The Living World: Ecosystems
Complete review of all 11 topics — explanations, comparison tables, high-frequency exam points, practice questions, and common mistakes analysis.
Introduction to Ecosystems
An ecosystem is a community of living organisms (biotic factors) interacting with non-living components (abiotic factors). Energy flows through ecosystems; matter is recycled.
Biotic vs. Abiotic
| Type | Definition | Examples |
|---|---|---|
| Biotic | Living or once-living components | Plants, animals, bacteria, fungi, decomposers |
| Abiotic | Non-living physical/chemical components | Temperature, water, sunlight, soil pH, salinity, wind |
Levels of Organization (Smallest → Largest)
Single individual — e.g., one wolf
All individuals of the same species in an area
All populations of different species — biotic only
Community + all abiotic factors
Large region defined by climate and dominant vegetation
All life on Earth plus its environments
The AP exam almost always tests community vs. ecosystem: community = biotic only; ecosystem = biotic + abiotic. Also tested: distinguishing biotic from abiotic factors in a scenario.
Which of the following is an abiotic factor limiting plant distribution in a terrestrial ecosystem?
- (A) Competition from neighboring plants
- (B) Soil pH and mineral content
- (C) Herbivory by deer
- (D) Decomposition by soil bacteria
❌ Community ≠ ecosystem. Community includes biotic components only; ecosystem adds all abiotic factors.
❌ Decomposers (bacteria, fungi) ARE biotic factors — they are living organisms.
Terrestrial Biomes
Biomes are defined primarily by climate — temperature + precipitation. The same biome can appear on any continent given the same climate conditions.
Major Terrestrial Biomes
| Biome | Temperature | Precip. | Key Plants | Key Adaptations |
|---|---|---|---|---|
| Tropical Rainforest | High, stable ~25–30°C | >200 cm/yr | Broadleaf trees, epiphytes | Buttress roots, drip-tip leaves |
| Tropical Savanna | Warm year-round | 25–75 cm, seasonal | Grasses, scattered acacias | Fire-resistant bark, deep roots |
| Desert (Hot) | Hot days, cold nights | <25 cm/yr | Cacti, succulents, shrubs | CAM photosynthesis, water storage |
| Temperate Grassland | Seasonal, cold winters | 25–75 cm/yr | Grasses, wildflowers | Deep roots, fire-tolerant |
| Chaparral | Hot dry summers, mild wet winters | 25–65 cm/yr | Drought-tolerant shrubs | Fire-adapted seeds |
| Temperate Deciduous Forest | 4 distinct seasons | 75–150 cm/yr | Oak, maple, beech | Leaf drop in autumn, dormancy |
| Boreal Forest (Taiga) | Long cold winters | 40–100 cm/yr | Conifers: spruce, fir, pine | Needle-leaves, conical shape |
| Tundra | Extremely cold | <25 cm/yr | Mosses, lichens, dwarf shrubs | Permafrost; low-growing form |
① Very low precip + extreme temp → Desert | ② High temp + very high precip year-round → Tropical Rainforest | ③ 4 seasons + moderate precip → Temperate Deciduous Forest | ④ Extreme cold + no trees + permafrost → Tundra
A region has annual precipitation of 15 cm and summer temperatures reaching 45°C. Identify the biome and describe TWO plant adaptations for this environment.
Adaptation 1 — CAM photosynthesis: Stomata open only at night to fix CO₂, then close during the hot day — dramatically reducing water loss while still allowing photosynthesis.
Adaptation 2 — Succulent water storage: Fleshy stems or leaves store large volumes of water absorbed during rare rainfalls, sustaining the plant through long dry periods.
❌ Taiga vs. Tundra: Taiga has conifer trees; Tundra has no trees and has permafrost — permafrost is the definitive identifier for Tundra.
❌ Biome geography: Chaparral exists in California, the Mediterranean, Chile, South Africa, and Australia — same climate = same biome, regardless of continent.
❌ Fire's role: Fire is a natural, necessary disturbance in Savanna, Chaparral, and Temperate Grassland — not purely destructive.
Aquatic Biomes
Aquatic biomes (~75% of Earth's surface) are characterized by salinity, depth, flow, and light availability rather than temperature + precipitation.
