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A Primer for Adults and Children
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Oct 1992, No. 108 ->
A Primer for Adults and Children
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This article is excerpted from John H. Storer's book The Web of Life. Attempts to gain permission to reprint portions of this book revealed that there was no record of this book, and that the author is dead.
Our forests, like other families, need the closeness of family members to support them, nurture them. When trees get a foothold in an open field, only those species can survive that are able to tolerate direct sunlight. Several kindsof pine, birch, and poplar have this ability.
As these trees grow, they provide shade for other species that require protection from the sunlight, such as tulip, poplar and many other species, including red oak and maple. These trees will grow until they crowd out the sun-loving trees that nursed them. Finally, their shade becomes so dense that their own seedlings die for lack of sunlight, and they in turn will be replaced by the shade-tolerant trees.
Trees of intermediate stages--the oaks, tulips and others-- eventually grow so big that their own seedlings cannot survive in their shade. They will be replaced by shade-tolerant trees like the beech, sugar maple and hemlock.
In a mature forest, the beech, sugar maple and hemlock, and other trees whose seedlings can grow in deep shade, become masters. They actually select and dominate all other life in the forest. Nothing can live that is not adapted to the shade, moisture and climate that they have established. Such a forest is known as a climax forest.
There are many types of climax communities of trees and plants, each one adapted to its own preferred conditions of soil and climate. This can be clearly seen on many mountainsides where forests are divided into bands of zones of trees, each one adapted to the conditions that prevail at its own preferred elevation. Douglas firs have small roots and great height. They have two requirements for survival: they must grow in thick stands for protection from the wind, and they must have enough moisture tosupport a thick stand. Farther down the mountainside where the soil is deeper and rainfall less, these trees will give way to a zone of ponderosa pines that have a deeper root system and require less rain. Below these there will be other successive zones of trees, bushes and grass.
Higher on the mountain, at timberline, the hardiest trees make their last stand against wind and cold. Spruces and white barked pine are among the last to succumb. Above them the low, flowering plants of the mountain meadows take over, and finally these give way to the lichens that grow on the bare rocky peaks.
Mountain forests play an important part in the world's water supply system; in many areas, mountains are the source of water for all the surrounding country. When a current of warm, moist air blowing inland from the pacific strikes a California mountain range and flows upward over the mountain it is chilled in the cold atmosphere above. When the air is chilled it loses much of its power to hold the moisture. The water vapor condenses to form a cloud, and under proper conditions, much of it will fallto the earth as rain or snow.
Trees shade the snow, holding it in deep, slow-melting drifts. They build an absorbent covering on the ground to hold the moisture as it becomes available, letting it out slowly to feed the streams and give life to the lowlands through summer droughts.
Plants on the ground under the trees protect the soil from the hammering force of the raindrops. They tie the soil into place and make it an absorbent storage reservoir for water.
The destruction of a forest may spell disaster to communities a thousand miles away. When trees are blown down in a windstorm, their broken roots and weakened sap flow can no longer withstand the attacks of bark beetles. Under a tangle of branches they are inaccessible to the woodpeckers that ordinarily catch most of the beetles that do gain entrance. The beetles, thus protected, multiply like an explosion, spreading out to attack and kill the healthy trees in the surrounding forest.
The dead trees dry out and turn to tinder, becoming far more vulnerable to fire than a normal forest. The catastrophe started by a local windstorm may spread out to ruin the forests on an entire water shed. The same thing may happen when man improperly cuts or grazes a forest.
After the fire, the land is dead, and from it death will spread for a thousand miles over the country below, carried there by floods and droughts.
When raindrops strike unprotected soil, the fine soil particles spatter into the air. Falling back, they fill the crevices between the larger particles, making the surface waterproof. Instead of being stored in the earth, the water now runs off ina flood, tearing away the unprotected soil as it goes.
The price of removing forest protection is great: Rich soil that once lay level with the tops of tree roots can be carried away by floods, leaving the roots exposed and sterile with the sand five feet below the base of its trunk.
