Unit 3: Populations is all about how organisms grow, interact, and respond to environmental limits. On the AP Environmental Science exam, population dynamics appear in both MCQs and FRQs, often with graphs, calculations, or comparative analyses.
In this RevisionDojo review, we’ll break down the key population ecology concepts, math formulas, and AP tips.
📌 Key Topics in APES Unit 3
- Population size, density, and distribution
- Survivorship curves (Type I, II, III)
- Growth models (exponential vs. logistic)
- Carrying capacity (K)
- Limiting factors (density-dependent & independent)
- Human population growth trends
- Age structure diagrams
- Demographic transition model (DTM)
- Population doubling time
📊 Population Characteristics
- Size (N) – Total number of individuals.
- Density – Number of individuals per unit area.
- Distribution – Random, uniform, or clumped patterns.
📈 Growth Models
- Exponential Growth (J-Curve)
- Occurs when resources are unlimited.
- Equation: r=(births−deaths)populationr = \frac{(births - deaths)}{population}
- Logistic Growth (S-Curve)
- Growth slows as population approaches carrying capacity (K).
- More realistic for natural populations.
🐾 Survivorship Curves
- Type I – High survival early, drop in old age (humans).
- Type II – Constant death rate (birds).
- Type III – High death early, survivors live long (fish, plants).
⚖ Limiting Factors
- Density-dependent – Competition, predation, disease.
- Density-independent – Natural disasters, temperature, pollution.
🌍 Human Population Growth
- Current global population → ~8 billion.
- Growth rate slowing in many developed nations.
- Age structure diagrams predict future growth trends.
📑 Demographic Transition Model (DTM)
- Stage 1: High birth & death rates.
- Stage 2: Death rates fall; population grows rapidly.
- Stage 3: Birth rates decline.
- Stage 4: Low birth & death rates, stable population.
🧮 Doubling Time Formula
Doubling Time=70Growth Rate (%)\text{Doubling Time} = \frac{70}{\text{Growth Rate (\%)}}
🧠 APES Exam Tips for Unit 3
- Be ready to read and interpret age structure diagrams.
- Practice growth rate and doubling time calculations.
- Know examples of r-selected vs. K-selected species.
- Link human population trends to resource use and environmental impact.
💡 FAQ – Unit 3: Populations
Q: Which survivorship curve do r-selected species follow?
A: Type III — many offspring, low survival early.
Q: What’s the difference between density-dependent and independent factors?
A: Dependent factors increase impact as population density rises; independent factors affect populations regardless of density.
Q: How much of the APES exam is Unit 3?
A: Around 7–10% of MCQs and often appears in FRQs involving data interpretation.
Final Word from RevisionDojo:
Unit 3 is all about patterns — in numbers, in survival, and in human history. The best way to prepare is practicing with real APES FRQs and population calculation problems.
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