Whether a human population grows or shrinks comes down to a few measurable things: birth rate, death rate (including infant mortality), and net migration. Social factors change those rates—access to family planning, good nutrition, education (especially for girls), and later ages at marriage all lower birth rates. Environmental limits matter too: Earth's carrying capacity and resource limits (Malthusian ideas) set long-term bounds. Populations also respond to density-independent events (storms, droughts) and density-dependent factors (clean water, disease spread, food availability). For quick math on growth, use the Rule of 70: doubling time ≈ 70 / (percent growth rate). On the AP exam you may be asked to explain causes of growth/decline or apply the Rule of 70 (Topic 3.8/EIN-1.C). Review this topic on Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Birth and death rates directly determine whether a human population grows, shrinks, or stays stable. If births (including high fertility and early marriage) exceed deaths (low mortality), population rises; if deaths exceed births, it declines. Infant mortality and overall death rates matter a lot—high infant mortality often leads to higher birth rates culturally. Access to family planning, better nutrition, education (especially for women), and postponement of marriage lower birth rates and slow growth (EK EIN-1.C.1). Density-independent events (storms, droughts) and density-dependent limits (food, clean water, disease) also change death rates and thus growth (EK EIN-1.C.3). Remember the Rule of 70: doubling time ≈ 70 ÷ growth rate (%)—useful on the exam for quick calculations (EK EIN-1.C.4). For more AP-aligned review, check the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Malthusian theory (from Thomas Malthus) argues that human populations grow exponentially while food production grows arithmetically, so eventually population outstrips food supply and limits (famine, disease, war) reduce population. In APES terms you should know it because EK EIN-1.C.2 lists Malthusian factors as basic limits to human population growth and it connects to Earth's carrying capacity and density-dependent limits (food, clean water, disease). On the exam, questions about population growth, carrying capacity, and limiting factors draw on this idea (Unit 3 content). Quick tip: Malthus shows why we study birth/death rates, family planning, nutrition, and education (EK EIN-1.C.1)—those affect whether the predicted collapse happens. For more review, check the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Density-independent factors affect population size regardless of how many individuals are present. Examples: major storms (hurricanes), wildfires, heat waves, or droughts—they can kill lots of people or disrupt resources no matter if the population is small or large (CED EK EIN-1.C.3). Density-dependent factors change their effect as population density changes. Examples: food availability, access to clean water and air, disease transmission, and territory size—when a population is dense, competition for food/water rises and disease spreads faster, increasing death rates (CED EK EIN-1.C.3; keywords: food availability, clean water, disease transmission). For humans, policies that change birth rates (family planning, education, postponing marriage) interact with these factors to drive growth or decline (EK EIN-1.C.1). On the AP exam, expect questions asking you to classify factors as density-dependent or -independent and to explain effects on population growth (Unit 3). For a quick review, see the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and grab practice questions at (https://library.fiveable.me/practice/ap-environmental-science).

The rule of 70 is a quick way to estimate how long it takes a population to double given a constant percent growth rate. Formula: doubling time (years) ≈ 70 ÷ (annual percent growth rate). Example: if a population grows 2% per year, doubling time ≈ 70 ÷ 2 = 35 years. If growth = 1.4%, doubling time ≈ 70 ÷ 1.4 ≈ 50 years. Use the percent (not decimal)—so divide by 2, not 0.02. This is the same AP idea in EK EIN-1.C.4 about population doubling time; you'll see similar math on the exam where Practice 6 (mathematical routines) is tested. For more Topic 3.8 review see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and extra practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Access to education—especially for girls and women—lowers population growth rates in several direct ways. Education increases knowledge of and access to family planning, encourages postponement of marriage and childbearing, and raises career and life goals so people choose to have fewer children. Educated parents also tend to have fewer infant deaths (better health care use and nutrition), so they don’t “replace” lost children, which reduces birth rates over time. These links are exactly what the CED lists under EK EIN-1.C.1 (birth rates, family planning, postponement of marriage, education access). For AP exam prep, be ready to explain these cause–effect relationships in free-response questions on human population dynamics (Unit 3); the Topic 3.8 study guide has a concise review (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1). For more practice applying these ideas to questions, see the Unit 3 overview (https://library.fiveable.me/ap-environmental-science/unit-3) and AP practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Infant mortality rate (IMR) strongly affects population dynamics because it changes how many births are needed to maintain or grow a population. High IMR often leads to higher birth rates (parents have more children to ensure some survive), which fuels rapid population growth seen in early demographic-transition stages. Lower IMR—usually from better nutrition, healthcare, clean water, and access to family planning and education—reduces the need for many births, so birth rates fall and population growth slows or even declines. IMR also affects age structure: high IMR skews populations younger and lowers life expectancy; low IMR shifts toward older age cohorts, changing dependency ratios and resource needs. For AP exam stuff, this connects directly to EK EIN-1.C.1 (birth rates, infant mortality, family planning) and demographic transition questions on Unit 3. Review this topic on the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and practice related problems at (https://library.fiveable.me/practice/ap-environmental-science).

Locally limiting factors are things that affect population size in a specific place and often depend on population density—e.g., access to clean water, food availability, disease transmission, territory size, or local storms and fires. Density-independent factors (major storms, droughts, heat waves) also act locally but affect populations regardless of size. Global limiting factors are broader: Earth's carrying capacity (total resources and space available worldwide) and limits described by Malthusian theory (resource limits causing famine, higher death rates). For APES, remember EK EIN-1.C.3 for density-dependent vs independent effects and EK EIN-1.C.2 for global limits like carrying capacity. On the exam you may need to contrast local (resource access, infant mortality, family planning—EK EIN-1.C.1) versus global constraints and use the correct terms. For a quick review, check the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) or the Unit 3 overview (https://library.fiveable.me/ap-environmental-science/unit-3). For extra practice problems, see (https://library.fiveable.me/practice/ap-environmental-science).

