What this chapter is about
This chapter explores how living things and their surroundings work together in a balanced way. You will learn how plants, animals, microorganisms and non-living things like air, water and soil depend on each other. Nature is not a collection of separate parts — it is a connected system where everything plays a role.
At Class 8, you are ready to understand how energy flows from the sun through plants to animals, and how materials like carbon and nitrogen move in cycles. You will see why cutting forests or polluting rivers affects not just one species but the whole system. This helps you make sense of environmental problems you hear about.
After studying this chapter, you should be able to explain food chains and food webs, describe how nutrients cycle through nature, and give reasons why protecting one part of nature protects everything else.
Key ideas
- An ecosystem is a community of living things (plants, animals, microorganisms) interacting with non-living things (sunlight, water, air, soil) in a particular area.
- Producers are green plants and some bacteria that make their own food using sunlight through photosynthesis; they form the base of every food chain.
- Consumers are organisms that cannot make their own food and must eat other organisms — herbivores eat plants, carnivores eat animals, omnivores eat both.
- Decomposers like fungi and bacteria break down dead plants and animals, releasing nutrients back into the soil for plants to use again.
- A food chain shows the path of energy from one organism to another; a food web shows many connected food chains in an ecosystem.
- The carbon cycle describes how carbon moves from air into plants, then into animals, and back into air through respiration and decomposition.
- The nitrogen cycle explains how nitrogen in air is fixed into soil by bacteria, taken up by plants, passed to animals, and returned to soil and air.
- Balance in nature means that the number of producers, consumers and decomposers stays roughly steady over time — until humans or disasters disturb it.
Formulas and facts to remember
1. Sunlight is the ultimate source of energy for almost all life on Earth.
2. Photosynthesis equation in words: carbon dioxide + water + sunlight → glucose + oxygen.
3. Respiration releases energy: glucose + oxygen → carbon dioxide + water + energy.
4. About 78 percent of air is nitrogen, but plants cannot use it directly; bacteria must first convert it to nitrates.
5. At each step of a food chain, only about 10 percent of energy passes to the next level; the rest is lost as heat.
6. Decomposers recycle nutrients; without them, dead matter would pile up and nutrients would stay locked away.
7. A balanced ecosystem can recover from small disturbances, but large disturbances like deforestation may cause permanent damage.
Worked examples
Example 1: Building a simple food chain
In a pond near a village, tiny green algae float on the surface. Small fish eat the algae. A kingfisher catches and eats the fish.
Step 1: Identify the producer — algae (makes food using sunlight). Step 2: Identify the primary consumer — small fish (eats algae). Step 3: Identify the secondary consumer — kingfisher (eats fish).
Food chain: Algae → Small fish → Kingfisher.
Example 2: Tracing carbon through an ecosystem
A mango tree absorbs carbon dioxide from the air during photosynthesis. A parrot eats the mango and digests the sugars. When the parrot breathes, it releases carbon dioxide back into the air.
Step 1: Carbon enters the living world when the tree uses CO₂ to make glucose. Step 2: Carbon moves to the parrot when it eats the mango. Step 3: Carbon returns to air when the parrot respires.
This is one loop of the carbon cycle.
Example 3: Why removing one species matters
In a grassland, grass is eaten by deer, and deer are hunted by tigers. Suppose hunters kill all the tigers.
Step 1: With no tigers, deer population grows quickly. Step 2: Too many deer eat too much grass. Step 3: Grass disappears, soil erodes, and many other species lose their habitat.
This shows how removing one link can upset the whole system.
Common mistakes
Thinking plants only give out oxygen → Plants release oxygen during photosynthesis but also take in oxygen during respiration, especially at night.
Believing nitrogen in air can be used directly by plants → Plants need nitrogen in the form of nitrates; bacteria must first fix atmospheric nitrogen.
Confusing a food chain with a food web → A food chain is a single path; a food web is many chains linked together in the same ecosystem.
Assuming energy increases as it moves up the food chain → Energy decreases at each level because most is lost as heat.
Thinking decomposers are unimportant because they are small → Without decomposers, nutrients would never return to soil and new plants could not grow.
Quick revision
- Producers make food; consumers eat; decomposers recycle.
- Energy flows in one direction: sun → producer → consumer → lost as heat.
- Carbon and nitrogen move in cycles — they are used again and again.
- A food web is many connected food chains.
- Disturbing one part of an ecosystem can upset the whole balance.