Science · Class 8 · Chapter 12

How Nature Works in Harmony

• Habitat — The place providing conditions for an organism to live.
• Population / Community — Same species / all species sharing a habitat.
• Ecosystem — Biotic and abiotic components interacting.
• Producers / Consumers / Decomposers — Make food / eat others / recycle waste.
• Food chain / web — Who eats whom / interlinked food chains.
• Trophic level — An organism's feeding position in a food chain.
• Mutualism / Commensalism / Parasitism — Both benefit / one benefits / one harmed.
• Monoculture — Growing the same crop repeatedly.

Chapter 12 · Concept 1 of 11

Habitats: Biotic and Abiotic

• Biotic components — The living things in a habitat — plants, animals, and microbes.
• Abiotic components — The non-living things — air, water, soil, temperature, and sunlight.

Chapter 12 · Concept 2 of 11

Population and Community

• Population — A group of organisms of the same kind, living together in a habitat.
• Community — All the different populations sharing the same habitat, interacting and depending on each other.

Chapter 12 · Concept 3 of 11

Ecosystems in Action

• In a pond study: fish reduce dragonfly larvae, which lets more pollinators (bees, butterflies) survive, which increases seed production nearby.
• Ecosystem — The whole interacting system of biotic and abiotic components in a habitat — one small change can ripple through many others.

Chapter 12 · Concept 4 of 11

Producers, Consumers, and Decomposers

• Producers — Make their own food via photosynthesis — green plants.
• Consumers — Depend on others for food — herbivores, carnivores, omnivores.
• Decomposers — Break down dead matter, recycling nutrients — fungi, bacteria.

Chapter 12 · Concept 5 of 11

A Food Chain

• Grass → Grasshopper → Frog → Snake → Eagle

Chapter 12 · Concept 6 of 11

Trophic Levels

• The number of organisms typically decreases going up each level — forming a pyramid shape.

Chapter 12 · Concept 7 of 11

Food Webs

• Food web — Real ecosystems rarely have just one food chain — many chains interlink into a network, because most animals eat (and are eaten by) more than one other kind.

Chapter 12 · Concept 8 of 11

Mutualism, Commensalism, and Parasitism

• Relationship | Who benefits | Example
• Mutualism — Both organisms — Honeybees and flowers
• Commensalism — One; the other unaffected — Orchids growing on trees
• Parasitism — One; the other harmed — Ticks on dogs

Chapter 12 · Concept 9 of 11

When One Change Cascades

• Pollution killing pond plants → less oxygen → fish die → fewer consumers → more insects → pesticide use → further harm.
• Balance is fragile — Competition for food, water, space, and sunlight normally keeps populations in check — but one disruption can set off a chain reaction.

Chapter 12 · Concept 10 of 11

Ecosystem Benefits: The Sundarbans

• Ecosystems give us fresh air, fertile soil, food, timber, medicines, and recreational value.
• The world's largest mangrove forest — The Sundarbans protects coastlines from storms and absorbs CO2 — but faces threats from logging, hunting, and pollution.

Chapter 12 · Concept 11 of 11

The Green Revolution and Monoculture

• The Green Revolution — Boosted food production in the 1950s–60s using tractors, fertilisers, and pesticides — but overuse now causes soil degradation and biodiversity loss.
• Monoculture — Growing the same crop repeatedly reduces biodiversity and harms the pollinators that ecosystems depend on.

Quick Recap

Check Your Understanding

1. 1Looking at a nested diagram of population, community, and ecosystem, which statement is wrong: (i) A community is larger than a population. (ii) A community is smaller than an ecosystem. (iii) An ecosystem is part of a community.
2. 2If all decomposers suddenly disappeared from a forest ecosystem, what would happen? Why are decomposers essential?
3. 3Selvam's village in Cuddalore was less damaged by the 2004 Tsunami than nearby villages, thanks to mangrove forests. How might mangroves have protected the village?