A leaf is eaten by a caterpillar. The caterpillar is eaten by a bird. The bird may then be eaten by a larger predator. Although this looks like a simple sequence of organisms eating one another, what is really being transferred through the sequence is energy stored in organic matter.
Food chains and food webs help us trace this movement of energy through an ecosystem. Once you understand what the arrows mean, what each organism is doing, and why less energy reaches higher trophic levels, many apparently complicated ecology questions become much easier to interpret.
Start with five questions to check whether the essential ideas about energy flow and feeding relationships are already clear.
- 5 diagnostic questions
- Food-chain interpretation and core ecology ideas
- Instant explanation for every answer option
How energy flows through an ecosystem
For most ecosystems, the original source of energy is the Sun. Green plants and other photosynthetic producers absorb light energy and use it during photosynthesis to manufacture organic substances.
When another organism eats a producer, some of the chemical energy stored in that producer is transferred to the consumer. If that consumer is then eaten, some of its stored chemical energy can be transferred again.
The central idea: energy enters an ecosystem, passes from one organism to another, and is eventually dissipated to the surroundings. It does not cycle back to the Sun.
This is why energy flow is described as non-cyclical. Nutrients can be recycled through ecosystems, but energy requires a continual input.
Producers, consumers and decomposers
Organisms play different roles in the transfer of matter and energy through an ecosystem. Three important groups are producers, consumers and decomposers.
| Role | What the organism does | Examples |
|---|---|---|
| Producer | Manufactures organic substances from inorganic substances, usually using light energy during photosynthesis. | Grass, trees, algae |
| Consumer | Obtains energy and nutrients by feeding on other organisms. | Grasshopper, rabbit, frog, hawk |
| Decomposer | Obtains nutrients from dead organisms and organic wastes and helps return mineral nutrients to the environment. | Many bacteria and fungi |
Primary, secondary and tertiary consumers
Consumers can be described according to their feeding position.
Primary consumer: feeds directly on a producer.
Secondary consumer: feeds on a primary consumer.
Tertiary consumer: feeds on a secondary consumer.
These descriptions refer to an organism's position in a particular feeding pathway. An omnivore may therefore occupy different positions in different food chains depending on what it is eating.
Food chains show one pathway of energy transfer
A food chain is a sequence showing how energy is transferred from one organism to another through feeding.
The direction of every arrow matters. The arrow points towards the organism receiving the energy.
In grass → grasshopper, the arrow can be read as “grass is eaten by grasshopper”, but its biological meaning is that energy is transferred from the grass to the grasshopper.
Trophic levels
Each feeding position in a food chain is called a trophic level.
| Trophic level | Typical role | Example |
|---|---|---|
| 1 | Producer | Grass |
| 2 | Primary consumer | Grasshopper |
| 3 | Secondary consumer | Frog |
| 4 | Tertiary consumer | Snake |
Always begin counting trophic levels with the producer as trophic level 1.
Food webs connect many food chains
Real organisms usually have more than one food source, and many are eaten by more than one type of predator. A single straight food chain therefore gives only part of the picture.
A food web is a network of interconnected food chains showing several feeding relationships within an ecosystem.
A food chain follows one pathway of energy transfer. A food web shows many interconnected pathways.
Food webs are useful when predicting how a change in one population may affect other populations. For example, if one prey species decreases, a predator may have less food available. However, if that predator feeds on several different species, alternative food sources may reduce the effect.
This does not mean that every food web is automatically stable. The effect depends on how strongly organisms depend on one another and whether suitable alternative food sources are available.
Why less energy reaches higher trophic levels
Energy transfer from one trophic level to the next is inefficient. Only part of the energy available at one trophic level becomes part of the biomass that is eventually eaten and assimilated by organisms at the next level.
