AP Environmental Science Unit 1: Ecosystems — Complete Guide
APES Unit 1 focuses on ecosystems, their structure, energy flow, and interactions. This unit lays the foundation for the entire AP Environmental Science course and appears consistently in both multiple-choice questions (MCQs) and free-response questions (FRQs).
If you master Unit 1 early, later units—especially biodiversity, population dynamics, and land use—become much easier to understand.
This guide breaks down every major concept you need, plus how they are tested on the exam.
Key Topics in APES Unit 1
Ecosystem Structure
- Biotic components: producers, consumers, decomposers
- Abiotic components: sunlight, temperature, water, nutrients
Ecosystems function through interactions between living and nonliving components.
Energy Flow
- Food chains and food webs
- Trophic levels
- 10% Rule of Energy Transfer
Only about 10% of energy is passed to the next trophic level; the rest is lost as heat through metabolism. This limits food chains to about 4–5 trophic levels.
Example:
Sun → Grass (producer) → Rabbit (primary consumer) → Fox (secondary consumer)
Biogeochemical Cycles
You must understand processes, reservoirs, and human impacts for each cycle.
- Water cycle: evaporation, condensation, precipitation, runoff, infiltration