A farmer can lose his entire farm, carried away by a flood caused by the destruction of a forest on a distant watershed that he had never seen. Where did his land go? To join other farms whose top soil had been carried away by floods, much of it mixed with gravel and entirely useless, some of it dropped in the ocean.
Several years ago the reservoir that was part of the water system that supplied the city of Santa Barbara, CA was completely filled with sediments washed off the burned hills after a fire on the watershed above it.
Each wild creature can multiply safely only up to the limit that the forest can feed and shelter it.
The food provided by plants is used to build the bodies of insects, animals and birds. Each of these in turn is eventually used as food by other living things. Thus it is passed along from mouth to mouth in a food chain to support a series of creatures, until at last it is returned to enrich the soil. A catbird will eat its own weight in insects every day. The forest could not survive without the help of birds. But to receive this help it must always provide enough insects. Properly controlled insects are a necessary part of a plant community. It is only when the balance between insects and controls is upset that some become a menace.
The forest has its own sanitary corps. A turkey vulture, for instance, feeds on dead fish. Many kinds of scavengers--birds, insects and animals--help to keep the land clean. All are necessary parts of the organization that keeps the community functioning.
The ability of the land to support animal life depends directly on the food and shelter that it can provide. In winter mice search for seeds among dead weeds. The winter landscape produces no food. It supports only those creatures that can survive on the food left over from summer.
Nature automatically adjusts the number of predators to the needs of the community. Well-fed deer have the strength to protect themselves and their young against the attacks of coyotes. But in the winter when the ground is covered with snow, they must depend chiefly on the buds of trees and bushes for food. Only the deer that can find enough food to maintain strength can survive.
All animals have the power to multiply faster than their normal death rate. For its own survival, a community must have policemen to hold the multiplication of population within the limits of its food supply. Red-shouldered hawks monitor the snake population.
On land where there are not enough predators to protect the community, deer multiply beyond the capacity of the land to support them. They then browse the tree branches bare as high as they can reach. The land loses its power to feed them the following winter, and the result is starvation.
The predator protects the community. When the deer multiply beyond the limits that the winter food supply can support, the coyote removes those that are weakened by hunger. Coyotes in turn can survive only so long as there is a supply of meat to support them. Thus through the help of the predators, nature maintains a balance between each species and its food supply.
Each creature is a specialist. A red-bellied woodpecker has feet adapted to walking on the bark of trees, a bill adapted to drilling through bark to reach the insects beneath, and tail feathers with tips fashioned to provide support while it works.
The nuthatch also has feet adapted to walk on bark. Its bill is adapted to probing among bark crevices for insects, but it is not strong enough to drill through bark.
The evening grosbeak has a big bill adapted to cracking seeds. Each of these different birds is equipped to find its own special food in its own way. Each may find its food on the same tree, but each would probably starve if it had to depend on using the food that supports any of the others. Thus, each plays a special role in the community.
Each tree is specially adapted to its own preferred environment. The saguaro cactus can store great quantities of water in its body and exist through long periods of drought. A twelve foot saguaro can live on as little as 1/50 of a quart of water per day. A date palm, with the help of irrigation, can grow in the same desert, but it will require 500 quarts per day, twenty-five thousand times as much.
Leaping salmon are adapted to excel in their environment. The dwellers in the streams are an integral part of the forest community. They depend on the flow of water that in turn is controlled by the conditions in the forest. Salmon spend most of their lifein the ocean, growing fat on food origination with salt water plants. They have the special ability to ascend streams that are inaccessible to most fish. So at spawning time they journey upstream to lay their eggs in the headwater, safe from the predators of the ocean. Through this adaptation the salmon has become one of the most successful of the fishes, threatened only by man changing its habitat.
A five-storied warbler's nest can tell the story of a struggle for survival--a contest between a yellow warbler and a cowbird. The cowbird lays its eggs in the nests of smaller birds that hatch and rear the young, often with fatal results to their own offspring. A cowbird can lay its eggs in the original nest with the warbler building a roof over it for a new start, and again the cowbird will lay an egg. Finally, after several tries, the warbler will lay its own smaller eggs to be hatched with the cowbirds. One warbler's nest had five stories built on. Few birds are as resourceful or persistent as this warbler was.
