Why is "Drawing food chain arrows in the wrong direction (from predator to prey, or bidirectional)." a common mistake in Ecosystems?

Why it happens: Students confuse 'eats' with the direction of energy flow. How to avoid it: Arrows in food chains represent the direction of **energy flow** — they always point FROM the organism being eaten TO the organism doing the eating: Grass → Rabbit → Fox. The arrowhead points towards the organism receiving the energy. Source: Consistently penalised in 0680 mark schemes — the arrow direction is specifically checked.

Why is "Saying energy 'cycles' in an ecosystem (like nutrients do)." a common mistake in Ecosystems?

Why it happens: Students know nutrients cycle and assume energy does too. How to avoid it: Energy **flows** in one direction — it enters as sunlight, flows through the food web, and is lost as heat. It does NOT cycle. Only **matter** (nutrients: carbon, nitrogen, phosphorus) cycles. This is a fundamental distinction in ecology.

Why is "Confusing denitrification with decomposition in the nitrogen cycle." a common mistake in Ecosystems?

Why it happens: Both involve bacteria breaking down nitrogen compounds. How to avoid it: **Decomposition (ammonification):** dead organic matter → ammonium (NH₄⁺) — releases nitrogen INTO the soil. **Denitrification:** nitrates (NO₃⁻) → nitrogen gas (N₂) — removes nitrogen FROM the soil ecosystem back to atmosphere. Denitrification happens in anaerobic (waterlogged) conditions and reduces soil fertility.

Why is "Assuming the producer must always be a plant." a common mistake in Ecosystems?

Why it happens: Most examples use grass or trees as producers. How to avoid it: Producers are any autotrophs that can photosynthesise. In aquatic ecosystems, producers are **phytoplankton** (microscopic algae) and **algae** — not plants. In some hydrothermal vent ecosystems, **chemosynthetic bacteria** act as producers using chemical energy rather than sunlight.

Ecosystems, Biodiversity and FieldworkCambridge IGCSE Environmental Management (0680)

Ecosystems

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Ecosystems — Cambridge IGCSE Environmental Management 0680

An ecosystem is the interacting system of living organisms and their physical environment. This subtopic covers the components of ecosystems, how energy flows through food chains and webs, nutrient cycling (especially nitrogen), and the process of ecological succession from bare ground to climax community.

At a glance

Ecosystem = biotic (living) community + abiotic (non-living) environment acting as a system.
Biotic factors: food availability, predators, competition, disease, parasitism.
Abiotic factors: temperature, light intensity, rainfall, soil pH, wind speed, humidity, altitude.
Energy flows one way through food chains; matter (nutrients) cycles — never confuse the two.
10% rule: only ~10% of energy at each trophic level passes to the next; 90% lost as heat, faeces, and metabolic waste.
Nutrient cycling: nitrogen cycle involves fixation (N₂ → NH₄⁺), nitrification (NH₄⁺ → NO₃⁻), assimilation, decomposition, and denitrification (NO₃⁻ → N₂).
Ecological succession: pioneer species → seral stages → climax community; primary (bare substrate) vs. secondary (disturbed soil).
Energy pyramids always have a true pyramid shape — pyramids of numbers or biomass can invert.

What you’ll learn

Mapped to the Cambridge IGCSE 0680 syllabus (2027-2029).

5.1a — Define the term 'ecosystem' and describe its biotic and abiotic components.
5.1b — Describe and give examples of biotic and abiotic factors affecting organisms.
5.1c — Construct and interpret food chains and food webs, including the direction of energy flow.
5.1d — Explain energy loss at each trophic level and interpret pyramids of numbers, biomass and energy.
5.1e — Describe the nitrogen cycle, including the roles of different bacteria and decomposers.
5.1f — Describe the carbon cycle and the role of decomposers.
5.1g — Describe the stages of primary and secondary ecological succession to a climax community.
5.1h — Define the term 'biome' and describe the main global biomes.

Components of an Ecosystem

Ecosystems are defined by the interaction of biotic and abiotic components.

An ecosystem is a community of interacting organisms (the biotic component) together with the physical and chemical environment they inhabit (the abiotic component), functioning as a system through which energy flows and matter cycles.

Biotic factors (living components that affect organisms):

Food availability — limits population size of consumers; if prey declines, predator populations fall
Predators — control prey population size through predation; predator-prey cycles (e.g. lynx-snowshoe hare in Canada)
Competition — organisms competing for the same limited resources (food, water, territory, mates, light)
Disease and parasitism — pathogens and parasites reduce host fitness and population size
Mutualism — both species benefit (e.g. nitrogen-fixing bacteria in legume root nodules; mycorrhizal fungi and tree roots)

Abiotic factors (non-living factors):

Factor	Effect on organisms
Temperature	Controls metabolic rates and enzyme activity; limits range of ectothermic species
Light intensity	Determines photosynthesis rate; limits plant distribution by depth in water
Rainfall / water availability	Limits primary productivity; determines biome type
Soil pH	Affects nutrient availability and species composition of plants
Wind speed	Increases water loss; shapes tree form in exposed sites
Humidity	Affects water loss from organisms; important for amphibians and insects
Altitude	Lower temperature, lower O₂, higher UV at high altitude

Key principle: biotic and abiotic factors interact — a drought (abiotic) kills plants (biotic change) → herbivores decline → predators decline. No factor acts in isolation.

