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AP Environmental Science · Unit 1 of 9
The Living World: Ecosystems
6–8% of the exam5 lessons · 68 min31 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 1. This unit is worth 6–8% of the exam, so budget your time against that rather than against how long the unit takes to teach.
BiomesEnergy flowBiogeochemical cyclesProductivity
Lessons in this unit
- Ecosystems & Biomes12 min · 3 objectivesDistinguish an ecosystem from a biome and describe how the two nest together · Explain how temperature and precipitation determine the global distribution of biomes · Identify major terrestrial and aquatic biomes from their climate and characteristic organisms
- Energy Flow & Trophic Levels13 min · 3 objectivesTrace energy from the sun through producers, consumers, and decomposers · Apply the 10% rule to calculate energy available at each trophic level · Explain why food chains rarely exceed four or five trophic levels
- Biogeochemical Cycles14 min · 3 objectivesTrace carbon, nitrogen, and phosphorus atoms through their major reservoirs · Explain why the phosphorus cycle has no significant atmospheric phase · Connect the water cycle’s processes to the movement of nutrients
- Primary Productivity12 min · 3 objectivesDefine gross and net primary productivity and relate them through respiration · Calculate NPP from GPP and respiration · Compare the productivity of major ecosystems and identify limiting factors
- Reading Energy and Matter Diagrams Quantitatively17 min · 3 objectivesApply the 10% rule across trophic levels and compute available energy · Distinguish gross from net primary productivity and use the relationship · Explain why matter cycles while energy flows one way
Formulas in Unit 1
What separates biomes
terrestrial biome ≈ f(temperature, precipitation)
Aquatic biomes instead key on salinity, depth, and flow. Same idea: the abiotic setting selects the community.
The 10% rule
energy at next level ≈ 0.10 × energy at current level
A rule of thumb, not a law — real efficiencies range from ~5% to ~20%. AP problems assume exactly 10% unless told otherwise.
Key difference among the cycles
C and N: large atmospheric reservoir · P: no atmospheric phase
Carbon (CO₂) and nitrogen (N₂) both cycle through the air; phosphorus moves through rock, water, and soil only — which makes it move slowly.
Net primary productivity
NPP = GPP − R
R is the energy producers spend on their own respiration. NPP is what remains as biomass; GPP is always larger than NPP.
Productivity and trophic transfer
NPP = GPP − R energy at next level ≈ 0.10 × energy at current level
NPP is what consumers can eat. The 10% figure is an average; real efficiencies run from about 1% to 20%.
Every term in Unit 1
All 31 terms we publish for The Living World: Ecosystems, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Ecosystem
- All organisms in an area plus the abiotic factors they interact with. Energy flows through it while matter cycles within it.
- Producer and primary productivity
- Autotrophs convert solar or chemical energy into biomass. Gross primary productivity is total energy fixed; net is what remains after their own respiration.
- Trophic levels and the 10% rule
- About 10% of energy passes to the next level, the rest lost as heat and used in respiration. This limits food chains to four or five levels.
- Food chain vs food web
- A chain is one linear path; a web shows the many overlapping paths that actually exist, which is why webs are more stable to a species loss.
- Carbon cycle reservoirs
- Atmosphere, oceans, soil, biomass and fossil deposits. Combustion moves carbon from a slow reservoir to the atmosphere faster than sinks remove it.
- Nitrogen fixation
- Conversion of atmospheric N₂ to ammonia by Rhizobium bacteria, lightning or the industrial Haber-Bosch process. Nitrogen is abundant but inert without it.
- Nitrification and denitrification
- Nitrification converts ammonia to nitrite then nitrate; denitrification returns nitrate to N₂ gas, completing the cycle.
- Phosphorus cycle
- Has no atmospheric phase, so it moves slowly through rock weathering, soil, organisms and sediment. Usually the limiting nutrient in fresh water.
- Hydrologic cycle
- Evaporation, transpiration, condensation, precipitation, infiltration and runoff. Paving increases runoff and reduces infiltration and groundwater recharge.
- Symbiosis types
- Mutualism benefits both, commensalism benefits one without affecting the other, parasitism benefits one at the other's cost.
- Predation, competition and resource partitioning
- Competition is reduced when species divide a resource by time, space or type, allowing coexistence in the same habitat.
- Terrestrial biomes
- Determined chiefly by temperature and precipitation. Tundra is cold and dry, taiga cold and moist, desert hot and dry, rainforest hot and wet.
- Aquatic biomes
- Freshwater, estuary, coral reef, open ocean. Estuaries and reefs are the most productive because of nutrient input and shallow sunlit water.
- Ecosystem services
- Provisioning (food, timber), regulating (flood control, pollination), supporting (nutrient cycling) and cultural. Costly or impossible to replace artificially.
- Net primary productivity units
- Measured as energy or biomass per area per time, e.g. kcal/m²/yr. Getting the units right is worth a point on APES free-response.
- Calculating net primary productivity
- NPP = GPP − respiration. Watch the units — APES answers are expected in energy or biomass per unit area per unit time.
