Key Idea: This topic answers one question: how do energy and matter move through an ecosystem? Two rules run through all of it: Energy flows one way and runs out. It enters as sunlight, is captured by producers, and passes up food chains — but only about 10% survives each step, so it is steadily lost (mostly as heat) and is never recycled. Matter (carbon, nutrients) is recycled. The same atoms move round and round between living things, the air, the soil and rocks — driven by photosynthesis, respiration, decomposition and combustion, and unlocked from dead matter by decomposers. It is a regular on Paper 1A (MCQs and diagram-reading), Paper 2 (explain how energy/carbon moves) and Paper 3 (data: pyramids, feed-conversion, nutrient-cycle diagrams).
🍽️ Modes of nutrition — autotrophs vs heterotrophs
Every organism is sorted by how it gets two things: its energy and its carbon. An autotroph makes its own organic food from inorganic CO₂; a heterotroph cannot, so it takes in ready-made organic food from other organisms.
The key classifications
- Photoautotroph — energy from light (photosynthesis); carbon from CO₂ (plants, algae)
- Chemoautotroph — energy from oxidising inorganic substances (chemosynthesis); carbon from CO₂ (vent bacteria)
- Heterotroph — energy and carbon from food (animals, fungi, most bacteria)
- Holozoic = heterotroph that ingests food and digests it internally (animals)
- Saprotroph = heterotroph that digests externally (enzymes onto dead matter, then absorbs)
- Mixotroph = uses both autotrophic and heterotrophic nutrition (e.g. Euglena)
| Mode of nutrition | Energy source | Carbon source |
|---|---|---|
| Photoautotroph | Light (photosynthesis) | Inorganic — CO₂ |
| Chemoautotroph | Oxidising inorganic substances (chemosynthesis) | Inorganic — CO₂ |
| Heterotroph | Chemical energy in food | Organic — other organisms |
| Mixotroph | Both (light/chemicals AND food) | Both (inorganic AND organic) |
Photo = light. Chemo = chemicals. Auto = makes its own. Hetero = eats others. Photoautotrophs and chemoautotrophs differ only in their energy source — both fix CO₂ for carbon. To read a table row, always check both columns.
🔗 Food chains, food webs & trophic levels
A food chain shows the one-way flow of energy as a line of organisms joined by arrows. Each step is a trophic level — a feeding position. The arrow always points from the organism being eaten TO the organism that eats it (the direction energy flows): read it as 'is eaten by'.
A food web links many food chains. Arrows point FROM the organism eaten TO the eater — the direction energy flows. Count arrows up from a producer to read any organism's trophic level.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Food chain: Shows **one** path of energy. Usually **one** organism per level. A simplified slice of the ecosystem. Arrows point **towards the eater**.
Food web: **Many** food chains linked together. Often **several** organisms per level. Closer to the **real** ecosystem. One organism can sit at **two** trophic levels.
| Trophic level | Name | What it does | Example |
|---|---|---|---|
| 1st | Producer | Makes its own food by photosynthesis | Grass, algae |
| 2nd | Primary consumer | Herbivore — eats producers | Rabbit, grasshopper |
| 3rd | Secondary consumer | Carnivore — eats primary consumers | Fox, frog |
| 4th | Tertiary consumer | Carnivore — eats secondary consumers | Hawk, eagle |
Count arrows up from the producer: 1 producer, 2 primary, 3 secondary, 4 tertiary consumer. In a web, an organism that feeds at different levels (e.g. a fox eating both a rabbit and a frog) can occupy more than one trophic level at once — a favourite exam point worth a mark.
📉 Energy losses & pyramids of energy
As energy passes up a chain, only about 10% reaches the next trophic level — roughly 90% is lost at every step. A pyramid of energy shows this: each bar is far smaller than the one below. Energy is not recycled, so ecosystems need a constant supply of sunlight.
A pyramid of energy: only about 10% passes to each higher level, so each bar is roughly a tenth of the one below; the other ~90% is lost as heat (respiration) and in waste — which is why food chains stay short.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Energy is lost in three main ways at each level: heat from respiration (by far the largest — it leaves the ecosystem and can never be eaten), undigested waste (faeces), and uneaten or dead material. Because so little is left after a few steps, there is not enough energy to support another level — so food chains rarely exceed 4–5 links, and top predators are rare.
| Where the ~90% goes | What happens | Goes up the chain? |
|---|---|---|
| Heat from respiration | Energy released to power the organism, lost as heat | No — the biggest loss |
| Undigested waste (faeces) | Some food is never absorbed and is egested | No — passes to decomposers |
| Uneaten / dead material | Not all of an organism is eaten | No — passes to decomposers |
| Built into biomass | The small remainder stored as body growth | Yes — only THIS can be eaten |
An extra trophic level wastes energy, so beef (plant → cow → human, two transfers) is less energy-efficient than eating the plants directly or raising better feed-converters like chickens or insects — a classic data-question theme.
