WHAT IS AN ECOSYSTEM?

When you walk through a meadow, or visit a wood, or look at a pond, you can see at once that each has different kinds of plants and animals living in it. In nature very little is accidental, or random. Always, although often modified by human interference, there are basic patterns of life, and the study of them is what ecology is all about. In an oak wood, for instance, besides the oak trees themselves, you are likely to find ash trees, and sycamores, and many smaller trees such as hazels, and maples; there will be primroses, bluebells, wood anemones, and violets; in sunny patches between trees you may see silver-washed fritillary butterflies, among a host of other insects; you may hear robins, tits, finches, and woodpeckers; and almost certainly there will be squirrels there. On moorland, by contrast, there will be bracken, and ling heather, and mountain ash trees; instead of fritillaries, the emperor moth is more characteristic; cuckoos, red grouse, and skylarks will be seen, or heard; there will be no squirrels, but very possibly hares. A pond is different again. Sticklebacks, waterlilies, moorhens, and frogs may be there. So these places are often called habitats, meaning places where different forms of life find the conditions they need, and can live in association with each other. A cave, an igloo, or a house may be a human habitat. Your dog may be a habitat for fleas or worms!

Ecology, however, is a comparatively new science, and the terms used in it are still being developed or expanded in meaning. Some of the places described as habitats, such as a wood or a pond, might now be described as ecosystems rather than habitats. The difference might be that an ecosystem has some power of self-renewal. The different parts of which it is composed operate in such a way as to keep it in being as a system.

Indeed, an ecosystem has four parts – the inorganic substances needed by plants, the primary producers or plants themselves, the consumers, and the decomposers; and through these parts a continuous circulation of food takes place by means of the biogeochcmical cycle. A wood is an ecosystem, because it may be able to maintain itself by regeneration, producing enough seedling trees to keep itself going as a wood with all the other plants and animals it shelters. A cowpat is a habitat, because although communities of animals and plants may find a home there, they do not produce another cowpat!

Briefly, then, an ecosystem is a community of plants and animals adapted to living in the ecosystem as a special environment, and maintained more or less as a self-contained unit by means of the continuous interchange of life-giving substances. In this sense, the world is itself an ecosystem, when you are thinking of life as a whole, but used in this way the word ceases to be useful. Some writers group all the major habitats of the world into ecosystems, recognizing only four types – that is, aquatic, terrestrial, soil, and artificial or man-made. Others would divide up the world rather differently, into Homes, meaning major areas of the world with their own climatic conditions and special communities of life – some of these being grassland, tropical forest, coniferous forest, and desert regions. Then the term ecosystem would refer to smaller environments, such as hedgerows, meadows, ponds, or woods, where the two elements are present – (a) that the environment in question is a unit supporting groups or associations of plants and animals needing that environment, and (b) that the different parts of the ecosystem can work together so as to maintain it as a system. In practice no ecosystem is completely self-perpetuating, and none is really self-contained or isolated as a unit. The one factor of bird migration means that there is considerable coming and going between different systems.

If you are a ‘muddy boot’ ecologist, that is, one who likes to go out and see things for himself, the best way to learn about ecosystems is to go out and look for one. Perhaps the most accessible and at the same time most fascinating is a perfectly ordinary pond. Of course ponds come in all sorts of shapes and sizes. It’s quite interesting just to dig a hole in the garden and keep it full of water, to see what happens – it will become an ecosystem in time! Best of all is an old, natural, undisturbed, never dried-up, rather shallow pond, with squelchy wet edges, where perhaps a cow sometimes stands, drinking and swishing her tail. Having found such a pond, you may see quite a lot by just sitting in the sunshine and watching. Making and using a bird hide in which you can sit and watch unseen will widen the possibilities. Or you can try to make a survey of the pond and the life it sustains. There are all sorts of different ways of tackling a project like this. One way is to get a length of rope and lay it down on the ground, one end on the bank, leading inwards to the pond. It will then lie across the bank, the marshy area, and then the swamp, and part of the shallow water. Then you can list every plant the rope touches, and every animal, big or small, you can see on either side of the rope. This will indicate the zonation, or the way life is organized in areas at different depths of water or different degrees of wetness at the edge.

