The environment: the four spheres, ecosystems and the flows that keep life going

What this chapter is for

This chapter answers the first question of the subject, which is what the environment is and how it works. It sets out the four great parts of the Earth, the air, the water, the rock and the living world, and shows how each works on the others. It then explains what an ecosystem is, how energy passes through one and is lost along the way, and how carbon, nitrogen, water and phosphorus go round and round between living things and their surroundings. Every later chapter stands on this one, because pollution, climate change and conservation are all disturbances of the flows taught here.

The chapter runs long, so here is its shape. It begins with what the word “environment” means, in plain words and in Pakistan’s law, and with the four spheres. It then builds the ecosystem from its parts, follows energy up a food chain, and follows four kinds of matter round their cycles. It ends with Pakistan’s own ecosystems from north to south, the critics of the ecosystem idea, and the state of the planet now.

The papers test this ground mostly in Part-I, with questions on biomes, the parts of an ecosystem, the share of oxygen in the air and food chains, and in Part-II whenever a question turns on a cycle or a food web.¹

Before you start

The words

Environment. Everything around a living thing that affects it: the air, the water, the land, other living things, and, for people, the buildings and the social and economic conditions they live in.

Sphere. In environmental science, one of the four great parts of the Earth: the atmosphere (air), the hydrosphere (water), the lithosphere (rock and soil) and the biosphere (living things).

Ecosystem. A community of living things together with the non-living surroundings they depend on, working as one system. A pond, a forest and a mangrove creek are ecosystems.

Biotic and abiotic. Biotic means living or produced by living things, such as plants, animals and bacteria. Abiotic means non-living, such as sunlight, water, temperature and minerals.

Producer. A living thing that makes its own food from simple chemicals, almost always by using sunlight. Plants, algae and some bacteria are producers.

Consumer. A living thing that gets its energy by eating others. A plant-eater is a primary consumer, and an animal that eats plant-eaters is a secondary consumer.

Decomposer. A living thing, mostly bacteria and fungi, that breaks down dead matter and waste into simple chemicals that plants can use again.

Trophic level. A step in a food chain: producers are the first level, plant-eaters the second, and so on.

Food web. All the feeding links in an ecosystem drawn together, showing who eats whom. A food chain is one line through a food web.

Detritus. Dead plant and animal matter, such as fallen leaves, that is broken down by small animals and decomposers.

Biome. A large region of the Earth with a similar climate and similar plants and animals, such as a desert, a grassland or a tropical rainforest.

Biogeochemical cycle. The path by which a chemical element, such as carbon or nitrogen, moves between living things, the air, the water and the rock, and back again.

The places

The Karakoram. The mountain range in Pakistan’s far north, in Gilgit-Baltistan, home to K2 (8,611 metres) and to some of the largest glaciers outside the poles.

The Indus. Pakistan’s great river, rising in Tibet and flowing south through the length of the country to the Arabian Sea. Most of Pakistan’s water comes from it and its tributaries.

The Indus delta. The fan of creeks and islands where the Indus meets the sea, south-east of Karachi. Its creeks are lined with mangrove forests.

Silver Springs, Florida. A spring-fed river in the United States where the ecologist Howard T. Odum measured, in the middle of the twentieth century, how much energy each feeding level of an ecosystem holds. This chapter uses his numbers.

Mauna Loa, Hawaii. The volcano where the United States’ National Oceanic and Atmospheric Administration (NOAA) has measured the carbon dioxide in the air since 1958, the longest such record.

The story in plain words

1. Everything on Earth sits in four great stores: the air, the water, the rock and the living world.
2. These four are always trading with each other. Water leaves the sea as vapour and falls on the mountains as snow.
3. Living things and their surroundings together make ecosystems, from a pond to a forest.
4. Plants catch the sun’s energy and turn it into food. Animals eat the plants, and other animals eat them.
5. At each step most of the energy is lost as heat, so there is always far less energy at the top than at the bottom.
6. Matter is not lost. Carbon, nitrogen, water and phosphorus go round and round between the living and the non-living.
7. People now push on these cycles harder than ever, by burning fuel, spreading fertiliser and clearing forests.
8. Pakistan holds almost every kind of ecosystem, from glaciers to deserts to mangroves.
9. Scientists now say that humans have pushed the planet past seven of nine safe limits, and the rest of this subject is about how to come back.

1. What “the environment” means

Begin with a place you know. Stand on the bank of a canal in the Punjab in July. The water is brown with silt from the mountains. A heron waits in the shallows for a fish. Wheat stubble from the last harvest lies in the field beside you, and a tubewell hums. Over your head the monsoon clouds are building. Everything in that scene, the water, the silt, the heron, the fish, the field, the machine and the clouds, is part of the environment of the farmer who works that field. Each of them affects the others, and the farmer is affected by all of them.

The environment is everything around a living thing that affects its life. For a fish it is the water, its temperature and oxygen, the plants and insects it eats, and the heron that eats it. For a human being it is also the house, the road, the factory, the market and the laws, because people live inside a world they have built as well as the one they were born into. The word comes from the French environner, to surround.