Biotic: food, predators, competition, disease, parasitism, mutualism.
Abiotic: temperature, light, rainfall, pH, wind, humidity, altitude.
Abiotic factors set physical limits; biotic factors determine which species occupy those conditions.

Energy Flow: Food Chains, Food Webs and Pyramids

Energy enters as sunlight and flows one-way through trophic levels, with ~90% lost at each step.

Food chains show feeding relationships and the direction of energy flow:

Producer → Primary consumer → Secondary consumer → Tertiary consumer

Arrow direction = direction of energy flow (FROM organism being eaten TO organism doing the eating).

Trophic levels:

Level 1: Producers (plants, algae, phytoplankton) — fix solar energy via photosynthesis
Level 2: Primary consumers (herbivores — caterpillars, rabbits, zooplankton)
Level 3: Secondary consumers (eat herbivores — frogs, small fish, foxes)
Level 4: Tertiary consumers (top predators — eagles, large sharks, polar bears)

The 10% Rule (energy transfer efficiency): Only approximately 10% of energy at each trophic level is transferred to the next. The remaining 90% is lost as:

Heat from respiration — metabolic processes generate heat that cannot be re-used
Undigested material — faeces, bones, fur, plant cell walls not absorbed
Metabolic waste — urine (nitrogen excretion) not consumed by the next trophic level

Example: Grass fixes 100,000 kJ → Grasshopper receives 10,000 kJ → Frog 1,000 kJ → Snake 100 kJ → Hawk 10 kJ

Each step up the trophic pyramid retains only ~10% of the energy from the level below — the rest is lost as heat and waste.

Food webs contain multiple interconnected food chains — more stable than a single chain because:

Multiple prey items allow dietary switching if one prey declines
More feeding connections = more resilience to species loss
Omnivores (feeding at multiple levels) act as stabilising buffers

Pyramids:

Pyramid type	Always pyramid-shaped?	Notes
Numbers	No — can invert	Inverts when producer is a large organism (e.g. one oak tree → thousands of caterpillars)
Biomass	Usually, rarely inverts	Can invert in aquatic ecosystems (phytoplankton low instantaneous biomass)
Energy	Always	Energy flow always decreases up the chain — cannot violate thermodynamics

Arrows in food chains show direction of energy flow — from eaten to eater.
10% rule: 90% of energy lost as heat (respiration), faeces (undigested), urine (metabolic waste).
Energy pyramids always pyramid-shaped; numbers pyramids can invert (e.g. one tree → many insects).
Food webs more stable than food chains: dietary switching; multiple pathways.

Nutrient Cycling: Nitrogen and Carbon Cycles

Unlike energy, matter cycles continuously — the nitrogen and carbon cycles are the most important.

The Nitrogen Cycle

Nitrogen makes up 78% of air but most organisms cannot use N₂ gas directly. The nitrogen cycle converts N₂ into usable forms.

Stage 1 — Nitrogen fixation (N₂ → NH₄⁺):

Rhizobium bacteria in root nodules of legumes (peas, beans, clover, alfalfa) — mutualistic; plant provides glucose, bacteria fix N₂
Azotobacter — free-living nitrogen-fixing bacteria in soil
Lightning — converts N₂ + O₂ → NO₃⁻ (small contribution)

Stage 2 — Nitrification (NH₄⁺ → NO₃⁻):

Nitrifying bacteria in soil oxidise ammonium → nitrites → nitrates (aerobic process — needs O₂)
Nitrates (NO₃⁻) are the main form plants absorb through roots

Stage 3 — Assimilation (NO₃⁻ → proteins):

Plants absorb nitrates and use them to synthesise amino acids, proteins, and nucleic acids (DNA, RNA)
Animals get nitrogen by eating plant or animal proteins

Stage 4 — Decomposition / Ammonification (proteins → NH₄⁺):

Decomposers (bacteria and fungi) break down dead organic matter and excretory products (urea)
Releases ammonium ions (NH₄⁺) back into the soil — making nitrogen available again

Stage 5 — Denitrification (NO₃⁻ → N₂):

Denitrifying bacteria in waterlogged (anaerobic) soils convert nitrates → N₂ gas
Releases nitrogen back to atmosphere — a loss from the soil ecosystem
Occurs in compacted, waterlogged soils — reduces soil fertility

Key memory hook: Fix → Nitrify → Assimilate → Decompose → Denitrify → back to atmosphere

The Carbon Cycle

Carbon flows between atmosphere, living organisms, oceans, and geological stores.

