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AP® Environmental Science
All nine College Board units, the seven science practices, and the four big ideas: built for the 80 MC and the 3 FRQs.
Start Unit 1 free. Unit 1: The Living World: Ecosystems is open to everyone, no account needed. Other topics are locked.
Unit 1: The Living World: Ecosystems
THE BIG PICTURE. Unit 1 (6–8%) opens APES with the foundational ecology toolkit: how energy and matter move through ecosystems. Three ideas dominate: biomes (the climate-defined regions that shape what lives where), energy flow (one-way, through trophic levels, governed by the 10% rule), and biogeochemical cycles (the cyclical movement of carbon, nitrogen, phosphorus, water, and other key elements).
ECOSYSTEMS, BIOMES, AND THE LIVING-NON-LIVING DIVIDE
An ecosystem is a community of living organisms (biotic) plus their non-living environment (abiotic), interacting as a system. Biotic: plants, animals, decomposers. Abiotic: temperature, water, sunlight, soil, minerals, air.
A biome is a large geographic region defined by characteristic climate (temperature, precipitation) and the dominant vegetation/animal communities adapted to it. Major terrestrial biomes:
| Biome | Climate | Key features |
|---|---|---|
| Tropical rainforest | Hot, very wet (>200 cm/yr) | Highest biodiversity; thin soils, tall canopies |
| Tropical seasonal forest | Hot, wet/dry seasons | Drop leaves in dry season |
| Savanna | Hot, seasonal rain | Grasslands with scattered trees |
| Desert | Very low rain (<25 cm/yr) | Hot or cold; xerophytes |
| Temperate grassland | Moderate rain | Deep fertile soils; prairie |
| Temperate seasonal forest | Moderate, four seasons | Deciduous trees; rich soils |
| Temperate rainforest | Mild, very wet coastal climate | Giant conifers, mosses, ferns (e.g., Pacific Northwest) |
| Boreal forest (taiga) | Cold winters, short summers | Coniferous trees; acidic soils |
| Tundra | Very cold; permafrost | No trees; mosses, lichens, shrubs |
| Shrubland (chaparral) | Mediterranean (mild wet winter, dry summer) | Fire-adapted shrubs |
Major aquatic biomes: freshwater (lakes, rivers, wetlands) and marine (ocean zones, estuaries, coral reefs).
Reading Climatographs Bars show monthly precipitation and the line shows mean temperature. Total the rain and note the temperature range, then match the pattern to a biome: Station A fits a tropical rainforest and Station B a hot desert (illustrative data).
WORKED EXAMPLE: IDENTIFY THE BIOME FROM A CLIMATOGRAPH
Two weather stations (figure above) report monthly averages. Identify the most likely biome for each and justify your answer with the data.
- Step 1, total the precipitation. Station A receives about 2,810 mm (281 cm) per year. Station B receives about 105 mm (10.5 cm).
- Step 2, read the temperature line. Station A stays between 26 and 27 °C all year, so it has no cold season. Station B swings from 13 °C in winter to 35 °C in summer.
- Step 3, match to a biome. A is hot and very wet year-round (far above 200 cm per year): a tropical rainforest. B gets well under 25 cm per year: a desert, and a hot one given its 35 °C summer.
- Step 4, connect to organisms. Expect tall broadleaf evergreen trees, thin nutrient-poor soils, and very high species richness at A. Expect xerophytes such as cacti, with waxy cuticles and water-storing tissue, at B.
Quote values from the graph with units ("about 2,800 mm per year") rather than writing "a lot of rain".
ENERGY FLOW AND TROPHIC LEVELS
Energy Pyramid and the 10% Rule Energy decreases at each higher trophic level because only about 10% transfers upward; the other 90% is lost as heat and metabolic activity. This is why the pyramid narrows toward the top and why apex predators are rare.
Energy enters ecosystems from the SUN, captured by producers (autotrophs) via photosynthesis:
Energy then flows through trophic levels:
- Primary producers (plants, algae, cyanobacteria).
