What it covers: The foundation of environmental science — energy flow through ecosystems, the world's major biomes, and the carbon, nitrogen, and phosphorus cycles.
Exam weight: About 6–8% of the AP Environmental Science exam.
The big question: How does energy flow through an ecosystem, and how do biogeochemical cycles move matter through the living and nonliving world?
Big Ideas covered: Energy Transfer (ENG), Interactions Between Earth Systems (IST), Interactions Between Species & Environment (EIN), Sustainability (STB).
Key topics at a glance
Energy Flow & Trophic Levels
Energy flows one way through producers → consumers → decomposers. Only about 10% of energy transfers between trophic levels — the rest is lost as heat.
Terrestrial Biomes
Temperature and precipitation define biomes: tundra (cold, low biodiversity), taiga (coniferous forest), temperate forest/grassland, desert (low precipitation), and tropical rainforest (highest biodiversity, nutrient-poor soil).
Aquatic Biomes
Freshwater (lakes, rivers, wetlands) and marine (estuaries, coral reefs, open ocean) biomes. Estuaries and wetlands are the most productive due to nutrient input and sunlight access.
The Carbon Cycle
Photosynthesis removes CO2 from the atmosphere; respiration and combustion return it. Fossil fuel burning is the main human disruption, adding "extra" ancient carbon.
No atmospheric gas phase — phosphorus cycles through rock weathering, soil, and water only, making it the slowest of the three cycles. Runoff causes eutrophication.
Limiting Nutrients
Phosphorus is usually the limiting nutrient in freshwater systems; nitrogen usually limits marine systems. Excess of either triggers algal blooms.
Matter Cycles, Energy Flows
The single most important rule of Unit 1: energy moves one way and is lost as heat, while matter (carbon, nitrogen, phosphorus) is recycled indefinitely through ecosystems.
The key terms you must know
Trophic level — a position in a food chain (producer, primary consumer, secondary consumer, etc.).
Net primary productivity (NPP) — energy available to consumers after producers' own respiration is subtracted from gross production.
Biome — a large geographic region defined by its climate, vegetation, and animal life.
Estuary — where a river meets the ocean; among the most productive ecosystems on Earth.
Biogeochemical cycle — the movement of a chemical element through living organisms and the physical environment.
Nitrogen fixation — converting atmospheric N2 into a biologically usable form (ammonia).
Eutrophication — nutrient overload in water causing algal blooms and oxygen depletion.
Limiting nutrient — the nutrient in shortest supply that restricts further growth.
Decomposer — an organism that breaks down dead matter, releasing nutrients back into the ecosystem.
Carbon sink — a reservoir (like a forest or ocean) that absorbs more carbon than it releases.
Key themes to remember
Energy flows; matter cycles. Energy is lost as heat at every trophic level and must be continually resupplied by the sun. Matter is reused indefinitely.
Climate shapes biomes. Temperature and precipitation are the two variables that explain almost all biome distribution.
Humans accelerate natural cycles. Fossil fuels and synthetic fertilizer push carbon and nitrogen through their cycles far faster than natural processes can balance.
Nutrient pollution causes eutrophication. The same nutrients (N and P) that fuel plant growth on land cause algal blooms and dead zones in water.
Productivity isn't the same as nutrient richness. Tropical rainforests are highly productive but have nutrient-poor soil — nutrients are stored in the living biomass, not the ground.
Common exam traps
Don't confuse GPP and NPP. GPP is total energy captured; NPP is what's left after the producer's own respiration — and NPP is what's available to consumers.
The 10% rule is an average, not a law. Actual transfer efficiency varies, but exam questions expect you to apply ~10% per trophic level unless told otherwise.
Phosphorus has no atmospheric phase. Don't describe a "phosphorus gas" step the way you would for nitrogen or carbon.
Eutrophication kills aquatic life by depleting oxygen, not by direct toxicity. The mechanism is decomposers consuming oxygen as they break down dead algae.
Rainforests aren't nutrient-rich soil — they're nutrient-rich biomass. Heavy rain leaches nutrients from soil quickly; most nutrients are locked in living plants.
Identify the correct limiting nutrient for the system in the question. Freshwater → usually phosphorus. Marine → usually nitrogen.