What is an ecosystem?
An ecosystem is a unit of nature where living things interact with each other and with the non-living things around them. A forest, a pond, a lake, a grassland and a desert are natural ecosystems. A garden, a crop field and an aquarium are artificial (human-made) ecosystems, because people plan and look after them.
Biotic and abiotic components
Biotic components are the living parts: plants, animals, fungi and bacteria. Abiotic components are the non-living parts: sunlight, temperature, water, air, soil and minerals. Neither can be removed without changing the other. Remove sunlight and plants die; remove plants and animals starve.
Producers, consumers and decomposers
Producers are green plants and some bacteria (like blue-green algae) that make food by photosynthesis. They are called autotrophs.
Consumers cannot make food, so they eat others. They are grouped as herbivores (eat plants, e.g. goat), carnivores (eat animals, e.g. lion), omnivores (eat both, e.g. crow, humans) and parasites (live on a host, e.g. lice).
Decomposers are bacteria and fungi that break down dead plants and animals into simple substances. This returns minerals to the soil so producers can use them again. Without decomposers, dead matter would pile up and the soil would run out of nutrients.
Food chains, food webs and trophic levels
A food chain is a series of living things where each one eats the one before it. Example: grass → grasshopper → frog → snake → eagle. In water: algae → small fish → big fish → heron.
Each step of a food chain is a trophic level: T1 = producers, T2 = primary consumers (herbivores), T3 = secondary consumers, T4 = tertiary consumers.
In nature, most animals eat more than one kind of food and are eaten by more than one enemy. So many chains join into a network called a food web. A food web makes an ecosystem more stable: if one food runs short, animals can switch to another.
Energy flow and the 10% law
Green plants capture only about 1% of the sunlight falling on their leaves. When a herbivore eats a plant, it gets only about 10% of the energy stored in that plant. The rest is used for the plant's own life processes, lost as heat, or stays in parts that are not eaten or digested. This is the 10 per cent law (given by Lindeman).
Why energy flow is one-way (unidirectional)
Energy moves from the Sun → producers → herbivores → carnivores. It never comes back: energy lost as heat cannot be used again by plants. So energy flow is unidirectional. Minerals, in contrast, go round and round with the help of decomposers.
Why food chains have only 3–4 levels
Energy drops to one-tenth at each step. By the fourth or fifth level so little energy is left that it cannot support a population. That is why top carnivores are few, and why a vegetarian diet (eating at level 2) wastes less of the Sun's energy.
Biological magnification
Pesticides like DDT are sprayed on crops. Some wash into soil and water and get into plants. These chemicals do not break down and are not passed out of the body; they get stored in fat. An animal eating many plants collects all their chemical. The next animal eats many of those animals, and so on.
So the amount of the chemical per kg of body goes up at every trophic level. This is biological magnification. Top consumers, including humans, get the highest dose. That is why food grains, vegetables, fish and even milk can carry pesticide traces.
Key formulas and definitions
- Energy at next level = 10% of energy at this level = E ÷ 10
- Energy at level n = E₁ × (0.1)ⁿ⁻¹ (E₁ = energy in producers)
- Energy at lower level = energy at higher level × 10 (for going down)
- Trophic levels: T1 producers → T2 herbivores → T3 carnivores → T4 top carnivores
- Energy flow: one-way (Sun → producers → consumers → lost as heat). Nutrients: cyclic.
- Biomagnification: chemical concentration increases up the trophic levels
Worked examples
1. Name two biotic and two abiotic components of a pond.
Biotic: water plants (hydrilla, algae), fish, frogs, bacteria. Abiotic: water, sunlight, dissolved oxygen, soil at the bottom, temperature.
2. In a food chain, the producers hold 20,000 J of energy. How much energy reaches the secondary consumer?
Producer → primary consumer: 20,000 × 10% = 2,000 J. Primary → secondary consumer: 2,000 × 10% = 200 J. Answer: 200 J.
3. A hawk (4th trophic level) gets 5 J of energy. How much energy was in the producers of its chain?
Going down one level multiplies by 10. Level 3: 50 J, level 2: 500 J, level 1: 5,000 J. The producers had 5,000 J.
4. In the chain grass → deer → tiger, if the grass has 1,00,000 kJ, how much energy does the tiger get, and what fraction of the grass's energy is that?
Deer: 1,00,000 × 0.1 = 10,000 kJ. Tiger: 10,000 × 0.1 = 1,000 kJ. Fraction = 1,000 / 1,00,000 = 1/100 = 1%.
5. Why will a human feeding on fish from a DDT-polluted lake have more DDT than the fish?
DDT does not break down and stays in body fat. The human eats many fish over time, so all their DDT collects in the human. Being at a higher trophic level, the human ends up with the highest concentration: biological magnification.
6. Why is a food web more stable than a single food chain?
In a web each animal has several food choices. If one prey becomes rare, the predator eats another, so the ecosystem does not collapse.
Common mistakes
- Drawing arrows the wrong way. The arrow points to the eater: grass → deer means the deer eats the grass (energy flows along the arrow).
- Saying 10% of energy is lost at each level. It is the opposite: about 90% is lost and only 10% passes on.
- Thinking decomposers are a trophic level in the chain. They act on dead matter of every level and are shown separately.
- Mixing up the two trends: energy DECREASES up the chain, but pesticide concentration INCREASES up the chain.