AP® Human Geography review sheet from Aim for Five (aimforfive.com/geo/units/2/2-1)
Unit 2 · Topic 2.1
2.1 Population Distribution
People are spread very unevenly across Earth, and both physical and human factors explain why. You also need the three ways to measure population density, because each one shows a different kind of pressure on the land.
Key terms
- population distribution
- arithmetic density
- physiological density
- agricultural density
- arable land
Where people live
Roughly nine in ten people live in the Northern Hemisphere, and more than half of humanity lives in South, East and Southeast Asia. The biggest clusters are East Asia (eastern China, Japan, Korea), South Asia (the Ganges and Indus river plains of India, Pakistan and Bangladesh), Europe, and Southeast Asia, especially the Indonesian island of Java. Smaller clusters include the northeastern United States, West Africa around Nigeria, and the Nile valley in Egypt.
Large areas are nearly empty: deserts like the Sahara, very cold lands like northern Canada and Siberia, high mountains, and dense rainforest.
Why: physical and human factors
- Climate: most people live in mild, mid-latitude climates or wet tropical lowlands where crops grow well, not in extreme heat, cold or dryness.
- Landforms: flat river valleys and coastal plains are easier to farm and build on than steep mountains.
- Water bodies: rivers and coasts provide drinking water, irrigation, fish and trade routes. Many of the world's largest cities sit on a coast or river.
- Economic factors: jobs pull people to cities, ports and industrial areas.
- History: long-settled farming regions, like the river valleys of China and India, built up dense populations over thousands of years.
- Politics: governments can move people or build new capitals, and conflict can empty an area.
- Culture: people often stay near family, religious sites or communities that share their language.
The pattern changes with scale
At the global scale, the big story is the Asian and European clusters. At the national scale, you see patterns inside countries: most Australians live near the coasts, especially in the southeast, because the interior is dry; most Canadians live in the south, close to the U.S. border, where the climate is milder; and nearly all Egyptians live in the Nile valley and delta. At the local scale, density can change block by block, from apartment towers to single-family homes.
Three ways to measure density
Arable land is land that can be used to grow crops. Arithmetic density can mislead because it treats deserts and mountains the same as farmland. Egypt shows this: its arithmetic density looks moderate, but its physiological density is very high because only the Nile valley and delta can be farmed.
A high agricultural density usually means many people farm small plots by hand, which is common in less developed economies. A low agricultural density usually means machines do much of the work, as in the United States or Canada.
| Measure | Formula | What it tells you |
|---|---|---|
| Arithmetic density | Total population ÷ total land area | The simplest average; useful for broad comparisons |
| Physiological density | Total population ÷ arable (farmable) land | How much pressure people put on the land that can grow food |
| Agricultural density | Number of farmers ÷ arable land | How many farmers work each unit of farmland; hints at how mechanized farming is |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Calculating and interpreting the three densities
Country X has 50 million people, 500,000 km² of land, 50,000 km² of arable land, and 10 million farmers. Calculate the arithmetic, physiological and agricultural densities, and explain what the physiological density reveals.
Show the solutionHide the solution
- Step 1: Arithmetic density: 50,000,000 ÷ 500,000 km² = 100 people per km².
- Step 2: Physiological density: 50,000,000 ÷ 50,000 km² = 1,000 people per km² of arable land.
- Step 3: Agricultural density: 10,000,000 ÷ 50,000 km² = 200 farmers per km² of arable land.
- Step 4: Interpret: the physiological density is ten times the arithmetic density because only 10% of the land is arable. Each square kilometer of farmland must feed 1,000 people, so the country may need to import food or farm very intensively.
Answer: Arithmetic: 100 people/km². Physiological: 1,000 people/km² of arable land. Agricultural: 200 farmers/km² of arable land. The high physiological density shows heavy pressure on the small amount of farmland.
Common mistakes
- Mixing up physiological and agricultural density. Physiological uses the total population; agricultural uses only farmers. Both divide by arable land.
- Using arithmetic density to argue a country is crowded. It hides where people actually live and how much land is farmable.
- Listing only physical factors. Human factors like jobs, history and politics are just as important to explain distribution.
On the exam
- Expect a question that gives a table of densities and asks which measure best shows pressure on farmland; that's physiological density.
- Free-response parts often ask you to explain one physical and one human factor behind a distribution; give a specific place for each.
Connected topics
Videos
Check yourself
4 questions on 2.1 Population Distribution. Pick an answer to see if you got it, and why.
| Country | Population (millions) | Total land (thousand km²) | Arable land (thousand km²) | Farmers (millions) |
|---|---|---|---|---|
| P | 100 | 1,000 | 30 | 25 |
| Q | 50 | 2,000 | 400 | 2 |
| R | 20 | 100 | 40 | 6 |
Hypothetical national data
Which country has the highest physiological density?
Country P's physiological density is far higher than its arithmetic density. This most likely indicates that
Country Q has a large amount of arable land and a very low agricultural density. Which conclusion is best supported?
Which of the following best explains why most of Canada's population lives within a few hundred kilometers of its southern border?
0 of 4 answered