☠️ Pollutants in food chains — bioaccumulation & biomagnification
Some pollutants — such as DDT or methyl mercury — are persistent (non-biodegradable) and not excreted, so they are stored (often in fat) instead of being broken down. This makes them build up and concentrate up the chain — the opposite of energy.
A persistent pollutant gets more concentrated up the chain: each predator eats many contaminated prey and keeps the pollutant, so it multiplies up the levels and is highest in the top predator — the opposite of energy.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Bioaccumulation (within one organism): Pollutant **taken in faster than it is removed**. It is **stored** (often in fat), not excreted. The amount **rises as the organism ages**. Happens in **every** organism in the chain.
Biomagnification (up the chain): Each consumer eats **many** contaminated prey. It keeps **all** of their pollutant. Concentration **multiplies at each level**. **Highest** in the **top predator**.
Energy decreases up a chain (lost as heat); a persistent pollutant increases up it (stored and passed on). To explain why it peaks at the top, link 'persistent / not excreted → stored' to 'each predator eats many prey → concentration multiplies' — naming 'biomagnification' alone is not enough. Classic effect: DDT thins birds' eggshells, causing population decline.
♻️ The carbon cycle
Carbon is recycled between the air, living things, oceans and rocks — never made or destroyed. One process removes CO₂ from the air (photosynthesis); three put it back (respiration, decomposition and combustion). Feeding passes organic carbon along the food chain.
The carbon cycle is a balance: photosynthesis is the only process that removes CO₂, while respiration, decomposition and combustion return it. Decomposers also recycle mineral nutrients back to the soil.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Process | In or out of the air | Effect on CO₂ |
|---|---|---|
| Photosynthesis | carbon OUT of the air (the only one) | lowers CO₂ |
| Respiration | carbon INTO the air | raises CO₂ |
| Decomposition | carbon INTO the air | raises CO₂ |
| Combustion | carbon INTO the air | raises CO₂ |
A carbon sink stores carbon (forests, peat, limestone, fossil fuels); a carbon source releases it (respiration, decomposition, combustion). Carbon is locked into a sink when decomposition is slowed by waterlogged, anaerobic, acidic conditions (peat). Aquatic autotrophs take their carbon as dissolved CO₂ and hydrogencarbonate (HCO₃⁻) ions from the water — not as CO₂ gas from the air.
🍄 Decomposers & nutrient cycling
Decomposers feed on dead organic matter and break it down, releasing inorganic nutrients (nitrate, phosphate) back to the soil so producers can reuse them. They close the recycling loop — without them, nutrients would stay locked in dead matter and producers would run out of raw materials.
Key Idea: Detritivores (animals like earthworms) ingest dead matter and digest it internally. Saprotrophs (mostly bacteria and fungi) secrete enzymes onto the dead matter and digest it externally, then absorb the products. They feed in different ways but do the same job — recycling nutrients — which is why both are decomposers.
| Feature | Detritivore | Saprotroph |
|---|---|---|
| What it is | an animal (earthworm, woodlouse) | mostly bacteria and fungi |
| Takes food in by | ingesting (swallowing) dead matter | secreting enzymes onto dead matter |
| Digestion | internal (in a gut) | external (outside the body), then absorbs |
| Shared role | recycles nutrients from dead matter | recycles nutrients from dead matter |
Boxes = stores of nutrients (soil, litter, biomass); arrows = transfers of nutrients between them (thickness = size of the transfer). An arrow into the soil store (decomposition, weathering, rainfall) adds nutrients; an arrow out of it (uptake by roots, leaching, runoff) reduces it. Read the diagram — don't recall a fact.
✍️ Worked examples
IB-style question — construct a food chain & name a level
A meadow contains clover, field mice, grass snakes and barn owls. Construct a food chain using all four, and identify the trophic level of the grass snake. [3]
Model answer:
Producer first. Clover photosynthesises, so it starts the chain (level 1).
Order by what eats what, arrows to the eater. clover → field mouse → grass snake → barn owl, each arrow pointing from prey to the organism that eats it.
Name the snake's level. Count arrows from the producer: it is two arrows up, so the grass snake is at trophic level 3 — a secondary consumer (it eats the primary consumer, the mouse).
clover → field mouse → grass snake → barn owl (arrows towards the eater); the grass snake is a secondary consumer (third trophic level).
IB-style question — why energy decreases up a chain
Explain why the amount of energy available decreases at each successive trophic level. [4]
Model answer:
Only about 10% of the energy at one level is passed to the next, so most is lost at each step.
Most is lost as heat from respiration, which leaves the ecosystem and cannot be passed on as food.
