AP Environmental Science Unit 3 Notes: Human Populations and Change Over Time

Age structure diagram (population pyramid)

A graph showing the number or percentage of people in different age groups of a population, typically split by sex (males left, females right), used to predict future population trends.

Cohort

An age-group category (e.g., 0–4, 5–9) used in age structure diagrams to compare population sizes across ages.

Population momentum

Continued population growth that occurs because a large proportion of the population is entering reproductive age, even if birth rates (fertility) later decline.

Rapid growth (expanding pyramid)

Age structure shape with a wide base and quick narrowing; indicates high birth rates and strong potential for future population increase.

Slow or stable growth (column-like/rectangular)

Age structure shape with similar-sized cohorts across many ages; suggests birth rates have fallen and roughly balance death rates, leading to stability or slow growth.

Declining/negative growth (narrow base)

Age structure shape with a narrow base compared to middle/older cohorts; indicates low birth rates and an aging population, often leading to long-run decline.

Bulge (in an age structure diagram)

An unusually large cohort that may reflect a baby boom, immigration wave, or a period of higher birth rates.

Dent (missing cohort)

An unusually small cohort that may indicate war, famine, disease outbreak, or emigration.

Dependents

Population groups typically aged 0–14 and 65+ who rely more on support from the working-age population.

Working-age population

The age group commonly defined as 15–64, often considered the primary labor force supporting dependents.

Dependency ratio

A comparison of the number of dependents (0–14 and 65+) to the working-age population (15–64); higher values can increase economic pressure on workers and budgets.

Total fertility rate (TFR)

The average number of children a woman is expected to have over her lifetime, assuming current age-specific birth rates remain constant.

Replacement-level fertility

The TFR needed for a population to replace itself from one generation to the next (assuming no net migration); often about 2.1 in many higher-income countries due to mortality and sex-ratio factors.

Crude birth rate (CBR)

The number of births per 1,000 people per year.

Crude death rate (CDR)

The number of deaths per 1,000 people per year.

Infant mortality rate

Deaths of infants under age 1 per 1,000 live births in a year.

Life expectancy

The average number of years a newborn is expected to live given current mortality conditions.

Net migration rate

The difference between immigration and emigration (often expressed per 1,000 people per year), affecting population size and age structure.

Population growth rate (percent)

An estimate of annual population change as a percentage; when CBR and CDR are per 1,000 per year, growth rate (%) ≈ (CBR − CDR) / 10 (with migration included if given).

Divide-by-10 conversion (per 1,000 to percent)

A unit conversion step: rates given “per 1,000” are converted to “per 100” (percent) by dividing by 10.

Rule of 70

A quick approximation for doubling time: Doubling time (years) ≈ 70 / growth rate (%).

Doubling time

The approximate time it takes a population to double in size if it grows at a steady percentage rate.

Human population dynamics

The study of how and why human populations change over time, driven by births, deaths, immigration, and emigration.

Demographic transition model

A model describing typical changes in birth and death rates as a country industrializes and develops, helping explain shifts in population growth and age structure.

Stage 2 (Transitional) of the demographic transition

Stage with high birth rates and rapidly falling death rates (due to sanitation, clean water, healthcare), producing very rapid population growth.