The Living World: Ecosystems
Fast-track review of all 11 topics — key concepts, high-frequency exam points, common mistakes, and question-type guidance. Designed for 2–4 week exam prep.
Introduction to Ecosystems
Levels of Organization
Organism → Population → Community → Ecosystem → Biome → Biosphere
Community = all populations (biotic only) | Ecosystem = community + all abiotic factors
① Community vs. Ecosystem — the most-tested distinction. Community = biotic only. Ecosystem adds abiotic.
② MCQ scenarios: "Which is abiotic?" — decomposers are BIOTIC even though they're microscopic; sunlight and soil pH are abiotic.
❌ Decomposers (bacteria, fungi) = biotic — they are living organisms.
❌ Don't confuse community and ecosystem. Community = biotic only; ecosystem includes abiotic.
Which is an abiotic factor limiting plant distribution?
- (A) Competition from neighboring plants
- (B) Soil pH and mineral content
- (C) Herbivory by deer
- (D) Decomposition by soil bacteria
Terrestrial Biomes
Biomes are defined by climate (temperature + precipitation) — NOT by geography or continent.
| Biome | Temp | Precip | Key ID Clue |
|---|---|---|---|
| Tropical Rainforest | High, stable ~25–30°C | >200 cm/yr | No dry season; broadleaf; highest biodiversity |
| Tropical Savanna | Warm year-round | 25–75 cm, seasonal | Distinct wet/dry seasons; fire-adapted grasses |
| Hot Desert | Hot days, cold nights | <25 cm/yr | Extreme temp swings; CAM plants; cacti |
| Chaparral | Hot dry summers, mild wet winters | 25–65 cm/yr | Mediterranean pattern; fire-adapted shrubs |
| Temperate Grassland | Seasonal; cold winters | 25–75 cm/yr | No trees; deep-root grasses; fire-tolerant |
| Temperate Deciduous Forest | 4 distinct seasons | 75–150 cm/yr | Leaf drop in fall; oak/maple; moderate precip |
| Boreal Forest (Taiga) | Long cold winters | 40–100 cm/yr | Conifers (spruce, fir); has trees (≠ tundra) |
| Tundra | Extremely cold | <25 cm/yr | Permafrost; no trees; mosses, lichens |
① Very low precip + extreme T → Desert | ② High T + high precip year-round → Tropical Rainforest
③ 4 seasons + moderate precip → Temperate Deciduous Forest | ④ Extreme cold + no trees + permafrost → Tundra
⑤ Hot dry summer + wet winter → Chaparral | ⑥ Warm + seasonal wet/dry → Savanna
Given precip and temp data: (1) Name the biome. (2) Describe TWO specific plant or animal adaptations — explain the mechanism, not just the name. Example: "CAM photosynthesis — stomata open only at night to fix CO₂, reducing daytime water loss."
❌ Taiga ≠ Tundra: Taiga has conifer trees. Tundra has NO trees + permafrost. Permafrost = definitive tundra ID.
❌ Chaparral exists on 5 continents — same climate = same biome. Location alone doesn't determine biome type.
❌ Fire is natural and necessary in savanna, chaparral, and temperate grassland — not purely destructive.
Aquatic Biomes
Aquatic biomes defined by salinity, depth, flow, and light — not temperature/precip like terrestrial biomes.
| Biome | Key Feature | Why It Matters on Exam |
|---|---|---|
| Wetlands | Shallow; water-saturated soil | 4 ecosystem services: flood control, water filtration, C storage, wildlife habitat |
| Estuary | Where river meets sea; brackish | Most productive — gets nutrients from BOTH river & ocean |
| Coral Reef | Warm, clear, shallow marine | Bleaching: warm temp → expels zooxanthellae → coral dies |
| Open Ocean (Pelagic) | Photic + aphotic zones | Low NPP per m² but ~50% of global O₂; major C sink |
| Abyssal Zone | No sunlight; cold; deep | Chemosynthesis at hydrothermal vents — no sunlight needed |
| Intertidal Zone | Alternately wet/dry | Organisms adapted to extreme change (e.g., barnacles, sea stars) |
① Flood control (absorbs storm water) ② Water filtration (removes N, P, sediment) ③ Carbon sequestration (stores organic C in waterlogged soil) ④ Wildlife habitat (nursery for fish, birds)
FRQs frequently ask: "What are the consequences of draining wetlands?" — address all 4 lost services.
❌ Wetland ≠ Estuary. Estuary = brackish zone where river meets sea. Wetlands can be entirely freshwater.
❌ Deep ocean has life — hydrothermal vent communities use chemosynthesis, independent of sunlight.