Natural disasters like hurricanes are classically density-independent factors—they affect populations regardless of how many individuals live in an area (CED EK EIN-1.C.3 lists “major storms” as density-independent). Density-independent factors change birth or death rates without depending on population size. By contrast, density-dependent factors (food availability, disease transmission, clean water access) get stronger as population density increases. Important nuance: hurricanes can cause secondary, density-dependent effects. For example, after a storm, crowded shelters can increase disease transmission (density-dependent), or damaged infrastructure can make clean water scarce—which interacts with population density to influence mortality. For AP exam framing, name the factor type (density-independent), give a density-dependent contrast, and note those indirect effects. For a quick review, see the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

The Rule of 70 is a quick way to estimate doubling time from a percent growth rate (CED EK EIN-1.C.4). Steps: 1. Get the annual percent growth rate (r%). Example: 2% per year. 2. Convert r% to a whole number (don’t change units)—you’ll use the percent directly in the formula. 3. Divide 70 by the growth rate: Doubling time ≈ 70 ÷ r. For 2%: 70 ÷ 2 = 35 years. 4. Interpret: at 2% annual growth, the population doubles in ~35 years. Notes: this is an approximation that works best for small, constant growth rates (exponential growth). On the AP exam you might show the setup and calculation (Practice 6: mathematical routines). For more review and practice problems on human population dynamics, see the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1) and Unit 3 page (https://library.fiveable.me/ap-environmental-science/unit-3).

Postponing marriage usually slows population growth because it delays the start of childbearing and reduces the total number of children a person can have. Later marriage often goes along with higher education, career goals, and better access to family planning—factors in the CED that lower birth rates (EK EIN-1.C.1). Fewer reproductive years means fewer births per woman, which lowers the birth rate and thus the population growth rate; over time this reduces population momentum. This is why policies or social changes that raise the average age at first marriage tend to move a country through the demographic transition toward lower growth. For AP review, connect this to EK EIN-1.C.1 (birth rates, family planning, education, postponement of marriage) and see the Topic 3.8 study guide on Fiveable for practice and examples (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1). For extra practice, check Fiveable’s APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Carrying capacity is the maximum number of people Earth can support long-term given available resources and technology. It’s not a fixed number—it depends on per-person resource use (food, water, energy), technology, waste management, and social choices (education, family planning). Scientists estimate it with models that combine biophysical limits (land, freshwater, fisheries, photosynthetic productivity), consumption rates, and assumptions about lifestyle and technology. Because those assumptions vary, estimates range widely: low values assume high-efficiency, low-consumption lifestyles; high values assume current high consumption per person. Malthusian ideas focus on resource limits and population pressure, while modern estimates add technology, trade, and density-dependent factors (disease, clean water access). For AP review, focus on how birth/death rates, resource access, and limiting factors affect whether population moves toward or beyond carrying capacity (see the Topic 3.8 study guide: https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1 and unit overview: https://library.fiveable.me/ap-environmental-science/unit-3). For extra practice, use Fiveable’s problems (https://library.fiveable.me/practice/ap-environmental-science).

Access to family planning lowers birth rates by giving people control over if and when they have children. Contraceptives and reproductive health services reduce unintended pregnancies, so fewer births happen. Family planning also lets people delay first births and space children, which lowers total fertility over a lifetime—this ties directly to EK EIN-1.C.1 (birth rates, postponement of marriage, and access to education all affect population growth). When women can plan pregnancies, they’re more likely to finish school or work, which further reduces birth rates. Also, reliable family planning often accompanies better maternal/infant healthcare, lowering infant mortality; paradoxically, lower infant mortality usually leads families to choose fewer children. For AP prep, you should be able to explain these cause–effect links on the exam (CED EIN-1.C). Review Topic 3.8 on Fiveable (study guide: https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1; unit: https://library.fiveable.me/ap-environmental-science/unit-3) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Density-dependent factors change their effect as population density changes. For disease transmission, higher human density means more close contact, shared water/air, and faster spread—so birth/death rates and overall death rate can shift depending on how crowded people are (this ties to EK EIN-1.C.1 and EK EIN-1.C.3). Dense populations also run into limits from food availability, clean water, and territory, which feed back on growth toward carrying capacity (Malthusian ideas). Density-independent factors, like droughts or major storms, affect populations regardless of how many individuals are present: a severe drought reduces water and crops across an area whether 10 people or 10,000 live there. On the AP exam you should be able to contrast examples and explain why disease is density-dependent while drought is density-independent (see the Topic 3.8 study guide for quick review: https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1). For more practice, check Unit 3 and practice questions (https://library.fiveable.me/ap-environmental-science/unit-3, https://library.fiveable.me/practice/ap-environmental-science).

Examples: Japan, South Korea, Italy, and (recently) China all have shrinking populations. Reasons tie directly to CED factors: very low birth rates (fewer births per woman) driven by postponement of marriage, high education and career focus (especially for women), high cost of housing/childcare, and easy access to family planning (EK EIN-1.C.1). Russia and some Eastern European countries show declines too because of higher death rates, lower life expectancy for some groups, and emigration out of the country. Density-dependent factors (limited resources, healthcare access) and density-independent shocks (pandemics, economic crises) can also accelerate declines (EK EIN-1.C.3). These patterns matter for the AP exam when you explain growth vs. decline using birth/death rates, infant mortality, and social factors. For a focused review, see the Topic 3.8 study guide (https://library.fiveable.me/ap-environmental-science/unit-3/human-population-dynamics/study-guide/g1a8MMUTre14Qa5OfsU1), the Unit 3 overview (https://library.fiveable.me/ap-environmental-science/unit-3), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).