Ecosystem — Part 3: Energy flow and ecological pyramids: visual lesson, CBSE Class 12 Biology

Lesson

Energy flow and ecological pyramids

Part 3 — where the energy goes, and how we picture it.

In this lesson you will

Trace the flow of energy through grazing and detritus food chains
Apply the 10 percent law
Construct and interpret pyramids of number, biomass and energy and state their limitations

Diagram

Unidirectional flow of energy

Energy flows one way: from the sun to producers and then to consumers. Ecosystems are not exempt from the second law of thermodynamics; they need a constant supply of energy to make the molecules they require and counteract the tendency towards disorder. At each step energy is lost as heat.

Concept

Grazing and detritus food chains

The grazing food chain (GFC) starts with living plants, for example grass → goat → man. The detritus food chain (DFC) starts with dead organic matter and is made of decomposers, mainly fungi and bacteria (saprotrophs), which secrete digestive enzymes and absorb the products.

Table comparing the main energy route in aquatic and terrestrial ecosystems
Ecosystem	Major route of energy
Aquatic	Grazing food chain
Terrestrial	Detritus food chain carries a much larger fraction of energy

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