Inside This Unit: The Full Breakdown
Populations covers how populations grow, what factors regulate their size, and how human population growth differs from other species. This unit explores exponential and logistic growth, carrying capacity, and demographic transition.
Why it matters
Population ecology is heavily tested on the AP Environmental Science exam. You must interpret population growth curves, calculate growth rates, and understand the demographic transition model. Human population growth underlies nearly every environmental issue covered on the exam.
Key concepts
- Exponential growth (J-curve) occurs when resources are unlimited: dN/dt = rN. Logistic growth (S-curve) slows as population approaches carrying capacity (K).
- Density-dependent factors (competition, predation, disease) regulate populations differently at different sizes. Density-independent factors (disasters, weather) affect populations regardless of size.
- K-selected species (large body, few offspring, long lifespan) and r-selected species (small body, many offspring, short lifespan) represent different reproductive strategies.
- The demographic transition model describes how nations shift from high birth/death rates to low birth/death rates as they industrialize.
Population Growth Models
Population growth depends on births, deaths, immigration, and emigration. When resources are unlimited, populations grow exponentially: each generation is larger than the last by a constant proportion, producing a J-shaped curve. The growth rate is dN/dt = rN, where r is the per-capita rate of increase and N is population size. In nature, resources are limited, so populations experience logistic growth: growth slows as the population approaches the environment's carrying capacity (K), producing an S-shaped curve. The logistic equation is dN/dt = rN(K−N)/K. Growth rate is fastest at N = K/2. Populations may overshoot K temporarily if there is a time lag in the effects of density-dependent factors, followed by a crash or oscillation around K.
Population Regulation
Population size is regulated by both density-dependent and density-independent factors. Density-dependent factors become more intense as population density increases: competition for food and space, predation, disease transmission, and parasitism all worsen in crowded populations. These factors create negative feedback loops that keep populations near carrying capacity. Density-independent factors — natural disasters, extreme weather, human disturbances — affect populations regardless of their size and can cause sudden declines. Reproductive strategies exist on a spectrum. R-selected species (insects, bacteria, annual plants) produce many offspring with little parental care and thrive in unpredictable environments. K-selected species (elephants, whales, humans) produce few offspring with extensive parental care and dominate in stable environments near carrying capacity.
Human Population and Demographic Transition
Human population has grown exponentially since the Industrial Revolution, reaching 8 billion in 2022. The demographic transition model describes four stages that countries pass through as they develop. Stage 1: high birth and death rates (pre-industrial, slow growth). Stage 2: death rates fall due to improved sanitation and medicine while birth rates remain high (rapid growth). Stage 3: birth rates decline as urbanization and education increase (growth slows). Stage 4: low birth and death rates (slow or zero growth). Age-structure diagrams (population pyramids) show the distribution of a population by age and sex, revealing whether growth will continue, stabilize, or decline. Countries with wide bases (many young people) will continue growing even if birth rates drop, due to population momentum.
AP exam tip
On the AP exam, be prepared to interpret age-structure diagrams and predict future population trends. A pyramid shape (wide base) means rapid growth; a column shape means stable; an inverted shape (wider top) means declining population. Always explain WHY using the demographic transition model.
Connections to other units
- Unit 1 (Ecosystems): Carrying capacity depends on ecosystem productivity and resource availability.
- Unit 5 (Land and Water Use): Human population growth drives demand for food, water, and land, intensifying environmental impacts.
- Unit 9 (Global Change): Total human population size directly affects global carbon emissions and resource consumption.
