How Nature Works in Harmony

Stand quietly beside a village pond for two minutes and just look. You will see water, mud at the bottom, sunlight lying flat on the surface and air moving above it. You will also see living things — a green film of algae floating on top, water plants rooted in the mud, insects skating on the surface, small fish turning below, a frog on the wet bank, a bird waiting patiently for that frog. It feels like two separate lists: things that are alive, and things that are not. The whole point of this chapter is that they are not separate lists at all. They are one working system, and every part of it leans on the others.

Scientists call such a working system an ecosystem. An ecosystem is all the living things of an area, together with the non-living surroundings they live in, and all the give-and-take that goes on between them. The living members are called the biotic components: plants, animals, fungi, bacteria and every other organism, from the largest tree down to the microbes in the mud. The non-living members are called the abiotic components: sunlight, air, water, soil, temperature, humidity and the dissolved minerals that plants pull up through their roots.

Do not think of the abiotic part as background scenery. It decides almost everything about who can live there. Sunlight sets how much food green plants can make. Temperature decides which seeds will sprout and which animals can survive the season. The amount of water decides whether you get a thick forest, a grassland or a desert. In the pond, one abiotic factor matters more than students usually expect: the oxygen dissolved in the water. Fish do not use the oxygen locked inside water molecules; their gills take in the separate oxygen gas that is dissolved in the water. If that dissolved oxygen falls, the fish die even though the pond is still full of water.

It helps to build the idea in steps. A single Indian rohu fish lives in the pond — that pond is its habitat, the natural home that supplies its food, water, shelter and space. All the rohu fish in that pond together form a population: one kind of organism, one place. All the populations living there — rohu, frogs, algae, snails, water beetles, bacteria — together form a community. Add the water, mud, sunlight and dissolved gases to that community, and you finally have an ecosystem.

Ecosystems come in every size. A rotting log with fungi and beetles inside it is a tiny ecosystem. A pond is a small one. A forest, a grassland, a desert, a river and the ocean are large ones. All the ecosystems of the planet added together make the biosphere, the thin skin of land, water and air where life exists. Ecosystems are also of two broad kinds. Natural ecosystems such as forests, grasslands, deserts, ponds, rivers and oceans run on their own. Artificial ecosystems such as a crop field, a garden, a park or an aquarium are made and maintained by people, and they usually collapse without us — stop weeding and watering a garden, or stop feeding and cleaning an aquarium, and each one changes completely within weeks.

So where does the word harmony in the chapter title come from? Nobody is in charge of a pond. No animal has been given a duty list. Yet the amount of algae, the number of fish and the number of birds stay roughly steady over long periods. That steadiness is not planned; it comes out of the countless links between the members. When algae grow fast, the animals that eat algae get more food and increase in number, and they graze the algae back down. When the fish become too many, food runs short and their numbers fall again. Each link quietly pulls the system back towards balance. This self-correcting behaviour is what we mean by balance in nature, and understanding it is the reason the rest of this chapter exists.

Every member of an ecosystem has to eat. How an organism gets its food is the single most useful way to sort living things, because food is really a parcel of energy and building material being passed from one body to another. On that basis, all organisms fall into three working groups: producers, consumers and decomposers.

Producers make their own food. Green plants take in carbon dioxide from the air and water from the soil, while algae and some bacteria take both from the water around them, and all of them use the energy of sunlight, trapped by the green pigment chlorophyll, to build sugar. This process is photosynthesis, and it releases oxygen as a by-product. Because producers build food from simple non-living substances, they are also called autotrophs, which means self-feeders. Here is the point worth pausing on: producers are the only doorway through which fresh energy enters an ecosystem. Sunlight falls on a tiger too, but a tiger cannot do anything with it. Nearly every mouthful eaten by any animal on Earth was, a few steps earlier, sunlight captured by a producer. Remove the producers and the whole system has no income at all.

