Population and Migration Patterns and Processes

People are spread very unevenly across Earth. Population distribution 人口分布 describes where people live; population density 人口密度 measures how crowded a place is. Most people cluster in a few favourable places: mid-latitudes, low-lying land, coasts, and river valleys with water and fertile soil.

Geographers use three density measures:

• Arithmetic density 算术密度 — total people ÷ total land area.
• Physiological density 生理密度 — people ÷ area of arable (farmable) land; a better guide to pressure on food supply.
• Agricultural density 农业密度 — farmers ÷ arable land; a low value suggests more efficient, mechanised farming.

Uneven distribution creates consequences: crowded regions strain housing, water, and services, while empty regions may struggle to provide roads and schools.

Population composition 人口构成 is the make-up of a population by age and sex. It is shown on a population pyramid 人口金字塔 — back-to-back bar charts of males and females in five-year age cohorts 年龄组.

• A wide base means high birth rates and rapid growth (many young people).
• Straight sides mean slow, stable growth.
• A narrow base with a wide middle means an aging, shrinking population.

The dependency ratio 抚养比 compares dependents (under 15 and over 64) to the working-age population — a high ratio strains the workers who support them.

Populations change through births, deaths, and migration. Key rates (per 1,000 people per year):

• Crude birth rate (CBR) 粗出生率 and crude death rate (CDR) 粗死亡率.
• Rate of natural increase (RNI) 自然增长率 = (CBR − CDR) ÷ 10, as a percentage — it ignores migration.
• Total fertility rate (TFR) 总和生育率 — average children per woman; about 2.1 is replacement level 更替水平.

The doubling time 倍增时间 (years for a population to double) ≈ 70 ÷ RNI%. Small changes in fertility have huge long-run effects.

Worked example (a real AP exam question). "Explain ONE reason why the rate of natural increase (RNI) in urban areas may vary significantly from RNI in rural areas in the same country." (2023) A full-mark answer: "In cities the cost of living — housing, schooling, childcare — is higher, so urban families tend to have fewer children. Fewer births lower the crude birth rate, so the urban RNI falls below the rural RNI, where children still help with farm work." Notice the shape: the command word Explain wants a cause-and-effect chain, so you must name the reason and follow it all the way to the RNI. A bare fact — "cities are expensive" — earns zero.

The Demographic Transition Model (DTM) 人口转变模型 shows how birth and death rates change as a country develops, through five stages.

1. Stage 1 — high, fluctuating CBR and CDR; slow growth (pre-industrial).
2. Stage 2 — CDR falls (better food, medicine, sanitation) while CBR stays high; population booms.
3. Stage 3 — CBR falls (urbanisation, education, women in work); growth slows.
4. Stage 4 — low CBR and CDR; population stable and large.
5. Stage 5 — CBR falls below CDR; population declines and ages.

The linked epidemiological transition 流行病学转变 describes how the main causes of death shift from infectious disease to chronic, degenerative disease as a country moves through the DTM.

In 1798 Thomas Malthus argued that population grows exponentially 指数式 while food supply grows only linearly 线性, so population would outrun food and be checked by famine, disease, and war.

Critics say Malthus underestimated technology: the Green Revolution and modern farming raised food output enormously. Neo-Malthusians 新马尔萨斯主义者 update the worry to include water, energy, and other finite resources. The debate is a favourite exam prompt.

Governments try to influence population growth with policies:

• Pro-natalist 鼓励生育 policies encourage births (cash bonuses, parental leave, childcare) where populations are aging or shrinking — e.g. France, Japan.
• Anti-natalist 限制生育 policies discourage births where growth is seen as too fast — e.g. India's family-planning campaigns.

Policies have unintended consequences: strong anti-natal policies can skew the sex ratio or speed up aging, which later needs a pro-natal response.

The status of women is one of the strongest drivers of fertility. As female education and employment rise, women marry later and have fewer children, so fertility falls — a key reason countries move through the DTM.

At the other end, aging populations 人口老龄化 (a rising share of over-65s) bring their own challenges: higher pension and healthcare costs, a shrinking workforce, and a rising dependency ratio. Countries respond with later retirement, immigration, or pro-natal policies.

Migration 迁移 is a permanent move to a new location. It is driven by push factors 推力 (reasons to leave: war, poverty, disaster) and pull factors 拉力 (reasons to arrive: jobs, safety, family).

• Voluntary migration 自愿迁移 is by choice, usually for economic reasons.
• Forced migration 强迫迁移 is against a person's will — refugees 难民 fleeing danger, or people displaced by disaster.
• Ravenstein's laws describe patterns: most migrants move short distances, in steps, and are drawn mainly by economic pull.

Migration can be international (between countries) or internal (within a country, such as rural-to-urban).

Migration reshapes both the origin and the destination.

• The destination gains workers and cultural diversity but may face pressure on housing and services and social tension.
• The origin may lose skilled workers (a brain drain 人才外流) but gain remittances 侨汇 — money migrants send home, a major income source for many countries.
• Migration changes the age and sex structure of both places, because migrants are often young working-age adults.

• Compute RNI = (CBR − CDR) ÷ 10 as a percent, and read doubling time as about 70 ÷ RNI.
• Link a population pyramid's shape to fertility and the DTM stage — a wide base means Stage 2.
• Remember the death rate falls before the birth rate; that is why Stage 2 grows fastest.
• Separate push from pull and forced from voluntary; a refugee crosses a border, an IDP does not.
• Weigh migration effects: remittances help the origin, but brain drain and aging hurt it.