Several processes account for this.
| What happens to the energy? | Why it is not transferred to the next consumer |
|---|---|
| Respiration | Energy is released during respiration. Much of this energy is eventually dissipated as heat to the surroundings. |
| Movement and other life processes | Energy used for muscle contraction and other cellular activities is ultimately dissipated as heat. |
| Parts are not eaten | Roots, bones, shells or other parts may not be consumed by the next organism in that particular food chain. |
| Food is not completely digested | Some material passes out as faeces through egestion rather than being absorbed into the consumer. |
| Organic material leaves the body | Some energy-containing substances may be lost in excretory products rather than becoming biomass available to the next consumer. |
Material that is not eaten may still be used by decomposers, so it has not necessarily disappeared from the ecosystem. The important point is that it has not been transferred to the next consumer in that particular feeding pathway.
Calculating energy-transfer efficiency
Energy-transfer efficiency tells us what percentage of the energy available at one trophic level is transferred to the next trophic level.
Suppose producers contain 12 000 kJ of energy and the primary consumers obtain 1 800 kJ.
Efficiency = (1 800 ÷ 12 000) × 100% = 15%
You may sometimes see examples in which roughly 10% of the energy is transferred to the next trophic level. Treat this as an approximation rather than a universal biological rule. The actual percentage varies between organisms and ecosystems.
Why food chains are usually short
Because energy is lost from the transferable pool at every trophic level, progressively less energy is available to support organisms higher in the food chain.
Eventually, too little energy remains to support another substantial trophic level. This helps explain why food chains usually contain only a limited number of feeding steps.
Pyramids of numbers and biomass
Ecologists can represent trophic levels using ecological pyramids. At O-Level, two important types are the pyramid of numbers and the pyramid of biomass.
Pyramid of numbers
A pyramid of numbers represents the number of organisms at each trophic level.
A grassland may contain many grass plants, fewer herbivores and still fewer predators, giving the familiar broad-base pyramid shape.
However, a pyramid of numbers does not have to look like a normal pyramid. One large tree, for example, may support many insects. There would be only one producer but many primary consumers.
Pyramid of biomass
A pyramid of biomass represents the total dry mass of organisms at each trophic level, usually for a defined area or volume at a particular time.
Biomass gives more information about the amount of living material present than simply counting organisms. One tree and one caterpillar both count as one organism in a pyramid of numbers even though their masses are enormously different.
| Feature | Pyramid of numbers | Pyramid of biomass |
|---|---|---|
| Measures | Number of individual organisms | Total dry mass of organisms |
| Accounts for organism size? | No | Yes |
| Can have an unusual shape? | Yes. A small number of large organisms can support many smaller organisms. | Biomass often decreases at successive trophic levels, although the measured shape can depend on the ecosystem and the time of measurement. |
| Useful for | Comparing the number of organisms | Comparing the amount of living material |
Persistent insecticides can become concentrated along a food chain
Food chains can transfer substances that are harmful to organisms as well as transferring nutrients and energy.
Some insecticides are persistent. If they are not readily broken down or removed from an organism, residues may accumulate within its tissues.
Bioaccumulation describes the build-up of a substance within an individual organism over time.
Biomagnification describes an increase in the concentration of a persistent substance at successive trophic levels in a food chain.
Consider a predator that eats many contaminated prey organisms. Each prey may contain only a small amount of insecticide, but the predator takes in residues from many individuals. If the substance is not easily broken down or removed, its concentration in the predator's tissues can become greater.
As a result, organisms near the top of the food chain may experience the greatest concentrations and therefore the greatest biological effects. Depending on the substance and dose, these effects can include poisoning, reduced reproductive success or death.
Notice that the insecticide itself is not providing useful energy. It is simply transferred alongside food through feeding relationships.
