AP Environmental Science The Living World: Ecosystems — Worked Answer Explanations

Unit 1 · 12 questions explained

Below is a complete answer key for our AP Environmental Science The Living World: Ecosystems practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed The Living World: Ecosystems practice test and come back here to review, or head back to the The Living World: Ecosystems unit overview.

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Question 1 · Easy
Which of the following is an example of a regulating ecosystem service?
- A
  Timber harvested from a forest
  Why not A: Timber is a provisioning service — a direct material product.
- B
  Flood control provided by a wetlandCorrect
- C
  Spiritual value of a mountain landscape
  Why not C: Spiritual or aesthetic value is a cultural service.
- D
  Soil formation from decomposing organic matter
  Why not D: Soil formation is a supporting service — foundational to all other services.
Explanation
The Millennium Ecosystem Assessment classifies ecosystem services into four categories: provisioning (food, water, timber), regulating (flood control, climate regulation, pollination), cultural (recreation, spiritual), and supporting (nutrient cycling, soil formation). Wetlands regulate water flow, reducing flood intensity.

Key takeaway
Regulating services include flood control, water purification, and climate regulation.
Question 2 · Easy
Which biome has the highest net primary productivity per unit area?
- A
  Open ocean
  Why not A: Open ocean has low NPP per unit area despite covering most of Earth's surface.
- B
  Tropical rainforestCorrect
- C
  Temperate grassland
  Why not C: Grasslands have moderate NPP, well below tropical forests.
- D
  Arctic tundra
  Why not D: Cold temperatures and short growing seasons limit tundra NPP severely.
Explanation
Tropical rainforests have the highest NPP per unit area (~2,000 g C/m²/yr) due to year-round warmth, high solar radiation, and abundant rainfall. Open oceans have very low per-area NPP but contribute substantially to global totals simply because of their vast area.

Key takeaway
Highest NPP per area: tropical rainforest. Wetlands and estuaries also rank very high.
Question 3 · Easy
In the carbon cycle, which human activity has contributed most to increased atmospheric CO₂ concentrations since the Industrial Revolution?
- A
  Increased respiration from a growing human population
  Why not A: Human respiration contributes trivially to CO₂ compared to fossil fuel combustion.
- B
  Combustion of fossil fuelsCorrect
- C
  Volcanic eruptions
  Why not C: Volcanoes emit CO₂ naturally, but annual volcanic emissions are far less than fossil fuel emissions.
- D
  Increased ocean evaporation releasing dissolved CO₂
  Why not D: Warmer oceans actually outgas some CO₂, but this is a feedback, not the primary driver of the increase.
Explanation
Burning fossil fuels (coal, oil, natural gas) releases carbon that was sequestered for millions of years back into the atmosphere as CO₂. Deforestation is the second major anthropogenic source. Together they have raised atmospheric CO₂ from ~280 ppm pre-industrial to over 420 ppm today.

Key takeaway
Fossil fuel combustion is the #1 anthropogenic source of atmospheric CO₂.
Question 4 · Medium
In a grassland ecosystem, grass produces 8,000 kcal/m²/yr. Grasshoppers consume the grass and assimilate 800 kcal/m²/yr. What is the ecological efficiency of energy transfer from grass to grasshoppers?
- A
  1%
  Why not A: 1% would be 80 kcal, not 800 kcal, transferred.
- B
  10%Correct
- C
  20%
  Why not C: 20% would be 1,600 kcal transferred; this overstates the efficiency.
- D
  100%
  Why not D: 100% efficiency would mean no energy is lost, which violates thermodynamic principles.
Explanation
Ecological efficiency = (energy at next trophic level) / (energy at current trophic level) × 100. Here: 800/8,000 × 100 = 10%. The 10% rule approximates typical efficiency between trophic levels due to metabolic heat loss, excretion, and unconsumed biomass.