Major Aquatic Biomes
| Biome | Salinity | Key Features | Ecological Role |
|---|---|---|---|
| Lakes & Ponds | Fresh | Littoral / Limnetic / Profundal zones | Freshwater storage, habitat |
| Rivers & Streams | Fresh | Headwaters (cold, fast) → lower reaches (warm, slow) | Nutrient transport, sediment flow |
| Wetlands | Fresh/brackish | Marshes, swamps, bogs, fens | Flood control, water filtration, C storage, habitat |
| Estuary | Brackish | Where river meets sea; extremely productive | Marine species nursery, nutrient cycling |
| Intertidal Zone | Marine | Alternately submerged and exposed by tides | High biodiversity, nutrient cycling |
| Coral Reef | Marine, warm, clear | Symbiotic zooxanthellae algae; shallow | Highest marine biodiversity; coastal protection |
| Open Ocean (Pelagic) | Marine ~35 ppt | Photic + Aphotic zones | ~50% of Earth's O₂; major carbon sink |
| Deep Ocean (Abyssal) | Marine | No sunlight; chemosynthesis at hydrothermal vents | Carbon storage; unique chemosynthetic ecosystems |
Wetlands — know all 4 services: (1) flood control, (2) water filtration/nutrient removal, (3) carbon sequestration, (4) wildlife habitat. FRQs frequently ask for consequences of wetland destruction.
Coral bleaching: Rising temp → coral expels zooxanthellae → turns white → loses food source → dies if temp doesn't recover. Linked to climate change questions.
Estuaries are highly productive because they receive nutrients from both river runoff AND marine tidal inputs.
An estuary is considered one of Earth's most productive ecosystems primarily because it
- (A) has very high salinity that prevents competition
- (B) receives nutrient inputs from both river and marine sources
- (C) is located in tropical regions with high solar radiation
- (D) supports chemosynthetic bacteria that form the food web base
❌ Wetland ≠ Estuary: Estuary = specific brackish zone where fresh/salt water mix; wetland is broader (can be entirely freshwater).
❌ Deep ocean has life: Hydrothermal vent communities use chemosynthesis, completely independent of sunlight.
The Carbon Cycle
The carbon cycle moves carbon between the atmosphere, biosphere, hydrosphere, and lithosphere. Unlike energy, carbon is continuously recycled.
Key Processes & Reservoirs
CO₂ + H₂O + sunlight → glucose + O₂. Removes CO₂ from atmosphere; stores C in organisms.
Glucose + O₂ → CO₂ + H₂O + energy. All organisms; returns C to atmosphere.
Burning fossil fuels/biomass → rapid release of stored ancient carbon as CO₂.
Bacteria/fungi break down dead organic matter → CO₂ (aerobic) or CH₄ (anaerobic).
Oceans absorb ~30% of human CO₂. CO₂ + H₂O → H₂CO₃ → ocean acidification (pH drops).
Geological release of CO₂ from lithosphere over millions of years — natural, slow process.
🔴 Fossil fuel combustion → releases ancient carbon → raises atmospheric CO₂ → enhanced greenhouse effect
🔴 Deforestation — double impact: (1) less photosynthetic uptake + (2) stored carbon released from burned/decomposing biomass
🔴 Ocean acidification: CO₂ → carbonic acid → lower pH → dissolves shells of corals, mollusks, pteropods
🔴 Permafrost thaw: releases stored CH₄ → positive feedback loop amplifying warming
Describe the impact of large-scale deforestation on the carbon cycle. Address at least TWO distinct effects.
Effect 2 — Carbon release from biomass: Trees burned or left to decompose rapidly release their stored carbon as CO₂ (combustion) or CO₂/CH₄ (decomposition), amplifying the greenhouse effect.
Bonus: Soil disturbance releases soil organic carbon; the ecosystem may shift from a carbon sink to a carbon source.
❌ Photosynthesis and respiration don't always balance — only in mature, stable ecosystems. Growing forests are net carbon sinks; deforested areas can become net sources.
❌ Methane (CH₄) is part of the carbon cycle too — from wetlands, cattle, and rice paddies. Its warming potential is ~28× that of CO₂ over 100 years.
