The foundation of every other unit. How ecosystems are organized, the major terrestrial and aquatic biomes, and the biogeochemical cycles — carbon, nitrogen, and phosphorus — that move matter through the living world. Master this and the rest of APES gets a lot easier.
Energy flow, trophic levels, food chains and food webs, and the laws of thermodynamics as they apply to ecosystems.
Topic 1.2
Terrestrial Biomes
How latitude, elevation, and precipitation define biomes like tundra, taiga, temperate forest, grassland, desert, and tropical rainforest.
Topic 1.3
Aquatic Biomes
Freshwater and marine biomes, including the layers of lakes, wetlands, estuaries, and the open ocean.
Topic 1.4
The Carbon Cycle
Photosynthesis, respiration, combustion, and the reservoirs that store and exchange carbon.
Topic 1.5
The Nitrogen Cycle
Nitrogen fixation, nitrification, denitrification, and how human activity (fertilizer, fossil fuels) disrupts the cycle.
Topic 1.6
The Phosphorus Cycle
Why phosphorus cycles through rock and water (not the atmosphere), and how runoff causes eutrophication.
About Unit 1
Unit 1 is the conceptual foundation for the rest of APES. You'll learn how energy flows one-way through an ecosystem — captured by producers, lost as heat at every trophic level — while matter cycles endlessly through the carbon, nitrogen, and phosphorus cycles. You'll also see how climate variables like temperature and precipitation sort the planet into distinct terrestrial and aquatic biomes.
This unit is roughly 6–8% of the AP Environmental Science exam and takes about 7–9 class periods. It's one of the lighter units by exam weight, but nearly every later unit — pollution, land use, global change — assumes you understand how energy and nutrients move through ecosystems first.
The College Board ties Unit 1 to all four Big Ideas:
Big Idea: ENG
Energy Transfer — one-way energy flow through trophic levels
Big Idea: IST
Interactions Between Earth Systems — biogeochemical cycles
Big Idea: EIN
Interactions Between Species & Environment — biome distribution
Big Idea: STB
Sustainability — human disruption of nutrient cycles