Читать книгу Life in Lakes and Rivers - T. Macan T. - Страница 7

It is interesting to speculate on the contents of an Atlantic Charter drawn up by any species of animal other than man. We may start by comparing the lot of man with that of the rest of the animal kingdom, which is separated by a lower grade of intelligence. People are not often drowned as a result of catastrophic floods, few are blown to destruction by strong winds, and death in a forest or heath fire is a rare calamity; nor do abnormal spells of hot or cold weather claim many victims. The same is true for all large animals. But for innumerable small ones such dangers are great, and the populations of many are seriously depleted at intervals by one or other of these causes. A final calamity, which does not often befall man, though it befalls other animals of all sizes, is death at the hands of some beast of prey.

Medical science has rendered the more advanced sections of the community secure against many of the disease-causing parasites which formerly destroyed them in large numbers; plague (the black death), typhus (gaol-fever), and cholera may be mentioned as diseases that once took a heavy toll of life in the British Isles, but do so no longer. In less advanced parts of the world, disease still brings death to enormous numbers of human beings, and in this respect there is not so much difference between man and the other animals. Indeed on theoretical grounds it can be argued that many other animals are better off than man. Most parasites can survive only in living tissue and therefore it is to their advantage that the host should remain not only alive but unhandicapped in the struggle for existence. Too virulent a strain will be as unsuccessful as the one that is not aggressive enough to gain a foothold in the face of the counter-measures taken by the host. There is continual selection of a strain that can establish itself but will not kill. Selection acts on the host too and specimens lacking resistance to a particular parasite are removed at an early age. The result is a state of tolerance, with the host carrying parasites but not inconvenienced by them. This state is not attained by man for two reasons. First, selection of resistant individuals does not take place because medicine prevents it. Secondly, whereas the total population of most animals is separated into numerous units between which there is little interchange, man travels all over the world and carries strains of parasites from a region where tolerance has developed to one where it has not.

Death from starvation may have two causes: the food of a particular species may fail on account of some climatic irregularity, or may be eaten up by some other animal. We avoid the first of these eventualities by means of a highly organized system of transport; when the harvest fails in one part of the world, food is brought from somewhere else. But the recollection of the Bengal famine will serve as a reminder that this danger is not wholly eliminated. Against animals which eat the same food as himself, man brings to bear all the resources of science, and wages a never-ending war on rabbits, rats, grain-weevils, slugs, caterpillars, and a host of smaller pests. Lower animals cannot attack their competitors on anything like the same scale; until recently it might have been stated that they could not do it at all, but now it is know that certain organisms can produce chemical substances that kill competitors – penicillin produced by the mould Penicillium is the obvious example – and this process is comparable with the steps that man takes to safeguard his food supplies.

Were we writing about philosophy and not natural history, we should of course have to insert a passage about the perils peculiar to civilization – death on the roads from motor vehicles, and other accidents with machines, destruction by high explosives and other weapons of war, and mass annihilation by atomic bombs. However, we are not. The purpose of what has been written is to stress that the life of a small animal – and it is with such that this book is mainly concerned – is a continual struggle of extreme severity. The physical environment, predators, parasites, and competitors all have to be contended with. The response has been steady change and continual modification. Some animals have won and held a place where physical conditions are easiest – but dangers from predators, parasites, and competitors consequently greatest. Others have become adapted to conditions where physical or chemical conditions make life difficult – but where accordingly there will be fewer other organisms to harass them. Fresh water provides some of the most striking examples of the latter.

It might appear at first sight that the gulf between land animals and water animals is great, and that the easiest way into fresh water is from the sea. But, although an animal or plant may pass from marine to freshwater conditions with no alteration of structure, the change confronts it with considerable functional or physiological problems. The concentration of salts is generally much lower in fresh water than in the sea and moreover liable to considerable variation according to rainfall and other factors; from the biological point of view the constancy of the marine environment is one of its most notable features. Further, conditions vary widely from one freshwater locality to another. An animal proceeding up the Hampshire Avon, for example, would find, if it turned aside into one of the tributaries coming from the New Forest, an acid water, poor in dissolved salts, very different from that which would surround it if it followed the main river to a source at the foot of the chalk downs. These chemical conditions have proved a barrier which only a few marine animals have surmounted. Some of the snails, the bivalve molluscs, the freshwater shrimps, and the fishes are the most familiar.

Actually it appears from an examination of the groups to which present-day freshwater plants and animals belong, that it has been easier to invade fresh water from land than from the sea.

The animals of marine origin occupy in fresh water the same sorts of situation that they occupied in the sea, and they have not changed greatly. As a result of isolation they are now quite distinct from their nearest marine relatives, but they present no peculiar freshwater facies. Some of the land-animals, too, have effected the change to fresh water with little alteration; others with no more than some general adaptation such as the conversion of appendages from legs to paddles. But some of the animals from the land, having once established themselves in fresh water, have become considerably modified to live in one particular and difficult part of the underwater world such as a torrent; others have achieved remarkable physiological adaptations, such as the ability to live without oxygen. It is among these specialists that we find the peculiar and characteristic freshwater types.