Photosynthesis: CO₂ absorbed by plants → organic carbon in biomass (carbon REMOVED from atmosphere)
Respiration: all organisms release CO₂ back to atmosphere when they respire
Decomposition: bacteria/fungi break down dead organic matter → CO₂ released
Combustion: burning fossil fuels and forests releases ancient stored carbon → rapid CO₂ increase
Ocean absorption: oceans dissolve CO₂ forming carbonic acid (H₂CO₃) — oceans are the largest carbon sink

Human disruption of carbon cycle:

Fossil fuel combustion releases carbon stored for millions of years
Deforestation removes carbon sinks and releases stored carbon
→ Atmospheric CO₂ increases → enhanced greenhouse effect → climate change

Nitrogen fixation: N₂ → NH₄⁺ by Rhizobium (legume roots) or Azotobacter (free-living in soil).
Nitrification: NH₄⁺ → NO₃⁻ by nitrifying bacteria (aerobic — needs oxygen).
Denitrification: NO₃⁻ → N₂ by denitrifying bacteria (anaerobic — waterlogged soils).
Decomposers (bacteria + fungi) release NH₄⁺ from dead organic matter.
Carbon: photosynthesis removes CO₂; respiration and combustion release it.

Ecological Succession

Communities change predictably over time from pioneer species to a stable climax community.

Ecological succession is the directional, predictable process by which the species composition of a community changes over time, from the first colonising organisms to a stable climax community.

Two types of succession:

Primary succession — begins on a completely bare substrate with no soil:

Examples: bare volcanic rock after eruption; bare sand dunes; land exposed by retreating glaciers
Must create soil from scratch before complex communities can develop

Secondary succession — begins on disturbed land where soil and seed bank still remain:

Examples: abandoned farmland, cleared woodland, land after a fire
Faster than primary succession because soil is already present

Stages of primary succession (bare rock example):

Bare rock — extremely harsh; no soil; few organisms can survive
Lichens (pioneer stage) — first colonisers; tolerate desiccation and extreme temperatures; secrete acids that chemically weather rock; die and contribute first organic matter
Mosses — establish in thin proto-soil formed from lichen decomposition and rock particles; hold moisture; add more organic matter
Herbaceous plants (grasses, ferns) — roots break up rock further; more organic matter accumulated; soil deepens; water retention increases
Shrubs — deeper soil supports woody plants; shade out grasses; nitrogen-fixing shrubs (e.g. broom, gorse) enrich soil; further modify microclimate
Pioneer trees — light-demanding trees (e.g. birch, rowan) establish; canopy begins to form; ground flora adapts to partial shade
Climax community — shade-tolerant species (e.g. oak, beech in temperate Britain) establish; self-sustaining; no further systematic change in species composition

Why each community replaces the previous: Each community modifies the abiotic environment (deeper soil, changed light, humidity, temperature, nutrient content) in ways that favour the next, more complex community — which then outcompetes the current one. Pioneer species are adapted to harsh conditions but are poor competitors in improved conditions.

Climax community characteristics:

Final stable end-point of succession — in dynamic equilibrium with regional climate
Greatest species diversity, total biomass, and ecosystem complexity of any seral stage
Self-sustaining — dominant species create conditions that favour their own regeneration
In temperate UK: oak woodland; in tropical equatorial regions: tropical rainforest

Seral stage = each temporary community in the succession sequence.

Primary succession: bare substrate (no soil) → lichens → mosses → grasses → shrubs → trees → climax.
Secondary succession: disturbed soil → faster recovery (seed bank + soil already present).
Pioneer species: first colonisers of bare/disturbed habitats; tolerate extreme conditions.
Climax community: stable end-point; greatest diversity and biomass; in equilibrium with climate.
Each seral stage modifies environment to favour the next — autogenic succession.

Sources: Cambridge IGCSE Environmental Management 0680 syllabus 2027-2029, Section 5.1; 0680/12 May/Jun 2022 — Q3 (food chains and energy); 0680/22 Oct/Nov 2022 — Q6 (ecological succession); 0680 Examiner Reports 2022-2024 — energy flow and nutrient cycling. Last reviewed 2026-05-15.

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Step-by-step worked examples — Ecosystems

Step-by-step solutions to past-paper-style questions on ecosystems, written exactly the way a tutor would explain them at the board.

