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No living thing in the world lives entirely on its own. Every living organism depends on its surroundings and on the other plants and animals around it. A community of living things in a particular area, along with the soil, water, and other non-living material, forms what scientists call an ecosystem. An ecosystem can be as small as a water-filled hollow in a tree, or as large as a forest. In a single ecosystem, there may be any number of species from just a few to hundreds or even thousands, in the case of an ocean or a tropical rainforest.



An ecosystem is powered by the Sun. This provides the light energy that allows green plants to make food by the process called photosynthesis. Apart from that, almost all the other food and energy supplies come from within the ecosystem. As well as plants, or their dead and decaying remain, all ecosystems contain animals.


In an ecosystem, all the components are balanced. The green plants provide food and oxygen needed by the animals. The waste products of the animals, and the dead and decaying remains of both plants and animals, are recycled in the soil to be used by new plants as they grow. The new plants, in turn, provide food and oxygen for more animals.


Sometimes people upset the balance of an ecosystem. In the Kaibab Plateau in Arizona in the United States, the number of deer was controlled by such wild animals as coyotes, bobcats, pumas, and wolves who hunted the deer. In 1906, hunters decided to kill all these animal predators. Without the predators, the number of deer increased from 4,000 in 1906 to between 60,000 and 100,000 by 1924. By then, there was not enough food for that many deer. They stripped all the plants, and without the plants, the soil was gradually eroded away. Eventually, most of the deer starved to death during two severe winters.


The living things in an ecosystem are linked together by their food in what scientists call food chains. Plants make their own food using sunlight, water, and carbon dioxide gas from the air. They are the only living things that can do this, so all animals depend directly or indirectly on plants for their food. For this reason, green plants are often called producers and the animals that feed on them—the herbivores (plant-eaters)—are called consumers. These consumers are in turn eaten by other animals called carnivores (meat-eaters).

All food chains begin with plants. A simple food chain in a garden might begin with a lettuce plant (a producer), which is eaten by a garden snail (a herbivore or first consumer). A song thrush (a first carnivore or second consumer) then eats the snail. Later, the song thrush is eaten by a sparrowhawk (a second carnivore or third consumer).

This chain of lettuce plant→garden snail→song thrush→sparrow hawk is an example of a simple food chain. The arrows in a food chain always point from food to feeder.


Plants and animals use food to produce energy. This helps them to grow, move, keep warm and reproduce. At each step in a food chain, some energy is passed on, but much is lost in the form of heat. This means that much more energy is available at the beginning of a food chain than at the end. That is why food chains rarely have more than four or five links and why there are fewer organisms the further you go along a food chain. It is also why no animal preys on large carnivores such as tigers, jaguars and polar bears. It is not because these animals are so big and fierce, but because there would not be enough energy available to keep such a super carnivore alive.


Another simple food chain is grass→field vole→weasel. But many other animals eat grass beside field voles. They include rabbits, grasshoppers, slugs, snails, sheep, cows, and horses. Voles may also be the prey of other animals besides weasels, including stoats, foxes, and owls. In their turn, voles, stoats, foxes, and owls may have fleas and other parasites on their bodies. Food chains are not usually simple straight lines. Instead, each plant or animal may link with and be part of many other food chains. This more complicated relationship is known as a food web. A food web summarizes the living part of an ecosystem.


Plants and animals are always increasing in number. If they lived forever, the Earth would soon be overcrowded. Instead, all living things die eventually, but the surface of the Earth is not knee-deep in the dead bodies of plants and animals. This is because a group of living organisms specializes in feeding on dead plants and animals.

Animals that feed on the dead remains of plants and animals, including beetles and flies (and their larvae or maggots), together with earthworms, millipedes, slugs, snails, and woodlice, are called detritivores. They break the waste materials down into smaller pieces that they expel in their droppings. These then rot away. Bacteria and fungi finish off the work of breaking down the plant or animal material into chemicals that plants and other living things can use again. Detritivores and decomposers are an important part of many food chains and food webs.


All living things are connected to each other in food webs and food chains. So it is not surprising that upsetting one part of an ecosystem can produce huge effects somewhere else. The way in which the numbers of animals and plants in an ecosystem depend on each other is often called the balance of nature.

Another example of how the balance of an ecosystem can be upset comes from India. In some European countries, the legs of edible frogs are eaten in restaurants. There is a shortage of European frogs because many of the ponds and ditches where they lived have been drained or polluted. Someone had the idea of catching the bullfrogs that live in the rice fields of India to sell to restaurants in Europe. But when the insect-eating frogs were removed from the rice fields, the insects were able to multiply unchecked. Before long the rice crop failed, as a result of being attacked by the swarms of insects.