Where trees find enough moisture they grow tall and shade out the grass. Where the moisture is insufficient, grass takes it all and starves out the trees. At the borderline between grass and tree zones, there is strong competition. In moist seasons the seedling trees may find a foothold in the grass, only to die in the dry seasons that follow.
Grass roots are the key to survival in dry country. Grass has a far larger root system than trees in proportion to leaf area, and its roots are nearer the surface. This gives it an advantage over trees in dry country. Its roots take all available moisture as it seeps into the ground, allowing none to filter down to the tree roots. Even in the struggle between grass and trees, it is the survival of the fittest.
he Grand Canyon of the Colorado River tells us a lot about the earth's history. The area of this canyon was once level, but the rock was slowly worn away. Rain softened and dissolved its minerals. Heat and cold expanded and contracted it, causing it to crack and break into fragments that piled up to form grey slopes below the cliffs.
On the surface of small glaciers are rock fragments that have fallen from the cliffs above. Mixed with the moving glacier ice, these act like teeth that slowly grind away the face of the mountain.
The many layers in the face of the cliff suggest the variety of minerals that have gone into the building of this rock. These are mixed together by the ice and later spread over the land below.
During the ice ages more than seventeen million miles of the earth's surface were buried under moving ice, and under its grinding a large share of the northern soils had their beginnings.
The first step in soil building: Slopes below mountain peaks are composed of rock fragments broken from the face of the cliffs by the forces of erosion. Slowly this broken rock will be moved down into the valleys by water, wind and gravity. On the journey the pieces will be ground together and broken into particles small enough to form the parent material of soil. Before it can support life, soil must be anchored into place, enriched and organized into a living entity by the action of plants and animals.
Life makes its start with lichens, moss, and ferns on rock. The pioneer lichens can exist on bare rocks with very little moisture. They secrete acid that dissolves minerals from the rocks, thus creating a seed bed that can absorb moisture and provide food and a foothold for higher plants. The plant roots spread out into the surrounding soil tying it into place. From this foundation of raw soil, the plant community develops.
The plants build the soil. Chlorophyll in green leaves is the only substance in nature that has the power to harness the sun's energy and combine it with elements from air, water and rock into living tissue. This plant tissue becomes the food that supports all animal life, and it is the basis for the organic matter that is an essential part of productive soil. Thus, the creative work of plants is the essential first step toward building fertility into soil. By a maze of rootlets and fungus- fibres, most of them invisible to the eye, plants tie the soil together. When these die, they make a sponge of absorbent rotting vegetation. Insects, worms and other animals burrow to reach this organic food, mixing it thoroughly in the process and enriching it with their own remains.
Nature Plows the soil. Today's catastrophe, a fallen tree, prepares riches for tomorrow. In a mature forest the soil may be corrugated with ridges and hollows that hold rain and melting snow and that show where trees have fallen and mixed soil with vegetation.
Small animals play a useful role as nature's farmers, plowing the soil. Some, such as prairie dogs, dig in it to make their homes; others, like the shrews and moles, tunnel through it in their search for insects and worms, thus helping to make it rich and absorbent.
Insects and worms do their share in soil building, plowing and mixing it with the vegetation, and enriching it with their remains as they burrow to seek shelter and food among the plant roots.Cicada wasps dig tunnels to make a home for their young.
A soil profile of the finished product: The light lower layer is the parent soil composed of rock particles. The dark upper layer is topsoil, built by the action of plants and animals. It may be as much as 40% organic matter--i.e., living and dead plant and animal life. This soil has the power to absorb water and the energy to produce crops.
A tiny black speck moves into the light on a lake's surface. As it moves, a trail streams out behind it. At last the shape grows into a speeding motor boat with the long lines of its wake sweeping out on either side, dark ribbons onthe glowing water.
Finally, as it nears one side of the lake, the motor boat circles twice in wide sweeps and then disappears behind a headland. On the lake behind it, the lines of the circling wakes come together in interweaving patterns. In some places they collide to efface each other. In others they reinforce one another, finally rolling off in long curves to the distant shores.
