Human Population Dynamics
Introduced| English |
|---|
| birth rate/bɜːθ reɪt/ |
| death rate/deθ reɪt/ |
| growth rate/ɡrəʊθ reɪt/ |
| doubling time/ˈdʌblɪŋ taɪm/ |
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 population's growth rate is set mainly by…
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?
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.
Doubling time ≈ 70 ÷ (growth rate in percent) — even a small rate doubles a population over time.
Even a small yearly growth rate can double a population within a lifetime.
At 2% per year, a population doubles in only about 35 years (70 ÷ 2).
Select all true statements about human population dynamics.
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.