Consumers cannot make their own food, so they eat other organisms. They are called heterotrophs, meaning other-feeders. Consumers are sorted by what they eat:

• Herbivores eat only plants — cow, goat, deer, rabbit, grasshopper, elephant. Because they eat producers directly, they are called primary consumers.
• Carnivores eat other animals — frog, snake, lizard, tiger, hawk. A carnivore that eats a herbivore is a secondary consumer; one that eats that carnivore is a tertiary consumer.
• Omnivores eat both plants and animals — crow, bear, most human beings.
• Scavengers such as vultures, kites, crows and hyenas feed on animals that are already dead. They clear away carcasses quickly, which helps stop disease from spreading.

Decomposers are the group people forget, and they may be the most important of all. Bacteria and fungi settle on dead leaves, dead animals, fallen fruit and animal waste, release chemicals that break the complex substances into simple ones, and absorb what they need. What they do not need — nitrogen compounds, phosphates, potassium and other minerals — is released back into the soil and water, where plant roots can take it up again. Earthworms, termites and many small soil animals help by tearing dead matter into smaller pieces, giving the bacteria and fungi far more surface to work on.

Students often confuse scavengers with decomposers, so hold the difference firmly. A vulture is a consumer that happens to eat dead flesh; it swallows large pieces and leaves bones behind. A decomposer works chemically, breaks matter right down to simple mineral substances, and hands those minerals back to the soil. Only the decomposer completes the circle.

Imagine decomposers stopping work for one year. Every leaf that fell would still be lying where it fell. Every dead animal, every bit of dung, every fallen fruit would pile up. Worse than the mess, the minerals inside all that dead material would stay locked up. Soil would slowly run out of the nutrients plants need, plants would weaken, and the animals depending on them would follow. Decomposers are the reason the same atoms of nitrogen and carbon can be used again and again for millions of years instead of being used once and lost.

This is why the three groups are said to depend on one another. Producers feed consumers. Consumers, when they die, feed decomposers. Decomposers return raw material to the soil so producers can build food again. Break any one of the three and the other two are in trouble within a season.

Once you know who eats whom, you can join the members of an ecosystem into a line. A food chain is a sequence of organisms in which each one is eaten by the next. In a grassland it may look like this:

grass → grasshopper → frog → snake → hawk

In a pond it may look like this:

algae → water flea → small fish → big fish → kingfisher

The arrow is not decoration and it is not pointing at dinner. The arrow always points from the organism that is eaten towards the organism that eats it, because it is showing the direction in which food and energy travel. Reading the first chain correctly, grass gives its energy to the grasshopper, the grasshopper passes what is left to the frog, and so on. Turning an arrow around changes the whole meaning, so get into the habit of saying the words is eaten by out loud as you write each arrow.

Each step in a food chain is called a trophic level, from a word meaning nourishment. The producer is always the first trophic level. The herbivore that eats it is the second. The carnivore that eats the herbivore is the third, and so on. So in the grassland chain, grass sits at the first trophic level, the grasshopper at the second, the frog at the third, the snake at the fourth and the hawk at the fifth. Notice that a trophic level is a position, not a species. A crow eating grain is at the second level that morning; the same crow eating a caterpillar in the afternoon is at the third.

Now the problem with food chains: they are far too tidy to be true. A frog does not eat only grasshoppers. It also eats beetles, moths and small worms. A grasshopper does not feed on only one kind of grass. A snake will take a rat or a bird just as happily as a frog. In real life the chains cross, join and branch, and the resulting network is called a food web. A food web is simply all the food chains of an ecosystem drawn together, showing every feeding link at once.

Why does this matter so much? Because a food web is what gives an ecosystem its strength. Picture an animal that eats only one kind of food. If a bad season wipes that food out, the animal has nowhere to go. Now picture an animal in a web, with four or five possible foods. When one of them becomes scarce, it shifts to another, and the system absorbs the shock instead of collapsing. The more links a food web has, the more stable the ecosystem is. A field growing a single crop across hundreds of hectares has very few links in it, and that is one reason a single pest can spread through such a field with alarming speed.

A food web also shows something a straight line hides: the same organism can sit at different trophic levels at the same time. A kite that eats a grasshopper is feeding at the third trophic level, while the same kite eating a snake that had eaten a frog is feeding at the fifth. Human beings are the clearest case of all. Eat rice and you are a primary consumer at the second level; eat a big fish that had eaten a smaller fish, which had itself eaten algae, and you are at the fourth level of the same web.