The main problem confronting an animal taking to the water is how to obtain its oxygen. Often the difficulty is not great, because many land animals live in damp places and have a moist surface. If they are quite small, this surface is all they require for respiratory purposes, and accordingly it does not matter greatly whether air or water is the medium beyond the layer of surface moisture. The problem is not quite as simple as this, but we need go no further for the present.

Other animals, whose land ancestors were probably less dependent on humid conditions, spend their lives in the water, but have developed a variety of methods whereby they can utilize atmospheric oxygen. The familiar water-beetles swim to the surface with the aid of their hind legs (which, with the transfer to water, have been modified into efficient paddles), and take in a bubble of air between their backs and their wing-cases. Some snails come to the surface to fill a lung. Mosquito larvae – the well-known wrigglers of the domestic water-butt – feed at the surface with breathing-tubes penetrating through to the air. The rat-tailed maggot has a remarkable telescopic appendage, so that it can walk on the bottom and keep in contact with the air at the same time. Other animals take in oxygen as a gas but nevertheless live perpetually submerged. Several groups – none of them very familiar to the layman – have a close-set pile of unwettable hairs. Withdrawal of oxygen for use by the body from the gas entrapped in this pile causes a partial vacuum, which is filled by oxygen dissolved in the water. Others tap the gas-filled tubes inside plants by means of some part of the body modified for the purpose.

Fresh water has presented invaders with a variety of problems, and no animal has solved all of them; none is found in all the habitats and most are confined within a rather restricted range.

The surface offers a rich hunting-ground, for many land animals fall into the water and are held there helpless by the force of surface-tension. Two groups of insects have mastered the art of living on the surface film, and are able to prey on these unfortunates. They are, however, confined to small pieces of water or sheltered bays, presumably because the effort of keeping station against the wind in the middle of a big sheet of water is too great. The pond-skaters have developed long legs and proceed somewhat after the manner of long-oared skiffs. The other group, the whirligig beetles, have shortened and flattened their legs till they resemble more the paddles of a canoe, by means of which they move over the surface with great rapidity. The way of life of these surface-dwellers calls to mind that of the wreckers who once gained a livelihood round our coasts, though they are not able to take measures to lure their victims to destruction.

Quiet shallow conditions, where mud settles and rooted plants – all incidentally of terrestrial origin – provide shelter, oxygen, and food, are probably the most easy to colonize; that is, they present the would-be inhabitant with least in the way of physical and chemical difficulties. But life is not easy in this habitat because there are so many different types of organism and competition between them is severe. The water-beetles and water-boatmen pursue their prey through the underwater jungle with the speed and grace of terrestrial felines; dragonfly nymphs lurk concealed like a crocodile in a waterhole; the tiny Hydra trails its tentacles in the water like a fisherman setting out his net. Against these marauders the caddis-larvae seek protection within a cumbersome house of stick or stone, and snails can withdraw into their more neatly made shell. But the snail’s shell, the caddis-larva’s house, the dragon-fly nymph’s protective coloration, and the beetle’s speed may alike prove unavailing when fish come nosing through the undergrowth in search of food. They are not the only enemies, and some birds and the water-shrew also hunt in this territory.

In deeper water, if it is too dark for plants to grow, the mud offers a substratum which is often rich, because the remains of animals and plants rain upon it from above. Here the chief inhabitants are mussels, worms, and midge-larvae, most of which are modified for burrowing and for feeding on minute particles. The diversity of form is not great, though the number of individuals may be colossal.

In yet deeper water conditions may be extremely difficult, because there is no oxygen for part of the year, but some animals, notably midge-larvae, have solved this physiological problem.

In the open water also the variety of form is not great, though numbers may be. Some small animals, such as the water-flea, resemble their counterparts in the sea, but the marine zoologist inspecting a freshwater catch is immediately struck by the lack of diversity. This is partly because many marine animals which lead a fixed or relatively sedentary life have a free-swimming young stage. This presumably serves the end of dispersing the species. In the circumscribed conditions of fresh water such a stage is of less advantage for this purpose, and might indeed prove a danger by being carried away to the sea before it was ready to settle. Freshwater animals which have come in direct from the sea have almost all lost the free-swimming stage, which their nearest marine relatives possess. Another striking feature of the animals of the open water is their small size. This is probably due to the absence of shelter and consequent vulnerability to predation. Some protection is obtained by transparency, a feature seen in both the sea and in fresh water, but the main defence is small size, rapid reproduction while conditions are favourable, and the formation of a resting stage as soon as they cease to be. Large animals could not reproduce fast enough to make good losses due to predation. It may be objected that some animals floating in the open sea attain a large size. In comparison with the sea most bodies of fresh water are very small and fishes feeding on the larger animals on the bottom in shallow water could easily make excursions to prey on any large organism that developed in the open water. Most of the sea is so far from land that this cannot happen, and predators and prey must develop in balance together.

