Ecosystems
1. What is an Ecosystem?
An ecosystem is a SELF-SUSTAINING system where LIVING organisms interact with each other and their NON-LIVING environment.
'The term 'ecosystem' was coined by A.G. Tansley in 1935. Every ecosystem has TWO interacting components — biotic and abiotic.'
2. Components of an Ecosystem
Abiotic Components (NON-LIVING)
| Component | Examples |
|---|---|
| Climatic factors | Sunlight, temperature, rainfall, wind, humidity |
| Edaphic factors | Soil type, pH, minerals, texture |
| Inorganic substances | Carbon, nitrogen, oxygen, water, CO₂ |
| Organic substances | Humus, dead organic matter |
Biotic Components (LIVING)
| Category | Role | Examples |
|---|---|---|
| Producers (Autotrophs) | Make their OWN food by PHOTOSYNTHESIS | Green plants, algae, phytoplankton |
| Consumers (Heterotrophs) | Eat OTHER organisms for food | All animals |
| — Primary consumers | Eat PRODUCERS (herbivores) | Cow, rabbit, deer |
| — Secondary consumers | Eat PRIMARY consumers (carnivores) | Frog, snake, fox |
| — Tertiary consumers | Eat SECONDARY consumers (top carnivores) | Lion, eagle, shark |
| — Omnivores | Eat BOTH plants and animals | Humans, bear, crow |
| — Scavengers | Eat DEAD animals | Vulture, hyena |
| Decomposers (Saprotrophs) | BREAK DOWN dead matter into SIMPLE substances | Bacteria, fungi |
3. Food Chain
A LINEAR sequence showing WHO EATS WHOM.
General pattern: Producer → Primary consumer → Secondary consumer → Tertiary consumer
Example 1 (Terrestrial): Grass → Grasshopper → Frog → Snake → Eagle
Example 2 (Aquatic): Phytoplankton → Zooplankton → Small fish → Large fish
'Each ARROW (→) means 'is EATEN BY.' The direction of the arrow shows the FLOW of energy.'
4. Food Web
A NETWORK of INTERCONNECTED food chains in an ecosystem.
'In nature, an organism eats MORE than one type of food and is eaten by MORE than one predator. A food web is therefore MORE REALISTIC than a single food chain.'
5. Energy Flow and the 10% Law
Source of energy in most ecosystems: SUNLIGHT
10% Law (proposed by Raymond Lindeman): Only about 10% of the energy at ONE trophic level is TRANSFERRED to the NEXT trophic level. The remaining 90% is:
- Used for RESPIRATION and METABOLISM
- Lost as HEAT
- Used for GROWTH and REPRODUCTION
Worked Example: If green plants capture 10,000 J of solar energy:
| Trophic Level | Energy Available |
|---|---|
| Producers | 10,000 J |
| Primary consumers | 1,000 J (10%) |
| Secondary consumers | 100 J (10% of 1000) |
| Tertiary consumers | 10 J (10% of 100) |
'This explains why food chains are LIMITED to 4–5 trophic levels — not enough energy remains beyond that.'
6. Ecological Pyramids
Pyramid of Numbers
Shows the NUMBER of organisms at each trophic level.
- UPRIGHT: Most ecosystems (many producers, few top carnivores)
- INVERTED: A single tree (one producer) supporting many insects (primary consumers)
Pyramid of Biomass
Shows the TOTAL DRY WEIGHT of organisms at each level.
- Usually UPRIGHT (biomass DECREASES at higher levels)
Pyramid of Energy
Shows the ENERGY at each trophic level.
- ALWAYS UPRIGHT (energy ALWAYS decreases at higher levels)
7. Biotic Interactions
| Interaction | Effect on Species A | Effect on Species B | Example |
|---|---|---|---|
| Predation | BENEFITS (eats) | HARMED (is eaten) | Lion → deer |
| Competition | HARMED | HARMED | Two species competing for food |
| Parasitism | BENEFITS | HARMED | Tapeworm in human intestine |
| Commensalism | BENEFITS | NEUTRAL | Barnacles on whale |
| Mutualism | BENEFITS | BENEFITS | Lichen (algae + fungi) |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Food chains show all feeding relationships' | Food chains are SIMPLIFIED. Food WEBS show the COMPLETE network of relationships |
| 'Decomposers are at the top of the food chain' | Decomposers are at EVERY level — they break down dead matter from ALL levels |
| 'Energy flows in a cycle' | Energy flows ONE WAY (sun → producers → consumers). It does NOT cycle. MATTER cycles (carbon, nitrogen) |
| 'The 10% law means 10% of energy is lost' | 90% is LOST. Only 10% is TRANSFERRED to the next level |
ICSE Exam Focus (5–7 marks)
- 2-mark questions: Define ecosystem, food chain, trophic level
- 3-mark questions: Distinguish between food chain and food web
- 4-mark questions: 10% law — energy calculations
- 6-mark questions: Ecological pyramids or biotic interactions
Self-Test
Q1. What are the two components of an ecosystem? A1. (1) BIOTIC — all living organisms (plants, animals, microbes). (2) ABIOTIC — non-living factors (sunlight, soil, water, temperature).
Q2. Construct a food chain with four trophic levels. A2. Grass (producer) → Rabbit (primary consumer) → Fox (secondary consumer) → Lion (tertiary consumer).
Q3. State the 10% law of energy transfer. If producers have 1000 J, how much energy will be available at the tertiary consumer level? A3. Only 10% is transferred at each level. 1000 J → 100 J → 10 J. Tertiary consumers get 1 J.
Q4. What is the difference between a food chain and a food web? A4. A food chain is a SINGLE, LINEAR path of energy flow. A food web is a NETWORK of MANY interconnected food chains showing MULTIPLE feeding relationships.
Q5. Name the type of ecological pyramid that is ALWAYS upright. A5. The PYRAMID OF ENERGY is always upright because energy ALWAYS decreases at each successive trophic level.
Q6. What is mutualism? Give an example. A6. Mutualism is an interaction where BOTH organisms BENEFIT. Example: Lichen — algae provide food (photosynthesis) and fungi provide structure and minerals.