1Distinguish biotic from abiotic factors with examples
Core• biotic factors, abiotic factors, ecosystem components
Question
Define the terms 'biotic' and 'abiotic'. Give THREE examples of each and explain how ONE abiotic factor affects the organisms in an ecosystem.
Step-by-step solution
1. Step 1
  Biotic factors are the living (biological) components of an ecosystem that affect organisms. Examples: (1) food availability — determines population size of consumers; (2) predators — control prey populations by predation; (3) competition — organisms compete for the same limited resources (food, territory, mates), limiting population growth.
2. Step 2
  Abiotic factors are the non-living physical and chemical components of the environment. Examples: (1) temperature — affects metabolic rates and enzyme activity; (2) light intensity — determines the rate of photosynthesis in producers; (3) rainfall — determines water availability, affecting plant growth and animal distribution.
3. Step 3
  Effect of ONE abiotic factor — light intensity: Light intensity is the primary abiotic factor controlling photosynthesis. As light intensity increases (up to the light saturation point), the rate of photosynthesis increases, producing more glucose and oxygen. This supports greater plant biomass, which in turn supports larger populations of primary consumers and therefore the entire food web. In dense forests, understory plants are adapted to low light (large, flat leaves to capture maximum light), while canopy trees dominate in full sunlight.
4. Step 4
  Interaction of biotic and abiotic factors: Abiotic factors set the physical limits within which organisms can survive, while biotic factors (e.g. competition, predation) determine which species actually occupy that space. Both types interact: a drought (abiotic) reduces plant cover (biotic change), which reduces food for herbivores, which reduces prey for predators — demonstrating how changes in one factor cascade through the ecosystem.
Answer
Biotic: living components — food availability, predators, competition. Abiotic: non-living — temperature, light intensity, rainfall. Light intensity effect: determines photosynthesis rate → controls plant biomass → supports all trophic levels in the food web.
Examiner tip
Always give specific, named examples — 'things in the environment' scores 0. The key distinction is living vs. non-living. Examiners frequently ask for the effect mechanism, not just a list.
2Construct a food chain and calculate energy transfer
Core• Adapted from 0680/12 May/Jun 2022• food chains, energy flow, 10% rule, trophic levels
Question
A grassland food chain contains: grass → grasshopper → frog → snake → hawk. The grass fixes 100,000 kJ of energy per year. (a) State which organism is the producer and which is the tertiary consumer. (b) Using the 10% rule, calculate the energy available to the hawk. (c) Explain what happens to the 90% of energy that is lost at each trophic level.
Step-by-step solution
1. Step 1
  Identifying trophic levels: The producer is grass — it fixes solar energy via photosynthesis into chemical energy in organic compounds. The tertiary consumer is the snake — it is the third animal in the food chain (frog is primary consumer eating grasshopper, frog is secondary consumer, snake eats frogs). The hawk is the quaternary consumer.
2. Step 2
  Calculating energy transfer using the 10% rule: At each trophic level, only approximately 10% of the energy is transferred to the next level.
  