When you build a food chain yourself, follow three simple checks. Start with a producer, never with an animal. Point every arrow from the eaten to the eater. Keep the organisms in an order that could really happen in one place — a shark cannot follow a grasshopper, because they never meet.

Something quiet and very important happens each time food passes along a chain: most of the energy never arrives. This one fact explains the shape of nearly every ecosystem on Earth.

Follow the energy from the beginning. The Sun pours energy onto a field. Green leaves capture only a small share of it — a great deal is reflected, misses the leaves altogether or is of a kind the leaf cannot use. What the leaf does capture is stored as food. Then a grasshopper eats some of that grass. Does all of the stored energy become grasshopper? Not at all. Part of the grass is never eaten. Part of what is eaten passes out undigested. Most of what is digested is burnt in respiration to power hopping, breathing, chewing and staying alive, and that energy leaves the body as heat. Only the small remainder is built into the body of the grasshopper, and only that remainder is available to the frog that eats it.

As a rough working rule, only about one-tenth — roughly ten per cent — of the energy at one trophic level is passed on to the next. Treat this as an average, not an exact law; the real figure differs from one ecosystem and one pair of organisms to another. Even so, the rule shows the scale of the loss clearly. If grass in a field holds 10,000 units of energy, grasshoppers may end up with about 1,000, frogs with about 100, snakes with about 10 and the hawk at the top with roughly 1.

Two big conclusions follow, and they are worth more than the numbers themselves.

First, food chains are short. Most have only four or five links, and chains longer than about five are rare. Look again at those figures: after four transfers there is almost nothing left to support another level. A predator that fed only on hawks would starve, because the energy needed to keep it alive simply is not there. The length of a food chain is set by physics, not by choice.

Second, energy flows one way and is gone. This is the sharpest difference between energy and matter in nature. The atoms of carbon and nitrogen in a leaf are used again and again for millions of years, cycling endlessly. Energy does not cycle. It enters as sunlight, moves up the levels, escapes at every step as heat, and is never reused by the ecosystem. That is exactly why the Sun must keep shining. Cut the sunlight off and no amount of recycling can keep the system alive.

The same loss explains the ecological pyramid. Draw a bar for the amount of energy at each trophic level, put the producers at the bottom and stack the levels above, and you always get a broad base narrowing to a small tip. A grassland shows the same shape in ordinary numbers: countless grass plants, fewer grasshoppers, still fewer frogs, a handful of snakes and perhaps one hawk patrolling the whole field. A pyramid drawn by counting individuals is not always neat — one large tree can feed a great many caterpillars, so the producer step ends up narrower than the step above it — but a pyramid drawn using energy is always widest at the producer level.

There is a practical lesson in all of this. Since roughly nine-tenths of the energy is lost at each step, a given piece of land feeds far more people when it grows grain that people eat directly than when it grows fodder for animals that people then eat. The energy pyramid is not only a diagram in a textbook; it quietly shapes how a country feeds itself.

Energy behaves in one particular way in nature: it arrives from the Sun, moves up the trophic levels and leaves as heat. Matter behaves in the exact opposite way. The atoms in your body are not new. They have been part of rocks, oceans, air, plants and other animals before this, and they will move on again. Nature runs on a fixed stock of material, so that material has to be used over and over. The paths along which it travels are called biogeochemical cycles — a long word that simply means life, earth and chemistry all taking part. Two of them, water and carbon, are described here.

The water cycle is the one you can watch from a rooftop. Heat from the Sun turns liquid water into water vapour and lifts it into the air; this is evaporation, and it happens from oceans, rivers, lakes, ponds and wet soil. Plants add a large amount too. Roots draw water from the soil, it travels up the stem, and it escapes as vapour through tiny pores in the leaves — a process called transpiration. That is why the air under a big tree feels cooler and moister than the air over an open road. High above, the air is colder, so the vapour cools and turns back into tiny droplets around specks of dust. This is condensation, and it is what a cloud actually is. When droplets join until they are heavy enough, they fall as rain, hail or snow, which we call precipitation. On the ground the water splits three ways: some runs over the surface into streams and rivers and eventually to the sea, some soaks down through the soil to become groundwater that we later draw from wells and hand pumps, and some is taken up by roots. Then the Sun starts the whole thing again. The monsoon that reaches the Indian coast is this cycle at a continental scale, driven by ocean water evaporating and winds carrying that vapour inland.