One representative of that enterprising group the insects inhabits open water, and provides one of the most remarkable examples of adaptation that the animal kingdom has to show. It is the larva of a mosquito-like fly, and is known as the phantom larva, on account of the transparency already mentioned. The breathing-tubes, in other insects continuous from back to front, are vestigial except for paired air-sacs fore and aft. These are hydrostatic organs by means of which the animal can maintain itself at any desired depth. The earliest workers supposed that it functioned like a submarine, pumping fluid in or out of ballast tanks as required. Later observers, noting that the bladders never contained fluid, postulated that they worked like the swim-bladders of fishes, and secreted or absorbed oxygen. Finally, when a technique for analysing the minute quantities of gas in the bladders was evolved, it was discovered that the walls of the bladders expand or contract as the result of nervous stimulation, and the gases dissolved in the body-fluid diffuse in or out accordingly.

Rocky shores of lakes are startlingly barren places compared with those of the sea; there is no canopy of weeds nor incrustation of barnacles and other sessile animals. The two are not strictly comparable, because in fresh water there is no tide and the water may sink slowly to a low level and stay there during a long spell of fine calm weather. But even below this level there are not many living organisms. Probably this is due to a poor food-supply, for hard rocks and waters that are not rich in nutrient substances commonly go together. Perhaps this is an environmental niche that has not been completely filled; the fact that neither rocks nor wood are attacked by boring organisms in fresh water, as they are in the sea, suggests that this idea is not as revolutionary as it may seem at first. Further, the zebra mussel, the only freshwater bivalve that can attach itself to a hard substratum as the marine mussels do, has only recently entered fresh-water.

On a rock-face in a lake the only plants are green algae, and the only animal the freshwater limpet. This is actually descended from terrestrial stock, though superficially it resembles the marine limpet closely except in size. Smooth rock in a stream may have some covering of moss, which harbours many animals. If it is bare, it may be covered by great numbers of buffalo-gnat larvae, which spin a web across its surface and attach themselves by means of a circle of hooks on a basal pad. They obtain food by straining the current with hairy mouth-appendages.

Smooth rock is not found very frequently, and, where wave action or running water prevents the settling of finer particles, the bottom usually consists of stones and boulders. Several animals have adapted themselves to these particular conditions. Some mayfly nymphs, such for example as those of the March Brown, have flat bodies and strong claws, and can crawl over the surface of a stone, where they graze on the attached algae, in such a way that the current cannot get beneath them and pluck them off. Certain caddis-larvae spin nets between the stones in a stream and subsist on the debris which these nets strain from the current.

These animals are specialists. They have solved the main problem of life in swiftly flowing water – anchorage; and two, having surmounted this difficulty, have turned the peculiarity of the medium to their own advantage – the constant flow brings them their food. The modifications of the specialists render them unable to compete with other animals except in the habitat to which they are adapted. Except in extreme conditions, specialists and non-specialists are found side by side. In streams, for example, many animals without any particular modifications for life in running water occur beneath the stones. One of them, the larva of the daddy-long-legs, is not greatly different in structure from its relative the leather-jacket, which lives in the soil and damages lawns and pastures by eating the grass-roots. Another, one of the commonest, is the freshwater shrimp (Gammarus pulex), a rather incompetent swimmer which is washed away at once if caught by the current. It is one of the most successful of all freshwater animals. It may be abundant in quiet weed-beds, and sometimes, apparently when fish are very few or absent, it may live in the open water of lakes. But it is not ubiquitous and some of the chemical problems posed by fresh water have proved too much for it; it is not found where the calcium concentration is very low, and it also requires a relatively large amount of oxygen in the water.

There is another difficulty with which fresh water confronts animals and plants that seek to live in it – it may dry up. Many freshwater organisms have, accordingly, developed a resting stage, which is resistant to desiccation and probably plays an important part in dispersal as well as survival. Some animals have specialized in the temporary habit, thereby gaining certain advantages, for they make a quick start when water reappears, and can exploit the resources of the pond before it fills up with competitors coming from permanent pieces of water. Many mosquitoes lay drought-resisting eggs in damp hollows, and no development takes place till the hollows fill with water. That beautiful animal, the fairy shrimp, is found only in temporary pools, and survives the dry periods in the egg-stage. In England it is rare, but in Iraq, for example, where there are innumerable pools that fill when the high river-level causes the water-table to rise in spring, but are dry throughout the rainless summer, it is one of the commonest pond animals.

Nobody in these islands has far to go to find a piece of fresh water, and his search will usually take him into pleasant surroundings. He will not find the gaily coloured, almost gaudy, creatures that the seaside naturalist encounters; most freshwater animals are rather drab. On the other hand neither land nor sea animals can so easily be kept and watched at home. Moreover a day’s pond-hunting is not rendered fruitless by rain, as is an excursion after butterflies and many other land creatures. The fauna is, as we have just seen, the product of a difficult environment, a severe struggle for existence, and the adaptability and plasticity of living organisms. In consequence it shows a fascinating diversity of form and function, which cannot fail to appeal to all who are interested in wild creatures and how they live.