  Grass → Grasshopper: 100,000 × 10% = 10,000 kJ
  
  Grasshopper → Frog: 10,000 × 10% = 1,000 kJ
  
  Frog → Snake: 1,000 × 10% = 100 kJ
  
  Snake → Hawk: 100 × 10% = 10 kJ
  
  The hawk has access to only 10 kJ of the original 100,000 kJ — just 0.01%.
3. Step 3
  What happens to the 90% of energy lost at each level? The 90% that is not transferred is lost in three ways: (1) Heat from respiration — organisms respire to fuel metabolic processes (movement, growth, reproduction), and energy is released as heat, which cannot be re-used by the next trophic level; (2) Undigested material — some organic matter (e.g. bones, fur, plant cell walls) is not digested and passes out in faeces, entering the decomposer pathway instead of the consumer food chain; (3) Metabolic waste (e.g. urea in urine) — nitrogen-containing compounds excreted rather than consumed by the next level.
Answer
Producer: grass. Tertiary consumer: snake. Energy to hawk: 100,000 × (0.1)⁴ = 10 kJ. Lost 90%: heat (respiration), undigested material (faeces), metabolic waste (urine).
Examiner tip
Show the step-by-step calculation — do not just write the final answer. Examiners award marks for each intermediate step. The three reasons for energy loss must be specific: 'heat', 'faeces'/'undigested' and 'metabolic waste'. Simply writing 'it is lost' scores 0.
3Describe the nitrogen cycle with named processes
Core• nitrogen cycle, nutrient cycling, bacteria, decomposition
Question
Describe the main stages of the nitrogen cycle. Name the bacteria involved and the process they carry out at each stage.
Step-by-step solution
1. Step 1
  Nitrogen fixation: Atmospheric nitrogen gas (N₂) makes up 78% of air but most organisms cannot use it directly. Nitrogen-fixing bacteria (e.g. Rhizobium in root nodules of legumes; free-living Azotobacter in soil) convert N₂ → ammonium ions (NH₄⁺) that plants can absorb. Lightning also fixes small amounts of nitrogen as nitrates.
2. Step 2
  Nitrification: Nitrifying bacteria in the soil convert ammonium (NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻) in a two-step oxidation process. Nitrates are the main form of nitrogen absorbed by plant roots. This process requires oxygen (aerobic).
3. Step 3
  Assimilation: Plants absorb nitrates (NO₃⁻) from the soil through their roots and use them to synthesise proteins and nucleic acids (DNA, RNA). Animals obtain nitrogen by consuming plant or animal proteins.
4. Step 4
  Decomposition: When organisms die, decomposers (bacteria and fungi) break down nitrogen-containing compounds (proteins, nucleic acids) in dead organic matter and excretory products (urea) → releasing ammonium ions (NH₄⁺) back into the soil. This process is called ammonification.
5. Step 5
  Denitrification: Denitrifying bacteria in waterlogged (anaerobic) soils convert nitrates (NO₃⁻) → nitrogen gas (N₂) which is released back to the atmosphere. This reduces soil nitrogen and is a loss from the ecosystem. Waterlogged soils promote denitrification, reducing soil fertility.
Answer
Nitrogen fixation: N₂ → NH₄⁺ (Rhizobium/Azotobacter). Nitrification: NH₄⁺ → NO₃⁻ (nitrifying bacteria, aerobic). Assimilation: plants absorb NO₃⁻ → proteins. Decomposition: proteins → NH₄⁺ (bacteria/fungi). Denitrification: NO₃⁻ → N₂ (denitrifying bacteria, anaerobic).
Examiner tip
Name specific bacteria (Rhizobium, Azotobacter, nitrifying bacteria, denitrifying bacteria) — generic 'bacteria' alone often loses marks. The conditions matter: nitrification is aerobic; denitrification is anaerobic (waterlogged soils). These are frequently tested with mark-scheme vocabulary.
4Describe ecological succession from bare rock to climax community
Extended• Adapted from 0680/22 Oct/Nov 2022• succession, pioneer species, climax community, seral stages
Question
A volcanic eruption creates new bare rock. Over time, this develops into a mature woodland ecosystem. (a) Name the type of succession occurring and define 'pioneer species'. (b) Describe the sequence of seral stages from bare rock to climax community, explaining why each community replaces the previous one. (c) Explain what is meant by a 'climax community' and why this represents a stable endpoint.
Step-by-step solution
1. Step 1
  Type of succession and definition: This is primary succession — succession that begins on a previously uncolonised substrate with no soil present (in contrast to secondary succession, which occurs on land that had vegetation but was disturbed, e.g. after a fire or abandoned farmland, where soil and seed banks still remain). A pioneer species is an organism that is the first to colonise a bare or disturbed area. Pioneer species can tolerate extreme, harsh conditions (e.g. little soil, temperature extremes, low nutrients) that other organisms cannot. They modify the environment, making it more hospitable for subsequent species. Examples: lichens on bare rock; marram grass on sand dunes.
2. Step 2
  Sequence of seral stages — bare rock to woodland:
  
  (1) Bare rock stage: Bare volcanic rock provides no soil or nutrients. Only lichens (a symbiosis of algae and fungi) can colonise — they tolerate desiccation and fix nitrogen; they secrete acids that chemically weather the rock.
  
  (2) Mosses: Lichen death adds the first organic matter, creating a thin proto-soil. Mosses establish in crevices, holding moisture and adding more organic matter. The lichen community is progressively replaced by mosses which can grow faster in the now slightly richer conditions.
  
  (3) Herbaceous plants (grasses and ferns): Continued weathering and accumulation of organic matter deepens the soil. Grasses, ferns, and annual herbaceous plants establish. Their roots further break up rock; dead plant matter increases soil depth, water retention, and nutrient content.
  
  (4) Shrubs: Deeper soils support woody shrubs (e.g. heather, bramble). Shrubs shade out grasses (competition for light) and their deeper root systems further improve soil structure. Nitrogen-fixing shrubs (e.g. broom, gorse) enrich soil nitrogen.
  
  (5) Pioneer trees (light-demanding): Light-demanding pioneer tree species (e.g. birch, rowan, pine) establish, growing taller than shrubs and shading them out. As the canopy closes, conditions beneath change dramatically (shade, humidity, shelter from wind).
  
