Lesson 7 of 11 · 6 min
Food chains and food webs
NCERT §12.4
Rohan watches a crow at the lake's edge eat a fallen fruit, then a dead fish, then a beetle. 'Which chain is it in?' he asks. 'All of them,' says Ma'am.
The lesson in notes
In short
Consumers (heterotrophs) depend on plants directly or indirectly for food.
Primary consumers eat producers, so they are herbivores. Examples: insects, birds and mammals on land; molluscs in water.
Secondary consumers eat herbivores; they are primary carnivores. Animals that eat primary carnivores are secondary carnivores (tertiary consumers).
A simple grazing food chain (GFC): grass (producer) → goat (primary consumer) → man (secondary consumer).
The detritus food chain (DFC) starts from dead organic material. Its decomposers, mainly fungi and bacteria, are heterotrophs also called saprotrophs.
Decomposers secrete digestive enzymes that break dead and waste materials into simple inorganic materials, which they then absorb.
In aquatic ecosystems the GFC is the main channel of energy flow. On land, a much larger share of energy flows through the DFC than through the GFC.
The DFC links with the GFC at some levels: some DFC organisms are eaten by GFC animals, and some animals, such as cockroaches and crows, are omnivores. These links make a food web.