Our Environment

A garden is not just a collection of plants sitting next to some animals — it is a working system, where every organism's growth, feeding and survival depends on every other organism and on the soil, air, water and sunlight around it. NCERT's Chapter 13, "Our Environment," is the chapter that finally names this system and traces what happens when a thread in it is pulled. It starts small: a garden, an aquarium, a pond, each one an ecosystem made of living (biotic) and non-living (abiotic) parts working together. From there it builds a precise vocabulary — producers, consumers, decomposers — for who does what inside that system, and shows how these roles link into food chains and branching food webs, with energy flowing through a sequence of trophic levels. The chapter's sharpest idea is that this flow of energy is neither free nor cyclic. At every step from one trophic level to the next, roughly ninety percent of the energy is lost as heat, movement and unused waste, and only about ten percent survives to become body mass available to the next consumer. That single number — the ten percent law — quietly explains why food chains rarely run past four or five links, why there are always far more producers than top predators, and why humans, sitting near the top of many food chains, end up absorbing the largest dose of any harmful chemical that enters that chain through biological magnification. The second half of the chapter turns outward, from natural ecosystems to human impact on the whole planet's system. It examines the protective ozone layer high in the atmosphere, how ultraviolet radiation both creates and destroys ozone, and how synthetic chemicals called CFCs have thinned this shield. It closes with the everyday but consequential problem of garbage: the difference between waste that microorganisms can break down (biodegradable) and waste that persists for decades or centuries (non-biodegradable), and why the choices we make about packaging and disposal ripple through the very ecosystems the chapter opened with.

Key takeaways

An ecosystem is all the interacting organisms in an area (biotic components) together with the non-living physical factors around them (abiotic components) — temperature, rainfall, soil, minerals, wind.
Three functional roles run every ecosystem: producers (green plants and some bacteria, which make food by photosynthesis), consumers (herbivores, carnivores, omnivores, parasites, who eat producers or other consumers), and decomposers (bacteria and fungi, which break down dead remains into simple substances the soil can reuse).
A food chain is a straight-line sequence of who eats whom; a food web is the more realistic, branching network formed when the many food chains of an ecosystem overlap.
Trophic levels number the steps: producers are the first trophic level, herbivores (primary consumers) the second, small carnivores (secondary consumers) the third, and larger carnivores (tertiary consumers) the fourth.
The ten percent law: at each transfer from one trophic level to the next, only about ten percent of the food energy consumed is converted into the consumer's own body and made available to the next level — the rest is lost as heat, in digestion, and in daily activity.
Energy flow is one-way; it never returns to the Sun or to an earlier trophic level, which is why food chains are usually limited to three or four steps, and why there are always more organisms (and more energy) at the lower trophic levels.
Biological magnification is the progressive build-up of non-degradable harmful chemicals, such as pesticides, at each successive trophic level, so organisms at the top of a food chain — often humans — carry the highest concentration.
The ozone layer ( $\text{O}_3$ ) high in the atmosphere absorbs harmful ultraviolet radiation from the Sun; it is formed and destroyed naturally by UV-driven reactions, but synthetic chemicals called chlorofluorocarbons (CFCs) have accelerated its depletion.
Waste is biodegradable or non-biodegradable: biodegradable waste is broken down by microorganisms' enzymes; non-biodegradable waste (like most plastics) resists this breakdown and persists in the environment for very long periods.
Key idea: an ecosystem's biotic and abiotic components are bound together by a one-way, ten-percent-per-step flow of energy from producers upward, and by human activities — like releasing CFCs or generating non-biodegradable waste — that can damage this system far beyond the point where the activity itself occurred.

The core insight

An ecosystem's producers, consumers and decomposers are linked by food chains and webs through which energy flows in one direction and shrinks to about ten percent at each trophic level, so short chains, magnified toxins at the top, a UV-shielding ozone layer under threat from CFCs, and the biodegradable/non-biodegradable split of our waste are all consequences of how tightly bound this system really is.