  (6) Climax community (mature woodland): Shade-tolerant climax species (e.g. oak, beech in temperate Britain) eventually replace pioneer trees. These dominant species modify the environment so effectively that no further species replacement occurs.
3. Step 3
  Climax community — definition and stability: A climax community is the final, stable end-point of succession — the community that is in dynamic equilibrium with the climate and soil conditions of that region. It is stable because: (1) The dominant species (e.g. oak trees) create conditions (shade, leaf litter depth, soil pH) that favour their own regeneration and exclude most competitors. (2) The species composition remains roughly constant over time — populations fluctuate but no single species can permanently displace the dominants. (3) The climax community has the greatest species diversity, complex food webs, and highest total biomass of any seral stage, making it resilient to small disturbances. In temperate Britain, the natural climax community is temperate deciduous oak woodland; in the tropical rainforest belt, it is tropical rainforest.
4. Step 4
  Why each community replaces the previous: Each community modifies the abiotic environment (deeper soil, more organic matter, changed light, temperature and humidity) in ways that favour the next, more complex community. The new community then outcompetes and shades out the previous — this is called autogenic succession (change driven by the organisms themselves). Pioneer species effectively 'engineer' conditions that lead to their own replacement — an ecological paradox. Each seral stage is less extreme in abiotic conditions but more complex biotically.
Answer
Primary succession (no pre-existing soil). Pioneer species: first colonisers of bare substrates, tolerating harsh conditions (e.g. lichens). Seral stages: lichens → mosses → grasses/ferns → shrubs → pioneer trees → climax woodland. Each replaces previous by modifying environment (deeper soil, changed light). Climax community: stable, self-sustaining end-state in equilibrium with regional climate; greatest diversity and biomass.
Examiner tip
Extended succession questions award marks for: (1) naming specific pioneer species (lichens) and climax species (oak), (2) explaining the mechanism of replacement (each community modifies abiotic conditions, outcompeted by next stage), (3) distinguishing primary from secondary succession. Describing the sequence without explaining WHY communities change scores only partial marks.
5Analyse a food web to explain stability and the effect of species removal
Extended• food webs, stability, trophic cascades, keystone species
Question
A grassland food web contains the following organisms: grass, clover, grasshopper, rabbit, vole, fox, hawk, owl. Grasshoppers eat grass; rabbits eat grass and clover; voles eat clover; foxes eat rabbits and voles; hawks eat rabbits, voles and grasshoppers; owls eat voles. (a) Explain why a food web is more stable than a single food chain. (b) If fox populations were eliminated by disease, predict and explain the consequences for the rest of the food web. (c) Explain the concept of a 'trophic cascade' using this food web.
Step-by-step solution
1. Step 1
  Why food webs are more stable than food chains: A food web contains multiple interconnected pathways for energy flow. If one species declines or is removed, predators with multiple prey items can switch to alternative prey — this is called dietary switching. For example, if grasshoppers decline, hawks can increase predation on rabbits and voles. In a single food chain with no alternatives, removing any one link collapses all species above it. The greater the number of food web connections (complexity), the more resistant the ecosystem is to perturbation — this is the diversity-stability hypothesis. Food webs also include omnivores (feeding at multiple trophic levels), which act as stabilising buffers.
2. Step 2
  Consequences of fox elimination: (1) Rabbits and voles are no longer preyed upon by foxes → their populations increase (removed top-down predation pressure). (2) Grass and clover experience increased grazing pressure from the now-larger rabbit and vole populations → grass and clover biomass decreases. (3) Hawks and owls have more prey (rabbits, voles more abundant) → hawk and owl populations may temporarily increase. (4) However, if grass/clover is severely depleted, the rabbit and vole populations may eventually crash as food becomes limiting — a delayed population collapse. (5) Grasshoppers may benefit initially if grass abundance increases (if rabbit pressure is initially unable to compensate for fox's absence), or be indirectly harmed if grass is overgrazed. (6) The overall effect is a trophic cascade that destabilises the ecosystem — reduced vegetation cover could lead to soil erosion and further habitat change.
3. Step 3
  Trophic cascade explanation: A trophic cascade is an indirect chain of ecological effects that begins at the top of the food web and cascades downward through multiple trophic levels. In this example: Foxes (top predator) ↓ → Rabbits and voles ↑ → Grass and clover ↓. The fox's effect on grass is indirect — foxes do not eat grass, but by controlling herbivore populations, they indirectly regulate vegetation. This is a top-down trophic cascade (predator-driven). An example from nature: wolf reintroduction in Yellowstone National Park caused elk populations to decline and change behaviour, allowing riverside vegetation (willows, aspens) to recover — even altering river channel morphology. Species that exert disproportionately large trophic effects relative to their abundance are called keystone species (e.g. wolves in Yellowstone, sea otters in kelp forests).