Notice what the cycle does not do. It does not make water. The quantity of water on Earth stays about the same; the cycle only moves it and changes its form. So when we say water is being wasted, we really mean it is being made dirty, or being moved to where it cannot easily be used, or being pumped out of the ground faster than rain can refill it. Cutting forests weakens the cycle from another direction: with fewer leaves transpiring, less vapour rises locally, and with no roots holding the soil, rainwater rushes away over the bare surface instead of sinking in to refill the groundwater.

The carbon cycle follows the element that every food, fuel and body part is built around. Carbon dioxide makes up only a very small part of air — roughly four hundredths of one per cent — yet all life on land depends on that trace. Green plants pull carbon dioxide out of the air during photosynthesis and lock the carbon into sugar, starch, cellulose and every other food substance. Animals eat plants and take that carbon into their own bodies. Carbon returns to the air by several routes. Every living thing, plants included, releases carbon dioxide during respiration. Decomposers release it as they break down dead bodies and waste. Fire and the burning of wood, coal, petrol, diesel and cooking gas release it fastest of all. Oceans absorb carbon dioxide and give it back, and some carbon is stored for very long periods in limestone rock and in the shells of sea animals.

Balance is the whole story here. For a long time the carbon leaving the air roughly matched the carbon returning to it. Coal and petroleum are the buried remains of organisms from millions of years ago, holding carbon that was taken out of circulation long before humans existed. Burning them puts that ancient carbon back into the air of today over a few hundred years instead of a few million. Photosynthesis and the oceans cannot absorb it that quickly, so the carbon dioxide content of the air keeps rising, and that rise traps more heat near the surface. Cutting forests pushes the same balance the same way, because fewer trees are left to take carbon dioxide out.

The nitrogen cycle deserves a section of its own, because it contains the strangest puzzle in this chapter. Air is about seventy-eight per cent nitrogen. Nitrogen is the commonest gas above your head by a wide margin, and yet plants growing in that air can starve for want of it. Farmers spend large amounts of money adding nitrogen to fields that are already bathed in it.

The reason is the shape of the gas. Nitrogen in the air travels as pairs of nitrogen atoms held together by an unusually strong bond. That bond is so hard to break that plants cannot pull the atoms apart, and neither can animals. Living things need nitrogen badly — it is a necessary part of every protein, of the DNA that carries inherited information, and of chlorophyll itself — but they can only use nitrogen that has already been broken out of the gas and joined to other elements. Getting nitrogen from the air into a usable form is called nitrogen fixation, and it happens in four main ways.

• Bacteria in root nodules. Plants of the pea family — gram, peas, beans, moong, arhar, groundnut and soybean, all grouped together as legumes — carry small swellings called root nodules. Inside those nodules live bacteria named Rhizobium. They break apart nitrogen gas and hand usable nitrogen compounds to the plant; the plant feeds and shelters them in return. Both sides gain, which makes this a fine example of two very different organisms living together for mutual benefit.
• Free-living soil bacteria and blue-green algae. Certain bacteria fix nitrogen in the soil without any partner plant, and some blue-green algae do the same in the standing water of flooded paddy fields, which is one reason rice can be grown in the same field season after season.
• Lightning. The enormous energy of a lightning flash breaks nitrogen apart in the air itself. The compounds formed dissolve in rain and reach the soil, though this route supplies only a small share of the nitrogen a crop needs.
• Factories. Nitrogen fertilisers such as urea are manufactured industrially by combining nitrogen from the air with hydrogen under high pressure and temperature.

Once nitrogen is fixed, soil bacteria change it step by step into nitrates, the form plant roots absorb most easily. The plant uses those nitrates to build proteins. Animals get their nitrogen only by eating — herbivores from plants, carnivores from other animals — and rebuild the plant proteins into animal proteins.