These rolling waves were set in motion by the energy of the moving boat that displaced the water in its passing. Each wave carried within itself a portion of that energy, finally expending it as it rolled a pebble on the shore, or rocked the roots of a floating derelict tree, or perhaps washed out the final handful of dirt from the undermined roots of a tree that would topple into the water to join the other derelict. Thus, the energy of the motor boat had its impact on the land far beyond the vision of the man who set it moving. Just so, the forces set in motion by every act of man or bird, animal, insect, or bacterium move out to affect the lives of many other creatures.
The principles that govern all these interrelationships are called the principles of ecology--the science that deals with the mutual relations between living organisms and theirenvironment. The subject of ecology is so vast and complex that no human mind has ever fathomed all its secrets. Many of them can probably never be unraveled, but the basic principles of ecology are known, and on the functioning of these known principles depends the future of all human lives.
Life is such a personal thing, wrapped up within the being of every living creature, that it is sometimes hard to realize how intimately each life is connected with a great many other lives.
Life is a flowing stream, forever passing away and as constantly being renewed. The energy that brings us life is supplied from many different sources, most of them beyond our vision of experience.
The bread that comes from the grocery shelf brings minerals and vitamins that may have been prepared by wheat plants in the soil of Kansas. Vegetables may bring their special contributions from the farms of Texas or California. Meat was built by cattle out of grass that may have drawn its special qualities from the soil of the western plains. It requires five thousand pounds of water to produce thegrass that goes into the making of each single pound of meat. To store this water and supply it to the grass as needed through summer droughts, the soil must first be prepared by many generations of earlier plants and animals.
Bread, vegetables, and meat are merely vehicles for transferring to us the special properties of soil, air and sunlight gathered and organized by the plants. With some minor variations, all these living things are composed of the same elements and in roughly the same proportions. But each differs in its methods of obtaining and using them. Plants and animals vary greatly in the amounts of light, heat and moisture they must have for their development and in the kinds of soil on which they thrive best. Each is adapted to its own special environment.
In our modern civilization we find the stream of our own lives flowing from many different sources, each adapted to make its own particular contribution. Why should the citizenof New York or Pennsylvania want to draw on the soils of Texas or Kansas, Wyoming, or other far-off places for the minerals and vitamins that go into the building of his life? The answer is that these soils are more richly suppliedwith the necessary elements. Some of them did not originally exist in these spots but were transported and placed there by a number of different agencies.
The story of their coming to make possible our own stream of life lies deep in the past history of the earth.
There is much that we do not know about the earth's early history, and our knowledge of its interior today is far from definite. But there is good evidence that the surface of the globe was at one time a mass of molten material. Within this molten mass the heavier elements such as iron and nickel sank deep below the surface, the lighter elements floating over them.
Slowly this liquid mass cooled, and as it did the elements that composed it tended to draw together, each to its own kind, to form crystals--silica forming quartz, for example--or they gathered into groups to form more complex minerals, such as mica or hornblende.
Over most of the earth's original crust, this grouping of crystals took the form of granite. Granite ordinarily carries few of the elements needed for life--potassium being one of the few--and is a poor environment for a plant. But this thin crust of granite lay over an uneasy foundation of molten rock. As the hot mass cooled and contracted, great ridges were forced up to form mountains, and in some places huge cracks were opened through which the molten rock from below flowed up to spread in sheets of lava over the lighter outer layers of rock. Smaller cracks were filled with molten material to form veins in the rock. In some cases, vaporized metals from below were cooled and solidified in these veins,and boiling water, carrying elements in solution, welled up from the inner earth, to deposit them in the cracks.
Thus, some parts of the earth's surface became rich with concentrations of rare minerals that were gifts from the deep interior of the earth. In many parts of the world, where hot springs may be seen boiling to the surface, this process is still going on.
