Lesson 14: Our Environment — Ecosystems

The environment is everything around us — living and non-living — interacting in systems that sustain life. This NCERT lesson (Class 10's environmental chapter) covers ecosystems and their components, food chains and webs, energy flow (the 10% law), ecological pyramids, and how human activities disrupt natural cycles: ozone depletion, waste, and biomagnification. It is short, conceptual, and heavily examined.

Ecosystems — components and types

An ecosystem is a functional unit of nature: biotic (living — producers, consumers, decomposers) interacting with abiotic (non-living — light, temperature, water, soil, air) components. Producers (green plants, algae — photosynthesis) capture solar energy; consumers (herbivores → carnivores → top carnivores) eat; decomposers (bacteria, fungi) recycle dead matter into nutrients. Without decomposers, nutrients would lock up in corpses and life would halt — the unglamorous essential workers. Types: natural (forests — the Western Ghats' shola; grasslands — the Deccan; deserts — the Thar; ponds, lakes, rivers, oceans) and artificial (crop fields, aquariums, gardens — human-managed, needing inputs). The aquarium is the model ecosystem in miniature — and the crop field shows human simplification: one species, needing fertiliser and pesticide subsidies. Ponds are the textbook example: phytoplankton → zooplankton → small fish → large fish → decomposers. Garden, park, and school-campus ecosystems are observable daily: the food chains in a hedge (leaves → caterpillars → birds → cats), the decomposers in compost pits, the microclimate under a banyan. NCERT's pedagogy: observe first, theorise after. The board wants the component classification (producer/consumer/decomposer/abiotic) applied to any example given.

Food chains, webs and trophic levels

A food chain shows who eats whom in sequence: grass → grasshopper → frog → snake → eagle (a grassland chain); phytoplankton → zooplankton → fish → kingfisher (a pond chain). Trophic levels: T1 producers, T2 primary consumers (herbivores), T3 secondary consumers (small carnivores), T4 tertiary (top carnivores). Each organism occupies a level; omnivores (humans, crows) span levels. Food webs interconnect chains — the realistic picture: the grasshopper also eats other plants, the frog eats many insects, the snake eats frogs and rats. Arrows point in energy-flow direction (food → feeder) — the board's diagram convention. Interdependence: removing one species ripples through the web — the classic case is wolves' reintroduction to Yellowstone (trophic cascade); India's vulture crisis (diclofenac killing vultures → carcasses rotting → feral dog populations rising → rabies) is the subcontinental example. Decomposers close the loop: bacteria and fungi return nutrients to soil for producers — the cycle that fertilisers supplement and that organic farming honours. Detritus food chains (dead leaves → earthworms → birds) run parallel to grazing chains. In exams, construct chains from given organisms, identify trophic levels, and predict the effect of removing one link — always reasoning through the web, not just the chain: "if the top carnivore is removed, the primary carnivores increase, herbivores decrease, producers increase" — the cascade logic step by step.

Energy flow — the 10% law

Energy enters as sunlight; producers capture ~1% in photosynthesis; herbivores eat producers; each transfer loses most energy as heat (respiration), waste, and uneaten parts. Lindeman's 10% law: only ~10% of energy transfers between trophic levels — 10,000 J in producers → 1,000 J in herbivores → 100 J in primary carnivores → 10 J in top carnivores. The calculation is mechanical: move the decimal one place per level. Run it on a pond: if phytoplankton store 20,000 J, zooplankton get 2,000 J, small fish 200 J, and the kingfisher just 20 J — 99.9% of the captured energy never reaches the top. Consequences: food chains are short (rarely beyond 4–5 levels — insufficient energy remains), top carnivores are rare (tigers need vast territories — the ecological reason for large reserves), and eating lower on the chain feeds more people (the vegetarian efficiency argument — relevant to India's food security). Biomass pyramids (mass per level) narrow upward for the same reason; numbers pyramids can invert (one banyan feeding thousands of insects) but energy pyramids never invert. Board numericals: "If 50,000 J reaches primary consumers, how much reaches tertiary consumers?" (50,000 → 5,000 → 500 J). "Explain why a forest supports fewer tigers than deer." (Energy loss per level.) The 10% law is the chapter's mathematical heart — practise until the decimal shifting is instant.

Ozone depletion and waste

The ozone layer (stratospheric O₃) absorbs UV radiation — the planet's sunscreen. CFCs (refrigerants, aerosols) released chlorine that catalytically destroyed ozone: one chlorine atom destroys ~100,000 ozone molecules — the Antarctic "hole" discovered in 1985. Effects of UV increase: skin cancer, cataracts, crop damage, plankton decline. The Montreal Protocol (1987) phased out CFCs — ratified universally, the hole is recovering. Waste: biodegradable (food scraps, paper — decomposers handle) versus non-biodegradable (plastics, glass, e-waste — persisting for centuries). India's waste crisis: metros generating thousands of tonnes daily, landfills (Ghazipur's mountain), the plastic menace (single-use bans in many states), e-waste (the growing toxic stream — formal recycling versus informal burning). The hierarchy: reduce, reuse, recycle, recover — in that order. Biomagnification: non-biodegradable toxins (DDT, mercury) concentrate up food chains. Follow the classic pattern (values are typical reported magnitudes): DDT at 0.003 ppm in water → 0.04 ppm in plankton → 0.5 ppm in small fish → 2 ppm in large fish → 25 ppm in fish-eating birds — a concentration rise of thousands of times, because the toxin is fat-soluble and not excreted while energy (and food mass) is lost at each level. The classic outcome: DDT thinning raptors' eggshells; Minamata's mercury poisoning. For board answers: define, give the mechanism (not excreted, fat-soluble, concentrating), and one example. It is the 10% law's dark mirror — energy dissipates upward, toxins accumulate.

