SCAVENGERS AND DECOMPOSERS

APART from the ones killed on the roads, it is really quite rare in nature to find the dead bodies of animals and birds, considering how many millions of them die every year. What happens to them? A naturalist decided to find out by watching. The body of a dead fallow deer was found in Epping Forest. It had been killed by poachers who took part of it for venison and abandoned the rest. It was left on the ground for observation. Within two days badgers started to eat the softer parts of the flesh, having broken open the stomach. Carrion crows, magpies, and a fox were seen near. Flies and maggots soon appeared; insects sheltering and feeding attracted other birds, such as a spotted flycatcher and a blackbird; and within six weeks the bones had been picked clean.

Animal droppings also disappear quite quickly, some less speedily than others. Horse droppings are attacked by flies, maggots and beetles. There is even a fungus which grows only on such droppings and helps to decompose them. A cowpat disappears in a rather different way, often eaten and decomposed from below so that a dricd-up skin may be left on top. Dead fish and any other dead bodies in water are soon used as larders by a host of hungry scavengers. So are dead trees and plants. You can test this process of decomposition for yourself. If you find a dead sparrow and can leave it on grass or earth (preferably where cats can’t get at it and spoil your experiment) you can record which species can be found eating there, and in what order they come, and how long it takes for the body to disappear. If you don’t like using a dead body, try something like half an orange. Doing this several times a year you would find great differences in the rate of disappearance; warm damp weather is the most favourable – guess why! An old orchard with windfall apples lying in it is very attractive to many forms of life. Even horses have been known to get drunk on fermented apples!

Many of nature’s scavengers can easily be seen – the bluebottles, earthworms, and burying beetles, for instance. Others are very small – mites, springtails, eel-worms, among them. Others again are microscopic. In one gram of soil there are literally millions of microorganisms, such as bacteria, fungi, algae and others, their work being to feed on and break down the remains of dead animals and plants. They are sometimes called nature’s dustmen, or refuse collectors. If you think this whole subject rather disgusting, think how much more unpleasant the world would be if remains were not dealt with in this way. It has been suggested that one day the whole island of New Zealand will sink under a load of accumulated ordure, because mammals have been introduced there comparatively recently and the resulting absence of dung beetles and decomposing bacteria has led to piles of dung. And this explains, too, why some forms of litter are so much worse than others. An apple core thrown down at a picnic, or a banana skin, will fairly soon be got rid of, but nature’s dustmen cannot deal with tin cans, glass bottles, or plastic bags. They will eventually tackle paper, but not tinfoil.

THE BI0CHEHEMICAL CYCLE

The scavengers and decomposers are very useful in keeping the world tidy, but their work is really much more important than that. They form an essential link in the cycle of life, by which materials essential for life and growth are sent circulating round and round, instead of just being used and then discarded.

In the old Yorkshire song, ‘On Ilkley Moor ‘baht ‘aht’, a young man is told to wear a hat when he goes courting Mary Jane on the moor. If he doesn’t, he will catch cold, and die; then the worms will eat him up; then ducks will eat the worms; then people will eat the ducks, and then, the man is told, ‘We shall ‘ave etten thee!’ This illustrates that nothing is lost in nature, becoming instead food for some other organism. But the song is not quite accurate in describing the cycle of life. It misses out the green element.

Green is a marvellous colour. If you look out of the window on a railway journey and watch the fields and woods flashing by, you can see hundreds of different shades of green, varying from blue black in the shadows to golden in the sunlight. If you try to paint out of doors you need to mix many different tints. But in plants green is also a marvellous working colour. The green colouring matter in leaves (chlorophyll) has the wonderful ability to absorb the light energy of the sun, and use it to start the process of photosynthesis (literally ‘building up by means of light’) by which carbon dioxide and water are combined to produce basic food substances. The carbon dioxide is taken in from the air into the leaves, and water, containing such chemicals as nitrogen, sulphur, calcium, magnesium, phosphorous, and iron, is absorbed by the roots. With energy derived from respiration, and the consumption of atmospheric oxygen, the plant produces the carbohydrates, proteins, and fats needed for body tissue. So green plants produce food from non-food. No wonder they have been called food factories; they are really rather elaborate chemical works. Without the light of the sun, and without green plants, our living world could not exist. Scientists today are thinking in terms of by-passing one or other or both of these two requirements for making food, but so far the idea of feeding the world in these new ways belongs to science fiction rather than to reality.

You can test for yourself the importance of sunlight for growing plants. Any plants kept in the dark or in artificial light soon die. Bulbs such as daffodils will begin to grow, because their first sources of food are stored inside them, but as soon as they begin to shoot they have to be brought into the light. If you leave them in the dark you will eventually find a nasty twisted mass of white shoots, just as you would if you left a bag of old potatoes in the dark in spring, when they are beginning to put out shoots from their eyes. To test the ability of plants to convert chemicals into tissue, you can put a plant in a jam jar of water, so that the plant itself rests on the rim and the roots are in water, and then see if you can keep the plant alive by adding drops of a general chemical fertilizer from a gardening shop. This may or may not work; it depends on whether the fertilizer is right for the plant, and whether you get the quantities right.

After the green plants, or primary producers, have made their food, they can build up their tissues and are then available as food for other organisms. Much of the food they produce, however, cannot be eaten by any hungry creature that comes along. Man can eat lettuce, or cabbage; but he can’t digest grass. Nebuchadnezzar, in the Bible, is said to have eaten grass, but it must have made him very sick, as it would a pet dog or cat. The primary consumers, or herbivores, are the grazing animals such as cattle and sheep. In turn these primary consumers are themselves eaten by flesh-eaters, the secondary consumers, that is animals such as lions, or wolves, or birds like owls or hawks. In their turn these creatures may become food for other flesh-eating animals which would then be called tertiary consumers. If you had a plate of cold meat and salad for your lunch, you would be a primary consumer when you ate the lettuce, and a secondary consumer if you were eating cold lamb. If you were eating cold pork, from a pig which might itself have been fed on meat scraps, you would also be a tertiary consumer.

And finally in the chain we come back to our dustmen – the scavengers and decomposers in nature, including the organisms that feed on decayed plants (being thus primary consumers) and the others that feed on the remains of animals (thus being secondary or tertiary consumers). When micro-organisms finally break down all the remains, they are returning to the soil, or to water, the mineral salts needed again by plants, thus completing the circle. They are ensuring that the essential substances required for growth will always be there for green plants to use.

Today the term biogeochemical cycle is used to describe this process. Bio is life; geo is earth or soil; chemical here means the substances needed by plants to make living tissue, by means of light energy, and cycle means a series of happenings that keeps repeating itself like the turning of a bicycle wheel. So the biogeochemical cycle is the process by which non-living or inorganic substances move from the non-living world through the living world and back again, in a continuous repeat performance.

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