Key takeaway
Ecological efficiency ≈ 10%; 90% of energy is lost as heat or waste at each trophic level.
Question 5 · Medium
A scientist measures the amount of carbon dioxide fixed by phytoplankton in a lake over one year and reports 500 g C/m²/yr. This figure represents the lake's:
- A
  Net secondary productivity
  Why not A: Secondary productivity refers to consumer (heterotroph) biomass accumulation, not photosynthesis.
- B
  Gross primary productivity (GPP)Correct
- C
  Net primary productivity (NPP)
  Why not C: NPP = GPP − plant respiration; total CO₂ fixed equals GPP before subtracting respiration.
- D
  Standing crop biomass
  Why not D: Standing crop is the total biomass present at a moment in time, not a rate of production.
Explanation
Gross primary productivity (GPP) is the total rate of photosynthetic carbon fixation by autotrophs, before any respiration losses. Net primary productivity (NPP) = GPP − autotrophic respiration (Ra), so NPP is less than GPP.

Key takeaway
GPP = total photosynthesis. NPP = GPP − plant respiration (energy available to consumers).
Question 6 · Medium
During the nitrogen cycle, which process converts nitrate (NO₃⁻) in waterlogged soils back into atmospheric nitrogen gas (N₂)?
- A
  Nitrification
  Why not A: Nitrification converts ammonium (NH₄⁺) to nitrite then nitrate; it moves nitrogen the opposite direction.
- B
  Nitrogen fixation
  Why not B: Nitrogen fixation converts N₂ into ammonia/ammonium — the reverse direction.
- C
  Ammonification
  Why not C: Ammonification converts organic nitrogen in dead matter to ammonium; it does not produce N₂.
- D
  DenitrificationCorrect
Explanation
Denitrification is performed by anaerobic bacteria in oxygen-poor (waterlogged) soils. They use nitrate as an electron acceptor and reduce it to N₂ (and some N₂O), returning nitrogen to the atmosphere. This is the step that returns fixed nitrogen to the atmospheric reservoir.

Key takeaway
Denitrification: NO₃⁻ → N₂ (anaerobic, completes the nitrogen cycle).
Question 7 · Medium
In a food web, a hawk eats rabbits, which eat grass. If a disease eliminated 50% of the rabbit population, which of the following is the most likely short-term outcome?
- A
  Hawk population increases; grass decreases.
  Why not A: With fewer rabbits, hawks lose a primary food source and would decline, not increase.
- B
  Hawk population decreases; grass increases.Correct
- C
  Hawk population is unaffected; grass remains the same.
  Why not C: Trophic cascades propagate effects up and down the food chain.
- D
  Hawk population decreases; grass decreases.
  Why not D: With fewer rabbits grazing, grass would be less consumed and would increase.
Explanation
This is a trophic cascade. Fewer rabbits → less food for hawks → hawk population declines. Fewer rabbits also means less grazing pressure on grass → grass biomass increases. This two-directional effect propagates through the food web.

Key takeaway
Trophic cascades: removing a middle trophic level reduces predators above and releases pressure on prey below.
Question 8 · Medium
The phosphorus cycle differs from the carbon and nitrogen cycles primarily because phosphorus:
- A
  Has no biological role in living organisms.
  Why not A: Phosphorus is essential — it is a component of ATP, DNA, and cell membranes.
- B
  Does not have a significant atmospheric reservoir.Correct
- C
  Is recycled entirely by microbial decomposers.
  Why not C: Decomposers play a role, but the primary reservoir is rock, not microbial activity.
- D
  Is primarily stored in the ocean as a dissolved gas.
  Why not D: Phosphorus does not form stable atmospheric gases; the ocean holds phosphate ions, not gas.
Explanation
Unlike carbon (CO₂ reservoir) and nitrogen (N₂ reservoir), phosphorus cycles mainly through rocks, soils, sediments, and organisms. It enters ecosystems primarily through weathering of rocks and is returned by decomposition. There is no phosphorus gas equivalent of CO₂ or N₂.

