Master The Living World: Ecosystems with AP Environmental Science review games.
This unit covers food webs, energy flow and biogeochemical cycles — essential concepts for AP Environmental Science. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers food webs, energy flow and biogeochemical cycles — essential concepts for AP Environmental Science. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Food Webs
Students must understand the structure of food webs, including producers, consumers (primary, secondary, tertiary), and decomposers, and how energy and matter flow through them. The exam tests ability to predict effects of removing or adding a species (trophic cascades) and distinguish food chains from food webs. Know the difference between a keystone species and a dominant species.
Key Points
- Producers (autotrophs) capture solar energy via photosynthesis; all consumer energy ultimately derives from them
- Trophic levels: producer → primary consumer → secondary consumer → tertiary consumer; each level loses ~90% of energy as heat
- Removing a keystone predator causes population explosions at the next trophic level down (trophic cascade)
- Decomposers (bacteria, fungi) break down dead organic matter and return nutrients to the soil, completing nutrient cycles
In Yellowstone, wolves were reintroduced in 1995. Predict what happened to elk populations, streamside vegetation, and stream bank erosion.
Wolves (tertiary consumer) preyed on elk (primary consumer), reducing elk populations and changing their grazing behavior. With less elk pressure, willows and aspens recovered along streamsides. Deeper root systems stabilized stream banks, reducing erosion — a classic trophic cascade demonstrating top-down regulation of ecosystems.
2 Energy Flow
Students must be able to calculate energy transfer between trophic levels using the 10% rule and explain why ecosystems support fewer organisms at higher trophic levels. The exam frequently asks students to use or interpret ecological pyramids (energy, biomass, numbers). Understand why energy is lost (heat from cellular respiration) and why matter cycles but energy flows in one direction.
Key Points
- Only ~10% of energy is transferred from one trophic level to the next; ~90% is lost as heat via respiration
- Gross Primary Productivity (GPP) = total energy fixed by photosynthesis; Net Primary Productivity (NPP) = GPP − plant respiration
- Ecological pyramids of energy are always upright; biomass pyramids can be inverted (e.g., aquatic systems with high phytoplankton turnover)
- Energy flow is unidirectional: sun → producers → consumers → decomposers; it cannot be recycled within the system
A grassland ecosystem fixes 10,000 kcal/m²/yr at the producer level. How much energy is available to a secondary consumer (e.g., a hawk eating mice)?
Apply the 10% rule at each transfer: producers have 10,000 kcal → primary consumers (mice) receive 1,000 kcal (10% of 10,000) → secondary consumers (hawks) receive 100 kcal (10% of 1,000). The hawk has access to only 100 kcal/m²/yr, or 1% of the original producer energy. This is why meat-based diets require more land than plant-based diets.
3 Biogeochemical Cycles
Students must know the carbon, nitrogen, phosphorus, and water cycles in detail — specifically the major reservoirs, key processes that move matter between reservoirs, and how human activities disrupt each cycle. The exam tests ability to identify which process occurs at each step (e.g., nitrification vs. denitrification) and predict consequences of disruption (e.g., eutrophication from excess nitrogen or phosphorus).
Key Points
- Carbon cycle: photosynthesis removes CO₂; respiration, decomposition, and combustion release it; oceans are the largest carbon sink
- Nitrogen cycle key steps: nitrogen fixation (N₂ → NH₃ by bacteria), nitrification (NH₃ → NO₃⁻), assimilation (plants absorb NO₃⁻), denitrification (NO₃⁻ → N₂ released back to atmosphere)
- Phosphorus cycle has no significant atmospheric component; it moves from rock → soil → organisms → water; mining and fertilizers accelerate release
- Excess nitrogen and phosphorus from agriculture causes eutrophication: algal blooms → decomposer bacteria consume oxygen → hypoxic dead zones
A farmer applies excess nitrogen fertilizer to fields near a lake. Describe the sequence of events that leads to a dead zone in the lake.
Nitrogen runoff enters the lake, acting as a limiting nutrient that triggers rapid algal bloom growth (eutrophication). When algae die, decomposer bacteria population explodes and consumes dissolved oxygen through aerobic respiration. Dissolved oxygen drops below levels fish and invertebrates can survive (hypoxia), creating a dead zone where most aerobic life cannot persist.
Questions, answered.
What is The Living World: Ecosystems?
The Living World: Ecosystems is Unit 1 of AP Environmental Science, covering food webs, energy flow and biogeochemical cycles.
How to study for AP Environmental Science Unit 1?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.