Lesson focus: Food chains and webs model feeding relationships and the transfer of biomassThe mass of living material, such as plants or animals. and chemical energy through ecosystems.
Construct and interpret food webA diagram of linked food chains showing who eats what in a habitat., using arrow direction and trophic-level language accurately.
Build on earlier learning
Anaerobic respirationRespiration explains why organisms need organic substrates. Food webs show how those substrates move between organisms.
You will be successful when you can…
- Identify producers and consumers.
- Use arrow direction correctly.
- Assign trophic levels cautiously.
- Explain why decomposers connect the whole web.
Words that carry the science
- producer
- An organism that makes organic molecules from inorganic materials, commonly by photosynthesis.
- consumer
- An organism that obtains organic material by feeding on other organisms.
- trophic level
- A feeding position in a chain or web.
- food web
- A network of interconnected feeding relationships.
- decomposer
- An organism, often a bacterium or fungus, that externally digests dead material and absorbs products.
- biomass
- The mass of living biological material, often measured as dry mass.
Read arrows as transfers from resource to consumer
01Producers introduce biomass into most food webs
Photosynthetic producerA living thing, usually a plant, that makes its own food using light energy. convert inorganic carbon into organic molecules and biomassThe mass of living material, such as plants or animals.. Primary consumerA living thing that gets its energy by eating other living things. feed on producers; higher consumers feed on other consumers. An omnivore may occupy different trophic levelThe position of a living thing in a food chain, such as producer or first consumer. in different chains.
Arrows point from the organism being eaten to the consumer because they represent transfer of biomass and chemical energy.
02A web reveals alternative pathways
Real consumers often use several food sources and have several predators. A food webA diagram of linked food chains showing who eats what in a habitat. therefore makes it possible to predict both direct and indirect effects, but it remains a simplified model.
Species abundance, seasonal diet, parasite relationships and detrital pathways may be omitted. An arrow confirms a feeding link, not how strong or frequent it is.
03Decomposers recycle matter
Dead organisms and waste contain organic matter. decomposerA living thing, such as a fungus or bacterium, that breaks down dead plants and animals. secrete enzymes, absorb soluble products and respire, returning carbon dioxide and mineral ions to the environment. Detritivores fragment material and increase surfaces for decomposition.
Matter cycles through ecosystems, whereas energy transferred into thermal stores eventually disperses and is not recycled in the same way.
Predator and prey populations
Foxes eat rabbits. More rabbits means more food for foxes, so the fox population rises after the rabbits do. Then the rabbits fall, and the foxes follow.
Rabbit (each dot ≈ 20) Fox (each ≈ 3)
Controls
Trace one chain through a web
A web contains grass → rabbit → fox and grass → grasshopper → frog → fox.
- Start with the producer: grass.
- Follow arrows in their direction to a consumer.
- One chain is grass → rabbit → fox.
- Another is grass → grasshopper → frog → fox; the fox occupies different numbered trophic positions in the two chains.
How to make the reasoning visible: Trophic level belongs to a position in a particular chain, so one species can feed at more than one level.
Interpret a meadow food web
Use the listed links as evidence; do not assume unlisted feeding relationships.
| Resource | Consumer | Consumer role in this link | Possible shared competitor |
|---|---|---|---|
| Grass | Rabbit | Primary consumer | Grasshopper |
| Grass | Grasshopper | Primary consumer | Rabbit |
| Grasshopper | Frog | Secondary consumer | Small bird |
| Rabbit | Fox | Secondary consumer | Hawk |
| Frog | Fox | Higher consumer | Hawk |
Why might fox numbers not fall immediately if rabbit numbers decrease?
Where should decomposers be added?
Check your understanding
Answer from memory, then check against the suggested answer.
Which way does a food-chain arrow point?
What is a producer?
Why are decomposers important?
A bird eats both seeds and caterpillars. Explain why it cannot be assigned one trophic level in every chain.
Explain why a food web cannot by itself predict the size of a population change.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The number of grasshoppers in a meadow food web falls sharply.
Explain the likely effect on the frogs that eat grasshoppers.
(2)Explain the likely effect on plants and other herbivores, and why it is hard to predict.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., follow at least two arrows and distinguish direct from indirect effects.