These concentrations of minerals, if left undisturbed, would not have supplied a very wide area with the essentials of life. But the forces of nature are constantly at work to break up the rocks and spread them over the earth's surface. Air, water, and sunlight all do their part. Air supplies carbon dioxide. Falling rain absorbs this gas from the air to form carbonic acid. Through the years this acid slowly dissolves the more soluble minerals from the rocks, leaving crevices where water may enter. The sun warms the exposed faces of the rock, causing them to expand and crack, making room for more water. When water freezes, it expands, so in cold weather the cracks are widened, sometimes breaking up huge blocks of rock. Thus the rock begins to disintegrate as soon as it is exposed to the weather until, at last, it is reduced to the small particles that go into the making of soil. Much of this soil material lies in place, where it is born. But a good share is scattered far and wide by other forces of nature.
Wind and flowing water sweep rock fragments from the ridges, drive them against other exposed rocks, grind them to smaller pieces, and sometimes carry them for many miles to drop them at last as sediment in distant valleys or in lake or ocean beds.
Now the water may change from destroyer to builder. As it carries the dissolved minerals in solution, it may deposit some of them among these sediments, thus cementing them into rock again.
Microscopic living organisms absorb minerals from this liquid solution, too, and turn them to solids within their substance. Through the ages their remains have settled on the bottom to form vast deposits of limestone and phosphates.
Later on, through movements of the earth's crust, some of these deposits have been lifted high above the water. Many of our western mountains are capped by rocks holding fossils of shells that were made of lime taken from the waterof an ancient sea. These rocks are now attacked in their turn by the sun and frost and rain, finally to be scattered over the earth by wind and water. In past ages enormous quantities of rock have been ground to dust by the ice of moving glaciers and carried many miles before being added to the material of the soil.
It is estimated that there is scarcely a square mile of the earth's surface that does not contain some ingredient from every other mile because of the action of wind and water. But, despite all this stirring and mixing, the soil usually draws most of its mineral content from the underlying parent rock that produced it. Where this rock is rich in important minerals, as in the case of limestone, the soil can usually provide a rich environment for life. But over rocks like granite or quartzite, which offer little mineral food, the life above is apt to find a much less nourishing environment.
The upheavals and varied contours on the earth's surface spread out their influence to affect our lives in many ways. They are among the important factors innature's water transportation system that makes life possible on the land.
Among its many useful qualities, water has one that is especially valuable to life. When it is warmed above a certain temperature, it turns to vapor and can be carried by the wind. When the air is cooled, some of thisvapor turns back to fine drops of water in the form of clouds and under certain conditions precipitates as rain.
The temperature of the air is constantly changing from day to night and with the seasons. While the sun's rays pass through the air without raising its temperature, they warm the surface of the earth and the oceans noticeably. Air coming in contact with these heated surfaces is then warmed.
The result is that air near the earth's surface is usually much warmer than the air high overhead. This added warmth has two dramatic effects on air. It not only gives it greater power to absorb moisture, but under its influence, air expands and becomes lighter. Light air rises through heavy, cooler air above, while cool air sinks to replace it at the surface. So there is a steady circulation of air, rising from the warmer parts of the earth only to be cooled in the upper atmosphere and returned to earth again.
But the air with its burden of moisture, moves over definite pathways before delivering the water back to earth because as the earth revolves around its axis, the air moves with it, just as the oceans move in unison with the land.
The warmest part of the earth is in the tropics, where the warm earth can give most heat to the surrounding air. So, around the earth at the equator the warmed air expands and rises to float over the cold air above, and then flows out over it north and south toward the poles; while down below, on the surface, the cooler air from north and south flows in toward the equator to fill its place. Thus there is established a vast system of air movement, first toward the equator, then upward and overhead back toward the poles.
As the returning air becomes chilled overhead, most of it drops to earth again before completing the poleward journey. On reaching the earth it will have lost some of its more than 1,000-mile-an-hour eastward motion; but it is still moving a good deal faster than the earth in its new location with the smaller "circumference." Thus, over a wide area of the earth's surface there is created a band of westerly winds, where the air is moving eastward faster than the earth. As this air moves back toward the equator it again reaches the faster moving part of the world, until at last it lags behind the earth's motion and so becomes a northeasterly wind. The world system of air transportation actually has two forces driving it: first, the motion from west to east given it by the spin of the earth, and second, the movement between the equator and the poles, given it by the heat of the sun.
