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Unit 1 · The Living World: Ecosystems Unit Hub Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice
Unit 1 · 6–8% of Exam

The Living World: Ecosystems

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.

11 topics
~14–15 class periods
2 Big Ideas covered
College Board aligned
← Back to AP Environmental Science

Choose your study tool

Six ways to master Unit 1 — pick whichever fits how you like to study.

Flashcards
25 interactive flashcards covering every key term from Unit 1. Tap to flip, shuffle, and use keyboard arrows.
Open flashcards →
Cheat Sheet
A one-page visual summary of Unit 1 — every key topic, term, and theme on a single screen.
Open cheat sheet →
Essentials
The big ideas plus a searchable glossary of every vocabulary term you need to know for the exam.
Open essentials →
Visual Review
A slide-by-slide walkthrough of Unit 1 with diagrams of biomes, food webs, and biogeochemical cycles.
Open visual review →
MCQ Practice
35 multiple-choice questions in College Board exam style — with full explanations of every answer.
Start practice →
FRQ Practice
A free-response question with model answers showing exactly how each part earns its point on the exam.
Start FRQ →

Topics in Unit 1

Eleven topics from the College Board CED, in order.

Topic 1.1
Introduction to Ecosystems
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.
Topic 1.7
The Hydrologic (Water) Cycle
Evaporation, transpiration, condensation, precipitation, and runoff — how water moves and is stored across Earth's reservoirs.
Topic 1.8
Primary Productivity
Gross vs. net primary productivity (GPP and NPP) and how productivity is measured across ecosystems.
Topic 1.9
Trophic Levels
Producers, consumers, and decomposers, and how energy and biomass are organized into trophic levels.
Topic 1.10
Energy Flow and the 10% Rule
Why roughly only 10% of energy passes to the next trophic level, and how to apply the rule in calculations.
Topic 1.11
Food Chains and Food Webs
How energy moves through food chains and webs, and how changes at one level ripple through the ecosystem.

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 14–15 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 two Big Ideas:

Big Idea: ERT
Interactions Between Earth Systems — biomes and biogeochemical cycles
Big Idea: ENG
Energy Transfer — productivity, trophic levels, and one-way energy flow
Up next
Unit 2: The Living World — Biodiversity
Start Unit 2 →