Review AP Environmental Science Unit 1 to build your foundation in how ecosystems function, from species interactions and biome distribution to biogeochemical cycles and energy flow. These concepts appear throughout the course and connect directly to pollution, land use, and global change units.
Use this page to review all 11 topics in Unit 1, check your understanding of key cycles and energy rules, and identify gaps before the exam.
Unit 1 asks you to think about Earth as a system where living organisms, physical processes, and chemical cycles are all connected. You will explain how resource availability shapes species interactions, identify biomes by their climate and organisms, trace carbon, nitrogen, phosphorus, and water through their cycles, and calculate how energy decreases as it moves up a food chain.
Topics 1.1-1.3 establish how resource availability drives predator-prey relationships, symbiosis, and competition, and how climate shapes the global distribution of terrestrial biomes (taiga through tropical rainforest) and aquatic biomes (freshwater and marine).
Topics 1.4-1.7 trace carbon, nitrogen, phosphorus, and water through sources, sinks, and reservoirs. Key distinctions include short-term vs. long-term carbon storage, the role of bacteria in the nitrogen cycle, the absence of an atmospheric phase in the phosphorus cycle, and the sun as the driver of the hydrologic cycle.
Topics 1.8-1.11 connect primary productivity (GPP and NPP), trophic levels, the 10% rule, and food web structure. Energy decreases at each trophic level due to the second law of thermodynamics, while matter is conserved and recycled through biogeochemical cycles.
Every concept in Unit 1 reflects the same principle: matter cycles and energy flows through interconnected living and nonliving components. Disrupting one part, such as burning fossil fuels to release stored carbon or adding excess nitrogen through fertilizers, ripples through the entire system. This systems thinking is the lens you need for every unit that follows.
Explains how resource availability shapes predator-prey relationships, three types of symbiosis (mutualism, commensalism, parasitism), intraspecific and interspecific competition, and resource partitioning as a strategy for coexistence.
Covers the nine major terrestrial biomes from tundra to tropical rainforest, their climate signatures, characteristic organisms, and why biome boundaries shift with global climate change.
Distinguishes freshwater biomes (streams, rivers, ponds, lakes, wetlands) from marine biomes (oceans, coral reefs, marshes, estuaries) and explains how salinity, depth, turbidity, nutrients, and temperature control resource distribution.
Traces carbon through sources and sinks, contrasts short-term reservoirs (living organisms, atmosphere) with long-term reservoirs (fossil fuels), and explains how fossil fuel combustion rapidly transfers stored carbon to the atmosphere as CO2.
Explains the five steps of the nitrogen cycle (fixation, nitrification, assimilation, ammonification, denitrification), the role of bacteria at each step, and why the atmosphere is the major nitrogen reservoir.
Describes phosphorus movement through rock, soil, water, and organisms with no atmospheric phase; explains why phosphorus limits freshwater productivity and how agricultural runoff causes eutrophication.
Traces water through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and groundwater storage; identifies the ocean as the primary surface reservoir and the sun as the energy driver.
Defines gross primary productivity (GPP) and net primary productivity (NPP), explains the formula NPP = GPP minus respiration, introduces units (kcal/m2/yr), and describes how light attenuation limits aquatic photosynthesis.
Organizes producers, primary consumers, secondary consumers, tertiary consumers, and decomposers into trophic levels and explains how energy flows upward while matter cycles through biogeochemical cycles.
Applies the 10% rule to calculate energy available at each trophic level, explains energy loss through the second law of thermodynamics, and uses energy pyramids to visualize why food chains rarely exceed four or five links.
Distinguishes linear food chains from complex food webs, explains how removing or adding a species triggers trophic cascades, and identifies positive and negative feedback loops in food web dynamics.
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Review Food Chains and Food Webs with attention to how the concept appears in AP-style source and evidence questions.
Review Terrestrial Biomes with attention to how the concept appears in AP-style source and evidence questions.
Review The Carbon Cycle with attention to how the concept appears in AP-style source and evidence questions.
Review Trophic Levels with attention to how the concept appears in AP-style source and evidence questions.
Resource availability determines how species interact. When resources are limited, species compete; when one species relies on another, symbiotic or predator-prey relationships form. Understanding these interactions explains population dynamics and ecosystem stability.
| Interaction | Species A outcome | Species B outcome | Example |
|---|---|---|---|
| Mutualism | Benefits (+) | Benefits (+) | Legume roots and Rhizobium bacteria |
| Commensalism | Benefits (+) | Unaffected (0) | Barnacles on a whale |
| Parasitism | Benefits (+) | Harmed (-) | Tapeworm in a host |
| Competition | Harmed (-) | Harmed (-) | Two plant species competing for soil nitrogen |
| Predation | Benefits (+) | Harmed (-) | Wolf eating a rabbit |
A biome is defined by its climate, especially temperature and precipitation, which determines the characteristic plants and animals found there. Biome boundaries are not fixed; they shift as climate changes. The global distribution of resources like water and timber also varies by latitude, altitude, and soil type.
