Class 8 › Science › Chapter 12
How Nature Works in Harmony
Habitats, populations and communities, ecosystems, food chains and webs, decomposers, relationships between organisms, how human actions disturb balance, and sustainable farming.
1. Habitat: Biotic and Abiotic Components
Biotic
Living components
Plants, animals and microorganisms (e.g. fish, frogs, algae, lotus, trees, birds).
Abiotic
Non-living components
Air, water, sunlight, soil, temperature (and rocks, minerals).
Habitat: the place where an organism lives. It can be as small as the bark of a tree. It provides food, oxygen, shelter and space to grow.
- Examples of habitats: pond, forest, farm, a large tree (banyan, mango, pilkhan).
- Different habitats have different biotic and abiotic components, hence different living conditions.
- Example (fish in a pond): biotic need (food) from small plants and animals; abiotic need (oxygen) from water.
- Organisms in one habitat use resources differently, so they coexist. Example: a night-active snake and a day-active rodent in the same forest.
2. Who Lives Together? Population and Community
Levels of organisation: each level is part of the next (population ⊂ community ⊂ ecosystem).
- Population: a group of the same type of organisms in a habitat at a given time (e.g. all fish of one kind in a pond). Found by counting organisms in a fixed area, e.g. a 1 m × 1 m plot.
- Community: different populations (plants, animals, microorganisms) sharing the same habitat, interacting and depending on one another.
- Why one kind of organism is not enough: they would have identical needs (food, water, space) and compete heavily, causing scarcity.
- Pollination (Ever heard of): a flower has sepals (green), petals (coloured), stamens (male, make pollen) and carpels (female). Wind, water, insects, bats and birds carry pollen from stamens to carpels. Pollination is needed for fruits and seeds.
3. Does Every Organism in a Community Matter?
Pond study: how fish affect seed production
Pond A (with fish) had more flowering plants; Pond B (no fish) had fewer.
1. Fish eat dragonfly larvae, so there are fewer dragonflies.
2. Dragonflies eat flies, bees and butterflies, so with fewer dragonflies there are more bees, flies and butterflies.
3. These insects pollinate flowers, so plants make more seeds.
- Direct effect: fish → fewer larval dragonflies. Indirect effect: fish → more seeds in plants.
- Biotic components (fish, dragonflies, pollinators, plants) and abiotic components (temperature, water, nutrients) affect each other. Overfishing by humans can upset this balance.
4. Ecosystem and Its Types
Ecosystem: the biotic components (plants, animals, microorganisms) and abiotic components (air, water, soil, sunlight, temperature) of an area interacting with each other.
| Interaction type | Examples |
|---|---|
| Biotic ↔ Abiotic | Earthworms live in moist soil; fish lay eggs in water; microbes in a pond. Plants need sunlight, CO₂ and water for food, soil for nutrients, air for oxygen. |
| Abiotic ↔ Abiotic | Bright sunlight raises day temperature; sunlight makes water evaporate fast; air currents make waves; soil near a pond is moist. |
| Biotic ↔ Biotic | A frog eats insects; a water snake eats fish; frogs and fish compete for insect larvae; fish lay eggs near vegetation for protection. |
Aquatic
In water
Ponds, rivers, lakes (also sea and ocean).
Terrestrial
On land
Forests, grasslands, deserts, farms, even a large tree.
- Ecosystems can be large or small and can overlap (e.g. a river with mountains, forests and farmland).
- Farmland is a human-made ecosystem.
- Abiotic factors are also influenced by biotic ones: plants release oxygen, roots hold soil and prevent erosion, plants retain soil moisture and help cool the atmosphere.
5. Producers, Consumers and Decomposers
| Group | Meaning | Examples |
|---|---|---|
| Producers (autotrophs) auto = self, troph = food | Make their own food by photosynthesis | Green plants, grass, trees |
| Consumers (heterotrophs) hetero = other | Cannot make food; depend on other organisms | Deer, hare, fox, vulture, mouse |
| Decomposers (saprotrophs) sapro = rotten | Break down dead matter and recycle nutrients | Bacteria, fungi (mushroom), beetles and flies on dung |
| Type of consumer | Eats | Examples |
|---|---|---|
| Herbivore | Only plants | Deer, hare |
| Carnivore | Only animals | Leopard |
| Omnivore | Both plants and animals | Crow, fox, mouse |
6. Who Eats Whom: Food Chain, Trophic Levels, Food Web
Food chain: a simple linear sequence showing 'who eats whom' in an ecosystem. Arrows point from the organism eaten to the one that eats it.
A grassland food chain with trophic levels.
- Trophic level: the position of an organism in a food chain. Level 1: producers; level 2: herbivores; level 3: small carnivores (e.g. frogs); next: large carnivores (e.g. tigers, vultures).
- Pyramid of numbers (crop field activity): counts decrease going up: many millet plants at the base, fewer mice, fewest eagles at the top.
- Food web: an organism can be eaten by two or more types of organisms, so food chains interlink into a network.
- Energy source for the whole chain: the Sun (through producers).
7. What Happens to Waste in Nature?
- Decomposition: microorganisms (fungi, bacteria) and small insects (beetles, flies on dung) break complex substances in dead plants, animals and droppings into simpler ones.
- This returns nutrients to the soil, which plants use. So decomposers recycle nutrients: in nature nothing is wasted.
- Migratory birds (Ever heard of): travel from far to escape harsh climate and find food. They act as pollinators, seed dispersers and predators of insect pests, linking two habitats (e.g. Demoiselle Cranes at Khichan, Jodhpur in winter).