❌ Coral bleaching = loss of algae (zooxanthellae), NOT death yet. Prolonged bleaching → death.
An estuary is highly productive primarily because it
- (A) has very high salinity preventing 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 at the base of its food web
The Carbon Cycle
CO₂ + H₂O + sunlight → glucose + O₂. Removes C from atmosphere; stores in biomass.
All organisms: glucose + O₂ → CO₂ + H₂O. Returns C to atmosphere.
Burning fossil fuels/biomass → rapid release of ancient carbon as CO₂.
Aerobic → CO₂. Anaerobic → CH₄ (methane). Both return C to cycle.
Oceans absorb ~30% of human CO₂. CO₂ + H₂O → H₂CO₃ → lower pH = ocean acidification.
🔴 Fossil fuel combustion → ancient C released → raises atmospheric CO₂ → greenhouse effect
🔴 Deforestation — double impact: (1) less photosynthesis to absorb CO₂, (2) burned/decomposing trees release stored C
🔴 Ocean acidification: absorbed CO₂ forms carbonic acid → lowers pH → dissolves calcium carbonate shells of corals, mollusks, pteropods
🔴 Permafrost thaw → releases CH₄ → positive feedback loop (warming accelerates warming)
❌ Ocean absorbing CO₂ does NOT raise pH — it lowers pH (acidification). This trips up many students.
❌ Methane (CH₄) is part of the carbon cycle — from wetlands, cattle, rice paddies. ~28× warming potency of CO₂.
❌ Deforestation has TWO separate carbon cycle effects — don't just name one.
The Nitrogen Cycle
N₂ = 78% of atmosphere but unusable directly by most organisms. Must be converted ("fixed") first.
| Process | What Happens | Agent |
|---|---|---|
| Nitrogen Fixation | N₂ → NH₃ (usable) | Rhizobium in legume roots; lightning; Haber-Bosch process |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ | Aerobic soil bacteria (Nitrosomonas, Nitrobacter) |
| Assimilation | NO₃⁻ → organic N (proteins, DNA) | Plant root uptake; animals eat plants |
| Ammonification | Organic N → NH₃ | Decomposer bacteria/fungi on dead matter |
| Denitrification | NO₃⁻ → N₂ (back to air) | Anaerobic bacteria in waterlogged soils |
Excess N & P fertilizer runoff → algal bloom → algae die → decomposers multiply, consume O₂ → hypoxia / dead zone → fish kill
Stopping at "algal bloom" = partial credit only. Must include O₂ depletion and fish kill for full marks.
❌ Fixation ≠ Nitrification. Fixation: N₂ from air → NH₃. Nitrification: NH₃ → NO₃⁻ within soil.
❌ Denitrification is not "bad" — it naturally removes excess N from soil, returning it to the atmosphere.
Which process converts organic nitrogen in dead organisms back to inorganic ammonia?
- (A) Nitrogen fixation
- (B) Nitrification
- (C) Ammonification
- (D) Denitrification
The Phosphorus Cycle
Phosphorus has no atmospheric gas phase — cycles only through rocks → soil → water. Makes it the slowest major biogeochemical cycle. Essential for DNA, RNA, ATP, cell membranes.
| Feature | Phosphorus | Nitrogen |
|---|---|---|
| Atmospheric gas? | ❌ None | ✅ N₂ (78%) |
| Cycle speed | Very slow — geological timescale | Faster — biological processes |
| Primary reservoir | Rocks (lithosphere) | Atmosphere |
| Limiting in… | Freshwater ecosystems | Terrestrial & marine |
| Human input | Phosphate mining; fertilizer | Industrial fixation (Haber-Bosch); combustion |
① P = no gas phase → no shortcut through atmosphere → slowest cycle
② P limits freshwater; N limits marine/terrestrial — this distinction is frequently reversed on MCQ distractors
③ Both P and N cause eutrophication — excess fertilizer runoff triggers algal blooms in both lakes and coastal zones
❌ Students often say P limits marine — it's the reverse. P limits freshwater; N (and Fe) limits marine.
❌ No bacteria "fix" phosphorus from the atmosphere — there's no atmospheric P to fix.