Pakistan’s law gives the word its widest meaning. The Pakistan Environmental Protection Act of 1997 says, in section 2, that “environment” means “air, water and land”, “all layers of the atmosphere”, “all organic and inorganic matter and living organisms”, “the ecosystem and ecological relationships”, “buildings, structures, roads, facilities and works”, “all social and economic conditions affecting community life”, and “the inter-relationships between any of the factors” in that list.³ Notice the last item. The law does not treat the environment as a list of separate things. It treats it as the relationships between them, which is exactly how an environmental scientist sees it.

There is an older way of saying the same thing. The Qur’an speaks of a balance set in creation: “And the heaven He raised and imposed the balance”, it says, and in the next verse, “That you not transgress within the balance.”⁴ It also says that “Corruption has appeared throughout the land and sea by [reason of] what the hands of people have earned”.⁵ Pakistani writers on the environment often begin from these verses, because they say in a few words what this chapter takes many pages to explain. The world is a system in balance, people can upset it, and the harm comes back to them.

Environmental science is the study of that system and of what people do to it. Ecology, a word older than environmental science, is the study of how living things relate to each other and to their surroundings. Environmental science is wider. It uses ecology, but also chemistry, physics, geology, economics, law and sociology, because a problem such as Lahore’s winter smog has a chemical cause, a physical one in the weather, an economic one in cheap fuel, a legal one in weak enforcement and a social one in how people live. Head III of this subject teaches each of those sciences in its own chapter.

2. The four spheres of the Earth

The easiest way to hold the whole environment in your head is to divide it into four stores, which scientists call spheres. Figure 1 stacks them as layers, though in truth they overlap everywhere.

The atmosphere is the layer of gases held round the Earth by gravity. Dry air is 78.08 per cent nitrogen and 20.95 per cent oxygen by volume, with argon at about 9,340 parts per million, which is just under one per cent.⁶ Carbon dioxide is tiny by comparison. NOAA’s measurements at Mauna Loa put it at 427.35 parts per million on average in 2025, or about 0.04 per cent of the air.⁷ Water vapour varies from place to place and is typically about one per cent.⁶ Small as they are, carbon dioxide and water vapour do most of the work of keeping the planet warm, which is the subject of the chapter on the greenhouse effect. The atmosphere also has layers by height, the troposphere where weather happens and the stratosphere where the ozone layer sits, which the chapter on environmental physics draws.

The hydrosphere is all the Earth’s water, in the sea, in ice, in the ground, in lakes and rivers and in the air. Almost all of it is salt. The United States Geological Survey’s figures, from the hydrologist Igor Shiklomanov, put 96.54 per cent of all water in the oceans and seas.⁸ Only about 2.5 per cent is fresh, and of that fresh water 68.7 per cent is locked in ice and glaciers and 30.1 per cent is in the ground. Rivers hold only 0.006 per cent of the world’s fresh water.⁸ This is worth remembering in Pakistan, where nearly everyone depends on one river system and the groundwater beneath it.

The lithosphere is the solid Earth: the rock of the crust and upper mantle, and the thin skin of soil on top. Soil is where the lithosphere meets the other three spheres. It is broken rock mixed with air, water, dead matter and countless living things, and it takes centuries to form a few centimetres of it. The chapter on environmental geology teaches the rock cycle and groundwater.

The biosphere is every living thing, together with the parts of the other three spheres where life is found. It reaches from a few kilometres up in the air to the deepest ocean floor, but most of it lives in a thin band near the surface. The biosphere is the smallest of the four by mass and the one that changes the others most. Plants made the oxygen in the air, shells and corals made much of the world’s limestone, and forests hold soil on hillsides.

3. How the spheres work on each other

The four spheres matter because they are always trading. No sphere keeps its matter or its energy to itself for long, and a change in one passes into the others. Pakistan’s summer monsoon shows this better than any textbook example, and Figure 2 draws it.

In early summer the land of South Asia heats faster than the sea. Warm air rises over the land, and moist air from the Arabian Sea and the Bay of Bengal is drawn in to replace it. That is the hydrosphere giving water to the atmosphere. The winds carry the moisture inland, and when it rises over the Himalaya, the foothills and the plains it cools, condenses and falls as rain. That is the atmosphere giving water to the lithosphere, where it soaks the soil, fills the rivers and carries silt down from the hills. Crops, forests and people then draw on that water, and plants breathe much of it back into the air through their leaves. The biosphere returns water to the atmosphere, and the loop is closed.

Every part of this loop can be changed by people. Warm the sea, and the air above it holds more moisture, so the rain can fall harder. Clear the forests on a hillside, and the soil washes into the river, silting the dams downstream. Pump the groundwater faster than the rain replaces it, and the water table falls. Nearly every chapter in this subject is about one such change in one sphere and how it passes into the others. Seeing the four spheres as one system is the first habit an environmental scientist learns.

