19.2 Food chains and food webs
- Syllabus
- 0610–2026–2027
- Topic
- 19.2
- Level
- —
Learning objectives
19.2.1Food chain as showing the transfer• Describe a food chain as showing the transfer of energy from one organism to the next, beginning with a producer19.2.2Construct and interpret simple food• Construct and interpret simple food chains19.2.3Food web as a network• Describe a food web as a network of interconnected food chains and interpret food webs19.2.4Producer as an organism that makes its• Describe a producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis19.2.5Consumer as an organism that gets its• Describe a consumer as an organism that gets its energy by feeding on other organisms19.2.6Consumers may be classed as primary• State that consumers may be classed as primary, secondary, tertiary and quaternary according to their position in a food chain19.2.7Herbivore as an animal that gets its• Describe a herbivore as an animal that gets its energy by eating plants19.2.8Carnivore as an animal that gets its• Describe a carnivore as an animal that gets its energy by eating other animals19.2.9Decomposer as an organism that gets• Describe a decomposer as an organism that gets its energy from dead or waste organic material19.2.10Food chains and food webs to describe• Use food chains and food webs to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat19.2.11Pyramids of numbers and pyramids• Draw, describe and interpret pyramids of numbers and pyramids of biomass19.2.12Pyramids of biomass• Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain19.2.13Trophic level as the position• Describe a trophic level as the position of an organism in a food chain, food web or ecological pyramid19.2.14Following as the trophic levels• Identify the following as the trophic levels in food webs, food chains and ecological pyramids: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers19.2.15Pyramids of energy• Draw, describe and interpret pyramids of energy19.2.16Pyramids of energy• Discuss the advantages of using a pyramid of energy rather than pyramids of numbers or biomass to represent a food chain19.2.17Transfer of energy from one trophic• Explain why the transfer of energy from one trophic level to another is often not efficient19.2.18Explain, in terms of energy loss, why• Explain, in terms of energy loss, why food chains usually have fewer than five trophic levels19.2.19It is more energy efficient for humans• Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants
Read a food chain as energy transfer
A food chain shows the transfer of energy from one organism to the next and begins with a producer.
Each arrow points from the organism being eaten to the organism that feeds on it: it shows the direction of energy transfer.
An arrow does not point towards the food. In grass → rabbit, energy moves from grass to rabbit.
Construct and interpret a simple food chain
- Start with the producer. 2. Place the organism that eats it next. 3. Continue in feeding order. 4. Draw every arrow from food to feeder.
To predict a population change, follow both directions: fewer prey can reduce its predator, while fewer predators can allow its prey to increase; then trace further effects along the chain.
A prediction needs a feeding link and a direction. Do not assume every population changes in the same direction.
Interpret an interconnected food web
A food web is a network of interconnected food chains. One organism may have several food sources or predators and may feed at more than one trophic level.
For any question, first trace arrows into the named consumer to find its food, then arrows leaving its prey or entering its predators. Predict direct effects before indirect ones and state the feeding reason.
A food web does not prove that every possible effect will occur. Use only the drawn feeding relationships and qualify indirect predictions when alternatives remain.
Define a producer by nutrient production
A producer is an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis.
Producers convert light energy into chemical energy stored in organic molecules and form the first trophic level of most food chains.
A producer is not merely an organism that provides food; it makes organic nutrients rather than obtaining them by feeding.
Define a consumer by feeding
A consumer is an organism that gets its energy by feeding on other organisms.
A consumer's classification depends on what it eats and its position in a particular food chain or route through a food web.
Consumer does not mean only carnivore: herbivores and animals at higher feeding positions are consumers.
Classify consumers by position
A primary consumer feeds on a producer; a secondary consumer feeds on a primary consumer; a tertiary consumer feeds on a secondary consumer; a quaternary consumer feeds on a tertiary consumer.
In a food web, trace one complete route from a producer. The same organism can occupy different consumer levels on different routes.
The label is determined by position, not by an organism's name or size. A predator can be secondary on one route and tertiary on another.
Define a herbivore
A herbivore is an animal that gets its energy by eating plants.
In a chain beginning with a plant producer, the herbivore is normally the primary consumer.
Herbivore describes the animal's food source; it is not another word for producer.
Define a carnivore
A carnivore is an animal that gets its energy by eating other animals.
A carnivore may be a secondary, tertiary or quaternary consumer, depending on the feeding route.
Carnivore does not identify one fixed trophic level; trace what the animal eats in the stated chain or web.
Define a decomposer
A decomposer is an organism that gets its energy from dead or waste organic material.