| Biome | Climate signature | Key vegetation |
|---|---|---|
| Tundra | Very cold, low precipitation | Mosses, lichens, sedges |
| Taiga | Cold winters, moderate precipitation | Coniferous trees (spruce, fir) |
| Temperate seasonal forest | Distinct seasons, moderate precipitation | Deciduous broadleaf trees |
| Tropical rainforest | Warm year-round, very high precipitation | Broadleaf evergreen trees, lianas |
| Desert | Low precipitation, extreme temperature swings | Cacti, xerophytes |
Aquatic biomes are divided into freshwater and marine systems. The distribution of organisms and resources in aquatic biomes depends on salinity, depth, turbidity, nutrient availability, and temperature. Algae are the primary photosynthetic organisms in most aquatic systems.
| System | Salinity | Key productivity factor | Example biome |
|---|---|---|---|
| Freshwater | Very low | Nutrient input, light | Lake, river, wetland |
| Estuarine | Variable (brackish) | Nutrient mixing, tidal flow | Salt marsh, estuary |
| Marine (coastal) | High | Upwelling, nutrient availability | Coral reef, continental shelf |
| Open ocean | High | Nutrient scarcity limits productivity | Pelagic zone |
The carbon cycle moves carbon between the atmosphere, living organisms, soil, oceans, and geological reservoirs. The key distinction for the exam is between short-term reservoirs (living organisms, atmosphere) and long-term reservoirs (fossil fuels, deep ocean sediments). Human combustion of fossil fuels rapidly transfers long-stored carbon into the atmosphere as CO2.
The atmosphere is the major nitrogen reservoir, holding nitrogen as N2 gas that most organisms cannot use directly. Bacteria are essential at every step of the nitrogen cycle, converting nitrogen into usable forms and returning it to the atmosphere. Most nitrogen reservoirs hold compounds for relatively short periods.
Phosphorus cycles through rock, soil, water, and living organisms but has no atmospheric phase. Rock weathering is the primary natural source of phosphate. Because phosphorus is often scarce in soils and water, it frequently limits plant and algae growth. Excess phosphorus from agricultural runoff causes eutrophication in aquatic systems.
| Cycle | Atmospheric reservoir? | Major long-term reservoir | Key limiting factor role |
|---|---|---|---|
| Carbon | Yes (CO2) | Fossil fuels, ocean sediments | Greenhouse gas; climate driver |
| Nitrogen | Yes (N2) | Atmosphere | Limits productivity in many terrestrial systems |
| Phosphorus | No | Rock and sediments | Limits productivity in most freshwater systems |
| Water | Yes (water vapor) | Oceans | Limits productivity in arid terrestrial biomes |
The hydrologic cycle is powered by solar energy and moves water between the atmosphere, land surface, and subsurface reservoirs. The oceans are the primary surface reservoir. Water moves through evaporation, transpiration, condensation, precipitation, surface runoff, infiltration, and groundwater flow.
Primary productivity measures how fast producers convert solar energy into organic matter. Gross primary productivity (GPP) is the total rate of photosynthesis; net primary productivity (NPP) is what remains after producers use energy for their own respiration. NPP is the energy available to the rest of the ecosystem.
Energy enters ecosystems at the producer level and flows upward through consumers, but only about 10% transfers from one trophic level to the next. The remaining 90% is lost as heat through metabolic processes, which is explained by the second law of thermodynamics. This limits food chain length and determines how much biomass each level can support.
A food chain shows a single linear path of energy transfer; a food web shows the full network of overlapping food chains in an ecosystem. Because species are interconnected, adding or removing one species triggers feedback effects throughout the web. Positive feedback amplifies change; negative feedback dampens it.
Try stimulus-based AP practice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| Predator-Prey Relationship | A dynamic interaction where the predator hunts and consumes the prey; predator and prey population sizes cycle together over time. |
| symbiosis | A close, long-term interaction between two species; includes mutualism (both benefit), commensalism (one benefits, one unaffected), and parasitism (one benefits, one harmed). |
| Resource Partitioning | The division of limited resources among species by using them differently in space, time, or method, reducing competition and allowing coexistence. |
| Biome | A large geographic region defined by its climate, characteristic plant communities, and animal populations; examples include tundra, tropical rainforest, and savanna. |
| Biogeochemical cycles | Natural processes that cycle elements such as carbon, nitrogen, phosphorus, and water through living organisms and their nonliving environment. |
| nitrogen fixation | The conversion of atmospheric N2 into ammonia by bacteria, making nitrogen available for plant uptake and incorporation into biological molecules. |
| Denitrification | The bacterial conversion of nitrate (NO3-) back to nitrogen gas (N2) in low-oxygen environments, returning nitrogen to the atmosphere. |
| Phosphorus | An element that cycles through rock, soil, water, and organisms with no atmospheric phase; often the limiting nutrient in freshwater systems and essential for DNA, RNA, and ATP. |
| net primary productivity | The rate of energy storage by producers after subtracting their own respiration; calculated as GPP minus respiration, measured in kcal/m2/yr. |
| Trophic efficiency | The percentage of energy transferred from one trophic level to the next; approximately 10%, meaning 90% is lost as heat through respiration. |
| Food Web | A model of interlocking food chains showing how energy and matter flow among producers, consumers, and decomposers in an ecosystem. |
| Feedback Loops | Processes in which a change in an ecosystem triggers a response that either amplifies the change (positive feedback) or dampens it (negative feedback), affecting food web stability. |
| Photic Zone | The upper layer of a water body where sunlight is sufficient for photosynthesis; depth is limited by turbidity and water clarity. |
| evapotranspiration | The combined water loss from soil and open water surfaces through evaporation and from plants through transpiration; a key process in the hydrologic cycle. |
| Keystone species | A species with a disproportionately large effect on ecosystem structure relative to its abundance; its removal can trigger a trophic cascade. |
GPP is the total rate of photosynthesis; NPP is what remains after producers use energy for their own respiration. NPP is always less than GPP. On the exam, questions about energy available to consumers require NPP, not GPP.