The environment that supports life extends far beyond the vision of experience of the things that live there. It's most important feature may lie in the distant mountain ranges, perhaps a thousand miles away. And events like forestfires, which affect the ability of the mountains to store and control their supply of water, may decide the issue of life or death for the creatures in the lowlands.
The influences of the mountain watersheds extend to farmlands far down the valley of the Mississippi. But here new forces begin to affect the water supply. The streams of air that flowed around the north and south ends of the mountains near the coast have by now begun to draw together again. In the winter the northern air stream sweeps as an icy blast through Canada and then down through the Dakotas. Part of the southern stream flows over the warm Gulf of Mexico, picking up a heavy supply of moisture. When the two streams come together over the central United States, the cold, heavy northern current acts like a barrier mountain range. The warm southern current rises over it, and like the air over the mountains it is chilled to produce rain or snow. Storms of this kind are familiar on every newspaper weather map that shows the meeting of warm and cold fronts. They are an important source of moisture in the Central United States in winter.
In warm weather the local thunderstorm takes its place as an important water producer. It comes chiefly as a result of temperature differences on earth's surface. There may be many causes for these differences. For example, the dark earth of a plowed field will absorb more heat than the surrounding forest, and over this warm field the air will rise. As it goes higher the moisture in the air begins to condense into water droplets, producing the towering cumulous clouds whose contours outline the movements of the rising air. Given the proper combination of heat, moisture and subsequent chilling, the cloud will at last build up to produce a thunderstorm.
The wide variations in rainfall over different parts of the country produce important effects on the quality of the soil. We might expect soil fertility to increase with abundant rainfall. But actually it often deteriorates. We have seen how rain combines with carbon dioxide from the air to become a mild acid that dissolves minerals from the rocks. If the rain is not too heavy these dissolved minerals stay near the surface where the plants can use them for food, and under such conditions the soil may become very fertile. Some of the most fertile soil in the United States, for example, is in the Arizona desert where these mineral solutions have accumulated for ages. With irrigation, a desert may become fabulously productive. But with too much irrigation, some have been ruined; for when the soil is given more water than it can hold, its dissolved minerals are washed away in a process know as leaching. They are carried out of reach of the plant roots. The water that brought out the productive power of the soil can thus ruin it and turn it back again to desert minus its former potentialities.
When the land receives too much rain it tends to rob the soil in the same way, leaching away the dissolved minerals. The dividing line between an annual rainfall that builds and one that leaches comes somewhere between 20 and 30 inches, depending partly on the temperature and the amount of moisture evaporation, and partly, too, on the quality of the soil and its ability to absorb water and to hold it. This ability in its turn depends very largely on the help of plants and animals that bring to the soil new qualities not possessed by the original rock particles.
A good soil is really a combination of three basic ingredients: first, the rock particles that are its foundation; second, the organic matter given it by dead plants and animals; and third, a community of living plant and animal organisms.
Air, rock, water, and sunlight--these are the four sources from which come all living things and their environment. On the bare sands of the desert the sun's rays strike in tiny units of energy moving with atomic speed. Some of them we can feel as heat or see as light. These speeding units impart some of their energy to the dead sands, which temporarily store it in the form of heat, but when the sun sinks, this newly acquired energy is radiated back into space and lost. The sand becomes as cold and dead as ever. But chlorophyll in the leaves of green plants exists as an agent for garnering these units of solar energy. It makes of the green leaf a laboratory in which nature creates food for living creatures and carries on unceasingly the magic of building life.
Like the sand, a field of grass absorbs the sun's rays, but when night comes the grass does not give back this newly gained energy. In its green laboratory the chlorophyll blends the sun's captured radiance together with elements taken from the air, the water, and the soil, and builds these dead materials into organized living form to make new blades of grass.
This grass is cool and quiet, giving no hint of the sunlight stored within its framework. But dry it out and touch a match to it. The blades of grass--these tiny bits of organized gas and sunlight--blaze up with a flame hot enough to kill a man. All of that fierce heat is merely a release of the same energy that the cool, moist plants have been quietly gathering from the sunlight and storing for later use.
