Population Dynamics
Introduced| English | Meaning |
|---|---|
| crude birth rate/kruːd bɜːθ reɪt/ | The number of live births during a year per thousand people in the population, usually using its midyear size. |
| crude death rate/kruːd deθ reɪt/ | The number of deaths during a year per thousand people in the population, usually using its midyear size. |
| rate of natural increase/reɪt ɒv ˈnætʃərəl ˈɪŋkriːs/ | The rate at which births exceed deaths in a population over a period, excluding migration. |
| total fertility rate/ˈtəʊtl fɜːˈtɪlɪti reɪt/ | The average number of children a woman would have if she experienced the current age-specific fertility rates throughout her reproductive life. |
| replacement level/rɪˈpleɪsmənt ˈlevl/ | The fertility level at which one generation replaces itself in the absence of migration, allowing for mortality; it does not imply that population size becomes stable immediately. |
| doubling time/ˈdʌblɪŋ taɪm/ | The time required for a growing quantity to become twice its initial size. |
How populations change
- A population changes through births, deaths, and migration.
- Rates are measured per 1,000 people per year.
- Crude birth rate 粗出生率 (CBR) and crude death rate 粗死亡率 (CDR) are the basics.
Natural increase
- The rate of natural increase 自然增长率 (RNI) = (CBR - CDR) ÷ 10, as a percent.
- It ignores migration — only births minus deaths.
- A positive RNI means the population is growing from within.
A schematic transition: the birth–death gap is natural change per 1,000; migration is excluded
CBR = 28 and CDR = 8 per 1,000. What is the rate of natural increase, as a percent?
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RNI = (28 - 8) ÷ 10 = 2.0%.
The rate of natural increase includes migration.
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RNI is births minus deaths only — it ignores migration.
Fertility and doubling
- The total fertility rate 总和生育率 (TFR) is the average children per woman.
- About 2.1 is replacement level 更替水平 — enough to keep a population steady.
- Doubling time 倍增时间 ≈ 70 ÷ RNI% (the "rule of 70").
Which population measure?
Match each description to the right population measure.
If a country's RNI is 2%, roughly how many years until its population doubles (rule of 70)?
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Doubling time ≈ 70 ÷ 2 = 35 years.
The total fertility rate that keeps a population steady (replacement level) is about...
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About 2.1 children per woman replaces the parents (allowing for child mortality).
Match each measure to its meaning.
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CBR = births/1,000; RNI = natural growth %; TFR = children per woman.
RNI ignores migration, so it is not the same as total population change. A country with RNI of 0 can still grow (from immigration) or shrink (from emigration). Do not treat natural increase as the whole story.
A country has CBR = 30 and CDR = 10 (per 1,000). Its RNI = (30 - 10) ÷ 10 = 2%. Its doubling time ≈ 70 ÷ 2 = 35 years — the population will double in about a generation.
CBR and CDR are births/deaths per 1,000/year. RNI = (CBR - CDR) ÷ 10 (%), ignoring migration. TFR is children per woman (2.1 = replacement). Doubling time ≈ 70 ÷ RNI%.
Write from a doubling time of 70 years
- The crude birth rate is 19 per 1000 and the crude death rate is 9 per 1000.
- Use the rule of 70. The lesson rates of 28 and 8 give a natural increase of 2 percent and about 35 years.
What is the doubling time?
Doubling time in years is about 70 divided by the percent rate.
How long until this population doubles?
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A 1 percent rate doubles in about 70 years. Other options — About 35 years (the lesson case); About 19 years (the birth rate).