4. Step 4
  Biomass pyramid and energy implications: With foxes removed, more energy is channelled into the rabbit and vole trophic level rather than into fox biomass. The biomass pyramid changes shape — the intermediate trophic levels (rabbits/voles) accumulate disproportionately more biomass relative to the vegetation below, which can lead to overgrazing and ecosystem collapse if vegetation cannot recover. This illustrates why apex predators — even if few in number — are essential for ecosystem stability.
Answer
Food webs more stable: multiple energy pathways allow dietary switching if one prey declines; single food chains collapse if any link is removed. Fox removal: rabbits and voles increase → vegetation decreases → hawks/owls temporarily increase → eventual crash if vegetation depleted. Trophic cascade: top predator removal has indirect cascading effects down to producers (fox controls herbivores; herbivores control vegetation). Yellowstone wolf reintroduction is the classic real-world example.
Examiner tip
Trophic cascade is an Extended concept — use it explicitly and name the mechanism. Always explain the indirect pathway (fox → rabbit → grass, not fox → grass). The Yellowstone wolf case study adds strong AO2 credit for application. Avoid vague answers like 'the food web is disrupted' — specify which populations increase, which decrease, and why.
6Interpret and compare pyramids of numbers, biomass and energy
Extended• Adapted from 0680/12 Oct/Nov 2024• energy pyramids, biomass pyramids, numbers pyramids, 10% rule
Question
A food chain: oak tree → caterpillar → blue tit → sparrowhawk. The following data are available: oak trees = 1; caterpillars = 10,000; blue tits = 100; sparrowhawks = 2. Total biomass: oak = 500 kg; caterpillars = 50 kg; blue tits = 6 kg; sparrowhawks = 0.8 kg. (a) Draw and explain the pyramid of numbers for this food chain. Comment on whether it follows the expected shape. (b) Draw and explain the pyramid of biomass and comment on its shape. (c) Explain why pyramids of energy always have a true pyramid shape, while pyramids of numbers and biomass sometimes do not.
Step-by-step solution
1. Step 1
  Pyramid of numbers: Plotting number of individuals at each trophic level: Oak (1) → Caterpillars (10,000) → Blue tits (100) → Sparrowhawks (2). This gives an inverted pyramid (narrowest at base, widest in the middle) — NOT the expected pyramid shape. This is because one very large oak tree supports thousands of caterpillars. The shape is unusual because the producer is a single large individual rather than many small plants. This is a common exception: when the producer is a large organism (tree, large plant), the numbers pyramid inverts at the base.
2. Step 2
  Pyramid of biomass: Plotting total biomass at each trophic level: Oak (500 kg) → Caterpillars (50 kg) → Blue tits (6 kg) → Sparrowhawks (0.8 kg). This gives the expected true pyramid shape — widest at base, narrowing at each successive trophic level. At each level, total biomass decreases because: only ~10% of energy (and thus biomass production capacity) is transferred from one level to the next; the rest is lost as heat, faeces, and metabolic waste. Note: even though there are more caterpillars than blue tits (by number), the total biomass of caterpillars (50 kg) exceeds blue tit biomass (6 kg), so the pyramid remains correctly shaped here.
3. Step 3
  Why pyramids of energy ALWAYS have a true pyramid shape: Pyramids of energy measure the flow of energy through each trophic level per unit time (kJ/m²/year), not the number or instantaneous biomass of organisms. Because energy transfer efficiency is always less than 100% (typically ~10%) at each trophic level — due to heat loss in respiration, undigested material, and metabolic waste — the total energy available ALWAYS decreases at each successive level. It is physically impossible for more energy to be present in the consumer than in the organism it eats (this would violate the First Law of Thermodynamics). By contrast, numbers pyramids can invert when producers are large; biomass pyramids can occasionally invert in aquatic ecosystems where phytoplankton (producers) are tiny, short-lived, but support large zooplankton populations (zooplankton biomass can exceed phytoplankton biomass at any given moment, even though phytoplankton productivity is higher annually).
4. Step 4
  The key distinction: Numbers and biomass pyramids represent a snapshot at a single point in time — they can be misleading because they do not account for production rates. A phytoplankton cell lives for hours and is immediately eaten; it may have low instantaneous biomass but very high annual production. Energy pyramids integrate production over time and always give the true picture of energy flow. This is why pyramids of energy are the most ecologically meaningful of the three types.
Answer
Numbers pyramid: inverted (1 oak supports 10,000 caterpillars) — exception to standard pyramid shape due to large producer. Biomass pyramid: true pyramid (500 → 50 → 6 → 0.8 kg) — biomass always decreases up the chain. Energy pyramids always pyramid-shaped: energy lost as heat/faeces at each level means energy availability always decreases — cannot violate First Law of Thermodynamics. Biomass pyramids can invert in aquatic systems (phytoplankton low instantaneous biomass but high productivity).
Examiner tip
The phytoplankton exception (inverted biomass pyramid in aquatic systems) is a high-level Extended point that distinguishes A* candidates. Always give the reason for the pyramid shape — 'the numbers decrease' is insufficient; explain WHY (10% rule, energy loss mechanisms). Numbers pyramid inversion at producer level (trees) is the most commonly tested exception.