Common Mistakes
| Common mistake | Why it is wrong | Correct approach |
|---|---|---|
| Thinking the arrow points towards the organism being eaten. | The arrow represents the direction of energy transfer. | Point the arrow from the organism being eaten towards the organism receiving the energy. |
| Starting trophic-level numbering with the primary consumer. | The producer is already the first trophic level. | Producer = trophic level 1; primary consumer = trophic level 2. |
| Saying decomposers recycle energy. | Decomposers help recycle nutrients, but energy continues to be dissipated through respiration. | Distinguish nutrient cycling from non-cyclical energy flow. |
| Assuming exactly 10% of energy is always transferred. | Energy-transfer efficiency varies between organisms and ecosystems. | Use the values supplied in the question and calculate the percentage when required. |
| Calling faeces an excretory product. | Faeces mainly contain undigested material that was never absorbed into the body's cells. | Loss of faeces is egestion; removal of metabolic waste is excretion. |
| Assuming a pyramid of numbers must always look like a normal pyramid. | A small number of large producers may support many smaller consumers. | Interpret the actual numbers rather than predicting the shape automatically. |
| Confusing numbers with biomass. | Counting organisms does not account for differences in their size. | Biomass refers to the mass of living material, whereas numbers refers to individual organisms. |
| Using bioaccumulation and biomagnification as though they mean exactly the same thing. | One describes build-up within an organism; the other describes increasing concentration across trophic levels. | Use bioaccumulation for an individual organism and biomagnification for the increase along a food chain. |
What You Should Be Able to Do
Describe energy flow
Explain why energy flow through an ecosystem is non-cyclical.
Identify ecological roles
Distinguish producers, consumers and decomposers in unfamiliar examples.
Read food chains and food webs
Interpret arrows correctly and identify feeding relationships and trophic levels.
Explain energy losses
Explain why only part of the energy at one trophic level reaches the next.
Interpret ecological pyramids
Describe and compare pyramids of numbers and pyramids of biomass.
Explain biomagnification
Explain why persistent insecticides may reach their highest concentrations in top carnivores.
These ten questions move beyond basic recall and test whether you can interpret energy transfer, trophic levels, ecological pyramids and changes within food webs.
- 10 mastery questions
- Calculations and unfamiliar ecological situations
- Instant explanation for every answer option
Frequently asked questions
Food Chains and Food Webs: Questions Students Often Ask
The details that cause the most confusion in this topic are usually arrow direction, trophic levels, energy losses and the difference between ecological pyramids. These answers bring those ideas together.
Which way should an arrow point in a food chain?
The arrow points from the organism being eaten towards the organism receiving the energy. In grass → rabbit, chemical energy stored in the grass is transferred to the rabbit when the rabbit eats it.
What is the difference between a food chain and a food web?
A food chain shows one pathway of energy transfer through feeding. A food web links several food chains together and shows the many feeding relationships that can occur within an ecosystem.
Why is energy flow non-cyclical if decomposers recycle material?
Decomposers help return nutrients to the environment, so matter can be reused. Energy behaves differently. Organisms respire, and energy is continually dissipated to the surroundings as heat. That energy is not recycled through the ecosystem.
Is exactly 10% of the energy always transferred to the next trophic level?
No. Values around 10% are sometimes used as simple examples, but the actual efficiency varies. In an examination question, use the values given and calculate the efficiency rather than automatically assuming 10%.
Is a primary consumer at trophic level 1?
No. Producers occupy trophic level 1. A primary consumer feeds directly on a producer and therefore normally occupies trophic level 2.
Why can a pyramid of numbers have an unusual shape?
A pyramid of numbers counts individual organisms without considering their size. A single large producer such as a tree may support many smaller herbivores, so the producer level can contain fewer individuals than the primary-consumer level.
What does a pyramid of biomass measure?
It represents the total dry mass of organisms at each trophic level, usually within a defined area or volume at a particular time. Because it takes organism size into account, it gives different information from a pyramid of numbers.
What is the difference between bioaccumulation and biomagnification?
Bioaccumulation is the gradual build-up of a persistent substance within one organism. Biomagnification is the increase in concentration of that substance at successive trophic levels. This is why top carnivores may contain particularly high concentrations of persistent insecticides.
Do decomposers belong to one particular trophic level?
Not neatly. Decomposers can obtain organic material from dead organisms originating at several trophic levels. They are therefore often discussed separately from the simple producer–consumer trophic-level sequence.
Why does less energy reach organisms near the top of a food chain?
At every trophic level, energy is used in respiration and eventually dissipated as heat. Some biomass is not eaten, and some ingested material is not digested or absorbed. As a result, only part of the energy at one trophic level becomes biomass available to the next consumer.
Food chains are really maps of energy transfer. Once you follow the arrows in the correct direction, identify the trophic levels and remember that progressively less energy is available at each transfer, food webs, ecological pyramids and biomagnification all fit into the same connected picture of how an ecosystem works.