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Ecosystems
ICSE Class 8 Biology • Chapter 4 (Detailed Master Notes)
Chapter Overview
No animal or plant can survive in total isolation. They need air to breathe, water to drink, and other living things to eat. This complex web of relationships between living organisms and their non-living environment is what biologists call an Ecosystem.
4.1 What is an Ecosystem?
Ecosystem: A self-sustaining functional unit of nature where living organisms interact with each other and also with their surrounding physical environment.
Ecosystems can be massive like a rainforest or an ocean, or very small like a single pond or a rotting log.
4.2 Components of an Ecosystem
Every ecosystem is built upon two major components interacting endlessly.
1. Abiotic (Non-Living) Components
These are the physical and chemical factors that define the environment. They determine what kind of life can survive there.
- Sunlight: The primary source of energy for almost all ecosystems. It drives photosynthesis.
- Temperature: Affects the survival of animals. Polar bears need freezing cold, whereas camels need dry heat.
- Water: Essential for all biochemical reactions. Ecosystems with abundant water (rainforests) support far more life than deserts.
- Soil and Air: Soil provides minerals and anchorage for plants. Air provides oxygen for respiration and carbon dioxide for photosynthesis.
2. Biotic (Living) Components
These include all the living organisms—plants, animals, and microorganisms. They are divided into three groups based on how they obtain their food:
| Category | Role | Examples |
|---|---|---|
| Producers (Autotrophs) | They make their own food using sunlight, water, and carbon dioxide through photosynthesis. They bring energy into the ecosystem. | Green plants, algae, certain bacteria. |
| Consumers (Heterotrophs) | They cannot make their own food. They must eat producers or other consumers to get energy. | Animals (Herbivores, Carnivores, Omnivores). |
| Decomposers (Saprotrophs) | They break down the complex organic matter of dead plants and animals into simple inorganic nutrients. They return these nutrients back to the soil. | Fungi, soil bacteria. |
4.3 Food Chains and Food Webs
Energy continuously flows through an ecosystem. This flow is always unidirectional (one way), starting from the sun.
Food Chain: A linear sequence showing who eats whom, transferring energy from one organism to the next.
Grass (Producer) $\rightarrow$ Grasshopper (Primary Consumer) $\rightarrow$ Frog (Secondary Consumer) $\rightarrow$ Snake (Tertiary Consumer) $\rightarrow$ Eagle (Top Carnivore)
In real life, animals eat many different types of food, not just one. For example, a hawk might eat a snake, a mouse, or a small bird. This creates a highly interconnected network of multiple food chains.
Food Web: A complex network of many interconnected food chains within an ecosystem. It provides stability. If one food source dies out, animals can rely on a different food chain to survive.
AI Image Prompt: A vibrant diagram of a forest food web. At the bottom, green plants (Producers). Arrows point up to Herbivores (rabbits, deer, insects). More arrows criss-cross upwards to Carnivores (frogs, foxes, snakes) and finally to an Eagle at the very top. The arrows represent the flow of energy.
4.4 Symbiosis (Close Relationships)
Sometimes, organisms from two different species live closely together. This relationship is called symbiosis.
- Mutualism: Both species benefit from the relationship. Example: A bee and a flower. The bee gets food (nectar), and the flower gets successfully pollinated.
- Commensalism: One species benefits, while the other is neither helped nor harmed. Example: An orchid growing high up on a large tree branch to get sunlight. The tree is unaffected.
- Parasitism: One species benefits (the parasite) at the cost of harming the other species (the host). Example: Tapeworms living inside the human gut, stealing nutrients and causing illness.
Q1. Why are decomposers considered the "cleaning agents" of nature?
Answer: If decomposers did not exist, dead plants and animal bodies would just pile up endlessly forever. Decomposers eat these dead bodies and convert them into simple raw nutrients. These nutrients mix back into the soil, acting as natural fertilizer for new plants to grow. They keep the cycle of life moving.
Q2. What happens to the amount of energy as it passes along a food chain?
Answer: The amount of available energy strictly decreases at every step. By the Ten Percent Law, an animal only stores about 10% of the energy from the food it eats. The remaining 90% is lost to the environment as body heat or used for daily movement and respiration. Because so much energy is lost, food chains rarely have more than four or five steps.