4. What an ecosystem is

The four spheres describe the whole planet. To study a piece of it, ecologists use a smaller unit, the ecosystem. An ecosystem is a community of living things together with the non-living surroundings they depend on, treated as one working system. The British ecologist Arthur Tansley gave the word in 1935, arguing that living things and their physical surroundings must be studied together, because neither makes sense without the other.⁹

An ecosystem has two kinds of parts. Its biotic parts are the living things, the plants, animals, fungi and bacteria. Its abiotic parts are the non-living conditions, such as sunlight, temperature, water, soil, minerals and the gases in the air or water. In a village pond in Sindh, the biotic parts are the algae, water plants, snails, fish, frogs, egrets and the bacteria in the mud. The abiotic parts are the water, the dissolved oxygen, the mud, the heat of the sun and the nutrients washed in from the fields. Change an abiotic part, such as by letting fertiliser drain in, and the biotic parts change too, often in ways nobody planned.

Ecosystems have no fixed size. A drop of water with its microbes can be studied as one, and so can a forest of thousands of square kilometres. The boundary is chosen by the person studying it, according to the question asked. This is a strength, because the same ideas apply at every scale, and it is also a weakness that the section on the critics returns to.

Large regions of the Earth with similar climates carry similar ecosystems, and these are called biomes. Deserts, grasslands, temperate forests, tundra and tropical rainforests are biomes. Pakistan holds several, from cold desert and alpine meadow in the north to hot desert in Thar and Cholistan, and the chapter on environmental biology describes the world’s biomes in full.

Every ecosystem, whatever its size, does two things. Energy flows through it, entering as sunlight and leaving as heat. Matter cycles within it, the same atoms of carbon, nitrogen and phosphorus used again and again. The rest of this chapter follows those two processes, beginning with the living things that make them happen.

5. Producers, consumers and decomposers

Every living thing in an ecosystem has a job in how food and energy pass through it, and there are three jobs.

Producers make their own food. A plant takes in carbon dioxide from the air and water from the soil, and uses the energy of sunlight to join them into sugar, giving off oxygen. This is photosynthesis, and it can be written in one line: carbon dioxide plus water, with light, gives sugar plus oxygen. Every leaf of wheat in the Punjab, every alga in the Arabian Sea and every mangrove leaf in the delta is a small factory doing this. Almost all the energy in every food chain on Earth begins as sunlight caught this way.

Consumers cannot make their own food, so they eat. A goat grazing in the Potohar eats grass, so it is a primary consumer, a plant-eater. A wolf that eats the goat is a secondary consumer. An animal that eats meat-eaters, such as a snow leopard taking a fox, is a tertiary consumer. Many animals, people among them, eat at several levels at once. Consumers also burn part of their food in respiration, the reverse of photosynthesis, which turns sugar and oxygen back into carbon dioxide and water and releases the energy the animal needs to live.

Decomposers break down what the others leave behind. Bacteria and fungi feed on dead leaves, dead animals and dung, and turn them back into simple chemicals, such as ammonium and phosphate, that plants can use again. Earthworms, termites and crabs help by breaking dead matter into small pieces, and these are called detritus feeders. Without decomposers, the nutrients in a forest would be locked up in fallen trees and the forest would starve. They are the reason matter can cycle at all.

6. Food chains and food webs

A food chain is a line of who eats whom: grass, then goat, then wolf. Each step is a trophic level. Producers are the first, plant-eaters the second, the animals that eat them the third, and so on up to the top predator. Real ecosystems are never so simple, because most animals eat more than one thing and are eaten by more than one thing. When all the feeding links are drawn together, they make a food web. Figure 3 draws a simplified food web for the mangrove creeks of the Indus delta.

The grey mangrove, Avicennia marina, is the most abundant mangrove of the Indus delta.¹⁴ Its leaves feed few animals directly. Most fall into the creeks, where bacteria and fungi soften them and shrimp, crabs and small fish feed on the detritus. Out in the open water, the producers are phytoplankton, microscopic algae eaten by zooplankton, which are eaten in turn by small fish. Larger fish eat the small fish and the shrimp, and herons, egrets and people take what they can. Ecologists call the two strands a grazing food chain, which starts with living plants, and a detritus food chain, which starts with dead matter. In a mangrove creek, the detritus chain carries most of the energy.

The web explains a fact every fisherman in the delta knows. Shrimp and many fish spend their young lives among the mangrove roots, which give them food and shelter. When the mangroves are cut for fuelwood or camel fodder, or die because too little fresh water reaches the delta, the detritus that fed the young shrimp disappears, and the catch falls a few years later, far out at sea. A food web shows how a cause in one place produces an effect somewhere that looks unconnected.

Webs also explain why removing one species can change a whole ecosystem. If the top predators are killed, the animals they ate can multiply and strip the plants. CE-2022 asked exactly this, “If the population of the top carnivores declines, what will happen to the landscape ecosystems?”¹ The chapter on environmental biology answers it with the classic case of the wolves of Yellowstone. The point to take from this chapter is that a food web is a system, so a change at any point travels along the links.

