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Unit 3 · Populations Unit Hub Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Environmental Science Unit 3 Essentials

The must-know terms and big ideas for Unit 3: Populations. Every vocabulary word and concept you need to master.

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Big Idea 1
Population growth is never unlimited — resources eventually constrain every species
Exponential growth describes what happens under ideal, unlimited conditions, but no real population experiences that forever. As a population approaches its environment's carrying capacity, limiting factors — food, water, space, disease — slow growth, producing the S-shaped logistic curve. This single idea underlies nearly every population question on the exam, from bacteria in a petri dish to human populations on Earth.
Carrying Capacity Logistic Growth Limiting Factors
Big Idea 2
Reproductive strategy predicts survivorship, and survivorship predicts population dynamics
r-selected species (many offspring, little care) and K-selected species (few offspring, high care) aren't just different reproductive strategies — they map directly onto distinct survivorship curves (Type III vs. Type I) and different responses to environmental disturbance. r-selected populations can rebound quickly after a crash; K-selected populations recover slowly, making them more vulnerable to extinction from habitat loss or overharvesting.
r/K Selection Survivorship Curves Extinction Risk
Big Idea 3
Human population dynamics follow predictable patterns tied to economic development
The demographic transition model shows that as countries industrialize, death rates fall first (due to healthcare and sanitation improvements), causing a population boom, before birth rates eventually fall too (due to education, family planning, and urbanization) as the country reaches Stage 4. Age structure diagrams and total fertility rate let you read off where any country is in this transition — and predict its population trajectory for decades to come.
Demographic Transition Age Structure Total Fertility Rate
Generalist species
A species that can survive across a wide range of conditions and resources, such as raccoons or rats.
Species Traits
Specialist species
A species adapted to a narrow set of conditions or a specific resource, making it more vulnerable to environmental change.
Species Traits
r-selected species
Species that produce many offspring with little parental care and mature quickly; populations can grow extremely fast.
Life History
K-selected species
Species that produce few offspring with high parental investment and mature slowly; populations grow slowly and stabilize near carrying capacity.
Life History
Survivorship curve
A graph showing the pattern of mortality across a species' lifespan, classified as Type I, II, or III.
Survivorship
Type I survivorship curve
Low mortality early and mid-life, with most deaths late in life — typical of K-selected species like humans.
Survivorship
Type II survivorship curve
A relatively constant mortality rate across all ages.
Survivorship
Type III survivorship curve
Very high early mortality, with the few survivors then living relatively long — typical of r-selected species like insects.
Survivorship
Carrying capacity (K)
The maximum population size an environment can sustain indefinitely given its available resources.
Growth Models
Exponential growth
Growth at a constant rate under unlimited resources, producing a J-shaped curve.
Growth Models
Logistic growth
Growth that slows as a population approaches carrying capacity, producing an S-shaped curve.
Growth Models
Limiting factor
A resource or condition that restricts population growth once in short supply, such as food, water, or space.
Growth Models
Density-dependent limiting factor
A limiting factor whose effect intensifies as population density increases, such as disease or competition.
Growth Models
Density-independent limiting factor
A limiting factor that affects a population regardless of density, such as a natural disaster.
Growth Models
Age structure diagram
A population pyramid showing the age and sex distribution of a population, used to predict future growth or decline.
Human Populations
Total fertility rate (TFR)
The average number of children a woman would have during her reproductive lifetime, given current birth rates.
Human Populations
Replacement-level fertility
The TFR (about 2.1 in developed countries) needed to keep a population stable across generations.
Human Populations
Crude birth rate
The number of live births per 1,000 people in a population per year.
Human Populations
Crude death rate
The number of deaths per 1,000 people in a population per year.
Human Populations
Rate of natural increase
Crude birth rate minus crude death rate, expressed as a percentage, excluding migration.
Human Populations
Demographic transition model
A four-stage model describing how birth and death rates change as a country industrializes and develops economically.
Human Populations
Stage 2 of demographic transition
Death rates fall quickly due to improved healthcare while birth rates remain high, causing rapid population growth.
Human Populations
Stage 4 of demographic transition
Both birth rates and death rates are low; population growth stabilizes or declines, typical of highly developed countries.
Human Populations