The 10% rule means energy decreases going up trophic levels. To find energy at a higher level, multiply by 0.1. To find the energy needed at a lower level to support a higher level, divide by 0.1. Students often multiply when they should divide.
Phosphorus has no atmospheric phase. It moves only between rock, soil, water, and organisms. Confusing phosphorus with nitrogen or carbon on a cycle question is a common error.
Nitrogen fixation converts N2 to ammonia; nitrification converts ammonia to nitrate; denitrification converts nitrate back to N2. Students often reverse the direction of denitrification or forget that each step requires specific bacteria.
Biome distributions are dynamic and have shifted in the past. Climate change is projected to shift biome boundaries again. Describing biomes as permanently fixed ignores a key essential knowledge point and a common exam application.
The AP exam frequently presents a diagram of a biogeochemical cycle and asks you to identify a specific process, label a reservoir, or explain how human activity alters the cycle. Practice tracing carbon, nitrogen, and phosphorus through their steps and naming the organisms or processes responsible at each stage.
Quantitative questions in Unit 1 most often involve calculating energy at a given trophic level using the 10% rule or solving for GPP or NPP given the other values. Show your work clearly, include correct units (kcal/m2/yr), and be prepared to explain why energy decreases using the laws of thermodynamics.
A common task asks you to read a food web, identify trophic levels, and predict the effect of removing or adding a species. Your response should name specific organisms affected, describe the direction of population change at each trophic level, and identify the type of feedback loop involved.
Open the individual guides for Unit 1 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 1 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAPES Unit 1 covers 11 topics built around how ecosystems function: Introduction to Ecosystems, Terrestrial Biomes, Aquatic Biomes, the Carbon Cycle, the Nitrogen Cycle, the Phosphorus Cycle, the Hydrologic Cycle, Primary Productivity, Trophic Levels, Energy Flow and the 10% Rule, and Food Chains and Food Webs. Together they show how energy and matter move through living systems. See APES Unit 1 for full topic breakdowns.
APES Unit 1 makes up 6-8% of the AP exam score. That weight covers everything from the carbon cycle and nitrogen cycle to terrestrial and aquatic biomes, energy flow, and food webs. It's a smaller unit by percentage, but the biogeochemical cycles and ecosystem concepts it introduces show up as context in later units, so a solid foundation here pays off across the whole exam.
The APES Unit 1 progress check includes both MCQ and FRQ parts drawn from all 11 unit topics. MCQ questions test recognition of biome characteristics, carbon cycle and nitrogen cycle pathways, trophic levels, and the 10% rule for energy flow. The FRQ portion typically asks you to explain a biogeochemical cycle or analyze a food web diagram. Practicing with those same topic areas on APES Unit 1 is the most direct way to prepare for the progress check.
APES Unit 1 FRQs most often focus on the carbon cycle, nitrogen cycle, and energy flow through food chains and food webs. A typical question gives you a scenario, then asks you to trace how matter or energy moves, explain a disruption, or calculate energy transfer using the 10% rule. To practice, write out full cycle diagrams from memory, then narrate each step in complete sentences the way a real FRQ demands. You can find matched practice prompts at APES Unit 1.
The best place to find APES Unit 1 practice questions, including multiple-choice and practice test sets, is APES Unit 1. The page organizes MCQ and FRQ practice by topic, so you can target specific areas like the carbon cycle, food chains and food webs, or biome identification. Working through topic-by-topic MCQs before taking a full unit practice test helps you spot gaps before they cost you points on the real exam.
Start APES Unit 1 by mapping out the carbon cycle from scratch, since it anchors the whole unit and connects to climate topics later in the course. Then work through the nitrogen cycle, phosphorus cycle, and hydrologic cycle the same way. For ecosystems and biomes, focus on the key abiotic factors that define each one. Once the cycles click, tackle trophic levels and energy flow together, since the 10% rule shows up in calculations on both the progress check and the exam. Use APES Unit 1 to check your understanding topic by topic as you go.