What this chapter covers

This chapter has two connected halves. The first half asks "how does an ecosystem work?" and builds a vocabulary for it: biotic and abiotic components, the three functional roles of producers, consumers and decomposers, and how these roles link into food chains, food webs and numbered trophic levels. It then follows energy as it flows through this structure, arriving at the ten percent law and the phenomenon of biological magnification, where harmful chemicals — rather than energy — travel the other way, concentrating as they climb. The second half asks "how do our activities affect this system?" and looks at two concrete, exam-important case studies. The first is the depletion of the ozone layer: how ozone is naturally formed and destroyed high in the atmosphere by ultraviolet radiation, why this layer matters for every organism below it, and how chlorofluorocarbons (CFCs) have disrupted the natural balance. The second is the garbage we generate every day: the distinction between biodegradable and non-biodegradable waste, why some materials simply will not break down under natural biological action, and what this means for managing waste responsibly.

The core ideas

Ecosystems: biotic and abiotic components

All organisms in a given area — plants, animals, microorganisms, human beings — interact with each other and with the physical surroundings, and together they maintain a rough balance. This entire interacting system is called an ecosystem. It has two kinds of components: biotic components, the living organisms themselves, and abiotic components, the physical and chemical factors around them, such as temperature, rainfall, wind, soil and the minerals dissolved in it. A garden, with its grasses, flowering plants, frogs, insects and birds, is an ecosystem; so are forests, ponds and lakes. Forests, ponds and lakes are natural ecosystems, formed without deliberate human design, while gardens, crop-fields and aquariums are human-made (artificial) ecosystems — an aquarium, for instance, only stays alive if it is stocked with the right balance of fish, aquatic plants, oxygen and food, and it will eventually need cleaning because waste and dead material accumulate faster than a small, closed system can process them on its own. Within any ecosystem, organisms play one of three functional roles based on how they obtain their food. Producers are green plants and certain bacteria that make organic compounds such as sugars and starch from simple inorganic substances — carbon dioxide and water — using the radiant energy of sunlight captured by chlorophyll; this process is photosynthesis. Consumers cannot make their own food, so they depend on producers, either directly or indirectly, and are classed as herbivores (eat plants only, like a deer), carnivores (eat other animals, like a lion), omnivores (eat both, like a bear or a human), and parasites (live on or in a host and draw nutrition from it without necessarily killing it). Decomposers — mainly bacteria and fungi — break down the complex organic substances in dead organisms and waste products into simple inorganic substances that return to the soil, where producers can use them again. Without decomposers, dead matter and garbage would simply accumulate indefinitely, and the soil's nutrients would never be replenished for the next generation of producers.

Food chains, food webs and trophic levels

When you arrange a set of organisms in the order in which one is eaten by the next — grass eaten by a deer, the deer eaten by a lion — you get a food chain. Each step or level in a food chain is a trophic level. Producers (autotrophs) occupy the first trophic level, because they alone can fix solar energy directly and make it available to every other organism in the chain. Herbivores, or primary consumers, occupy the second trophic level; small carnivores that eat herbivores, or secondary consumers, occupy the third; and larger carnivores that eat other carnivores, or tertiary consumers, occupy the fourth. Real ecosystems rarely contain a single, isolated food chain. Because most organisms are eaten by more than one kind of predator, and most predators eat more than one kind of prey, the many food chains of an ecosystem overlap and branch into a food web — a far more realistic picture of who depends on whom.

Figure

A food chain diagram running from grass through a grasshopper a frog a snake and an eagle with each organism labelled with its trophic level number from first to fourth and an arrow showing the direction of energy flow between consecutive organisms

Energy flow and the ten percent law

Green plants in a terrestrial ecosystem capture only about one percent of the solar energy that falls on their leaves and convert it into food energy through photosynthesis — this small captured fraction is the entire energy budget that supports every other organism in the ecosystem. When a herbivore eats a green plant, a great deal of that stored energy is lost as heat to the environment, some is used up in digestion and daily activity, and only a fraction goes toward the herbivore's own growth and reproduction. Careful study of many ecosystems shows that, on average, only about ten percent of the food energy consumed at one trophic level is converted into body mass and made available to the next trophic level — this is the ten percent law. Because so little usable energy survives each transfer, food chains generally consist of only three or four steps; by the time you reach a fifth trophic level, so little energy remains that supporting an additional level of predators becomes impossible.