- Energy transfer calculation
- Multiply by 0.1 for each trophic level climbed. 10,000 kcal at producers leaves 10 kcal at the tertiary consumer.
- Why food chains are short
- Roughly 90% of energy is lost at each transfer, so there is not enough left to support a fifth or sixth level.
- Biomass pyramid inversions
- In open ocean, rapidly reproducing phytoplankton support a larger standing mass of zooplankton, so the biomass pyramid can appear inverted while the energy pyramid never is.
- Where the lost 90% goes
- Most is respired as heat by the organisms at that level; the rest is never eaten, or is eaten but not digested and leaves as waste. It is not destroyed — it leaves the food chain as low-grade heat, which is why energy flows through rather than cycling.
- Why the 10% rule is only a rule of thumb
- Real ecological efficiencies range from about 2% to 20%. Endotherms transfer less because they burn energy holding body temperature; aquatic ectotherms transfer more. Use 10% unless the question gives you numbers.
- Detritivores vs decomposers
- Detritivores (earthworms, millipedes, vultures) physically consume dead material; decomposers (fungi, bacteria) break it down chemically and release inorganic nutrients back to the soil. Both return matter to the cycle.
- Fast carbon pool vs slow carbon pool
- The fast pool — atmosphere, surface ocean, living biomass, soil — exchanges carbon on years to decades. The slow pool — limestone, fossil fuels, deep ocean — takes millions of years. Combustion moves slow-pool carbon into the fast pool at a rate the fast pool cannot absorb.
- Ammonification
- Decomposers convert organic nitrogen in dead tissue and waste into ammonium. It is the step that returns nitrogen from bodies to soil, before nitrification converts it to the nitrate most plants take up.
- How humans doubled the nitrogen cycle
- Haber-Bosch fertilizer synthesis plus combustion and legume cultivation now fix roughly as much nitrogen as all natural processes combined. The excess runs off to cause eutrophication and volatilizes as N₂O, a potent greenhouse gas.
- Residence time of a reservoir
- The average time a molecule stays in a pool: reservoir size divided by flux in or out. Water spends about nine days in the atmosphere and thousands of years in the deep ocean, which is why a pulse of CO₂ absorbed by the deep ocean stays absorbed.
- Sulfur cycle
- Moves through rock, ocean, and the atmosphere as SO₂ from volcanoes and, far more now, from burning coal. Atmospheric sulfate both causes acid deposition and reflects sunlight, so cutting sulfur emissions cleans the air and unmasks a little warming.
- Competitive exclusion principle
- Two species cannot occupy exactly the same niche indefinitely — one outcompetes the other. Coexistence requires resource partitioning, which is why closely related species in the same habitat usually differ in feeding height, timing, or prey size.
- Fundamental vs realized niche
- The fundamental niche is every condition a species could tolerate; the realized niche is the smaller slice it actually occupies once competitors, predators, and disease are present. Removing a competitor expands the realized niche toward the fundamental one.
- Lake zones
- Littoral (shallow, rooted plants), limnetic (open sunlit water, plankton), profundal (below light penetration), benthic (bottom sediment, decomposers). Productivity concentrates in the littoral and limnetic zones because that is where the light is.
- Reading a climatogram
- Temperature on one axis, precipitation on the other, twelve months across. The overall precipitation total and how evenly it falls identify the biome; a summer-dry, winter-wet pattern with mild temperatures is chaparral, not desert.
What examiners penalize here
- AP questions frequently give you a climate description or a climatograph (a bar-and-line graph of monthly rainfall and temperature) and ask for the biome. Read the precipitation first — it eliminates the most options fastest — then use temperature to decide between the remaining candidates.
- On free-response questions, showing the chain of 10% multiplications earns the calculation points even if the final number is off. Always write the units (kcal·m⁻²·yr⁻¹) — AP awards a point for correct units, and dropping them is an easy way to lose it.
- Expect a question that asks you to name the *process* at a specific arrow in a cycle diagram. Memorize the direction of each: photosynthesis pulls CO₂ down, respiration/combustion push it up; fixation pulls N₂ down, denitrification pushes it up.
- Watch the wording on GPP/NPP problems. "Total energy captured" or "total photosynthesis" means GPP; "energy stored as biomass" or "available to consumers" means NPP. If a question gives you NPP and R and asks for GPP, rearrange to GPP = NPP + R.
- Start every trophic calculation from **NPP, not GPP**. Using GPP inflates the answer by the amount of plant respiration and is the most common error on this question type. If the prompt gives you GPP and respiration, the subtraction is the first thing the rubric looks for.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 1?
Unit 1, The Living World: Ecosystems, is worth 6–8% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it a middling share, roughly what an even split across units would give.
What topics are covered in Env. Science Unit 1?
The Living World: Ecosystems covers Biomes, Energy flow, Biogeochemical cycles and Productivity. We publish 31 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 1?
Read the 5 lessons below first — about 70 minutes — then drill the 31 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.