❌ Ocean absorbing CO₂ does NOT raise pH — it forms carbonic acid, lowering pH (acidification).
The Nitrogen Cycle
N₂ = 78% of atmosphere but cannot be used directly by most organisms. It must be converted ("fixed") into NH₃ or NO₃⁻ through biological or industrial processes.
Five Key Processes
| Process | Transformation | Agents |
|---|---|---|
| Nitrogen Fixation | N₂ → NH₃ | Rhizobium (legume nodules), Azotobacter, lightning, Haber-Bosch |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter (aerobic soil bacteria) |
| Assimilation | NO₃⁻ → organic N (amino acids, DNA) | Plants uptake through roots; animals eat plants |
| Ammonification | Organic N → NH₃/NH₄⁺ | Decomposer bacteria and fungi |
| Denitrification | NO₃⁻ → N₂ (back to atmosphere) | Anaerobic bacteria (Pseudomonas); waterlogged soils |
Excess N & P from fertilizer runoff → algal bloom → algae die → decomposers multiply, consuming O₂ → hypoxia (O₂ depletion) → fish kill / dead zone.
Also: NOₓ from combustion + water → HNO₃ → acid rain → damages forests, acidifies lakes.
Which nitrogen cycle process converts organic nitrogen in dead organisms back to inorganic ammonia (NH₃)?
- (A) Nitrogen fixation
- (B) Nitrification
- (C) Ammonification
- (D) Denitrification
❌ Fixation ≠ Nitrification: Fixation brings N₂ from atmosphere into the biological cycle (N₂→NH₃). Nitrification oxidizes ammonia within the soil (NH₃→NO₃⁻).
❌ Denitrification is not harmful — it naturally returns excess nitrogen to the atmosphere, preventing dangerous accumulation.
❌ Stop eutrophication chain too early: must include O₂ depletion → fish kill. Algal bloom alone is not the final answer.
The Phosphorus Cycle
Unlike C and N, phosphorus has no significant atmospheric reservoir. It cycles only through rocks, soil, and water — making it the slowest major biogeochemical cycle. Essential for DNA, RNA, ATP, and cell membranes.
Phosphorus Cycle Pathway
Rain and erosion release PO₄³⁻ from phosphate rocks → enters soil water
Plants absorb H₂PO₄⁻ through roots → enters food web via herbivory
Decomposers release phosphate from dead organic matter back to soil
Runoff carries P into waterways → settles in ocean sediment → uplifted geologically
Phosphorus vs. Nitrogen — Critical Differences
| Feature | Phosphorus | Nitrogen |
|---|---|---|
| Atmospheric gas? | ❌ None | ✅ N₂ (78%) |
| Cycle speed | Very slow (geological) | Faster (biological) |
| Primary reservoir | Rocks (lithosphere) | Atmosphere |
| Limits which system? | Freshwater ecosystems | Terrestrial & marine |
| Main human impact | Mining; agricultural runoff | Industrial fixation; combustion |
❌ P cycle is NOT fast — it depends on rock weathering, not atmospheric gas exchange.
❌ In freshwater lakes, phosphorus is usually the limiting nutrient — not nitrogen. In marine/terrestrial, nitrogen is typically limiting.
The Hydrologic (Water) Cycle
The hydrologic cycle moves water continuously through the biosphere, atmosphere, and lithosphere. Solar energy drives upward movement (evaporation, transpiration); gravity drives downward movement (precipitation, runoff).
Key Processes
| Process | Description | Direction |
|---|---|---|
| Evaporation | Liquid water → vapor from oceans, lakes, soil | Surface → Atmosphere |
| Transpiration | Water vapor released through plant stomata | Plants → Atmosphere |
| Evapotranspiration | Evaporation + transpiration combined | Land → Atmosphere |
| Precipitation | Water falls as rain, snow, sleet, hail | Atmosphere → Surface |
| Infiltration | Water soaks into soil → recharges groundwater | Surface → Underground |
| Surface Runoff | Water flows overland into streams | Land → Waterways |
🔴 Deforestation → reduced transpiration → less local precipitation; increased surface runoff and soil erosion (no roots)
🔴 Urbanization / Impervious surfaces → less infiltration → more flooding; less groundwater recharge
🔴 Groundwater over-extraction → aquifer depletion (e.g., Ogallala Aquifer); land subsidence
🔴 Irrigation → soil salinization from evaporation leaving salts behind
A forested watershed is clear-cut. Describe TWO changes to the water cycle that would result.