7. How energy flows through an ecosystem

Energy does something in an ecosystem that matter does not. It flows one way, in at the bottom and out as heat, and it is never recycled. This is the most important mechanism in the chapter, because it explains why food chains are short, why there are few tigers and many deer, and why a meat diet needs far more land than a diet of grain.

The cause is a law of physics. Whenever energy changes from one form to another, part of it is lost as heat. A goat that eats a kilogram of grass does not turn that kilogram into goat. It burns most of the energy to stay warm, to walk, to breathe and to digest, and some passes through it as dung. Only a small part is built into new goat tissue that a wolf could later eat. The American ecologist Raymond Lindeman described this in 1942 in a paper on the flow of energy through a lake, and it became the basis of how ecologists measure ecosystems.¹⁰

The best-known measurements come from Silver Springs in Florida, studied by Howard T. Odum. Each year a square metre of the spring’s plants captured 20,819 kilocalories of energy. The plant-eaters took in 3,368 kilocalories of that, the animals that ate them took in 383, and the top consumers only 21.¹¹ Each level then burned most of what it took in, as the goat does, so only a part was left in its body for the next level to eat. Figure 4 draws these four numbers to scale, and the top bar is too thin to see.

Work out the share that passes from each level to the next. From plants to plant-eaters it is 3,368 divided by 20,819, which is 16.2 per cent. From plant-eaters to the next level it is 383 divided by 3,368, or 11.4 per cent. From there to the top it is 21 divided by 383, or 5.5 per cent. Textbooks round this to a “ten per cent rule”: on average about a tenth of the energy at one level reaches the next. The Silver Springs numbers show that the real figure varies, here between about five and sixteen per cent, but the lesson holds. After four to six transfers of energy there is too little left to support another level.¹²

Now take the lesson to a question of policy. A field of wheat that could feed ten people directly would feed only about one person if the wheat were first fed to animals and the people ate the meat, because about nine-tenths of the energy would be lost on the way. This is why a growing population puts more pressure on land when its diet shifts towards meat, a point the chapter on food security returns to. The same law also tells you that the top predators in any ecosystem, such as the snow leopard in the Karakoram or the Indus river dolphin in the river, are always rare. They live at the end of a long and leaky chain, and anything that weakens the lower levels reaches them first.

8. Ecological pyramids

The loss of energy at each step gives ecosystems a shape. If each trophic level is drawn as a bar, with the producers at the bottom, the bars make a pyramid. Ecologists draw three kinds, and Figure 5 shows them side by side.

A pyramid of energy measures the energy each level takes in over a year, as in Silver Springs. It is always widest at the bottom, because energy is lost at every step and is never recycled. No ecosystem can have more energy flowing through its plant-eaters than through its plants.

A pyramid of numbers counts the individuals at each level. It is usually upright, with many grass plants, fewer goats and fewer wolves. It can stand on its point when one large producer feeds many small consumers. A single mango tree in an orchard may feed thousands of insects, and those insects feed a few dozen birds.

A pyramid of biomass weighs the living matter at each level at one moment. It too is usually upright, but it can be inverted. In the English Channel, the phytoplankton at one moment weigh about 4 grams a square metre while the zooplankton that eat them weigh 21 grams.¹² The phytoplankton are not short of production. They reproduce so fast and are eaten so quickly that little of them is standing at any one time. This is the commonest trap in a question on pyramids. Only the pyramid of energy must always be upright, because it measures a flow over time, while the other two measure what happens to be standing at one moment.

9. The carbon cycle

Energy flows through an ecosystem and is lost, but matter stays and goes round. The paths by which elements move between living things and the air, water and rock are called biogeochemical cycles, a long word made of bio (life), geo (earth) and chemical. Four of them matter most for this subject: carbon, nitrogen, water and phosphorus. Figure 6 draws the carbon cycle as stores and flows.

Carbon is the backbone of every living molecule. Its main stores are the air (as carbon dioxide), living things, the soil, the oceans and, far larger than all of these, rock and fossil fuels. Plants pull carbon dioxide out of the air by photosynthesis and build it into sugar. Animals eat the plants, and both plants and animals return carbon dioxide to the air by respiration. When they die, decomposers break them down and release more. The oceans swap carbon dioxide with the air at their surface, and sea creatures build carbon into their shells, which settle on the sea floor and over millions of years become limestone. Some dead plants are buried before they rot and, under heat and pressure over millions of years, become coal, oil and gas.¹³

For most of human history the flows into the air and out of it were roughly in balance. Burning fossil fuels broke that balance. It takes carbon that was buried over millions of years and returns it to the air in a few decades. The result can be measured. At Mauna Loa the yearly average was 315.98 parts per million in 1959 and 427.35 in 2025.⁷ Clearing forests adds to the rise, because it removes plants that would have taken carbon in and releases the carbon stored in their wood and soil. The chapters on the greenhouse effect and on climate change follow what that extra carbon dioxide does.

