How populations grow, decline, and stabilize. Generalist vs. specialist species, r- and K-selected life history strategies, survivorship curves, carrying capacity, age structure diagrams, and the demographic transition that explains how human population growth is changing worldwide.
Generalists thrive across many conditions; specialists rely on narrow conditions or resources and are more vulnerable to environmental change.
Topic 3.2
K-Selected & r-Selected Species
Two reproductive strategies: K-selected species invest in few offspring with high parental care; r-selected species produce many offspring with little care.
Topic 3.3
Survivorship Curves
Type I, II, and III curves describe the pattern of mortality across a species' lifespan.
Topic 3.4
Carrying Capacity
The maximum population size an environment can sustain indefinitely, given its available resources.
Topic 3.5
Population Growth & Resource Availability
Exponential vs. logistic growth models, and how limiting resources slow growth as populations approach carrying capacity.
Topic 3.6
Age Structure Diagrams
Population pyramids that reveal a population's growth trajectory based on the distribution of ages and sexes.
Topic 3.7
Total Fertility Rate
The average number of children a woman would have in her lifetime, and how it relates to replacement-level fertility.
Topic 3.8
Human Population Dynamics
Birth rates, death rates, and the factors — economic development, healthcare, education — that drive human population change.
Topic 3.9
Demographic Transition
The four-stage model describing how birth and death rates change as a country industrializes and develops.
About Unit 3
Unit 3 shifts focus from communities to populations — how they grow, what limits them, and how to predict their future size. You'll learn the math and models behind exponential and logistic growth, how to read survivorship curves and age structure diagrams, and how the demographic transition model explains why some countries' populations are still booming while others are shrinking.
This unit is roughly 10–15% of the AP Environmental Science exam and takes about 12–15 class periods — one of the most heavily weighted units, and one where you should expect quantitative, calculation-based questions in addition to conceptual ones.
The College Board ties Unit 3 to two Big Ideas:
Big Idea: ERT
Interactions Between Earth Systems — life-history strategies, carrying capacity, and population growth
Big Idea: EIN
Interactions Between Different Species & the Environment — human population dynamics and the demographic transition