Key Definitions and Keywords — Ecosystems

Definitions to memorise and the exact keywords mark schemes credit for ecosystems answers — sharpened from recent examiner reports for the 2026 0680 sitting.

Ecosystem
Examiner keyword
A community of interacting organisms (the biotic component) together with the physical and chemical environment they inhabit (the abiotic component), functioning as a system through which energy flows and matter cycles.
Related: biotic factors, abiotic factors, community
Food chain
A linear sequence showing the feeding relationships between organisms, with arrows indicating the direction of energy flow from producer to consumers. Each stage is a trophic level.
Related: food web, trophic level, energy flow
Producer
Examiner keyword
An organism (typically a green plant or alga) that synthesises organic compounds from inorganic sources (mainly CO₂ and H₂O) using solar energy via photosynthesis. Producers form the first trophic level of all food chains.
Related: photosynthesis, autotroph, trophic level
Trophic level
A feeding level in a food chain or web. Producers occupy level 1; primary consumers (herbivores) level 2; secondary consumers level 3; tertiary consumers level 4.
Related: food chain, energy pyramid, 10% rule
Decomposer
Examiner keyword
Organisms (mainly bacteria and fungi) that break down dead organic matter and waste products, releasing inorganic nutrients (e.g. ammonium ions, phosphates) back into the soil or water for re-use by producers.
Related: nutrient cycling, nitrogen cycle, carbon cycle
Ecological succession
Examiner keyword
The directional, predictable change in the species composition of a community over time, from pioneer species through seral stages to a stable climax community.
Related: pioneer species, climax community, primary succession, secondary succession
Climax community
The final, stable community reached at the end of ecological succession, in dynamic equilibrium with the regional climate and soil. It has the greatest biodiversity, biomass and ecological complexity of any seral stage.
Related: ecological succession, temperate deciduous woodland
Nitrogen fixation
Examiner keyword
The conversion of atmospheric nitrogen gas (N₂) into ammonium ions (NH₄⁺) or nitrates (NO₃⁻) that plants can absorb. Carried out by nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules; free-living Azotobacter in soil) and by lightning.
Related: nitrogen cycle, Rhizobium, denitrification

Common Mistakes and Misconceptions — Ecosystems

The traps other students keep falling into on ecosystems questions — taken from recent Cambridge IGCSE 0680 examiner reports and mark schemes — and how to avoid them.

✕Drawing food chain arrows in the wrong direction (from predator to prey, or bidirectional).
Consistently penalised in 0680 mark schemes — the arrow direction is specifically checked.
Why it happens
Students confuse 'eats' with the direction of energy flow.
How to avoid it
Arrows in food chains represent the direction of energy flow — they always point FROM the organism being eaten TO the organism doing the eating: Grass → Rabbit → Fox. The arrowhead points towards the organism receiving the energy.
✕Saying energy 'cycles' in an ecosystem (like nutrients do).
Why it happens
Students know nutrients cycle and assume energy does too.
How to avoid it
Energy flows in one direction — it enters as sunlight, flows through the food web, and is lost as heat. It does NOT cycle. Only matter (nutrients: carbon, nitrogen, phosphorus) cycles. This is a fundamental distinction in ecology.
✕Confusing denitrification with decomposition in the nitrogen cycle.
Why it happens
Both involve bacteria breaking down nitrogen compounds.
How to avoid it
Decomposition (ammonification): dead organic matter → ammonium (NH₄⁺) — releases nitrogen INTO the soil. Denitrification: nitrates (NO₃⁻) → nitrogen gas (N₂) — removes nitrogen FROM the soil ecosystem back to atmosphere. Denitrification happens in anaerobic (waterlogged) conditions and reduces soil fertility.
✕Assuming the producer must always be a plant.
Why it happens
Most examples use grass or trees as producers.
How to avoid it
Producers are any autotrophs that can photosynthesise. In aquatic ecosystems, producers are phytoplankton (microscopic algae) and algae — not plants. In some hydrothermal vent ecosystems, chemosynthetic bacteria act as producers using chemical energy rather than sunlight.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Ecosystems

Detailed Notes

What you’ll learn

1Distinguish biotic from abiotic factors with examples

2Construct a food chain and calculate energy transfer

3Describe the nitrogen cycle with named processes

4Describe ecological succession from bare rock to climax community

5Analyse a food web to explain stability and the effect of species removal

6Interpret and compare pyramids of numbers, biomass and energy

Ecosystem

Food chain

Producer

Trophic level

Decomposer

Ecological succession

Climax community

Nitrogen fixation

✕Drawing food chain arrows in the wrong direction (from predator to prey, or bidirectional).

✕Saying energy 'cycles' in an ecosystem (like nutrients do).

✕Confusing denitrification with decomposition in the nitrogen cycle.

✕Assuming the producer must always be a plant.

Past paper style quiz