The Living World: Ecosystems
Unit 1 of AP Environmental Science, worth 6–8% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Ecosystem structure, biomes, carbon, nitrogen, phosphorus and water cycles, primary productivity, trophic levels, energy flow and food webs.
How this unit is tested
What you have to know
14 practice questions
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An ecosystem has GPP of 6,200 kcal/m²/yr and plant respiration of 2,700 kcal/m²/yr. What is the NPP?
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Answer. 3,500 kcal/m²/yr
NPP = GPP − R = 6,200 − 2,700 = 3,500 kcal/m²/yr. This is the energy stored as new biomass and available to consumers and decomposers. -
Which biome is characterized by low annual precipitation (often under 25 cm), extreme daily temperature fluctuations, and vegetation adapted to conserve water such as cacti and shrubs?
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Answer. Desert
Deserts are defined primarily by very low precipitation regardless of whether they are hot or cold. Plants show adaptations like CAM photosynthesis and thick cuticles to minimize water loss. -
Producers in a lake generate 10,000 kcal/m²/yr of NPP. Using the 10% rule, how much energy is available to tertiary consumers (three trophic transfers above producers)?
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Answer. 10 kcal/m²/yr
10,000 × 0.10 = 1,000 kcal (primary consumers); 1,000 × 0.10 = 100 kcal (secondary consumers); 100 × 0.10 = 10 kcal (tertiary consumers). Each transfer loses about 90% of the energy as heat and unused matter. -
Distinguish a food chain from a food web, and explain why food webs more accurately represent real ecosystems.
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Answer. A food chain is a single linear pathway of energy transfer from producer to consumer; a food web is an interconnected network of many overlapping food chains.
Real ecosystems rarely have organisms that eat or are eaten by only one species — most consumers have multiple food sources and predators, so a web better captures these overlapping, interdependent relationships than a simple chain. -
Which process directly converts atmospheric N2 gas into a form usable by plants (NH3/NH4+)?
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Answer. Nitrogen fixation
Nitrogen-fixing bacteria, such as Rhizobium in root nodules or free-living soil and cyanobacteria, convert inert N2 gas into biologically available ammonia, the first step that lets nitrogen enter the food web. -
Explain why phosphorus is often the limiting nutrient in freshwater aquatic ecosystems, and describe how the phosphorus cycle differs from the carbon and nitrogen cycles.
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Answer. Phosphorus has no significant atmospheric gas phase and cycles mainly through rock weathering, soil, and water on slow geologic timescales, making it naturally scarce and frequently the nutrient that limits primary production in freshwater systems.
By contrast, carbon and nitrogen have large atmospheric reservoirs (CO2 and N2) that allow much faster cycling and replenishment, so they are less often the limiting factor in aquatic productivity. -
Fertilizer runoff introduces excess phosphorus and nitrogen into a lake. Which sequence correctly describes the resulting process?
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Answer. Algal bloom, algae die and decompose, bacterial decomposition consumes oxygen, hypoxia and fish kill
This is eutrophication: nutrient enrichment triggers rapid algal growth; when the algae die, decomposer bacteria consume dissolved oxygen breaking them down, producing hypoxic 'dead zones' that kill fish and other aerobic organisms. -
Define transpiration and explain its role in the water cycle.
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Answer. Transpiration is the release of water vapor from plant leaves, mainly through stomata, into the atmosphere.
Plants absorb water through their roots, but most of it is lost as vapor rather than used for growth. Transpiration combines with evaporation from soil and water bodies to form evapotranspiration, a major flux moving water from land back to the atmosphere. -
Identify two major human activities that shift carbon from long-term sinks into the atmosphere, and explain the mechanism for one.
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Answer. Burning fossil fuels and deforestation. Burning fossil fuels releases carbon that has been stored in geologic reservoirs for millions of years, converting it to CO2 through combustion and rapidly moving it from a slow, long-term sink into the fast-cycling atmospheric pool.
Deforestation similarly releases stored carbon (through burning or decomposition of cleared vegetation) and reduces future carbon uptake by removing photosynthesizing trees. -
Using the second law of thermodynamics, explain why energy pyramids are always narrower at higher trophic levels while biogeochemical cycles are not similarly 'used up.'
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Answer. Energy transformations are never 100% efficient, so most energy is lost as heat at each trophic transfer, causing usable energy to shrink up the pyramid; matter, unlike energy, is conserved and can be recycled repeatedly through biogeochemical cycles rather than dissipating as heat.
This is why ecosystems require a continuous input of solar energy but can reuse the same finite pool of carbon, nitrogen, phosphorus, and water molecules indefinitely. -
A biome has average annual precipitation of 200–400 cm and an average temperature around 25–30°C with minimal seasonal variation. Which biome does this describe, and what is one characteristic feature?
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Answer. Tropical rainforest; it has extremely high biodiversity and rapid nutrient cycling, even though its soils are nutrient-poor because most nutrients are stored in living biomass rather than the soil.
Consistently warm temperatures and abundant year-round rainfall support continuous plant growth, giving tropical rainforests the highest NPP of any terrestrial biome. -
Rank these terrestrial biomes from highest to lowest average net primary productivity: tundra, temperate grassland, tropical rainforest, desert.
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Answer. Tropical rainforest, then temperate grassland, then tundra, then desert
Productivity is driven by temperature, precipitation, and growing-season length. Tropical rainforests have optimal warmth and moisture year-round; deserts have the least productivity because extreme water scarcity limits photosynthesis more severely than the cold but moderately moist conditions of the tundra. -
A food web includes producers, primary consumers, secondary consumers, and decomposers. Explain the ecological role of decomposers in nutrient cycling.
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Answer. Decomposers such as bacteria and fungi break down dead organic matter and waste, releasing nutrients like nitrogen, phosphorus, and carbon back into the soil, water, or atmosphere in inorganic forms that producers can absorb.
Without decomposers, nutrients would remain locked inside dead organisms and waste, halting the biogeochemical cycles that supply producers with the raw materials for new growth. -
Explain why food chains rarely extend beyond four or five trophic levels.
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Answer. Because only about 10% of energy transfers between successive trophic levels, the amount of usable energy remaining after four or five transfers becomes too small to support additional consumer populations.
For example, starting with 10,000 kcal of producer energy, only about 1 kcal remains after four transfers (10,000 → 1,000 → 100 → 10 → 1), which is far too little energy to sustain a viable population at a fifth trophic level.
What people get wrong
- Confusing GPP and NPP — students forget to subtract respiration and treat GPP as the energy available to consumers. Always compute NPP = GPP − R before doing any consumer-level energy calculations.
- Treating the 10% rule as an exact law rather than an average — actual transfer efficiency varies by ecosystem and species. State it as 'approximately 10%' or 'on average' rather than an exact universal constant.
- Mixing up nitrogen fixation, nitrification, and denitrification — students use these terms interchangeably. Keep them straight: fixation converts N2 to NH3/NH4+; nitrification converts NH4+ to NO2−/NO3−; denitrification converts NO3− back to N2 gas.
- Assuming the phosphorus cycle has a large atmospheric gas phase like carbon and nitrogen — it does not. Remember phosphorus moves mainly through rock weathering, soil, and water, which is exactly why it's so often the limiting nutrient in freshwater systems.
- Equating high standing biomass with high productivity — a forest can have huge biomass but slow turnover, while phytoplankton have low biomass but very high productivity (rapid reproduction/turnover). Distinguish 'amount present' (biomass) from 'rate of production' (productivity).
- Forgetting decomposers when describing trophic structure or food webs — decomposers/detritivores are essential recyclers at every trophic level, not a separate 'extra' level tacked on at the end.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.