10. The nitrogen cycle

Nitrogen is in every protein and in the DNA of every cell. The air is about 78 per cent nitrogen, yet plants are often short of it, because the nitrogen in the air is a gas whose two atoms are bound tightly together and plants cannot use it. The nitrogen cycle is the story of how it is unlocked and returned, and Figure 7 draws its steps.

The first step is fixation, turning nitrogen gas into ammonium or nitrate that plants can take up. Certain bacteria do this, some living freely in soil and water and some, such as Rhizobium, living in the root nodules of legumes like peas, beans, gram and lentils.¹³ Farmers in Pakistan who rotate wheat with gram or berseem are using these bacteria to put nitrogen back into the soil. Lightning fixes a little more, and since the early twentieth century fertiliser factories have fixed vast amounts by the Haber-Bosch process, which makes ammonia from the nitrogen of the air.

Next, in nitrification, soil bacteria turn ammonium into nitrite and then nitrate, the form most plants take up best. Plants build the nitrogen into proteins, and animals get it by eating plants. When plants and animals die or excrete waste, ammonification by bacteria and fungi turns the organic nitrogen back into ammonium. Finally, in denitrification, other bacteria, mostly in waterlogged soil and mud, turn nitrate back into nitrogen gas, which returns to the air.¹³

Fertiliser factories add large amounts of fixed nitrogen to farmland every year, and much of it is not taken up by crops. It washes into canals, rivers and lakes, where it feeds algae, and the chapter on eutrophication follows what happens next. Some escapes as nitrous oxide, a powerful greenhouse gas. The planetary boundaries, in the section on where the environment stands now, count this overload of nitrogen and phosphorus as one of the limits the world has already crossed.

11. The water cycle

The water cycle is the one every student has drawn, and it is the one Pakistan depends on most directly. Water evaporates from the sea and from land, is carried by winds, condenses into clouds, and falls as rain or snow. It then runs off into rivers, soaks into the ground to become groundwater, or is taken up by plants and breathed out through their leaves, which is called transpiration. Figure 8 draws the cycle on Pakistan’s own ground, from the Arabian Sea to the Karakoram and back down the Indus.

The cycle is driven by the sun’s heat, which lifts water as vapour, and by gravity, which brings it down again. It also moves heat. When water evaporates it takes heat with it, and when it condenses in a cloud it releases that heat higher in the air, which is part of what powers storms and the monsoon. In Pakistan two parts of the cycle carry the country’s water. The summer monsoon brings rain from the south-west, and the winter rains and snow from the west build the snow and ice of the northern mountains, whose melt feeds the Indus through the spring and summer. Groundwater, filled by rain and by seepage from rivers and canals, is the store that farmers and cities fall back on when the rivers run low.

Water also stays in each store for very different lengths of time. A molecule of water stays only days in the air and weeks in a river, but it can stay for centuries or longer in deep groundwater or a glacier. This is why groundwater pumped out faster than it is refilled is effectively lost to the present generation, and why melting glaciers give a river more water for a few decades and then less. The chapters on water resources and on waterlogging and salinity build on this.

12. The phosphorus cycle

Phosphorus is the fourth cycle, and it differs from the other three in one way that matters. It has no gas. Carbon, nitrogen and water all spend part of their cycle in the air, so they can travel round the world quickly. Phosphorus moves only through rock, soil, water and living things, and slowly.

The cycle begins with rock. As phosphate rock weathers, phosphate dissolves into soil water, and plants take it up to build their DNA, their cell walls and the molecules that carry energy in their cells. Animals get it by eating plants, and it returns to the soil in their dung and their remains, where decomposers release it again. Some phosphate washes into rivers and the sea and settles as sediment, which over millions of years becomes rock again and may be lifted back onto land.¹³

Because phosphorus moves so slowly, it is often the nutrient in shortest supply in lakes and rivers, which is why ecologists call it the limiting nutrient in fresh water.¹³ People now mine phosphate rock to make fertiliser and add phosphate to detergents. When fertiliser and sewage carry the extra phosphate into a lake, the shortage that had kept the algae in check is lifted, and the algae multiply. This is the starting point of eutrophication, asked in five of the eleven papers, and its own chapter in head IV.¹

13. Pakistan’s ecosystems, from the Karakoram to the delta

Pakistan is one of the few countries that holds, within one border, ecosystems running from permanent ice to tropical mangrove. Height and rain decide what lives where, and the change can be seen in a single journey down the Indus. Figure 9 draws the profile.

The north is the land of the high mountains. In the Karakoram, the Himalaya and the Hindu Kush, the peaks carry permanent snow and great glaciers. K2, at 8,611 metres, is the highest point in Pakistan and the second highest in the world.¹⁵ The Pakistan Economic Survey 2025-26 notes that these ranges “contain the largest non-polar glacial reserves”.¹⁶ Below the ice lie alpine meadows, grazed in summer by the yaks and goats of herders and by wild animals such as the ibex, the markhor and the snow leopard that hunts them. Here the producers are short grasses and herbs with a brief growing season, and the food webs are simple and fragile.

