Human Population Dynamics
Introduced| English | Meaning |
|---|---|
| birth rate/bɜːθ reɪt/ | The number of live births per thousand people in a population per year. |
| death rate/deθ reɪt/ | The number of deaths per thousand people in a population per year. |
| growth rate/ɡrəʊθ reɪt/ | The percentage change in population per year. |
| doubling time/ˈdʌblɪŋ taɪm/ | The time required for a growing quantity to become twice its initial size. |
A crowded planet
- For most of history, humans were few and grew slowly.
- Then, in the last 200 years, our numbers exploded.
- From one billion to eight billion in a couple of centuries.
- Understanding why is the study of human population dynamics.
Births versus deaths
- The birth rate 出生率 is how many are born per 1000 people each year.
- The death rate 死亡率 is how many die per 1000 each year.
- The growth rate 增长率 is the birth rate minus the death rate, plus migration.
- When births outnumber deaths, the population grows.
A crowded city crossing: human population dynamics depend on birth rates, death rates and migration
A population's growth rate is set mainly by…
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Growth rate = (birth rate − death rate) plus net migration.
Why growth exploded
- For most of history, high birth rates were matched by high death rates.
- Then medicine, sanitation, and food supply cut the death rate sharply.
- Birth rates stayed high for a while, so the population surged.
- The gap between falling deaths and high births drove the explosion.
Human population growth
growth rate = births - deaths (+ migration)
Growth depends on the gap between birth rate and death rate; a small rate still doubles the population over time.
Why did the human population grow so fast in the last century?
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Falling death rates (medicine, sanitation, food) with high birth rates caused rapid growth.
Doubling time
- Even a small growth rate compounds over time.
- The doubling time 倍增时间 is how long a population takes to double.
- A handy shortcut: doubling time ≈ 70 ÷ (growth rate as a percent).
- At 2% per year, a population doubles in only about 35 years.
The time it takes a population to double in size is its ____ time.
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Doubling time ≈ 70 ÷ (growth rate in percent) — even a small rate doubles a population over time.
A growth rate of 2% per year takes several centuries to double a population.
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False. Using the rule of 70, doubling time = 70 ÷ 2 = about 35 years, well within a lifetime.
Select all true statements about human population dynamics.
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A growing population puts rising pressure on resources. The other three are correct.
A "small" growth rate is not small over time. Because growth compounds, even 2% per year doubles a population in about 35 years. This is why a growth rate that sounds tiny can lead to enormous numbers within a single lifetime.
The rule of 70 in action:
- Suppose a country grows at 2% per year.
- Doubling time ≈ 70 ÷ 2 = 35 years.
- A country of 50 million would reach 100 million in just 35 years — and 200 million in 70 — from a rate that sounds modest.
Human population dynamics depend on the growth rate = birth rate − death rate (plus migration). Rapid growth came from falling death rates while birth rates stayed high. Doubling time ≈ 70 ÷ (growth rate percent), so even a small rate can double a population within a lifetime.