- Primary consumers / herbivores (grasshoppers, cows, zooplankton).
- Secondary consumers / carnivores (snakes, small fish).
- Tertiary consumers (hawks, sharks).
- Apex predators (no natural predators).
Also: decomposers (bacteria, fungi) recycle dead organic matter, returning nutrients to soil/water.
The 10% rule: only ~10% of energy at one trophic level is incorporated into biomass at the next; the other ~90% is not passed on: most is used in respiration and lost as heat, and some is never eaten or is egested as undigested waste that goes to decomposers.
Implications:
- Energy pyramid: broader at bottom (producers, large biomass); narrower at top (predators, small biomass).
- Why apex predators are RARE (limited energy reaches the top).
- Why eating low on the food chain is energetically efficient (fewer trophic transfers).
The 10% Rule With Numbers Each trophic level stores only about a tenth of the energy of the level below it, so three transfers turn 10,000 kcal into 10 kcal. Most of the rest is used in respiration and lost as heat.
WORKED EXAMPLE: THE 10% RULE (FRQ QUESTION 3 STYLE)
A grassland's producers store 10,000 kcal/m²/yr as new biomass. (a) How much energy reaches the snakes, the tertiary consumers? (b) How much energy per year is available to the frogs in a 2,000 m² field?
- (a) Count the transfers. Producers to snakes is three transfers, and each passes on about 10%: .
- (b) Find the frogs' share, then scale up. Frogs are two transfers up: . Then , or kcal/yr.
- (c) Explain the loss. Most of the other ~90% at each step is used in cellular respiration and released as heat; some biomass is never eaten or passes through undigested.
GPP, NPP, and respiration:
- Gross Primary Productivity (GPP): total energy captured by producers via photosynthesis (g C/m²/yr).
- Net Primary Productivity (NPP) = GPP − R, where R is respiration losses. NPP is the energy available to consumers.
- Highest NPP biomes: tropical rainforests, estuaries, swamps.
- Lowest NPP: deserts, tundra, open ocean.
WORKED EXAMPLE: NPP FROM GPP AND RESPIRATION
Plants in a forest plot fix 2,500 g C/m²/yr by photosynthesis (GPP) and use 1,400 g C/m²/yr in respiration. Find NPP and the percent of GPP left for consumers.
- Step 1. .
- Step 2. of what the plants captured is stored as new biomass that herbivores and decomposers can use.
- Interpret. A drought that cuts photosynthesis lowers GPP; if respiration stays the same, NPP falls even faster, and less energy moves up the food web.
FOOD CHAINS AND FOOD WEBS
A Food Web A food web links many food chains into a network of feeding relationships. Energy flows one way from producers upward, while detritivores and decomposers return matter from every level to the soil and water.
Food chain: linear sequence (grass → grasshopper → frog → snake → hawk). Food web: interconnected network of food chains showing the flow of energy and matter. More realistic; shows multiple feeding relationships. Detritivores and decomposers return nutrients to the soil in both chains and webs.
Keystone species: disproportionate effect on ecosystem structure relative to its abundance. Removing it causes ecosystem cascade. Classic examples: sea otters (control sea urchins, protect kelp forests), wolves (Yellowstone, control elk, protect riparian vegetation).
CASE STUDY: WOLVES RETURN TO YELLOWSTONE (1995 TO 1997)
Gray wolves had been missing from Yellowstone National Park for about 70 years when managers released 41 wolves there between 1995 and 1997 (14 from Alberta, 17 from British Columbia, and 10 from northwest Montana). The northern elk herd, the wolves' main prey, declined in the years that followed. Some researchers reported that willows and aspen along streams grew taller as elk browsing eased, a widely cited example of a trophic cascade. Other scientists dispute how strong or widespread that cascade is: a 2010 USGS-led study found that aspen outside fenced plots were still not regenerating, and researchers continue to debate how much of the elk decline and plant recovery came from wolves rather than hunting, bears and cougars, and drought.
- Concept: a keystone predator exerting top-down control on a food web.