Key takeaway
The phosphorus cycle lacks an atmospheric reservoir — phosphorus moves through rock, soil, water, and organisms.
Question 9 · Medium
A forest has GPP of 2,000 g C/m²/yr and plant respiration of 800 g C/m²/yr. What is the NPP available to consumers?
- A
  2,800 g C/m²/yr
  Why not A: Adding respiration to GPP double-counts energy; respiration is subtracted.
- B
  800 g C/m²/yr
  Why not B: 800 g is the plant respiration, not NPP.
- C
  1,200 g C/m²/yrCorrect
- D
  2,000 g C/m²/yr
  Why not D: 2,000 g is GPP; NPP requires subtracting plant respiration.
Explanation
NPP = GPP − plant respiration = 2,000 − 800 = 1,200 g C/m²/yr. NPP represents the organic matter available for consumers (herbivores, detritivores) and is the ecologically meaningful measure of energy input to higher trophic levels.

Key takeaway
NPP = GPP − Ra. It represents net biomass accumulated and available to consumers.
Question 10 · Medium
A coral reef ecosystem provides habitat for hundreds of fish species and protects coastlines from wave erosion. Which pair of ecosystem service categories does this represent?
- A
  Provisioning and cultural
  Why not A: Provisioning refers to direct material goods (food, timber); cultural refers to non-material human benefits.
- B
  Supporting and regulatingCorrect
- C
  Cultural and provisioning
  Why not C: Neither habitat provision for biodiversity nor physical wave buffering fits these categories.
- D
  Regulating and provisioning
  Why not D: Wave protection is regulating, but habitat for species is a supporting service, not provisioning.
Explanation
Habitat provision for biodiversity is a supporting service (it underpins all other services). Coastal protection from wave erosion is a regulating service — the reef physically buffers energy. Supporting + regulating is the correct pair for these two functions.

Key takeaway
Supporting services: habitat, nutrient cycling. Regulating services: flood/wave control, climate regulation, pollination.
Question 11 · Hard
Deforestation of a watershed causes increased sediment runoff into a nearby river. Which biogeochemical cycle is MOST directly disrupted by the loss of plant root systems holding the soil?
- A
  Carbon cycle — increased CO₂ from decomposing roots
  Why not A: Decomposing roots do release CO₂, but the direct physical mechanism here is soil erosion, affecting the phosphorus and mineral cycles.
- B
  Nitrogen cycle — excess nitrogen gas lost to atmosphere
  Why not B: Denitrification can increase but it is not the primary direct effect of root loss causing sediment runoff.
- C
  Phosphorus cycle — phosphorus-laden sediment leaches into the riverCorrect
- D
  Water cycle — reduced transpiration only
  Why not D: Transpiration is reduced, but the question specifically asks about the biogeochemical cycle disrupted by sediment runoff.
Explanation
Plant roots anchor soil and absorb phosphate. When forests are cleared, runoff erodes topsoil laden with phosphate-rich particles into rivers and streams. This excess phosphorus can trigger eutrophication downstream. The phosphorus cycle has no atmospheric buffer — once lost via erosion, it is difficult to replace.

Key takeaway
Deforestation accelerates phosphorus loss via soil erosion; there is no atmospheric safety valve for phosphorus.
Question 12 · Hard
A food chain shows: phytoplankton → zooplankton → small fish → tuna. If phytoplankton store 100,000 kcal of energy, approximately how many kcal are available at the tuna level (assuming 10% efficiency at each step)?
- A
  1,000 kcal
  Why not A: 1,000 kcal represents two transfers (10% × 10%), not three.
- B
  100 kcalCorrect
- C
  10 kcal
  Why not C: 10 kcal represents four transfers; tuna is the fourth trophic level (three transfers from phytoplankton).
- D
  10,000 kcal
  Why not D: 10,000 kcal is only one transfer step (10% of 100,000).
Explanation
Tuna is at the 4th trophic level. Three energy transfers occur: phytoplankton→zooplankton (×0.1), zooplankton→small fish (×0.1), small fish→tuna (×0.1). Total: 100,000 × 0.1 × 0.1 × 0.1 = 100,000 × 0.001 = 100 kcal. This illustrates why large predatory fish (like tuna) are energetically expensive to produce.

Key takeaway
Energy available at trophic level n = starting energy × (0.10)^(n−1). Three transfers = (0.1)³ = 0.001.