Some is lost in undigested waste (faeces), passing to decomposers rather than up the chain.
Not all of an organism is eaten (uneaten or dead material), so that energy is also lost. (One mark per distinct point.)
Only ~10% passes on; most energy is lost as heat from respiration; some in undigested waste (faeces); and not all of an organism is eaten — so less energy is available at each higher level.
IB-style question — why the top predator is worst affected
A persistent insecticide is sprayed on a lake. Explain why its concentration is far higher in the fish-eating birds than in the algae. [3]
Model answer:
The insecticide is non-biodegradable and not excreted, so once taken in it is stored, not broken down or lost.
Each consumer eats many contaminated organisms and keeps all of their insecticide, so its concentration is higher than in any single prey item (biomagnification).
This repeats at every level, so the concentration multiplies up the chain and is highest in the top predator (the fish-eating birds).
It is persistent and not excreted, so it is stored; each consumer eats many contaminated prey and keeps it, so the concentration multiplies up the chain and is highest in the top predator.
IB-style question — describe how carbon is recycled
Describe how carbon is recycled within an ecosystem. [4]
Model answer:
Photosynthesis fixes CO₂ from the air into organic carbon (glucose) in producers, removing it from the air.
Feeding passes that organic carbon from producers to consumers along the food chain.
Respiration in all organisms breaks down organic carbon and releases CO₂ back to the air.
Decomposition (decomposers respiring as they break down dead matter) and combustion of wood/fossil fuels also return carbon as CO₂. (One mark per process.)
Photosynthesis fixes CO₂ into organic carbon; feeding passes it along the food chain; respiration releases CO₂; and decomposition plus combustion return the rest as CO₂.
✅ Quick self-check
Tap each card to check yourself.
Autotroph vs heterotroph — and how do photo- and chemo-autotrophs differ? An autotroph makes its own organic food from inorganic CO₂; a heterotroph takes in ready-made organic food. Photoautotrophs and chemoautotrophs differ only in energy source (light vs oxidising inorganic substances) — both fix CO₂.
Which way does a food-chain arrow point, and how do you read a trophic level? From the organism eaten TO the eater (the direction energy flows — 'is eaten by'). Count arrows up from the producer: 1 producer, 2 primary, 3 secondary, 4 tertiary consumer.
About what % of energy passes up, and where does the rest go? About 10% passes to the next level. The other ~90% is lost mainly as heat from respiration, plus in faeces and uneaten/dead material — which is why food chains rarely exceed 4–5 links.
Bioaccumulation vs biomagnification? Bioaccumulation = build-up of a pollutant inside ONE organism over time; biomagnification = the rise in its concentration BETWEEN trophic levels, so it is highest in the top predator. A toxin rises up the chain while energy falls.
Which carbon-cycle process removes CO₂, and which add it back? Photosynthesis is the only process that removes CO₂ (producers fix it into organic carbon). Respiration, decomposition and combustion all return it. Aquatic autotrophs take dissolved CO₂ and HCO₃⁻ from water.
What do decomposers do, and how do detritivores and saprotrophs differ? Decomposers break down dead matter and recycle inorganic nutrients to the soil for producers. A detritivore ingests dead matter and digests it internally; a saprotroph secretes enzymes onto it and digests externally — same job, different route.
Exam Tips
- Two big rules: ENERGY flows one way and runs out (~10% per step); MATTER (carbon, nutrients) is recycled. Keep them separate.
- Classify nutrition on TWO axes — energy source AND carbon source; photo- vs chemo-autotrophs differ only in energy.
- Food-chain arrows point TO the eater; count arrows from the producer to name a level; in a web one organism can occupy two levels.
- For 'why energy decreases', give SEPARATE points: ~10% passed on, heat from respiration (the biggest), faeces, uneaten material — not 'energy is lost' four times.
- To EXPLAIN biomagnification, link 'persistent / not excreted → stored' to 'many prey eaten → concentration multiplies up the chain'. Energy falls, toxin rises.
- Name the carbon PROCESSES (photosynthesis, respiration, decomposition, combustion) — photosynthesis is the only one that removes CO₂. Aquatic autotrophs use dissolved CO₂ / HCO₃⁻.
- Role of decomposers = break down dead matter AND recycle nutrients to producers (the recycling mark is the one most often missed). On a Gersmehl diagram, arrows = transfers; an arrow out of the soil store (uptake, leaching) reduces it.
Key Idea: Energy enters as sunlight, flows up food chains losing ~90% per step (mostly as heat), and is never recycled — so pyramids of energy shrink and chains stay short. Matter is recycled: carbon via photosynthesis/respiration/decomposition/combustion, and mineral nutrients via decomposers. Persistent pollutants buck the trend, concentrating up the chain to peak in the top predator.