- What it shows: restoring a top predator can ripple down through herbivores to producers, but real ecosystems have several interacting causes.
- Exam link: on an FRQ, lay out the mechanism step by step (more wolves, fewer or warier elk, less browsing, taller streamside plants) and avoid claiming it is proven everywhere.
BIOGEOCHEMICAL CYCLES
Elements move cyclically between living organisms and non-living reservoirs (atmosphere, ocean, rock).
The Carbon Cycle Carbon moves between the atmosphere, biosphere, oceans, and fossil reservoirs. Photosynthesis pulls CO in; respiration, decomposition, and combustion release it. Burning fossil fuels adds carbon faster than natural sinks can absorb it.
Carbon cycle:
- Reservoirs: atmosphere (CO), ocean (dissolved), biosphere (living), fossil fuels (long-term).
- Fluxes: photosynthesis (atmosphere → biosphere), respiration (biosphere → atmosphere), combustion (fossil fuels → atmosphere), ocean uptake/release.
- Human disruption: fossil fuel burning, deforestation → net CO accumulation in atmosphere (Unit 9).
The Nitrogen Cycle Atmospheric N is unusable until fixation (by bacteria, lightning, or the Haber-Bosch process) converts it to ammonium. Nitrification then yields nitrate for plants; denitrification returns N to the air. Synthetic fertilizer now drives excess runoff and eutrophication.
Nitrogen cycle:
- Reservoir: atmosphere (78% N, but N is unusable by most organisms).
- Nitrogen fixation: bacteria (Rhizobium in legume roots), lightning, and the Haber-Bosch industrial process convert N to NH/NH.
- Nitrification: bacteria convert NH to NO to NO (usable by plants).
- Assimilation: plants take up NO, build proteins.
- Ammonification: decomposers return N to soil as NH.
- Denitrification: bacteria convert NO back to N, returning to atmosphere.
- Human disruption: synthetic fertilizers → excess N runoff → eutrophication (Unit 8).
Phosphorus cycle:
- Reservoir: ROCK (no significant atmospheric phase). Phosphorus is a limiting nutrient in many ecosystems.
- Cycle: weathering of rock → soil → plants → consumers → decomposition → returns to soil/sediment → eventually back to rock over geologic timescales.
- Human disruption: mining, fertilizer use, detergents → P pollution of waterways → eutrophication.
The Phosphorus Cycle Phosphorus has no gas phase: it moves from rock to soil, organisms, water, and sediment, and back to rock only over geologic time. Its slow release by weathering is why phosphorus often limits plant and algal growth.
Water (hydrologic) cycle:
- Evaporation → atmospheric water → condensation → precipitation → surface and groundwater → evaporation again.
- Includes transpiration (water vapor from plants), runoff, infiltration, percolation.
The Hydrologic Cycle Solar energy drives evaporation and transpiration; condensation forms clouds; gravity returns water as precipitation, runoff, infiltration, and groundwater flow. The ocean is by far the largest reservoir.
PRACTICE: NAME THE CYCLE AND THE STEP
| Process described | Cycle | Step and effect |
|---|---|---|
| Rhizobium bacteria in soybean root nodules convert N to NH | Nitrogen | Nitrogen fixation: makes atmospheric N usable |
| Bacteria in waterlogged soil convert NO to N gas | Nitrogen | Denitrification: returns N to the atmosphere |
| Rain slowly weathers phosphate rock | Phosphorus | Releases PO from the rock reservoir into soil |
| Water vapor exits through leaf stomata | Water | Transpiration |
| Rainwater soaks into the ground and down to an aquifer | Water | Infiltration and percolation |
| Marine organisms build CaCO shells that sink and become limestone | Carbon | Long-term storage in sedimentary rock |
| A forest is burned to clear land for pasture | Carbon | Combustion releases stored carbon as CO |
WORKED EXAMPLE: PREDICT THE EFFECT OF A CHANGE IN A CYCLE
A farmer plants soybeans (a legume) instead of corn and stops applying nitrogen fertilizer. Predict what happens to soil nitrogen, and explain one effect on a nearby stream.
