Ecosystem — Part 2: Energy flow and ecological pyramids: visual lesson, ISC Class 12 Biology

Lesson

Energy in ecosystems

Part 2 — how energy moves through food chains and webs, and how it shapes ecological pyramids.

In this lesson you will

Distinguish grazing and detritus food chains, and explain food webs and trophic levels
Apply the 10 percent law to energy flow
Describe pyramids of number, biomass and energy, with upright and inverted examples

Concept

There are two main kinds of food chain.

Table comparing grazing and detritus food chains
Feature	Grazing food chain (GFC)	Detritus food chain (DFC)
Begins with	Living producers (green plants)	Dead organic matter (detritus)
Example	Grass → goat → man	Dead leaves → earthworm → bird
Main conduit in	Aquatic ecosystems	Terrestrial ecosystems (a larger fraction of energy)
Organisms	Herbivores, carnivores	Decomposers (saprotrophs: fungi, bacteria) and detritivores

Concept

Natural food chains are interconnected into a food web. Each organism occupies a trophic level according to its source of food.

Table of trophic levels and members
Trophic level	Members
First	Producers (plants, phytoplankton)
Second	Primary consumers (herbivores: grasshopper, cow, zooplankton)
Third	Secondary consumers (primary carnivores: frog, fish)
Fourth	Tertiary consumers (top carnivores: snake, hawk)