Considering the pond as an ecosystem, you will find the four parts present that actually define it as an ecosystem. First and foremost, in the water of the pond and in the soil round it, and the sediment at the bottom, there will be the oxygen all plants need (the amount of it will greatly affect the life of the pond) and the mineral salts such as the phosphates, ammonium salts, and other nutrients, often enough to form a very rich and soupy brew which will help to form a very productive system. Much of this material is ready for use by the plants that need it, and it is continually being added to by the decomposers of plant and animal remains. Secondly, there are the plants growing round and in the pond, some in soil at the edge, some floating on top of the water but with their roots at the bottom, like water lilies, some floating aquatic plants with their roots dangling in the water, and some submerged aquatics, such as the Canadian pondweed. Thirdly, there is the animal life of freshwater, including the herbivores such as tadpoles and caddis larvae, and the carnivores such as frogs and toads, fish like perch and pike, most water beetles, water spiders, and dragonfly nymphs. And lastly there are the scavengers, such as freshwater shrimps and water worms, and the decomposers such as the bacteria and fungi feeding on dead plants and animals and helping to release chemical substances to be re-used.

Another approach to the study of ponds as ecosystems is to consider the degree of pollution to be found there. One cause of pollution is too much organic matter getting into the water. Suppose a cesspool from a house leaked into the pool, or drainage from a farmyard somehow reached it — or even suppose a very heavy leaf fall in autumn filled up a pond. If there is this excess of organic matter, the bacteria decomposing it may use up too much oxygen – more than is replaced by seeping through the surface of the water and that given off by submerged plants. Then the bacteria produce marsh gas (methane) and substances with a bad smell, such as ammonia and hydrogen sulphide. This can be tested by experiment as well as by the nose. If you can get a bit of turmeric paper from a chemistry set try dipping it in the water. It will turn brown if ammonia is present. To test for hydrogen sulphide, which smells like bad eggs, use lead acetate paper, because sulphides turn it black. Try holding a piece over a pan of boiling cabbage, or over a newly-opened bad egg. To test the water, half fill a glass tube with it, then wedge the paper in with the cork, so that the paper is held above the water. If it darkens, the water is giving off the gas. animals. For streams and rivers (not stagnant water in ponds) they have been classed in this way: stonefly nymph mayfly nymph caddis fly larva freshwater shrimp water louse ‘bloodworm’ (chironomid larva sludge worm and only those breathing air, e.g. rat-tailed maggot

A If these animals are present, the water is clean.

B These can tolerate more pollution than A.

G Finds of these animals only (and D) indicate pollution fairly serious.

D Finds of these only indicate highly polluted water.

This gives a scale of pollution for running water, once you have learnt to identify the animals. You will need a magnifying glass to do so, but you can soon put together a do-it-yourself kit for this kind of water pollution and make notes of the results in different streams. It is more difficult to work out indicator animals for ponds, but if you found no animals there, you could certainly conclude the pond was highly polluted.

FOOD CHAINS AND FOOD WEBS

Within an ecosystem, there is a whole complex of feeding relationships. The simplest form of such a relationship is a chain, with two, three or four links in it. As an example, rabbits eat grass, and foxes eat rabbits.

In practice, feeding relationships are seldom as simple as this. Each link in the chain interconnects with other chains. Take for instance the simple chain grass^rabbit ~*fox. What else eats grass besides rabbits? and what do they eat? and what eats them? what else eats rabbits, besides foxes? and what eats them? what else do foxes eat? Suppose they eat chickens. What do chickens eat, and what eats them? If you think these relationships are simple, try working out some for yourself. (To find out what animals cat, you can analyse their droppings, or if you find the bodies of dead animals while they are still fresh, you can slit their stomachs and examine the contents.) In fact, isolated food chains do not occur in nature, and the pattern of feeding relationships within an ecosystem is better described as a food web.