E_{n+1} \approx 0.10 \times E_n \qquad (1)

where: $E_n$ is the energy (or biomass) available at trophic level $n$ , and $E_{n+1}$ is the energy available at the next trophic level. The ten percent law also explains a pattern you can see just by counting: there are generally far more individual organisms at the lower trophic levels of an ecosystem than at the higher ones, with the greatest number always being the producers, because it takes a very large amount of biomass at the base to support even a small population of top predators.

Biological magnification

Energy is not the only thing that moves up a food chain — so, unfortunately, do certain harmful chemicals. Pesticides and other chemicals used to protect crops are washed into the soil or into water bodies, where they are absorbed by plants along with water and minerals, or taken up by aquatic plants and animals. Because many of these chemicals are not biodegradable, they are not broken down or excreted efficiently by the organisms that absorb them; instead they accumulate in body tissues. As each consumer eats many organisms from the level below, the chemical becomes progressively more concentrated at each successive trophic level — a phenomenon called biological magnification. Because human beings typically occupy one of the topmost levels in many food chains, the maximum concentration of such chemicals tends to build up in human bodies, which is why food grains, vegetables, fruits and even meat can contain pesticide residues that washing alone cannot remove.

The ozone layer and its depletion

High in the atmosphere, well above the layer where we breathe, a small fraction of the oxygen there exists as ozone ( $\text{O}_3$ ), a molecule made of three oxygen atoms rather than the two-atom oxygen ( $\text{O}_2$ ) we normally breathe. While $\text{O}_2$ is essential for aerobic life, concentrated ozone is actually a poison; at ground level it is a harmful pollutant. But at the higher levels of the atmosphere, ozone performs an essential protective function: it absorbs and shields the Earth's surface from ultraviolet (UV) radiation from the Sun, radiation that is highly damaging to living organisms and is known to cause skin cancer in humans. Ozone at these heights is itself a product of UV radiation acting on ordinary molecular oxygen, splitting some $\text{O}_2$ molecules into free oxygen atoms that then combine with more $\text{O}_2$ to form ozone.

\text{O}_2 \xrightarrow{\ \text{UV}\ } \text{O} + \text{O}, \qquad \text{O} + \text{O}_2 \rightarrow \text{O}_3 \qquad (2)

where: the first reaction is UV-driven photolysis of molecular oxygen into free oxygen atoms, and the second is the combination of a free oxygen atom with an intact $\text{O}_2$ molecule to form ozone. This same UV radiation that creates ozone also naturally destroys some of it, so under normal conditions formation and destruction stay roughly balanced. Beginning sharply in the 1980s, however, the total amount of ozone in the atmosphere began to drop, a change linked to synthetic chemicals called chlorofluorocarbons (CFCs), once widely used as refrigerants and in fire extinguishers. In 1987 the United Nations Environment Programme (UNEP) brokered an international agreement to freeze CFC production at 1986 levels, and it is now mandatory worldwide for manufacturers to make CFC-free refrigerators.

Figure

A cross section of the atmosphere showing sunlight and ultraviolet rays reaching a shaded ozone layer band above the Earths surface with some ultraviolet radiation absorbed by the ozone layer and only a small amount reaching the ground and a separate inset showing the reaction of oxygen molecules splitting under ultraviolet light and recombining into ozone