Change 2 — Increased surface runoff: No roots to hold soil or intercept rain → water runs overland rather than infiltrating → greater flood risk, accelerated erosion, reduced groundwater recharge.
❌ Transpiration ≠ Evaporation: Transpiration is specifically water vapor from plant stomata; evaporation is from open water and moist soil surfaces.
❌ Forgetting energy sources: Solar energy drives upward water movement; gravity drives precipitation and runoff downward.
Primary Productivity
Primary productivity is the rate at which producers convert solar energy into organic matter. It determines how much energy is available to all consumers in an ecosystem.
GPP vs. NPP
| Term | Definition | Formula | Ecological Meaning |
|---|---|---|---|
| GPP (Gross Primary Productivity) | Total photosynthesis — all energy fixed | GPP = NPP + R | Total photosynthetic output |
| NPP (Net Primary Productivity) | Energy available to consumers after plant respiration | NPP = GPP − R | Biomass available to herbivores |
| Plant Respiration (R) | Energy plant uses for its own metabolism | R = GPP − NPP | Plant's "overhead cost" |
NPP = GPP − Plant Respiration
Example: GPP = 10,000 kcal/m²/yr; R = 4,000 kcal/m²/yr → NPP = 6,000 kcal/m²/yr available to consumers.
NPP Ranking Across Biomes (Commonly Tested)
| Biome | NPP (g C/m²/yr) | Limiting Factor |
|---|---|---|
| Tropical Rainforest | ~2,000 (highest/area) | None — high T, water, & light year-round |
| Estuary / Wetland | ~1,500–2,000 | High nutrients; shallow water |
| Temperate Deciduous Forest | ~600–1,000 | Seasonal growing period |
| Boreal Forest | ~300–500 | Cold; short season |
| Open Ocean | ~100–150 (low/area) | Nutrient-poor; N and Fe limited |
| Desert / Tundra | <100 (lowest/area) | Water (desert) or temperature (tundra) |
A grassland has GPP = 8,500 kcal/m²/yr. Plants use 3,200 kcal/m²/yr for cellular respiration. Calculate NPP and explain its ecological meaning.
This represents the energy stored in plant biomass that is available to primary consumers (herbivores) and decomposers — the net energy gain of the producers after their own metabolic costs.
❌ Always start 10% Rule calculations from NPP, not GPP. GPP includes what the plant uses itself.
❌ Open ocean is low per m² but covers 70% of Earth → contributes ~half of global NPP and O₂ production.
❌ Tropical rainforest soils are very nutrient-poor — nutrients are locked in living biomass, not in the soil.
Trophic Levels
A trophic level is a feeding position in a food chain. Energy decreases at each level; organism count and biomass generally decrease too.
Trophic Level Classification
| Level | Name | Energy Source | Examples |
|---|---|---|---|
| 1st | Producers (Autotrophs) | Sunlight/chemicals | Plants, algae, phytoplankton, cyanobacteria |
| 2nd | Primary Consumers (Herbivores) | Eat producers | Deer, cattle, caterpillars, zooplankton |
| 3rd | Secondary Consumers | Eat primary consumers | Frogs, foxes, small fish, songbirds |
| 4th | Tertiary Consumers | Eat secondary consumers | Hawks, sharks, killer whales |
| — | Decomposers | Dead organic matter (all levels) | Bacteria, fungi, earthworms |
Three Types of Ecological Pyramids
| Type | What It Shows | Can Be Inverted? | Example of Inversion |
|---|---|---|---|
| Energy | Energy at each level | ❌ Never | Thermodynamics prevents it |
| Biomass | Total mass at each level | ✅ Sometimes | Open ocean: phytoplankton fast turnover → less standing biomass than zooplankton |
| Numbers | Count of organisms | ✅ Often | One tree → thousands of insects; one host → many parasites |
❌ Decomposers belong to NO specific trophic level — they decompose organisms from ALL levels simultaneously.
