third of its people aged 65 or over.

Two cautions when you use these labels. Income groups are national averages and hide enormous differences inside a country: the richest fifth of a middle-income country may live like an HIC while the poorest fifth has no piped water. And the boundaries between the groups move, because the World Bank updates the thresholds each year and countries are reclassified as their income changes, so an LIC today may be a middle-income country in ten years. Use the group as a starting description of population structure, then look at the actual pyramid before you draw your conclusion.