Lower down, on the wetter southern slopes of the Himalaya and the hills of Khyber Pakhtunkhwa and Azad Jammu and Kashmir, grow conifer forests of deodar, blue pine and fir. These forests hold the soil on steep slopes and slow the run-off of rain, so their loss is felt downstream as floods and as silt in the reservoirs of Tarbela and Mangla. Further south, on the drier foothills and the Potohar plateau, the forest thins to scrub of olive, acacia and phulai.

Out of the hills, the Indus and its tributaries spread across the plains of the Punjab and Sindh. This was once a land of grassland, scrub and riverine forest, which the chapter on the agricultural revolutions follows as it was turned into farmland. Today it is one of the most intensely farmed and irrigated regions on Earth. Its ecosystem is largely made by people, wheat, cotton, rice and sugarcane fed by canals and tubewells, with strips of riverine forest along the river and wetlands such as the lakes near Haleji and Keenjhar.

To the east and west lie the deserts. Thar and Cholistan are hot sandy deserts where plants survive on deep roots and short rains, and animals such as the chinkara gazelle are adapted to go long periods with little water. The Balochistan plateau to the west is a cold, dry upland of juniper forest near Ziarat and wide rangelands grazed by flocks. At the river’s mouth, finally, the Indus delta carries mangrove forests, the nursery of the coastal fisheries described in the section on food webs.

This variety is a resource and a responsibility. It means Pakistan faces nearly every kind of environmental problem this subject teaches, from melting glaciers to desertification to the loss of mangroves, often in the same year. It also means that a Pakistani example exists for almost every idea in this chapter. The examiner rewards a candidate who uses them.

14. The critics of the ecosystem idea

The ecosystem is the central idea of this chapter, and it has had serious critics. A candidate who knows them can write the critical paragraph an examiner hopes to find.

The first criticism is about boundaries. An ecosystem is a system chosen by the person studying it, and nature draws no line round a pond or a forest. Birds fly in and out, water flows through, and seeds blow across. Critics say the idea makes nature look neater and more separate than it is. The defence is that the boundary is a tool chosen for a question, and that every science draws such lines. A geographer studying the Indus basin draws a different line from one studying a single lake, and both are useful.

The second criticism goes deeper and is about balance. For much of the twentieth century, ecologists and the public pictured nature as tending towards a stable balance, which people disturb. The ecologist Daniel Botkin, one of the authors on FPSC’s own reading list, attacked this picture in Discordant Harmonies in 1990. He argued that ecosystems are always changing through fire, flood, drought and the arrival of new species, and that there is no single natural state to protect or return to.¹⁷ Today most ecologists accept that disturbance and change are normal. This matters for policy. A conservation plan that tries to freeze a forest as it was in one year may fail, while one that keeps the forest able to recover from disturbance has a better chance.

A third idea, often confused with ecology, is the Gaia hypothesis of the British scientist James Lovelock, set out in Gaia: A New Look at Life on Earth in 1979.¹⁸ Lovelock argued that life, air, water and rock behave together like a single self-regulating system that keeps the planet fit for life. Many biologists objected that natural selection works on individuals, not on a planet, and so could not produce such regulation, and that the idea came close to treating the Earth as having a purpose. The scientific part of Lovelock’s idea, that life and the physical Earth regulate each other through feedbacks, has since become part of Earth system science, while the stronger claims remain disputed. In an answer, Gaia is best used as an example of thinking about the whole Earth as one system, with its critics named.

None of these criticisms removes the ecosystem from the subject. They refine it. The working view today is that ecosystems are open, changing systems whose parts are tied together by flows of energy and matter. That is the view this chapter has taught.

15. Where the environment stands now

Scientists now try to measure how far human activity has pushed the whole Earth system, and the most widely used measure is the idea of planetary boundaries. First proposed in 2009 by a group led by the Swedish scientist Johan Rockström,¹⁹ it names nine processes that keep the planet stable, such as the climate, the integrity of living systems and the flows of nitrogen and phosphorus, and sets a safe limit for each. Figure 10 shows their state in the latest assessment.

The Planetary Health Check of 2025, led by the Potsdam Institute for Climate Impact Research, found that seven of the nine boundaries have now been crossed: climate change, biosphere integrity, land system change, freshwater use, biogeochemical flows, novel entities (such as plastics and synthetic chemicals) and, for the first time, ocean acidification. Only ozone depletion and aerosol loading remain in the safe zone. The report found that the ocean’s surface pH has fallen by about 0.1 units since the start of the industrial era, “a 30-40% increase in acidity”.²⁰ It also offered a hopeful note. The ozone layer is healing and aerosol pollution has fallen, showing that the direction can be turned when the world acts, a story the chapter on the Montreal Protocol tells.