Biodegradable and non-biodegradable waste

Every day, homes, schools and industries generate large amounts of waste — kitchen scraps, paper, packaging, old clothes, broken items — and what becomes of this material once it is thrown away depends on one key property. The enzymes inside microorganisms such as bacteria and fungi are specific in their action: a given enzyme can only break down a particular kind of substance, which is why we cannot digest coal for energy even though it too is made of carbon compounds. Substances that natural biological processes — chiefly the action of decomposer microorganisms — can break down are called biodegradable; substances that resist this breakdown are called non-biodegradable. Kitchen waste, paper, cotton cloth and most natural materials are biodegradable and disappear over weeks or months when buried in moist soil. Many human-made materials, especially plastics, are non-biodegradable: they are not acted upon by the enzymes of bacteria or other decomposers, and although physical processes like heat and pressure can eventually alter them, under ordinary environmental conditions they persist for a very long time, sometimes decades or centuries. Non-biodegradable materials may simply sit inertly in the environment, or they may actively harm the members of an ecosystem — for instance, by choking waterways, entering the food chain, or releasing toxic substances as they slowly break down. Changes in lifestyle, more disposable products, and packaging that favours convenience over degradability have all increased the share of our waste that is non-biodegradable, which is why waste segregation and responsible disposal matter as much as reducing the total amount of waste generated.

Derivation AWhy food chains rarely extend beyond four trophic levels

The goal of this reasoning chain is to show, using nothing more than the ten percent law and simple arithmetic, exactly how little usable energy survives by the time it reaches a fourth or fifth trophic level, and why this — not any biological limit on how many predators could theoretically exist — is the real reason food chains stay short. We start from a fixed amount of energy captured by producers and track what a realistic ten percent transfer efficiency leaves behind at every subsequent step.

Step 1. Suppose the producers (green plants) in a patch of grassland capture and store $E_1 = 10{,}000$ units of energy through photosynthesis. This is the entire energy budget available to every consumer that will ever feed, directly or indirectly, within this food chain — nothing beyond this amount can ever be passed on.

Step 2. Apply the ten percent law once, moving from producers to primary consumers (herbivores). Only about ten percent of $E_1$ is converted into the herbivores' own body mass and made available to the next level: $E_2 = 0.10 \times 10{,}000 = 1000$ units. The remaining ninety percent was lost as heat during metabolism, used up in digestion and daily activity, or never eaten at all.

Step 3. Apply the law a second time, moving from herbivores to secondary consumers (small carnivores). Again only about ten percent of $E_2$ survives the transfer: $E_3 = 0.10 \times 1000 = 100$ units. Notice the energy available has now fallen to just one percent of what the producers originally captured.

Step 4. Apply the law a third time, moving from secondary to tertiary consumers (large carnivores): $E_4 = 0.10 \times 100 = 10$ units — a mere one-thousandth of the original producer energy. If a fifth trophic level tried to exist above this, it would receive only about $1$ unit out of the original $10{,}000$ , an amount too small to support a viable, self-sustaining population of any real predator, especially a large one with high energy needs.

Step 5. Generalise the pattern algebraically. After $n$ transfers, the energy remaining is $E_{n+1} = E_1 \times (0.10)^n$ , which shrinks by a factor of ten at every single step. This is exactly why NCERT states that the loss of energy at each step is so great that very little usable energy remains after four trophic levels, why ecosystems always support far more producers than top predators by sheer numbers, and why removing or damaging the producer base of an ecosystem is catastrophic for every level built on top of it — there is no reserve of energy anywhere else in the system to fall back on.

Go deeper · Derivation B · 2 minHow biological magnification concentrates a pesticide up a food chain

Derivation BHow biological magnification concentrates a pesticide up a food chain

The goal here is to show numerically why a chemical that starts out at a barely detectable concentration in water or soil can end up at a dangerous concentration in a top predator, purely because of how much biomass each consumer must eat to sustain itself, and why this makes biological magnification a completely different kind of "flow" from the one-way loss of energy described by the ten percent law. Where energy shrinks by a factor of ten at every step, a non-biodegradable, fat-soluble chemical does the opposite: it survives digestion nearly unchanged and simply concentrates as one organism eats many of the organisms below it.

Step 1. Start at the base of an aquatic food chain. Suppose pesticide-contaminated water carries a pesticide at a low concentration, and this is absorbed by algae and other aquatic plants (producers) directly from the water, along with the water and minerals they need. At this stage the concentration inside the plant tissue is already comparable to, or slightly above, the concentration in the surrounding water, because the plant has no efficient way to excrete this non-biodegradable chemical once absorbed.