8. How Does One Change Lead to Another?
Pollution in a pond can trigger a chain of changes (farmers then use more pesticides).
Case study: Indian bullfrog and frog-leg export (1980s)
Large-scale harvesting of frogs for export reduced their numbers.
Frogs eat insects, so agricultural pests increased.
Farmers used more synthetic pesticides, harming environment, soil, water and human health.
The Government of India banned export of frog legs to prevent further ecological damage.
- Balance: an ecosystem is in balance when interactions keep populations and resources stable. The balance is dynamic, not fixed, and can be disrupted by natural or human-made changes.
- Elephant corridor (chapter opener): loss of forest cover and changed rainfall dry up waterholes and shrink habitats, so elephants raid farms. Marked corridors connect forests so animals can move safely without conflict with people.
9. How Interactions Maintain Balance
- Competition: organisms compete for food, water, space or sunlight. It controls population size; without it one species could multiply too much.
| Relationship | Effect | Example |
|---|---|---|
| Mutualism | Both benefit (+ +) | Honeybees and flowers (nectar and pollination) |
| Commensalism | One benefits, other unaffected (+ 0) | Orchids growing on trees |
| Parasitism | One benefits, other harmed (+ −) | Ticks on dogs |
Be a scientist: A.J.T. Johnsingh, Indian wildlife biologist, studied Bandipur (Karnataka) and showed that tigers and leopards depend on prey such as deer and wild boar, so a healthy prey population is key to predator survival.
10. Benefits of Ecosystems and Threats to Them
- Benefits to humans: forests give fresh air, fertile soil, food, fibres, timber, medicines; aquatic ecosystems give water and food; also climate regulation and aesthetic and recreational value.
- Sundarbans: world's largest mangrove forest (Ganges and Brahmaputra delta, India and Bangladesh). Slows strong winds and waves in storms and floods, absorbs CO₂ and releases oxygen. UNESCO World Heritage Site in 1987.
- Threats: cutting mangroves for fuelwood and farming, illegal hunting, overuse of resources, industrial waste and untreated sewage. Elsewhere: deforestation, invasive species, unsustainable land use, pollution.
- Protected areas: land or water set aside to conserve wildlife and habitats: national parks, wildlife sanctuaries, biosphere reserves, community conserved areas. Examples: Jim Corbett (Uttarakhand), Manas (Assam), Nilgiri Biosphere Reserve, Chilika Lake (Odisha), Eaglenest (Arunachal Pradesh), Hemis (Leh), Keibul Lamjao (Manipur), Pirotan Island Marine NP (Gujarat).
11. Human-made Ecosystems and Sustainable Farming
- Human-made ecosystems: fish ponds, farms, parks. If well designed they can reduce pollution, support biodiversity and give recreation, but they need human care and management.
- Green Revolution: India faced a food crisis in 1950 to 1965. Tractors, machines, synthetic fertilisers and pesticides raised food production in the mid-20th century.
Problems
Why it is unsustainable
Overuse of synthetic chemicals; excessive groundwater extraction; monoculture (same crop repeatedly) lowers biodiversity and hurts pollinators; pests develop resistance; natural predators of pests decrease.
Soil
Effects on soil
Fewer friendly microorganisms and less humus (organic matter that binds soil particles), so lower fertility and more erosion. Heavy irrigation and repeated ploughing disturb earthworms and snails.
Solutions
Better practices
Organic and natural farming, less synthetic fertiliser, natural pest control by predators, crop diversity, composting.
Our scientific heritage: the ancient text Vrikshayurveda stresses soil health and nourishing soil with organic manure such as Kunapa Jala (liquid fertiliser made by fermenting animal and plant waste) and compost.
12. Quick Revision
| Term | Meaning |
|---|---|
| Habitat | Place that gives an organism the right conditions to live |
| Population | Same kind of organisms in a habitat at a given time |
| Community | All populations (biotic components) in a habitat |
| Ecosystem | Community + abiotic components interacting |
| Food chain / web | Linear 'who eats whom' / interlinked network of chains |
| Trophic level | Position of an organism in a food chain |
| Decomposition | Breakdown of dead matter into simpler substances, recycling nutrients |
| Mutualism / Commensalism / Parasitism | + + / + 0 / + − |
Exam tip
Chain reasoning
For "what happens if X disappears?" write the chain step by step: prey of X increases, predators of X decrease, then effects on the whole web.
Remember
Why decomposers matter
Without decomposers, dead matter would pile up and soil would lose nutrients, so producers (and everyone else) would suffer.
Common mistakes:
• Saying a population includes different species (that is a community).
• Thinking an ecosystem has only living things (abiotic components are part of it).
• Drawing food chain arrows from the eater to the food (arrow points to the eater).
• Forgetting that decomposers are not consumers in the usual food-chain sense but recycle nutrients.
• Mixing up commensalism (+ 0) with parasitism (+ −).
• Calling a food chain a food web (a web is many linked chains).
• Treating ecosystem balance as fixed instead of dynamic.
• Saying a population includes different species (that is a community).
• Thinking an ecosystem has only living things (abiotic components are part of it).
• Drawing food chain arrows from the eater to the food (arrow points to the eater).
• Forgetting that decomposers are not consumers in the usual food-chain sense but recycle nutrients.
• Mixing up commensalism (+ 0) with parasitism (+ −).
• Calling a food chain a food web (a web is many linked chains).
• Treating ecosystem balance as fixed instead of dynamic.