Some scientists argued that human change has gone so far that the Earth has entered a new geological epoch, the Anthropocene, the age of humans. In March 2024 the International Union of Geological Sciences approved a vote of its Subcommission on Quaternary Stratigraphy to reject the proposal for an Anthropocene epoch as a formal unit of the geological time scale.²¹ The word remains in wide use outside formal geology to mean the period in which human activity has become a main force shaping the planet.

Pakistan’s own record shows the same pressures. The Pakistan Economic Survey 2025-26 records that the annual mean temperature has risen by about 0.5 °C over the last fifty years and that 2025 was the second-warmest year in sixty-five, at 23.9 °C.¹⁶ Its forests are thin, its rivers carry less fresh water to the delta than they once did, and its cities have some of the dirtiest air in the world. The chapters that follow take each pressure in turn, and the ones on governance and on assessment and management show what Pakistan has done about them and what more could be done.

16. What students confuse about the environment

Environment, ecology and ecosystem. The environment is everything around a living thing. Ecology is the science that studies how living things relate to each other and their surroundings. An ecosystem is one working unit of living things and their surroundings. Writing “ecology” when you mean the environment, as in “we must protect the ecology”, is a common slip that examiners notice.

Food chain and food web. A food chain is one line of feeding, grass to goat to wolf. A food web is all the feeding links together. Real ecosystems are webs, and a question that asks about the effect of removing a species expects you to reason through the web.

Energy cycles and matter cycles. Energy does not cycle. It flows in as sunlight and out as heat, and is lost at every step. Matter cycles. Carbon, nitrogen, water and phosphorus are used again and again. Writing “the energy cycle” in an ecology answer is a mistake.

Biosphere and biome. The biosphere is all life on Earth and the places it lives. A biome is one large region with a similar climate and life, such as the tundra. There is one biosphere and many biomes.

The pyramid that can be inverted. Pyramids of numbers and of biomass can be inverted. The pyramid of energy never can.

Oxygen and carbon dioxide in the air. Oxygen is about 21 per cent (20.95) and carbon dioxide about 0.04 per cent (427 parts per million in 2025). A Part-I question in CE-2024 offered 0.004 and 0.40 per cent as wrong answers beside 0.04.¹

17. What the examiners have asked

This chapter’s ground is tested most often in Part-I. CE-2024 asked what a wide variety of living organisms is called, what percentage of oxygen and carbon dioxide exists, what “the set of ecosystems” is called, which component is not biotic, and what a relationship benefiting two organisms is called. CE-2025 asked which term means the variety of life, which biome has permafrost and no trees, and what biomagnification is.²

In Part-II, the ideas taught here are the tools for other questions. CE-2022 asked what happens to landscape ecosystems when top carnivores decline, which needs food webs and energy flow. CE-2017 asked how forest burning and land clearing affect the global climate, which needs the carbon cycle. CE-2019 asked about the reasons for algal blooms, which begin in the nitrogen and phosphorus cycles.¹ No paper has yet asked “What is an ecosystem?” on its own, but a twenty-mark question on the structure and functioning of an ecosystem, or on the biogeochemical cycles, is entirely within the syllabus line “Environment and Sustainable Development”.

18. Facts to take into the examination hall

• Definition in law. Pakistan Environmental Protection Act 1997, section 2: environment includes air, water and land, the atmosphere, living things, “the ecosystem and ecological relationships”, buildings and works, social and economic conditions, and “the inter-relationships” between them.³
• The air. Nitrogen 78.08 per cent, oxygen 20.95 per cent, argon about 0.93 per cent (9,340 parts per million), by volume of dry air.⁶ Carbon dioxide 427.35 parts per million at Mauna Loa in 2025, against 315.98 in 1959.⁷
• The water. Oceans 96.54 per cent of all water; about 2.5 per cent fresh; of fresh water, 68.7 per cent ice and glaciers and 30.1 per cent groundwater.⁸
• The ecosystem. Word given by Arthur Tansley in 1935.⁹ Energy flow first measured as a whole by Raymond Lindeman, 1942.¹⁰
• Silver Springs. Energy taken in by each level: producers 20,819, primary consumers 3,368, secondary 383, tertiary 21 kcal a square metre a year; 16.2, 11.4 and 5.5 per cent passed on.¹¹
• Pyramids. Energy always upright; numbers and biomass can be inverted (English Channel, 4 against 21 grams a square metre).¹²
• Nitrogen cycle. Fixation, nitrification, assimilation, ammonification, denitrification; Rhizobium in legume roots.¹³
• Phosphorus. No gas phase; the limiting nutrient in most fresh water.¹³
• Pakistan. K2 8,611 metres.¹⁵ “The largest non-polar glacial reserves”; mean temperature up about 0.5 °C in fifty years; 2025 at 23.9 °C, the second-warmest in sixty-five years.¹⁶ Grey mangrove (Avicennia marina) the most abundant in the Indus delta.¹⁴
• Planetary boundaries. Proposed 2009; seven of nine crossed in 2025, ocean acidification the newest; ozone and aerosols within the safe zone.²⁰
• Anthropocene. Rejected as a formal epoch, March 2024.²¹

