Lesson 1 of 5 / Organisms and their environment
Reading food chains and food webs
What does a feeding arrow tell you?
In this lesson: Identify producers, consumers and decomposers and interpret feeding relationships.
About 5 min
The key ideaFood-chain arrows point from food to consumer. A food web connects several feeding relationships, while decomposers act on dead material and waste from many levels.
A trophic level is a feeding level. Producers occupy the first level; primary consumers eat producers, and secondary consumers eat primary consumers.
Explore the idea
One bird, two possible feeding levels
Grass is the producer, the grasshopper a primary consumer and the frog a secondary consumer. Each arrow points to the eater.
Original food web. Arrows show transfer from food to consumer. Bacteria and fungi receive dead material and organic waste from all these levels; decomposers are not a final predator.
Explanation
Green plants and algae are producers: they use light energy to make organic food by photosynthesis. A consumer obtains food from other organisms. A grasshopper eating grass is a primary consumer; a frog eating that grasshopper is a secondary consumer.
Draw the arrow from the organism eaten to the organism that eats it. It shows the transfer of chemical energy and matter in food. In grass to grasshopper to frog, the grass does not chase the grasshopper; the arrow has a nutritional meaning.
Real organisms often eat several types of food. A food web combines these feeding links. An organism can occupy different trophic levels in different chains: an omnivore eating a plant is a primary consumer in that link, but eating a herbivore places it at a higher level.
Bacteria and fungi act as decomposers. They break down organic material from dead organisms and wastes, obtaining nutrients and releasing substances that can be reused in the ecosystem. They receive material from many trophic levels, not only from the last carnivore.
A change in one population can affect several others. If frogs decline, their prey may experience less predation, while predators that eat frogs may have less food. Predictions should allow for alternative prey, competition and other conditions; a web does not give a guaranteed numerical population change.
Step by step
- 1
Follow the arrow
Read the organism at its tail as food and the organism at its head as the consumer.
- 2
Name the role in that chain
Start with the producer, then count consumer levels. A role can depend on which feeding path you follow.
- 3
Trace more than one link
For a population change, consider its food, its predators and available alternatives.
Worked example
A bird eats seeds and caterpillars
A bird eats seeds directly and also eats caterpillars feeding on the plant. Explain its consumer level in each chain.
One way to explain it
When it eats seeds, the bird feeds on a producer and is a primary consumer. When it eats a caterpillar that fed on the plant, it is a secondary consumer. Its trophic level depends on the feeding path considered.
Why this answer works
- Identify what the bird eats in each case.
- Trace each route back to a producer.
- Do not assign one permanent trophic level to every omnivore.
Is this true? "Decomposers eat only the final predator in a food chain."
Dead material and waste arise at every trophic level. Decomposers can act on material from producers and consumers throughout the web.