Whether chain or web, the pattern is of tremendous importance for conservation, because if a poison gets into the pattern at any time, it may be passed on to different parts of it, with quite unforeseen results. Some years ago in Britain farmers dressed their seed corn with dieldrin, to prevent it from being attacked by wheat bulb fly larvae in the fields. Pigeons began to eat the seed – it was said they even learnt to dig it up from the ground and eat it in large quantities. As pigeons are a great agricultural pest, perhaps no one would have worried very much about this, but the pigeons were eaten by other animals. Predatory birds such as hawks, which fed on poisoned pigeons, were also killed, and their numbers in Britain fell alarmingly. There were cases of particularly intelligent badgers lurking beneath pigeon roosts, having learnt that some would fall down dead; but as the ones that fell first were those affected by the poison, the badgers were also killed.

Stable poisons like dieldrin and other organo-chlor-ines have had other effects. Dieldrin used in sheep dips may have poisoned golden eagles in Scotland, not always killing them, but affecting their fertility and breeding success, because the eagles fed on dead sheep carcases lying on the hills. And suppose a farmer casually threw an empty tin of sheep clip into a ditch or pond, not thinking any harm, that poison in water could do immense damage, poisoning fish, and then the birds that fed on the fish. Some organisms, particularly fish, seem to have the ability of concentrating poison in their tissues, so that although not killed, they retain some of the poison without excreting it, so that it remains ready to poison the next link in the chain. A heron eating many affected fish with small quantities in the tissues would then die.

Examination of poisoned birds or fish is a very technical matter requiring skill and equipment. But accidental poisoning of water can sometimes be indicated by experiment. If you can get some Universal Indicator paper (perhaps from a chemistry set), cut a piece into strips, and dip them in various liquids. Vinegar, which is an acid, turns it red; ammonia, an alkali, turns it blue. When you dip the paper into pure water, it will not change colour much. In water with too much acid in it, the paper will turn pinkish, brownish, and red, at the extreme. In water with too much of any alkali, the paper will turn green, and then blue. Gardening shops often sell small kits for testing soil or water, and they are very convenient because they include a colour chart.

SUCCESSION AND CONSERVATION

When an unforeseen element begins to affect one part of an ecosystem, many other parts will also be affected. The new factor may be a poison, such as dieldrin in seed dressings, but is sometimes quite ‘natural’. In the autumn of 1953, the disease of myxomatosis appeared in Britain, probably as the result of a mosquito infected with the virus being blown across the Channel from France, where the disease had been introduced, and starting the outbreak in Kent. Rabbits began to die in their thousands, and the rabbit population of the country was reduced by at least ninety per cent. The ecological effects – on other animals, and especially on vegetation – were immense; we still do not quite know what the total effect has been. Again, sometimes foreign elements are introduced into an ecosystem, with significant results. When farms were started in East Anglia to breed coypu for their fur, some of these animals escaped, and began to breed in the dykes and on river banks. Large scale programmes of trapping and killing had to be initiated to prevent widespread damage. Introduced animals (and even plants) can become ‘aggressive’, flourishing at the expense of native flora and fauna, and sometimes becoming a real menace. Sometimes, too, within an ecosystem, one type of organism may become too successful. In some way the controls over its numbers cease to operate; the population increases rapidly; the animal is said to ‘swarm’ and the process is finally brought to an abrupt end by some kind of disaster (the suicidal rush of lemmings to the sea may possibly be one of the effects of swarming).

Even without these dramas in nature, communities of plants and animals are continuously changing, adapting to change in and outside their own communities, often very slowly, but noticeably, and according to recognizable stages, and this process of orderly change is called ecological succession. It is of tremendous importance for nature conservation and as it can be observed at many different stages it is worth considering one or two of them.

Primary succession is said to start with a piece of land that has not previously supported life at all. Suppose you have a plain rock surface, with nothing growing on it. Gradually it may be weathered, by rain, frost, and wind. In the small cracks and hollows made in this way on the surface water will collect and dissolve some of the minerals on the rock surface. Then simple plants may establish themselves; lichens may be first, blown in spore form. Eventually there may be pockets for windborne dust and soil, plus organic matter from dead lichens. This provides some sort of habitat for animals such as mites, ants, and spiders; their dead remains add to the soil; mosses may move in; soon there are fresh animal arrivals, such as mites and other groups. Moss holds down the soil; plants decay; more soil is formed. Birds and insects may visit the pockets of soil; soon grasses appear; more animals and nematodes; more soil; then small shrubs and trees finally germinate there. The order of species appearing depends on local conditions. Finally you may get fairly stable conditions described as climax vegetation.

