Unit 8: Ecology explores how organisms interact with each other and their environment. It makes up about 8–11% of the AP Biology Exam and often appears in data-driven MCQs and experimental FRQs.
In this RevisionDojo Unit 8 Review, we’ll cover:
- Ecosystem organization and energy flow
- Population ecology
- Community interactions
- Nutrient cycles
- Human impacts on the environment
- Common AP exam question types
- Study strategies to master the unit
Unit 8 Overview
The College Board divides Unit 8 into five key themes:
- Ecosystem organization and energy flow
- Population ecology and growth models
- Community ecology and species interactions
- Nutrient cycling
- Human impacts and conservation biology
1. Ecosystem Organization
Levels of organization:
- Organism → Population → Community → Ecosystem → Biome → Biosphere
Energy Flow:
- Producers (autotrophs): Convert solar energy into chemical energy via photosynthesis
- Consumers (heterotrophs): Primary, secondary, tertiary
- Decomposers: Break down dead matter, recycle nutrients
AP Tip: Be ready to interpret energy pyramids—only ~10% of energy transfers between trophic levels.
2. Population Ecology
Key Concepts:
- Population size (N)
- Density and distribution patterns: Clumped, uniform, random
- Growth models:
- Exponential (J-shaped) – unlimited resources
- Logistic (S-shaped) – limited resources, carrying capacity (K)
Population Regulation:
- Density-dependent factors: Competition, predation, disease
- Density-independent factors: Natural disasters, climate
3. Community Interactions
- Competition: Species compete for the same resources
- Predation: Predator-prey relationships shape populations
- Mutualism: Both species benefit (e.g., pollinators and plants)
- Commensalism: One benefits, other unaffected
- Parasitism: One benefits at expense of other
4. Nutrient Cycles
Water Cycle: Evaporation → Condensation → Precipitation → Runoff
Carbon Cycle: Photosynthesis, respiration, combustion
Nitrogen Cycle: Nitrogen fixation, nitrification, denitrification
Phosphorus Cycle: Rock weathering, absorption by plants
5. Human Impacts
- Climate change from greenhouse gases
- Habitat destruction and fragmentation
- Invasive species disrupting ecosystems
- Pollution (air, water, soil)
- Overexploitation of resources
Conservation Biology Strategies:
- Habitat restoration
- Captive breeding programs
- Protected areas and biodiversity hotspots
6. Connections to Other Units
- Unit 3: Energy flow connects to cellular energetics
- Unit 7: Natural selection pressures in ecosystems
- Unit 5 & 6: Genetic diversity affects population resilience
Common AP Biology Exam Questions for Unit 8
Multiple Choice Examples:
- Calculate carrying capacity from a logistic growth graph
- Predict effects of removing a top predator on ecosystem stability
FRQ Examples:
- Analyze the impact of an invasive species on nutrient cycling
- Design an experiment to test the effect of temperature on plant growth rates
Study Tips for Unit 8
- Know the Cycles: Draw and label each nutrient cycle.
- Practice Graph Analysis: Especially logistic vs. exponential growth curves.
- Learn Interaction Examples: Be able to match examples to interaction types.
- Review Human Impact Case Studies: Coral bleaching, deforestation, climate change.
Frequently Asked Questions
1. How much of the AP Bio exam is Unit 8?
About 8–11% of the test.
2. Do I need to memorize every species interaction example?
No—know definitions and at least one clear example for each type.
3. Are math calculations in ecology common?
Yes—be ready for growth rate and energy transfer percentage problems.
4. What’s the most challenging part of Unit 8?
Interpreting multi-variable ecosystem data in FRQs.
Call to Action
Unit 8 closes out AP Biology with big-picture thinking—how life interacts at every scale. Master it, and you’ll be ready for synthesis questions that pull from multiple units.
👉 Use RevisionDojo’s ecology practice sets and data interpretation FRQs to secure every last point on test day.