Step 2. A small fish (primary consumer) must eat a very large number of these plants over its lifetime to grow and survive, since only about ten percent of the food energy it eats becomes its own body mass. Every one of those many plants contributes its small dose of pesticide, and because the pesticide — unlike food energy — is not broken down or used up, nearly all of it accumulates in the fish's body fat rather than being lost. The result is that the pesticide concentration inside the fish becomes noticeably higher than the concentration in any single plant it ate.

Step 3. A larger fish (secondary consumer) must, in turn, eat many of these small fish to sustain itself, and inherits the accumulated pesticide load from every one of them. Because this larger fish is again converting food into body mass at roughly the same low, ten-percent-like efficiency, while retaining nearly all the absorbed pesticide, the concentration in its body climbs again, well above what was present in any single small fish.

Step 4. A bird or human at the top of this food chain eats several of these larger fish over time, and inherits the cumulative pesticide load of every fish, every small fish, and every plant beneath it in the chain — a magnification effect that compounds at every single trophic transfer, in the opposite direction to the loss of usable energy. This is why apex organisms, including humans, consistently show the highest measured pesticide concentrations of any organism in a contaminated ecosystem, even though they themselves were never directly exposed to a large dose of the original chemical.

Step 5. Contrast the two flows explicitly. Energy shrinks by roughly a factor of ten at each trophic transfer because most of it is metabolised, respired away as heat, or otherwise used up — it obeys the ten percent law. A non-biodegradable toxin does not get used up or metabolised in the same way, so instead of shrinking, its concentration compounds upward through the same chain. The two phenomena move through the identical structure — the food chain — in the same direction, but with opposite outcomes, which is precisely why biological magnification is such a dangerous and counter-intuitive consequence of releasing non-biodegradable chemicals into an ecosystem.

Worked examples

Board level - energy available at each trophic level

In a food chain, producers capture and fix $20{,}000$ kilojoules of energy through photosynthesis in a given area. Using the ten percent law, calculate the energy available to (a) the primary consumers, (b) the secondary consumers, and (c) the tertiary consumers.

Step 1. Energy available to primary consumers (herbivores) is ten percent of the energy fixed by producers: $E_2 = 0.10 \times 20{,}000 = 2000$ kJ.

Step 2. Energy available to secondary consumers (small carnivores) is ten percent of what the primary consumers received: $E_3 = 0.10 \times 2000 = 200$ kJ.

Step 3. Energy available to tertiary consumers (large carnivores) is ten percent of what the secondary consumers received: $E_4 = 0.10 \times 200 = 20$ kJ. Answer: primary consumers get 2000 kJ, secondary consumers get 200 kJ, and tertiary consumers get only 20 kJ — a thousandth of the original energy fixed by the producers.

Board level - classifying components of an ecosystem

A pond ecosystem contains algae, small fish that eat algae, a heron that eats the small fish, dissolved oxygen in the water, and bacteria that break down dead fish. Identify the abiotic component, and classify each biotic organism as a producer, consumer, or decomposer, naming its trophic level where applicable.

Step 1. The dissolved oxygen in the water is the abiotic component — it is a physical/chemical factor of the environment, not a living organism, though it is essential for the living organisms to survive.

Step 2. Algae are producers, occupying the first trophic level, because they make their own food from inorganic substances using sunlight, exactly like green plants on land.

Step 3. The small fish are primary consumers (herbivores), occupying the second trophic level, since they feed directly on the producers (algae). The heron is a secondary consumer, occupying the third trophic level, since it feeds on the primary consumers (small fish).

Step 4. The bacteria that break down dead fish are decomposers — they do not occupy a numbered trophic level in the usual sense, since they act on dead organic matter from every level and return simple inorganic substances to the pond for the algae to reuse. Answer: abiotic component — dissolved oxygen; producer — algae (1st level); primary consumer — small fish (2nd level); secondary consumer — heron (3rd level); decomposer — bacteria.

