Ecosystems

A community never lives in a vacuum. The pond’s creatures depend on things that are not alive: the water itself, the mud on the bottom, the sunlight reaching the surface, the temperature, the oxygen dissolved in the water. A community together with its non-living surroundings is an ecosystem. The British botanist Arthur Tansley introduced the word in 1935 to make the point that the two cannot be understood apart. An ecosystem can be as small as a rotting log or as large as a rainforest.

Within an ecosystem, each species has its own address. The place where an organism naturally lives is its habitat. For a frog it is the pond’s shallow margin; for a woodpecker, old trees with soft wood. A species can only survive where its habitat does, which is why the loss of habitat is the main reason species disappear.

Every living thing needs energy, and in almost every ecosystem it arrives the same way: as sunlight. Green plants, algae and some bacteria capture a little of that light and use it to build sugar out of carbon dioxide and water. This is photosynthesis, and an organism that makes its own food in this way is called a producer. Producers are the entrance through which energy gets into the living world.

Everything else lives on what producers have made. An organism that cannot make its own food and must get its energy by eating other living things is a consumer. A caterpillar eating a leaf is a consumer, and so is the bird that eats the caterpillar, and so are you. Trace any meal back far enough and it began as light falling on a leaf.

Consumers are sorted by what they eat. A herbivore eats plants: rabbits, cows, caterpillars and deer are herbivores. A carnivore eats other animals: lions, hawks, spiders and sharks. An animal that eats both, as bears, crows and people do, is an omnivore.

A carnivore that hunts and kills its food is a predator, and the animal it hunts is its prey. The roles depend on who is looking. A frog is a predator to a fly and prey to a heron. Follow the eating upward and you eventually reach an animal that nothing hunts, such as an eagle or a killer whale. This is a top predator. It is safe from other hunters, but it depends on every living thing beneath it, and it is often among the first to suffer when something lower down goes wrong.

The simplest way to draw who eats whom is a single line. Grass is eaten by a rabbit, and the rabbit is eaten by a fox. Such a sequence, starting with a producer and running upward through one consumer after another, is a food chain. Each arrow in it shows where the energy goes, from the eaten to the eater.

A chain is tidy, but real life is not. The fox also eats voles, beetles and fallen fruit. The rabbit is also eaten by buzzards and stoats, and it eats far more than grass. Draw every one of these connections for one habitat and the chains cross and tangle into a food web. The web is the truer picture, and it explains something a chain cannot: why a change to one species can be felt by others that seem to have nothing to do with it.

A rabbit eats grass all day, yet very little of the energy in that grass ever becomes rabbit. Some of the grass passes through undigested. Most of what is absorbed is spent on staying alive: moving, breathing, keeping warm. All of that spent energy ends up as heat, which drifts away and cannot be used again by anything.

Only what the rabbit builds into its own body is there for a fox to eat, and it is a small share. As a rough rule, about 10 per cent of the energy at one level of a food chain is passed on to the next. The real figure runs from a few per cent to about twenty, depending on the ecosystem and the animals, but it is always a small share. The fox, in turn, burns most of what it gets from its rabbits.

The ten per cent rule shapes every ecosystem. Start with 100 units of energy stored in plants. The herbivores that eat them can build only about 10 units into their own bodies, and the carnivores that eat the herbivores get about 1. Stack the levels and they form a pyramid, wide at the bottom and narrow at the top.

This is why a grassland holds a vast weight of grass, a smaller weight of antelope, and only a handful of lions. It is also why food chains are short. After four or five steps so little energy is left that nothing could live on it. Top predators are rare for the same reason. There is nothing wrong with them. They are simply at the end of a line where the energy has almost run out.

Energy and matter behave differently in an ecosystem. Energy makes a single trip. It flows one way: in as sunlight, up through the food chain, and out as heat. It has to be supplied afresh every day.

Matter is another story. The atoms in a leaf, the carbon, the nitrogen, the minerals, are not used up when the leaf dies. They are used again, and the organisms that make this possible are the decomposers: the bacteria and fungi that feed on dead plants, dead animals and droppings. As they break this material down, they release its minerals into the soil, where plant roots take them up to build new leaves. Without decomposers, dead matter would simply pile up, and the supply of minerals that life depends on would stay locked inside it.

Carbon is the element that living bodies are built on, and it travels in a loop. It starts in the air, as carbon dioxide. Plants take that gas in and, by photosynthesis, build its carbon into sugar and then into leaves, wood and roots. When an animal eats the plant, the carbon moves into the animal.

It returns to the air by respiration. Every living thing, plants included, breaks down sugar for energy and gives off carbon dioxide as it does. Decomposers do the same to the dead. For thousands of years before industry the two halves of the loop roughly matched, and the carbon dioxide in the air held nearly steady. Burning coal, oil and gas adds a third route. These fuels hold carbon taken out of the air and buried long ago, and burning them adds carbon dioxide faster than the loop can take it back.

Water is recycled on an even grander scale, and here the Sun does the lifting. Its warmth turns liquid water at the surface of seas, lakes and wet ground into invisible vapour that rises into the air. This is evaporation. Plants add to it: water drawn up by their roots escapes as vapour through their leaves.

High up, where the air is cold, the vapour turns back into tiny droplets, and these gather as clouds. When the droplets grow heavy enough they fall as rain or snow. Some of that water soaks into the ground, where roots can reach it. The rest runs into streams and rivers and makes its way back to the sea, and the journey begins again. The water you drink today has been round this loop countless times.

Eating is not the only way species affect each other. When two species need the same thing and there is not enough of it, they are in competition. Trees in a forest compete for light, and the taller ones shade out the rest. Lions and hyenas compete for the same prey. Competition is fiercest between species with similar needs, and it helps decide which species can live where.

Other species help one another. A relationship from which both partners benefit is called mutualism. A bee visiting a flower gets nectar to eat, and the flower gets its pollen carried to another flower, which it could not manage alone. Most plants have a similar pact underground: fungi living in and around their roots collect water and minerals for the plant, and are paid in sugar.

In a food web, removing one species can change everything around it. Along the Pacific coast of North America, sea otters eat sea urchins, and sea urchins eat kelp, the giant seaweed that forms underwater forests. Where otters were hunted out for their fur, the urchins multiplied and grazed the kelp forests down to bare rock, and the fish that sheltered there dwindled. A species that holds a whole ecosystem in shape like this, far beyond what its numbers suggest, is called a keystone species.

An unchecked population can also undo itself. In 1944, 29 reindeer were put on St Matthew Island in Alaska, where nothing hunted them. By 1963 there were about 6,000, and they had eaten out the slow-growing lichen they depended on in winter, destroying their own food supply. One hard winter then starved them, and by 1966 only 42 were left.

People are part of ecosystems too, and the most powerful part. The clearing of forests for timber and farmland is called deforestation, and it does double damage: it removes the habitat of everything that lived there, and it releases the carbon the trees stored. Nearly eleven million hectares of forest are still cleared every year, an area a little larger than Iceland.

The answer is to use nature at a rate it can bear. Taking fish, timber or water no faster than they are replaced is called sustainable use. Protecting habitats and species so that they survive is conservation. The idea is old. In Arabia the ḥimā, a pasture where grazing and tree-cutting are restricted, was a chieftain’s private reserve until the Prophet and the first caliphs allowed such land to be set aside only for the common good, and a few are still kept today.