Human impact and conservation

The chapter's moral: human activities disrupt the systems studied — deforestation breaks water cycles, pollution poisons food webs, CFCs thinned ozone, plastics choke oceans and cattle (the stray-cow plastic problem in Indian cities). But the ozone success proves repair is possible: science identified the cause, policy acted globally, and the atmosphere heals. It is NCERT's chosen parable of hope. Conservation strategies: protected areas (national parks, sanctuaries, biosphere reserves — the legal shields), pollution control (effluent treatment, emission norms), waste management (segregation at source — the Swachh Bharat infrastructure), and lifestyle (the 3Rs, LiFE). Community conservation (Bishnoi, sacred groves, Joint Forest Management) shows Indians as conservers, not just consumers. Exam synthesis: "What can you do?" — the board's citizenship question. Segregate waste, refuse single-use plastic, save water and electricity, use public transport, plant natives. Small actions, aggregated across 1.4 billion people, are systemic. A last conceptual anchor: the ecosystem services framework — provisioning (food, timber), regulating (climate, water purification), supporting (nutrient cycling, pollination), cultural (recreation, spiritual). Classifying any conservation argument into these four services organises evaluation answers beautifully. "Why save the mangroves?" — provisioning (fisheries), regulating (storm protection, carbon), supporting (nursery habitat), cultural (tourism). Four services, complete answer.

Diagrams and definitions drill

Must-draw: a food web (6+ organisms, arrows in energy-flow direction), the energy-flow diagram with 10% labels, an ecological pyramid (numbers/biomass/energy), and the ozone-depletion mechanism (CFC → Cl → O₃ destruction) — see Lesson 18 for technique. Must-define, in one precise sentence each: ecosystem, biotic/abiotic, producer/consumer/decomposer, trophic level, food chain/web, 10% law, biodegradable/non-biodegradable, biomagnification, ozone depletion. Must-explain: why food chains are short (energy loss), why tigers are rare (10% law), why DDT concentrates upward (non-biodegradable, fat-soluble), why the ozone hole is healing (Montreal Protocol). Four explanations, each a short paragraph — the chapter's analytical core. Master these and the environment chapter is mastered.

Common mistakes

• Drawing food-web arrows backwards — arrows point from food to feeder (the direction energy flows).
• Claiming the numbers pyramid can never invert either — only the *energy* pyramid never inverts; one tree can feed thousands of insects.
• Applying the 10% law to toxins — energy falls by ~90% per level; non-biodegradable toxins rise in concentration instead.
• Confusing the ozone hole with the greenhouse effect — CFCs and UV versus greenhouse gases and infrared: different gases, different physics.
• Calling plastic biodegradable because it fragments — breaking into microplastics is not decomposition by organisms.
• Forgetting decomposers in food webs — every chain ends with them returning nutrients to producers.

Exam Experience

MCQ 1. Producers in an ecosystem store 20,000 J of energy. By the 10% law, the energy available to secondary consumers (third trophic level) is: (a) 2,000 J (b) 200 J (c) 20 J (d) 18,000 J Key: (b). 20,000 → 2,000 (herbivores) → 200 J (secondary consumers): one decimal place lost per transfer. MCQ 2 (Assertion–Reason). Assertion (A): The pyramid of energy is always upright. Reason (R): At each trophic level, about 90% of energy is lost as heat, waste and uneaten parts, so less energy reaches the next level. (a) Both A and R are true and R is the correct explanation of A (b) Both A and R are true but R is not the correct explanation of A (c) A is true but R is false (d) A is false but R is true Key: (a). Monotonic energy loss at every transfer guarantees the energy pyramid can never invert — unlike pyramids of numbers. Short answer (3 marks). In a pond study, DDT was measured at 0.003 ppm in water, 0.04 ppm in plankton, 0.5 ppm in small fish and 25 ppm in fish-eating birds. Name the phenomenon and explain in two points why the concentration rises. *Marking scheme:* Biomagnification (1). DDT is non-biodegradable and fat-soluble, stored rather than excreted (1). Energy/food mass falls ~90% per level while the toxin is retained, so concentration per unit mass rises (1). Long answer (5 marks). (a) Construct a five-link food chain from: grass, snake, grasshopper, eagle, frog (2). (b) "If all frogs are removed, grass increases." Evaluate this prediction using food-web reasoning (3). *Marking scheme:* (a) Grass → grasshopper → frog → snake → eagle, arrows in energy-flow direction (2). (b) Removing frogs releases grasshoppers from predation → grasshoppers increase → grass decreases (the cascade runs opposite to the prediction) (2); snakes shift to other prey/decline, qualifying the simple chain (1).