❌ Omnivores occupy multiple trophic levels at once (bears, humans).
❌ Energy pyramids can NEVER be inverted — only biomass and numbers pyramids can be.
Energy Flow & the 10% Rule
Energy flows one direction only — from sun through producers to consumers, then lost as heat. Unlike matter, energy is not recycled. At each trophic transfer ~90% is lost and only ~10% passes to the next level.
• Heat from cellular respiration (metabolic activity — the largest share)
• Undigested material passed as feces (goes to decomposers, not next consumer)
• Energy for movement and growth not stored as harvestable biomass
Energy Calculation — Step by Step
| Trophic Level | Energy | Calculation |
|---|---|---|
| Producers (start from NPP) | 100,000 kcal | Starting value |
| Primary Consumers | 10,000 kcal | 100,000 × 10% |
| Secondary Consumers | 1,000 kcal | 10,000 × 10% |
| Tertiary Consumers | 100 kcal | 1,000 × 10% |
A marine ecosystem has NPP = 500,000 kcal/yr. Using the 10% rule, how much energy is available to secondary consumers? Show your work.
Primary → Secondary Consumers: 50,000 × 10% = 5,000 kcal
Only 1% of original NPP reaches secondary consumers after two trophic transfers.
❌ Use NPP (not GPP) as the starting value — GPP includes what the plant spends on its own respiration.
❌ 10% is an approximation — real efficiency ranges 5–20%; AP defaults to 10% unless another value is given.
❌ Energy is NOT recycled — it flows one direction only and is permanently lost as heat at each level.
Food Chains & Food Webs
A food chain is a linear energy pathway (A→B→C). A food web is the realistic network of overlapping chains. Arrows point from prey to predator — in the direction of energy flow.
Food Chain vs. Food Web
| Feature | Food Chain | Food Web |
|---|---|---|
| Structure | Linear: A → B → C → D | Complex network, multiple pathways |
| Realism | Simplified; useful for energy math | More realistic |
| Stability | Fragile — lose one species → chain collapses | Resilient — redundant pathways buffer loss |
Removing/adding one species triggers chain reactions. Classic example: wolves reintroduced to Yellowstone → elk behavior changed → riverbank vegetation recovered → rivers changed course.
Disproportionately large impact relative to abundance. Sea otters → control urchins → preserve kelp forests. Removal = ecosystem collapse.
Persistent toxins (DDT, PCBs, Hg) increase ~10× in concentration at each trophic level. Apex predators accumulate the highest doses.
Build-up of toxins in an individual organism's tissues over time. Different from — but related to — biomagnification across a food chain.
Why concentrations multiply at each level: each consumer eats large quantities of prey, accumulating all the toxin stored in their prey's fat tissues. Classic AP example: DDT → bald eagle eggshell thinning → near extinction (recovered after DDT ban). Mercury highest in tuna, swordfish, shark, orcas.
Which organism would have the HIGHEST tissue concentration of DDT in this food web: phytoplankton → zooplankton → small fish → large fish → osprey?
- (A) Phytoplankton
- (B) Small fish
- (C) Large fish
- (D) Osprey
❌ Arrow direction: arrows point FROM prey TO predator (direction of energy flow).
❌ Simpler food webs are NOT more stable — greater complexity provides redundancy and resilience.
❌ Bioaccumulation ≠ biomagnification: bioaccumulation = within one individual over time; biomagnification = increasing concentration across trophic levels.
Comprehensive Practice Questions
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
A farmer applies excess nitrogen fertilizer near a lake. Which sequence best describes the subsequent changes in the lake?
- (A) Nitrogen fixation → denitrification → increased fish populations
- (B) Algal bloom → increased decomposer activity → oxygen depletion → fish kill
- (C) Increased primary productivity → increased dissolved oxygen → higher fish populations
- (D) Phosphorus limitation → reduced algae → clearer water → increased biodiversity
A grassland has NPP = 200,000 kcal/yr. How much energy is available to tertiary consumers?