19. What loses marks on this topic

• Calling energy a cycle. Energy flows; matter cycles. One wrong word tells the examiner you have not understood the central idea.
• A food chain where the question asks about a web. The effect of removing a species travels through all its links.
• Drawing the carbon cycle with no human arrow. The examiner wants burning and land clearing shown as the flows that have upset the balance.
• “The ten per cent rule” as an exact law. It is an average. Give a real range, such as five to sixteen per cent at Silver Springs.
• Saying every pyramid is upright. Only the pyramid of energy must be.
• A general answer with no Pakistani ecosystem. Every idea in this chapter has a Pakistani case: the monsoon, the delta’s mangroves, the Karakoram’s glaciers, the Potohar’s legumes.
• Treating “balance of nature” as settled science. Name Botkin’s critique, and say that ecosystems change naturally.

20. A worked answer plan

The question

How do forest burning and land clearing affect the global climate? (CE-2017, Q.6(B), 8 marks)¹

The tasks

It is one task worth eight marks, part of a three-part question, so about fifteen minutes. The examiner wants a chain from cutting or burning forests to a warmer climate, which runs through the carbon cycle, plus the other effects of clearing on climate.

The headings

1. Forests in the carbon cycle (a store and a sink).
2. What burning and clearing release (the carbon in wood and soil goes into the air).
3. What is lost (the forest no longer takes carbon in).
4. Other effects on climate (less transpiration, darker or lighter land, local rainfall).
5. Pakistan and the way forward.

A sketch

A small carbon cycle with two arrows marked heavier than the rest, “burning” from forest to air, and the “photosynthesis” arrow from air to forest crossed out.

A model introduction

Forests are among the largest stores of carbon on land and, while they grow, a sink that draws carbon dioxide out of the atmosphere through photosynthesis. Burning and clearing them reverses both roles at once: the carbon held in wood, leaves and soil is returned to the air as carbon dioxide, and the land loses its capacity to absorb more. Deforestation therefore adds to the enhanced greenhouse effect and warms the climate, while also altering rainfall and surface temperature in the region cleared.

The support, under each heading

Under the first heading, explain that photosynthesis pulls carbon from the air and respiration and decay return it, roughly in balance in a mature forest. Under the second, say that burning releases the carbon in minutes and that ploughing exposes soil carbon to decay. Under the third, add that the cleared land no longer absorbs carbon each year. Under the fourth, explain that forests return water to the air by transpiration, so clearing can reduce local rainfall and raise temperatures, and that the colour of the land changes how much sunlight it reflects. Under the fifth, bring it home: Pakistan’s forests are few and under pressure from fuelwood, timber and grazing, and planting and protecting forests is part of its climate policy, as the chapters on deforestation and on REDD+ show.

A model conclusion

Forest burning and land clearing warm the climate twice over, by releasing stored carbon and by removing a living sink, and they disturb regional rainfall as well. Protecting existing forests is therefore a cheaper and surer form of mitigation than planting new ones, and for Pakistan, whose forest cover is thin, it is also a form of adaptation.

What to remember from this chapter

• The environment is everything around a living thing that affects it; Pakistan’s 1997 Act defines it widely, including “the inter-relationships” between its parts.
• The four spheres, air, water, rock and life, are always trading matter and energy; the monsoon is the clearest Pakistani example.
• An ecosystem is a community of living things with its non-living surroundings, working as one system (Tansley, 1935).
• Producers make food from sunlight; consumers eat; decomposers return nutrients to the soil.
• Energy flows one way and is lost at each step, roughly ten per cent passing on (5.5 to 16.2 per cent at Silver Springs); food chains are therefore short.
• Only the pyramid of energy is always upright.
• Matter cycles: carbon (photosynthesis, respiration, burning), nitrogen (fixation, nitrification, ammonification, denitrification), water and phosphorus (no gas phase).
• Burning fossil fuels has raised carbon dioxide from about 316 to 427 parts per million at Mauna Loa between 1959 and 2025.
• Pakistan holds ecosystems from glaciers to mangroves; the grey mangrove dominates the Indus delta.
• Seven of the nine planetary boundaries were crossed in 2025.

Check yourself

Try each question from memory, in your own words, before you open the answer.

1. What are the four spheres of the Earth, and how does the monsoon link them? (The sections on the four spheres and how they work on each other)

2. Why do food chains rarely run beyond four to six transfers of energy? (The section on how energy flows)

3. Can a pyramid of biomass be inverted? Give an example and explain it. (The section on ecological pyramids)

4. Describe the steps of the nitrogen cycle, and say how fertiliser changes it. (The section on the nitrogen cycle)

5. Why does phosphorus often limit the growth of algae in lakes, and why does that matter? (The section on the phosphorus cycle)

6. Give one criticism of the ecosystem idea and one of the “balance of nature”. (The section on the critics)

7. From the chapter on how to sit the paper: what are the five parts of a twenty-mark answer, and which one sets this paper apart from most others?