Food chains and food webs

Who eats whom?

A goat eats grass. A person eats the goat. Grass, goat, person: this is a food chain. It is a line that shows how food passes from one organism to the next. The arrow means "is eaten by".

Every food chain starts somewhere. NCERT shows two starting points, and so there are two kinds of food chain.

The grazing food chain

The grazing food chain (GFC) starts with a green plant, a producer. A herbivore grazes on it, a carnivore eats the herbivore, and so on.

• Grass, grasshopper, frog, snake.
• Phytoplankton, zooplankton, fish.

The detritus food chain

The detritus food chain (DFC) starts with dead organic matter, also called detritus. This is made of dead leaves, bark, flowers, the dead bodies of animals and the faecal matter.

The first members of a detritus chain are the decomposers, which are mainly fungi and bacteria. They are saprotrophs (from the Greek sapro, which means to decompose). They secrete digestive enzymes onto the dead matter, break it down into simple inorganic substances, and then absorb what they need. The detritivores, such as the earthworm, eat the detritus and break it into smaller bits. Animals that eat the decomposers or the detritivores continue the chain.

• Dead leaves, earthworm, bird.
• Dead matter, fungi, a small animal, a larger carnivore.

Which chain carries more energy?

NCERT answers this in one place and in one sentence for each kind of ecosystem.

• In an aquatic ecosystem, the grazing food chain is the major route for the flow of energy.
• In a terrestrial ecosystem, a much larger fraction of the energy flows through the detritus food chain than through the grazing food chain.

Think of a forest. Very little of the leaf area is eaten by insects and deer. Most leaves fall to the ground and rot. In open water, the phytoplankton are eaten while they are alive, and the dead cells are few. The model below lets you move the share that is eaten alive.

You should find that a small grazed share, as in a forest, leaves most of the energy to the detritus chain. A large grazed share, as in open water, sends most of it to the grazing chain.

From chains to a web

In nature, a chain is rarely a single line. A sparrow eats seeds and also eats grasshoppers. A hawk eats snakes, sparrows and rabbits. So the chains join. This natural interconnection of food chains is a food web.

The detritus and the grazing chains join too. A bird can eat a grasshopper from the grazing chain and an earthworm from the detritus chain. The animals that eat both plants and animals, such as a crow or a cockroach, are omnivores.

Try it in Python

A food web is easy to store. For each animal, write the list of the things it eats. The program turns the list round and follows every path from the grass to an animal that nothing eats.

Practise with new numbers

Common mistakes

• Reading the arrow the wrong way. The arrow points from the food to the eater. It shows the direction of the flow of energy.
• Saying that decomposers start the grazing chain. They start the detritus chain. The grazing chain starts with a green plant.
• Giving the grazing chain to the terrestrial ecosystem as the larger one. On land, more energy goes through the detritus chain. In water, the grazing chain is the major route.
• Forgetting that the two chains meet. Some animals eat from both, so the chains are joined in a web.

Remember these

• Grazing food chain: starts with a green plant (producer). Detritus food chain: starts with dead organic matter.
• Decomposers (fungi and bacteria) are saprotrophs. Detritivores (such as the earthworm) fragment the detritus.
• Aquatic ecosystem: the grazing food chain is the major route of energy. Terrestrial ecosystem: the detritus food chain carries a much larger part.
• Interconnected food chains make a food web. Omnivores eat both plants and animals.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does a detritus food chain differ from a grazing food chain at the start?

2. Name three kinds of detritus.

3. Which chain carries the larger part of the energy in a forest? Which one in a pond?

4. What does the word saprotroph mean?

5. In a food web, a sparrow eats seeds and grasshoppers. What kind of consumer is it?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on energy flow.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on energy flow through ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy.