Unit 1: The Living World: Ecosystems
AP Environmental Science: 52 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 1 builds the energy and matter framework for the whole course: how ecosystems move energy one-way and cycle nutrients, and how productivity is measured.
Energy flow
Energy enters as sunlight, is fixed by producers, and flows up food chains with only about 10% transferred per trophic level. The rest leaves as heat — energy is never recycled.
Trophic structure
Producers → primary consumers → secondary consumers → tertiary consumers. Food chains simplify; food webs show the real tangle of feeding links. Ecological pyramids depict energy, biomass, or numbers — energy pyramids are always upright.
Productivity
Gross primary productivity (GPP) is total photosynthesis. Net primary productivity (NPP) = GPP − plant respiration — the energy actually available to consumers. NPP varies with sunlight, water, temperature, and nutrients.
Nutrient cycles
Matter cycles through reservoirs: carbon (atmosphere ↔ biosphere ↔ ocean ↔ rocks), nitrogen (fixation, nitrification, assimilation, ammonification, denitrification), phosphorus (rock weathering, no atmospheric phase), and water (evaporation, transpiration, condensation, precipitation, runoff).
Quantitative skill-set
NPP = GPP − respiration; 10% rule for energy transfer; reading pyramid shapes and cycle diagrams.
Exam traps
Energy flows one-way; only matter cycles. Nitrogen fixation is done by bacteria, not plants directly. The phosphorus cycle has no significant atmospheric component. Decomposers are not a "level above" tertiary consumers — they recycle at every level.