The Hydrologic (Water) Cycle
Solar energy drives upward movement (evaporation, transpiration); gravity drives downward movement (precipitation, runoff).
| Process | What It Does |
|---|---|
| Evaporation | Liquid water → vapor from oceans, lakes, soil (abiotic) |
| Transpiration | Water vapor released through plant stomata (biotic) |
| Evapotranspiration | Evaporation + transpiration combined (used to measure land water loss) |
| Precipitation | Rain/snow/sleet falls from atmosphere to surface |
| Infiltration | Water soaks into soil → recharges groundwater aquifers |
| Surface Runoff | Water flows overland into streams/rivers when infiltration is exceeded |
🔴 Deforestation → less transpiration (drier local climate) + more runoff & erosion (no roots)
🔴 Urbanization/Impervious surfaces → less infiltration (more flooding) + less groundwater recharge
🔴 Irrigation → soil salinization (evaporation leaves salts behind)
🔴 Over-extraction → aquifer depletion (e.g., Ogallala Aquifer) + land subsidence
❌ Transpiration ≠ Evaporation. Transpiration = specifically from plant stomata. Evaporation = from open water/soil.
❌ Deforestation reduces local precipitation — fewer trees = less transpiration = less moisture in air = less rain.
Primary Productivity
NPP = GPP − Plant Respiration (R)
GPP = all photosynthesis. NPP = energy available to consumers after plant uses its share. R = plant's own metabolic cost.
Example: GPP = 10,000 kcal; R = 4,000 kcal → NPP = 6,000 kcal available to consumers.
| Biome | NPP Level | Limiting Factor |
|---|---|---|
| Tropical Rainforest | Highest per m² | None — high T, water, light year-round |
| Estuary / Wetland | Very high | High nutrients, shallow → productive |
| Temperate Forest | Moderate | Seasonal growing period |
| Open Ocean | Low per m² | Nutrient-poor (N, Fe limited); low per area but covers 70% of Earth → ~50% of global NPP total |
| Desert / Tundra | Lowest per m² | Water (desert) or temperature (tundra) |
❌ Always start 10% Rule calculations from NPP, not GPP.
❌ Open ocean is low per m² but covers 70% of Earth — contributes ~half of global NPP and O₂ production.
❌ Tropical rainforest soils are nutrient-poor — almost all nutrients are locked in living biomass, not the soil.
A grassland has GPP = 8,500 kcal/m²/yr; plants use 3,200 kcal/m²/yr for respiration. Calculate NPP.
- (A) 11,700 kcal/m²/yr
- (B) 5,300 kcal/m²/yr
- (C) 3,200 kcal/m²/yr
- (D) 8,500 kcal/m²/yr
This is the energy available to primary consumers (herbivores) — the net biomass gain after the plant's own costs.
Trophic Levels
| Trophic Level | Role | Examples |
|---|---|---|
| Producers (TL1) | Make own food via photosynthesis/chemosynthesis | Plants, algae, phytoplankton, cyanobacteria |
| Primary Consumers (TL2) | Eat producers — herbivores | Deer, grasshoppers, zooplankton, rabbits |
| Secondary Consumers (TL3) | Eat primary consumers — carnivores/omnivores | Frogs, small fish, foxes |
| Tertiary Consumers (TL4) | Eat secondary consumers | Hawks, sharks, large predatory fish |
| Decomposers | Break down dead matter at ALL levels | Bacteria, fungi — shown separately, no fixed level |
Bioaccumulation = toxin builds up within ONE individual over its lifetime (e.g., a fish accumulates mercury over years).
Biomagnification = toxin concentration increases at each higher trophic level across the food chain (e.g., apex predators have 1000× more DDT than producers).
Reason: fat-soluble toxins (DDT, PCBs, mercury) are NOT excreted — they accumulate in fat tissue. Top predators consume many lower-level organisms → concentrate toxins.
❌ "Bioaccumulation" ≠ "biomagnification" — these must be defined precisely on FRQs.
❌ Decomposers have NO fixed trophic level — they feed on ALL levels simultaneously. Never put them in a pyramid.
❌ Omnivores can occupy multiple trophic levels depending on what they eat.
Energy Flow & the 10% Rule
Only ~10% of energy transfers to the next trophic level. The other ~90% is lost as heat (through cellular respiration, movement, metabolic processes).
Energy at TL(n+1) = Energy at TL(n) × 0.10
Example: 10,000 kcal producers → 1,000 kcal primary consumers → 100 kcal secondary → 10 kcal tertiary
MCQ: Given energy at one level, calculate energy available at a level 2–3 steps higher. Apply 0.10 per step.
FRQ: "Explain why a top predator population is small relative to producers." Must mention: 90% energy lost as heat at each level → insufficient energy to support large predator populations.
A grassland ecosystem has 200,000 kcal/yr of NPP. 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 (NPP) → Primary (×10%): 20,000 → Secondary (×10%): 2,000 → Tertiary (×10%): 200 kcal
Food Chains & Food Webs
Top-down cascade: Remove apex predator → prey population explodes → vegetation collapses. Example: remove wolves → deer overpopulate → overgrazing → habitat degradation.
Bottom-up cascade: Remove producers → all higher levels lose energy source → entire food web collapses.