- (A) 20,000 kcal/yr
- (B) 2,000 kcal/yr
- (C) 200 kcal/yr
- (D) 20 kcal/yr
Producers → Primary: 200,000 × 10% = 20,000
Primary → Secondary: 20,000 × 10% = 2,000
Secondary → Tertiary: 2,000 × 10% = 200 kcal
The phosphorus cycle differs from the nitrogen cycle in that phosphorus
- (A) is not required by living organisms
- (B) has no significant gaseous phase in its cycle
- (C) can be fixed from the atmosphere by bacteria
- (D) cycles rapidly through marine ecosystems
The Florida Everglades freshwater wetland is affected by agricultural runoff from surrounding farms.
(a) Identify ONE biogeochemical cycle disrupted by agricultural runoff and describe the disruption. [3 pts]
(b) Explain how this disruption affects the Everglades food web. [3 pts]
(c) Propose ONE management strategy to reduce impacts and explain its effectiveness. [2 pts]
Fertilizers containing PO₄³⁻ and NO₃⁻ run off into Everglades waterways at far above natural concentrations, artificially accelerating nutrient cycling and causing chronic nutrient overloading in a system adapted to low-nutrient conditions.
(b) Food Web Effects [3 pts]:
Excess N/P → eutrophication → algal blooms and cattail invasion replace native sawgrass → altered primary producers → disrupts food availability for primary consumers (apple snails, fish, wading birds) → cascades through food web to apex species (wood storks, alligators) → biodiversity decline across all trophic levels.
(c) Strategy — Constructed wetland treatment marshes [2 pts]:
Large, shallow constructed wetlands are built between farms and the Everglades. Aquatic plants absorb excess N and P; microbial denitrification removes nitrate; sediments settle. Effective because it mimics natural wetland biogeochemical cycling at scale, removing >70% of excess phosphorus before water enters the natural system.
High-Frequency Common Mistakes — Full Unit 1
- ⚡Energy flows; matter cycles — they're not the sameEnergy enters as sunlight and exits permanently as heat — never recycled. Nutrients (C, N, P, H₂O) cycle continuously. Mixing these up is the #1 Unit 1 error.
- 🌿Starting 10% calculations from GPP instead of NPPAlways use NPP as the starting value. GPP includes the plant's own respiration cost — unavailable to any consumer.
- 🌊Stopping eutrophication chain too earlyFull chain required: excess N/P → algal bloom → algae die → decomposers consume O₂ → hypoxia → fish kill. Stopping at "algal bloom" earns partial or no credit on FRQs.
- 🌡️Identifying biomes by location instead of climateBiomes are defined by climate, not geography. Chaparral exists on 5 continents — same climate conditions = same biome type.
- 🔬Using "bioaccumulation" and "biomagnification" interchangeablyBioaccumulation = toxin build-up within one individual over time. Biomagnification = increasing toxin concentration across trophic levels in a food chain. Both terms must be defined and used correctly.
- 🧪Thinking phosphorus has an atmospheric gas phaseNo significant phosphorus gas exists. Phosphorus cannot take an atmospheric shortcut — it must move through rock weathering, making it the slowest major cycle.
- 🌲Tropical rainforest soils are nutrient-richParadox: highest-productivity biome has some of Earth's poorest soils. Almost all nutrients are locked in living biomass; soils lose fertility rapidly after deforestation.
- 🔄Placing decomposers at a specific pyramid levelDecomposers feed on all trophic levels simultaneously — they have no fixed position in a food pyramid and are always shown separately.
- 💧Ocean absorbing CO₂ = higher pHWrong. CO₂ + H₂O → carbonic acid → LOWER pH. Ocean acidification means more acidic (lower pH), which dissolves calcium carbonate shells.
- 🌾Phosphorus limits marine ecosystems; nitrogen limits freshwaterIt's the reverse: phosphorus typically limits freshwater; nitrogen (and iron) typically limits marine and terrestrial systems.
- 🐺Trophic cascades only go top-downTop-down: remove apex predator → herbivores explode → vegetation collapses. Bottom-up: remove producers → energy unavailable to all levels above. Both are testable on the AP exam.
Unit 1 = ~6–8% of the AP exam, but its concepts underpin Units 2–9. Highest-yield topics: eutrophication chain, GPP/NPP calculations, 10% Rule math, biome climatograph reading, biomagnification, and deforestation effects on carbon and water cycles.