Board HOTS / foundation JEE-NEET - biological magnification and trophic-level removal

A lake is contaminated with a non-biodegradable pesticide at a concentration of $0.02$ parts per million (ppm) in the water. Algae in the lake concentrate the pesticide to $0.2$ ppm, small fish that eat the algae concentrate it further by a factor of $10$ , and large fish that eat the small fish concentrate it by a further factor of $10$ . (a) Calculate the pesticide concentration in the large fish. (b) A fishing community that eats only these large fish notices the concentration in their own bodies is even higher still — explain why, using the reasoning of biological magnification. (c) If all the large fish in this lake were suddenly removed, explain what would happen to the small fish population and to the algae population, and state whether this impact would be the same if instead all the algae were removed.

Step 1. Concentration in small fish $= 0.2 \times 10 = 2$ ppm. Concentration in large fish $= 2 \times 10 = 20$ ppm — already a thousand-fold increase over the original water concentration of $0.02$ ppm, purely from three trophic transfers.

Step 2. Humans who eat only these large fish sit at an even higher trophic level than the large fish themselves, and since a person eats many large fish over a lifetime while excreting very little of the accumulated, non-biodegradable pesticide, the same magnification logic applies once more: the concentration in human tissue exceeds even the $20$ ppm found in a single large fish, exactly as biological magnification predicts for any additional trophic transfer.

Step 3. If all the large fish were removed, the small fish they used to prey on would lose their main predator and its population would likely increase sharply (at least temporarily), while heavier grazing by this larger small-fish population could reduce the algae population, since the top-down check on their numbers is now gone.

Step 4. If instead all the algae — the producers — were removed, the effect would be far more severe and would cascade through the entire chain rather than shifting a balance: with no producers left to fix solar energy in the first place, there would be no energy source at all for the small fish or the large fish, so both populations would collapse rather than merely change size. Answer: large fish concentration = 20 ppm; humans accumulate even more because they add one more trophic transfer; removing a top predator shifts the balance of levels below it, but removing the producer base collapses the entire chain, because producers are the sole entry point for energy into the ecosystem. Timing note: about four to five minutes for a full written answer. Examiners specifically reward distinguishing "shifts a balance" (removing a consumer level) from "collapses the whole chain" (removing the producer level) — a common HOTS trap is treating every trophic-level removal as equally damaging, when the position of the removed level in the chain matters enormously.

Common traps

The most common slip is treating energy flow through an ecosystem as if it were cyclic, the way nutrients like carbon and nitrogen are — energy is not recycled; it flows in one direction only, from producers to consumers to decomposers, and is ultimately lost as heat, never returning to be reused by the Sun or an earlier trophic level. A second frequent error is misreading the ten percent law as meaning ten percent of the organisms move up a level, rather than about ten percent of the food energy consumed being converted into new body mass; the law is about energy transfer efficiency, not about counting individuals. Students often also assume that ozone (O3) and the oxygen (O2) we breathe are simply "more of the same good thing," when in fact concentrated ozone is toxic to breathe and only serves a protective role because of where it sits — high in the atmosphere, shielding against UV radiation, not down at ground level where we live. Another common confusion is thinking that "biodegradable" automatically means "harmless" — a large amount of biodegradable waste (such as untreated sewage) dumped into a water body can still cause serious problems, including depleting the oxygen dissolved in that water as decomposers work through it. Finally, students frequently forget that biological magnification and the ten percent energy law move through the very same food chain in opposite directions at the same time — energy shrinks by about ninety percent at each step while a non-biodegradable toxin's concentration grows at each step — and conflating the two, or assuming both quantities shrink together, is a very common source of incorrect answers.

Common trapEnergy flows through an ecosystem only once, in one direction — it is never recycled the way matter is

The single biggest misconception in this chapter is imagining that energy, like carbon or nitrogen or water, cycles endlessly through an ecosystem and can be reused. It cannot. The Sun continuously supplies fresh energy, producers capture a small fraction of it, and at every subsequent trophic transfer roughly ninety percent is permanently lost as heat to the environment — it never flows back to an earlier trophic level and never returns to the Sun. This is precisely why food chains cannot be indefinitely long, no matter how favourable the conditions: there is a hard, one-way, shrinking energy budget behind every single link in the chain, and every exam question about the length of food chains, the relative number of organisms at each trophic level, or the impact of removing a trophic level ultimately traces back to this one unidirectional fact.