MCQ provides a food web diagram. You're asked: "If species X is removed, what happens to species Y?" — trace energy pathways. Multiple steps may be required. Look for: competing prey species (one may increase if a shared predator is removed), and keystone predator removal causing cascades.
❌ Arrow direction: arrows go FROM prey TO predator (direction of energy flow, not "who eats whom" intuitively).
❌ Food webs are more stable than food chains — more connections = more resilience when one species is lost.
❌ Trophic cascades go both top-down AND bottom-up. Don't only describe one direction.
Top Common Mistakes — Full Unit 1
- ⚡Energy flows; matter cycles — not interchangeableEnergy enters as sunlight, exits permanently as heat. Nutrients (C, N, P, H₂O) cycle continuously. This distinction is tested every year.
- 🌿Starting 10% Rule from GPP instead of NPPAlways use NPP as your base. GPP includes the plant's own respiration cost — unavailable to any consumer.
- 🌊Stopping eutrophication chain at "algal bloom"Full chain required: excess N/P → algal bloom → algae die → decomposers consume O₂ → hypoxia → fish kill. Stopping early = partial credit.
- 🌡️Identifying biomes by continent, not climateBiomes are defined by climate. Chaparral exists on 5 continents. Same climate = same biome, regardless of location.
- 🔬Confusing bioaccumulation and biomagnificationBioaccumulation = toxin builds in one individual over time. Biomagnification = toxin concentration increases across trophic levels. Both need precise definitions on FRQs.
- 🧪Thinking phosphorus has an atmospheric phaseNo significant P gas exists. P moves only through rock → soil → water → rock. No atmospheric shortcut = slowest major cycle.
- 🌊Ocean absorbing CO₂ = higher pHWrong direction. CO₂ + H₂O → carbonic acid → LOWER pH. Ocean acidification means MORE acidic, which dissolves calcium carbonate shells.
- 🌾P limits marine; N limits freshwater — reversed!It's the opposite: P typically limits freshwater; N (and Fe) typically limits marine and terrestrial systems.
- 🌲Tropical rainforest soils are nutrient-richParadox: highest biodiversity, lowest soil fertility. Nutrients are locked in living biomass — soils are poor and lose fertility fast after deforestation.
- 🐺Taiga has trees; Tundra does notTaiga = conifer forest (spruce, fir). Tundra = no trees + permafrost. Permafrost is the definitive tundra identifier on climatographs.
Unit 1 Exam Strategy & High-Yield Topics
Highest-Yield Topics (Prioritize These First)
Almost always appears. Practice multi-step energy calculations. Start from NPP — not GPP.
Must know complete chain: N/P runoff → algal bloom → O₂ depletion → fish kill. Partial chains = partial credit.
Affects both carbon cycle (less uptake + more release) AND water cycle (less transpiration + more runoff). Know both.
Read temp + precip patterns. Key IDs: permafrost = tundra; hot dry summers / wet winters = chaparral; >200cm precip = tropical rainforest.
Two distinct terms — must define precisely. Biomagnification = increasing concentration across trophic levels.
Know all 4: flood control, water filtration, C storage, wildlife habitat. FRQs about wetland destruction test these.
MCQ vs. FRQ Pattern Guide
| Topic | MCQ Angle | FRQ Angle |
|---|---|---|
| Biomes (1.2) | Identify biome from climate data / climatograph | Name biome + describe 2 specific adaptations with mechanisms |
| Carbon Cycle (1.4) | Which process releases/absorbs CO₂? | Describe 2 effects of deforestation on carbon cycle |
| Nitrogen Cycle (1.5) | Which process matches the N transformation shown? | Trace eutrophication from fertilizer runoff to dead zone |
| Phosphorus Cycle (1.6) | How does P differ from N? Which limits freshwater? | Rarely standalone — usually paired with eutrophication |
| Water Cycle (1.7) | What process is described? | Describe 2 water cycle changes from deforestation/urbanization |
| NPP / GPP (1.8) | Rank biomes by NPP; calculate NPP | Calculate NPP and explain its ecological significance |
| 10% Rule (1.10) | Energy at higher trophic level after X transfers | Explain why top predators are rare; support with 10% rule |
| Food Webs (1.11) | Predict population change from diagram when species removed | Describe trophic cascade; explain top-down vs. bottom-up |
Unit 1 concepts appear again in later units — especially eutrophication (Unit 5), deforestation (Unit 5, 9), and energy flow (Unit 6). Getting Unit 1 solid pays dividends throughout the entire exam.
For FRQ writing: always define the term, describe the mechanism, and state the consequence. Vague answers like "it harms the ecosystem" earn zero points.