A different type of succession, sometimes called secondary succession, may begin in an area where there has been some catastrophic or dramatic event. After a forest fire, for instance, life gets itself going again. You might be able to watch the process if you could find an area of heathland that had been ravaged by fire, and see how and in what order plants and trees re-establish themselves, and how insects and animals move back into a devastated area. Again, everything depends on local conditions, but somehow, plants and animals will re-colonize the most unpropitious places; nature is very tough. But the area will not be quite the same, at any rate for a very long time, if left alone.

You can watch quite another kind of succession in a habitat such as a tree. When it is flourishing, it gives food and shelter to a multitude of other organisms, from lichens and mosses on its trunk, to birds in its branches and perhaps badgers at its roots. As it grows old, and dies, the birds have to find new homes, and the insects that have lived there; an old stump may be colonized by quite a different group of birds, and may support fungi; finally the wood of the tree will decompose, and a host of bacteria and decomposers will return substances to the soil.

But why should this worry conservationists? It is a fascinating process to observe, over periods of time, but it may convert interesting and delightful areas into something much less attractive. One of the most delightful parts of the English natural scene is the meadows and pasture of grasslands on chalk. Wild flowers there may include cowslips, birdsfoot trefoil, the horseshoe vetch, the rock rose, and the wild thyme. Brilliant blue butterflies will be there; and skylarks also. Rarities such as different orchids may be found. So if a stretch of this type of area can be preserved, we may well rejoice. But, and this is a big but, left alone such an area may lose all its attractions. If it is not grazed by rabbits, or sheep, or cows, untidy and unattractive scrubby bushes and brambles will grow there, and trees such as willow, maples and elms; a thicket will develop. Other plants and birds will enter, and grass will disappear as sun and moisture are taken up by trees. The community will change, and all that was valued in the meadow may disappear. So, such an area has to be managed, to retain its interest; and this is the sort of problem the practical conservationist has to consider.

MAN THE MENACE

If ecology has anything to teach, it is that nature is indivisible. Everything in nature is intricately meshed together. Many of us were brought up to believe that nature is ‘red in tooth and claw’, as if every part of nature were at war with every other part. It is true there may be intense competition, at different times and in different places, but generally the different parts of nature are interdependent. Plants and animals share the same conditions, they help each other, they cannot live without each other.

Into this complex and delicately-balanced pattern of life on earth, man has come blundering with little regard for anything but his own immediate needs. He has hunted animals to extinction, over-grazed large areas and turned them into dustbowls, hacked down forests, and dammed up rivers. Today science and technology have given him almost unlimited power. Vast schemes are being discussed. Should we melt polar icecaps – blast mountain ranges – drain seas – flood deserts? We are taking enormous risks. Our efforts to produce more and more food are having far-reaching effects on nature and vegetation. We may be poisoning land, sea, and air. We are certainly exhausting some of the world’s resources, using up fossil fuels and some minerals at a disastrous rate. We are even allowing our own numbers to ‘swarm’, and soon there may be far more people on earth than can well be supported. It has even been suggested, among other forecasts of doom, that supersonic aircraft may disturb the upper atmosphere to such an extent that dangerous radiation from the sun might reach the earth, with unimaginable effects on life there.

In this situation, nature can act as an early warning system. If trees die because of a sulphur-laden atmosphere, or fish are found to be full of mercury because of wastes poured into the sea, this may be an indication that we are making mistakes. Dead birds, disappearing butterflies – these too may be signs that we have gone too far. Nature conservation is far from what some people imagine it to be – the mere protection of a few animals and flowers. It becomes a question of whether man can survive at all, in the world he is creating.

It has been said that if you wish to see a specimen of the most dangerous and destructive animal on earth, all you have to do is look into a mirror. But this is only one side of the question. If you wish to see a specimen of the most intelligent, the most successful, and the most powerful animal in the world, again you can do so by looking into a mirror. Man has done great harm to the living world, but in many ways he has made the earth a more beautiful and interesting place. Once he sees the need for an intelligent policy of nature conservation, he will cease to be quite so dangerous to the world or to himself.

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