You should be able to…

Define an ecosystem and distinguish its biotic components from its abiotic components, with examples of natural and human-made ecosystems.
Classify any given organism in a food chain as a producer, consumer (herbivore, carnivore, omnivore or parasite), or decomposer.
Draw a simple food chain, label its trophic levels from producer through tertiary consumer, and explain the difference between a food chain and a food web.
State the ten percent law and use it to calculate the energy or biomass available at any trophic level, given the energy fixed by producers.
Explain why energy flow through an ecosystem is one-directional and why this limits food chains to about three or four trophic levels.
Explain biological magnification, work through a numeric example of concentration increasing up a food chain, and explain why humans are often the most affected.
Write the reactions by which ozone is formed and describe, in outline, how CFCs disrupt the natural ozone balance high in the atmosphere.
Distinguish biodegradable from non-biodegradable waste with examples, and explain why "biodegradable" does not automatically mean "harmless."
Predict, with reasoning, what happens to an ecosystem if a given trophic level — especially the producer level — is removed or damaged.

Ask the tutor: Want to work through a longer food web (not just a single chain) and see how removing one organism can ripple through several different chains at once?

Why do decomposers matter so much if they are not producers or consumers?

Decomposers close the loop that keeps an ecosystem's nutrients available for reuse. When plants and animals die, or when organisms produce waste, decomposers such as bacteria and fungi break down these complex organic substances into simple inorganic substances that go back into the soil. Producers then take up these same substances again as raw material for making new food. Without decomposers, dead matter and waste would simply pile up indefinitely, the soil would gradually run out of the nutrients producers need, and the whole ecosystem's ability to sustain new life would grind to a halt over time, even though energy itself would still be arriving fresh from the Sun every day.

If the ten percent law says so much energy is lost why doesn't nature just make the transfer more efficient?

The loss at each trophic level is not a design flaw that could simply be fixed — it is an unavoidable consequence of how living things use energy. Some of the energy a herbivore eats is lost as heat during normal metabolism, some is used up doing the work of digestion itself, some powers movement and other daily activities, and some is present in parts of the food that are never eaten or digested at all. Only what is left over after all of this goes toward the animal's own growth and reproduction, which is the only portion actually available to whatever eats it next. Every living organism, in every ecosystem on Earth, faces this same set of unavoidable energy costs, which is why the roughly ten percent figure shows up so consistently across very different food chains.

Is all plastic non-biodegradable, and are newer biodegradable plastics actually better for the environment?

Most conventional plastics are non-biodegradable because the bacteria and fungi found in soil and water simply do not have the right enzymes to break down their particular chemical structure, so they persist in the environment for a very long time, sometimes many decades. Newer materials marketed as biodegradable plastics are generally designed to break down faster under specific conditions, such as being exposed to particular microorganisms, sunlight, or industrial composting facilities, but this does not automatically mean they are harmless in every environment they end up in — how well they actually degrade, and what they break down into, depends heavily on the specific material and the conditions it is exposed to. This is exactly why NCERT encourages finding out more about specific materials rather than assuming a "biodegradable" label always means no environmental impact whatsoever.

Why does human activity get blamed for ozone depletion when ozone is also destroyed naturally by ultraviolet radiation?

Ozone formation and destruction by ultraviolet radiation happen naturally and continuously, and under normal conditions these two processes stay roughly balanced, keeping the total amount of ozone in the stratosphere fairly stable over long periods. The concern with human activity is not that we introduced ozone destruction where none existed before, but that synthetic chemicals like CFCs pushed this natural balance sharply toward destruction, because a single chlorine atom released from a CFC molecule can go on to destroy a very large number of ozone molecules in a repeating cycle before it is finally removed from the atmosphere. This is why the amount of ozone measured in the atmosphere began dropping sharply starting in the 1980s, well beyond what the natural formation-and-destruction balance alone would predict, and why international agreements specifically targeted CFC production rather than trying to stop the ozone layer's entirely natural, and otherwise harmless, background chemistry.