Dead material and waste can come from producers, herbivores or carnivores, so decomposers receive energy from material originating at every trophic level.
A decomposer is defined by obtaining energy from dead or waste organic material, not simply by being small or living in soil.
Trace human impacts through a food web
Overharvesting removes a food species faster than it is replaced. Its predators may lose a food source and decline, while organisms it ate may increase; further effects can spread through alternative links.
An introduced foreign species may become a new predator, prey or competitor. Trace which native population it directly changes, then follow the arrows to predict indirect effects.
Do not state that every introduced species causes the same outcome. A justified prediction must identify the new feeding or competition relationship and its direction.
Draw and interpret numbers and biomass pyramids
A pyramid of numbers uses bar width to represent the number of organisms at each trophic level. A pyramid of biomass uses bar width to represent their total biomass, usually per unit area.
| Representation | What each bar measures | Possible shape |
|---|---|---|
| numbers | count of individuals | may be inverted or irregular, for example one tree supporting many insects |
| biomass | total mass of living material | usually decreases at higher trophic levels |
Place the producer at the bottom and higher trophic levels above it. Use a common scale, make bar widths proportional to the data, and label every level.
Bar width represents the stated quantity, not the physical size of one organism.
Explain the advantage of biomass pyramids
A biomass pyramid accounts for the mass of organisms, so one large producer is not represented as equivalent to one tiny consumer merely because both count as one individual.
Biomass gives a better indication of how much living material—and therefore potential food—is present at each trophic level than numbers alone.
Biomass is more informative than organism count, but it is still a snapshot and does not directly show energy transferred over time.
Define trophic level as feeding position
A trophic level is the position of an organism in a food chain, food web or ecological pyramid.
Assign it by tracing a feeding route from the producer, which occupies the first trophic level.
An organism can feed at more than one trophic level in a food web if different routes place it in different positions.
Identify named trophic levels
Producer = trophic level 1; primary consumer = level 2; secondary consumer = level 3; tertiary consumer = level 4; quaternary consumer = level 5.
Start at a producer and count each energy-transfer arrow along one route. Name a consumer from the organism immediately before it: eating a primary consumer makes it a secondary consumer.
Do not count the Sun as a trophic level, and do not assign a web organism one level without checking every relevant route.
Draw and interpret a pyramid of energy
A pyramid of energy represents the energy available at each trophic level over a stated area and time. The producer bar forms the base and each higher bar is narrower.
Order trophic levels from producer upward, use one scale for proportional bar widths, include energy units with area and time when supplied, and read transfer as the difference or ratio between adjacent levels.
A pyramid of energy is always upright because less energy is available at each successive trophic level.
Compare the evidence in energy pyramids
A pyramid of energy shows actual energy transfer and includes a time interval, so it accounts for organisms with different sizes, life spans, reproduction rates and turnover.
| Pyramid | Main limitation avoided by energy data |
|---|---|
| numbers | treats every individual as one regardless of size |
| biomass | gives a standing mass at one time and may miss rapid replacement |
| energy | measures flow per area per time and is always upright |
Energy pyramids need more measurement, but they give the most direct representation of transfer efficiency.
Account for inefficient energy transfer
Transfer is inefficient because not all parts are eaten; some eaten material is not digested or absorbed and leaves as faeces; energy is used in respiration, movement and other life processes and is transferred to the environment as heat; waste products also carry energy away.
Efficiency of transfer (%) = energy transferred to the next trophic level ÷ energy available at the previous trophic level × 100.
Less energy is available at each higher level, so higher trophic levels usually support less biomass and fewer organisms.
Energy is transferred, not destroyed; energy unavailable to the next consumer has moved to waste, decomposers or the environment.
Explain why food chains stay short
Because energy transfer at every trophic step is inefficient, successive losses leave progressively less energy available to support biomass at the next level.
After several transfers, too little energy remains to support another stable consumer population. Food chains therefore usually have fewer than five trophic levels.
Food chains are not short because predators choose to stop feeding; accumulated energy loss limits the biomass that another level could support.
Compare crops with livestock for human food
Humans eating crop plants feed at a lower trophic level than humans eating livestock that first ate those crops.
The livestock uses crop energy in respiration, movement and other processes, and loses energy in waste and uneaten or indigestible material. Eating the crop directly avoids this additional inefficient transfer.
For the same crop energy input, a larger proportion can reach humans through crop → human than through crop → livestock → human.
The claim is specifically about energy-transfer efficiency, not a statement that every crop or diet has the same yield, nutrition or environmental impact.