Inside This Unit: The Full Breakdown
The Living World: Ecosystems introduces how ecosystems function, including energy flow through trophic levels, nutrient cycling, and the interactions between biotic and abiotic factors that sustain life.
Why it matters
Ecosystem ecology is foundational for the entire AP Environmental Science course. Understanding energy flow, food webs, and nutrient cycles is essential for analyzing environmental problems like eutrophication, climate change, and habitat loss.
Key concepts
- Ecosystems consist of biotic (living) and abiotic (nonliving) components that interact to sustain life. Energy flows one way; matter cycles.
- Primary productivity (GPP and NPP) measures the rate at which producers convert solar energy to chemical energy. NPP limits the energy available to consumers.
- The 10% rule: only about 10% of energy at one trophic level is transferred to the next. This limits food chain length.
- Biogeochemical cycles (carbon, nitrogen, phosphorus, water) move matter through biotic and abiotic reservoirs.
Ecosystem Structure
An ecosystem includes all the living organisms (biotic factors) and nonliving components (abiotic factors like temperature, water, sunlight, and soil) in a defined area. Biomes are large-scale ecosystems classified primarily by climate and dominant vegetation — tropical rainforests, deserts, tundra, temperate forests, grasslands, and aquatic systems. Within each ecosystem, organisms are organized by trophic levels: producers (autotrophs) form the base, followed by primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers. Decomposers (bacteria, fungi) break down dead organic matter at every level, recycling nutrients back into the ecosystem. Food webs show the complex feeding relationships in an ecosystem and are more realistic than simple food chains.
Energy Flow and Productivity
Energy enters ecosystems as sunlight and is converted to chemical energy by producers through photosynthesis. Gross primary productivity (GPP) is the total rate of photosynthesis; net primary productivity (NPP = GPP − respiration) is the energy actually stored in plant biomass and available to consumers. At each trophic level, organisms use about 90% of the energy they consume for metabolism (cellular respiration) and lose it as heat. Only about 10% is converted to biomass that the next level can consume. This 10% rule explains why ecosystems can support fewer organisms at higher trophic levels and why top predators are rare. It also explains why eating lower on the food chain (more plant-based diets) can support more people with the same amount of primary production.
Biogeochemical Cycles
Matter is recycled through biogeochemical cycles that move elements between living organisms and the physical environment. The carbon cycle moves carbon through the atmosphere (as CO₂), organisms (as organic compounds), oceans (dissolved CO₂ and carbonates), and fossil fuel deposits. Photosynthesis removes atmospheric carbon; respiration and combustion release it. The nitrogen cycle converts atmospheric N₂ into usable forms through nitrogen fixation (by bacteria), then cycles through nitrification, assimilation, and denitrification. The phosphorus cycle has no significant atmospheric component — phosphorus moves from rocks to soil to organisms and back. The water cycle drives the movement of other nutrients. Human activities — burning fossil fuels, applying synthetic fertilizers, deforestation — are disrupting these cycles, causing problems like climate change and eutrophication.
AP exam tip
On the AP Environmental Science exam, you will frequently need to trace energy or matter through an ecosystem. Always be specific: name the process (photosynthesis, decomposition, nitrogen fixation) and identify the reservoirs (atmosphere, biomass, soil, ocean) involved.
Connections to other units
- Unit 2 (Biodiversity): Species diversity within ecosystems depends on energy availability and habitat structure.
- Unit 9 (Global Change): Disruptions to carbon and nitrogen cycles are directly responsible for climate change and ocean acidification.
- Unit 5 (Land and Water Use): Agricultural practices alter nutrient cycles through fertilizer use and land clearing.