- Identify the step. Soybeans host Rhizobium bacteria in root nodules, which carry out nitrogen fixation: N from the air becomes ammonia the plant can use.
- Predict. Plants can get enough nitrogen without synthetic fertilizer, and less surplus nitrate is left in the soil after harvest.
- Connect to water. Less nitrate leaches or runs off into the stream, so the risk of an algal bloom and falling dissolved oxygen (eutrophication) goes down.
- Contrast with phosphorus. No organism can pull phosphorus from the air, because the phosphorus cycle has no gas phase. It enters ecosystems only as rock weathers, which is why P is often the limiting nutrient in freshwater.
EXAM CONNECTIONS. Unit 1 is foundational: every later unit borrows energy flow and biogeochemical cycles. Common MC items: identify trophic level, compute 10%-rule energy passed up, identify a biome from a climate diagram. FRQ: explain how a disturbance propagates through a food web, or how N or P cycling is disrupted by human activity. Memorize: 10% rule; the four cycles (C, N, P, HO); difference between GPP and NPP.
Key Terms
Ecosystem
Community of living (biotic) organisms plus their non-living (abiotic) environment, interacting as a system.
Biome
Large geographic region defined by climate (temperature + precipitation) and characteristic dominant vegetation. Examples: tropical rainforest, desert, tundra.
Autotroph (Producer)
Organism that produces its own food via photosynthesis (most) or chemosynthesis (some bacteria). Base of all food chains.
Heterotroph (Consumer)
Organism that obtains energy by eating other organisms. Includes herbivores, carnivores, omnivores, decomposers.
10% Rule
Only ~10% of energy at one trophic level is transferred to the next; ~90% lost as heat. Limits food chain length; explains energy pyramid.
Gross Primary Productivity (GPP)
Total energy fixed by producers via photosynthesis (e.g., g C/m²/yr). Includes energy used by producers for respiration.
Net Primary Productivity (NPP)
NPP = GPP − R (respiration). The energy available to consumers. Highest in tropical rainforests, estuaries, wetlands.
Food Web
Network of interconnected food chains showing all feeding relationships in an ecosystem.
Keystone Species
Species with disproportionately large effect on ecosystem relative to its abundance. Removal causes cascading change. Examples: sea otters, wolves.
Carbon Cycle
Movement of C between atmosphere (CO), biosphere (organic compounds), oceans (dissolved C), and rocks/fossil fuels. Disrupted by fossil fuel burning and deforestation.
Nitrogen Fixation
Conversion of atmospheric N to NH/NH. Done by bacteria (Rhizobium in legumes), lightning, and industrial Haber-Bosch process.
Nitrification
Bacterial conversion of NH → NO → NO (usable by plants).
Denitrification
Bacterial conversion of NO back to N, returning nitrogen to the atmosphere.
Phosphorus Cycle
Rock weathering → soil → plants → consumers → decomposition → soil/sediment → rock (geologic timescale). NO significant atmospheric phase. Often the limiting nutrient.
Hydrologic (Water) Cycle
Evaporation, transpiration, condensation, precipitation, runoff, infiltration. Powered by solar energy.
Eutrophication
Excess nutrient runoff (especially N and P) → algal blooms → oxygen depletion when algae die and decompose → fish kills. Unit 8 detail.
Exam Tips
- 10% rule: each trophic transfer keeps ~10%. Use to compute energy at any level, and to explain why apex predators are rare.
- Memorize the major biomes by climate (temperature + precipitation) and dominant vegetation. Climate diagrams (Walter diagrams) appear in MC.
- Four biogeochemical cycles (C, N, P, HO) recur throughout the course. Know reservoirs, key fluxes, and human disruptions.
- Phosphorus has NO atmospheric phase: this is a frequent MC distractor.
- Eutrophication = excess N + P from agricultural runoff → algal blooms → oxygen depletion → fish kills. Connect to Unit 8.
- GPP vs NPP: NPP = GPP − R. NPP is the food available to consumers.
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