Then the return journey begins. Animals pass out nitrogen-rich waste such as urea. Plants and animals eventually die. Decomposer bacteria and fungi break down all of it and release simple nitrogen compounds back into the soil, where they can be taken up once more. Finally, another set of bacteria in the soil takes some nitrogen compounds and releases nitrogen gas back into the air, closing the loop and keeping the air supplied. The nitrogen atoms in the dal you eat tonight may have been inside soil bacteria last year and inside the air the year before.

Farmers have used this cycle for a very long time without needing the chemistry. Growing a legume such as gram or moong in a field, or planting it between rows of another crop, leaves the soil richer in nitrogen for the crop that follows, because of the Rhizobium bacteria working in the nodules. This is why crop rotation with pulses is such an old practice across India. Modern nitrogen fertiliser does the same job faster, but it brings its own trouble. Spread more of it than the crop can absorb and the rest washes into ponds, lakes and rivers with the next rain. There it acts as fertiliser for algae, which multiply until they blanket the surface. When that mass of algae dies, decomposers multiply too and use up the oxygen dissolved in the water. The fish, which need that dissolved oxygen, suffocate. It is a sharp reminder that nutrients are helpful in a cycle and harmful in a flood.

Everything so far describes a system in balance. Now for the part that makes this chapter urgent: what happens when a link is pulled out. An ecosystem is not a chain of separate parts but a net, and cutting one thread loosens the whole net in ways that are hard to predict and slow to repair.

Take the simplest case. Frogs are caught in large numbers in some places. Frogs eat insects, so with the frogs gone, insect numbers rise. Many of those insects feed on crops, so harvests fall and farmers spray more pesticide. The pesticide kills more than the pest — it also kills the remaining frogs, the harmless insects and the bees that pollinate the crop. Snakes and birds that fed on frogs go hungry and move away or die. One removal, several failures. The ecosystem did not simply lose frogs; it lost the work frogs were doing.

The same story repeats in bigger forms:

• Deforestation. Cutting a forest removes producers, homes and shade at once. Animals lose habitat, less water vapour rises from the leaves, less rainwater soaks into the ground, bare soil is carried off by rain, and less carbon dioxide is taken out of the air.
• Overfishing and overhunting. Taking fish faster than they can breed empties a level of the water food web. Species that fed on those fish decline; species the fish used to eat suddenly increase.
• Pollution. Sewage and factory waste poured into a river kill the small organisms at the bottom of the web first, and everything above them thins out afterwards. Excess fertiliser produces the algae-and-oxygen collapse described earlier.
• Introduced species. A plant or animal brought from another region can spread unchecked in its new home because the predators and diseases that controlled it are not there. Water hyacinth, a plant not native to India, spread across many ponds and lakes, covered the surface, blocked sunlight from reaching the plants below and choked the water body.

Food chains carry one more hidden danger. Some pesticides and industrial chemicals do not break down easily and are not passed out of the body. Producers take in a little. A herbivore eats a large quantity of producers and stores what all of them contained. A carnivore eats many herbivores and stores what all of them contained. So the concentration of the chemical climbs at each trophic level, and the animals at the top — big fish, birds of prey and human beings — carry the highest load of all, even though none of them ever swallowed the chemical directly. Energy shrinks as you climb a food chain, but these chemicals grow.

Some damage can be repaired. A cleaned river refills with life; a protected forest patch grows back. But when the last member of a species dies, that species is extinct, and no effort can bring it back. Every extinction cuts permanent threads out of the net.

The useful response is not despair but ordinary, repeatable care. Protect habitats, which is the reason for national parks, wildlife sanctuaries and protected wetlands. Plant and look after trees rather than only counting saplings. Use water and electricity carefully, since both are drawn from the same shared systems. Compost kitchen and garden waste and let decomposers do the work they are built for. Keep plastic and untreated waste out of drains, ponds and rivers. Use pesticides and fertilisers sparingly and only where needed. Support local pulses and mixed cropping, which keep soil and food webs varied.

The thread running through this entire chapter is a single sentence worth carrying with you: in nature nothing stands alone. Sunlight becomes grass, grass becomes a grasshopper, the grasshopper becomes a frog, the frog becomes soil, and soil becomes grass again. Human beings are not standing outside that circle, watching it turn. We are inside it, and every choice we make is a tug on one of its threads.