APES Unit 1 is all about ecosystems — their structure, function, and interactions. This unit lays the foundation for the entire AP Environmental Science course, making it essential for both MCQs and FRQs.
In this RevisionDojo guide, we’ll break down all the major concepts you need to know, along with tips for how to study them effectively.
📌 Key Topics in APES Unit 1
- Ecosystem Structure
- Biotic components (producers, consumers, decomposers)
- Abiotic components (temperature, sunlight, water, nutrients)
- Energy Flow
- Food chains and food webs
- Trophic levels
- 10% Rule of Energy Transfer
- Biogeochemical Cycles
- Water cycle
- Carbon cycle
- Nitrogen cycle
- Phosphorus cycle
- Ecosystem Productivity
- Gross Primary Productivity (GPP)
- Net Primary Productivity (NPP)
- Factors affecting productivity
- Biodiversity
- Species diversity, genetic diversity, ecosystem diversity
- Importance of biodiversity in stability
- Ecological Tolerance & Niche
- Fundamental vs. realized niche
- Law of tolerance and limiting factors
🌱 Energy Flow in Ecosystems
- Energy enters ecosystems through photosynthesis in producers.
- Only ~10% of energy moves from one trophic level to the next; the rest is lost as heat.
- This limits the length of food chains — usually 4–5 trophic levels.
Example:
Sunlight → Grass (Producer) → Rabbit (Primary Consumer) → Fox (Secondary Consumer)
🔄 Biogeochemical Cycles You Must Know
- Water Cycle – Evaporation, condensation, precipitation, infiltration, runoff.
- Carbon Cycle – Photosynthesis, respiration, combustion, decomposition.
- Nitrogen Cycle – Nitrogen fixation, nitrification, assimilation, ammonification, denitrification.
- Phosphorus Cycle – Weathering of rocks, absorption by plants, transfer through food webs.
📊 Ecosystem Productivity
- GPP = Total energy captured by producers.
- NPP = GPP – Respiration (energy available to consumers).
- Tropical rainforests have the highest NPP, deserts have the lowest.
🧠 APES Exam Tips for Unit 1
- Know your cycles inside out — FRQs often involve diagrams of these.
- Practice 10% rule calculations — quick points on MCQs.
- Use diagrams to visualize food webs and cycles for better recall.
- Review real-world examples — deforestation, eutrophication, climate change impacts.
💡 FAQ – Unit 1: Ecosystems
Q: How many MCQs on the APES exam come from Unit 1?
A: Around 6–9% of MCQs are from Unit 1.
Q: What’s the most tested concept?
A: Energy flow (10% rule) and biogeochemical cycles.
Q: How should I study for Unit 1?
A: Use active recall with flashcards, practice FRQs, and draw cycle diagrams repeatedly.
Final Word from RevisionDojo:
Unit 1 is your foundation for APES. Master it early and you’ll find later units much easier to understand. Focus on energy flow, cycles, and biodiversity — these appear on the exam every year.