If the grass is not burned, the energy will remain stored within its substance. If it is eaten by an animal, its life force is transferred with it into the body of the animal to sustain the spark that we call life.
But, for all its abilities as a life builder, the plant is not entirely self-sufficient. Its principal building material is carbon, which it takes from the air. This carbon, once it is locked up in the plant, becomes useless for other plants unless it is later released into the air again. Throughout the ages that they have existed on earth, plants have locked up enormous quantities of carbon in this manner and stored it in the ground, often in the form ofcoal. If we could picture the thousands of tons of coal that were once used every day throughout the world and could realize that, with all that had been burned, there was enough left to last us for another thousand years, we could appreciate something of the amount of carbon that has been withdrawn from the air. But even this is only a drop compared with the amount that is used by plants and then returned to the air again by the animals and bacteria that eat the plants, decompose them, and release their elements. Withoutthe help of this cycle of use and return, the plants might before now have eliminated all life from the earth by using up all of their basic food.
Some scientific evidence seems to indicate that in the early days of the world there was not enough free oxygen available in the air to support animal life. But there is oxygen locked up in the plant's two chief foods: carbon dioxide and water. From these the plants draw, respectively, carbon and hydrogen, and in the process they release oxygen in a form that animals can use. Through this action of vegetation, the world's supply of free oxygen has been built and maintained.
So, even before our environment could start to support us, it had to be built up by a multitude of interacting forces, including living things. And the hierarchy of life had to start from the beginning with the simplest microscopic single cells that, step by step, prepared the way for higher forms.
There exist dramatic examples of land where man has destroyed all life. In them we can see some of the futile attempts of our common plants to make a fresh start and can visualize something of the slow process by which an environment must build up from its simplest beginnings before it can support life as we know it.
There is a spot in the woodlands of southeastern Tennessee that can never be forgotten by one who has seen it. To reach it, one may travel for a hundred miles through forest-covered hills, rich with laurel, azalea, and rhododendron, and along springs and brooks and ravines that sometimes open up into green meadows where cattle graze.
Suddenly this green world disappears. The forest gives way to a hundred square miles of desert as dead as the Sahara. The rolling hills are cut into rows of low-steep-sided ridges, sterile and bare of any life. The soil is dry, the springs and brooks are gone. In this area the annual rainfall is less than in the surrounding country. The winds are stronger. It is hotter in summer and colder in winter. Here and there on this desert there stand in rows the dead skeletons of small trees, planted by people who hoped to start a new forest.
The soil in the nearby woodland is dark, rich and spongelike. That on the desert is coarse, hard and yellow. This desert was once covered by a forest and by rich forest soil. But afterwards that soil lay five miles down the valley at the bottom of a reservoir, and the shoals of coarse desert soil grew deeper, year by year, as every rain washed its fresh quota down to the reservoir.
This all happened because many years ago, a copper smelter was built here, and the fumes from the smelter killed the surrounding trees, thereby setting in motion a train of events that finally produced the desert. The owners of the smelter had long since learned to control these fumes, which no longer poisoned the air so seriously; but the harm had been done.
After the fumes were controlled, many attempts were made to restore the forest. Desert grasses were planted in the hope of furnishing some green cover to hold the soil in place; for rich soil cannot exist without the help of plants to build and protect it. But with the killing of the forest, the living soil that gave it life had also been killed. There were no roots to hold the soil in place, no litter to absorb the rain, and the grass seeds washed away;the seedling trees withered, and the dead soil continued to wash from the desert and drift down to fill the reservoir.
A root system is a really incredible thing. Many studies have been made of its extent. In one study, a plant of winter rye grass was grown for four months in a box with less than two cubic feet of earth. In that time the plantgrew twenty inches high, with about 51 square feet of surface above the ground. But underground the root system had developed 378 miles of roots and an additional6,000 miles of root hairs! This meant an average growth of three miles of roots and 50 miles of root hairs for each day of the four-month growing season. The growth rate varies with different plants, of course, but this gives us some idea of the activity that goes on under the surface of a quiet- looking meadow, while the grass prepares food that will later become milk and meat and butter for us.
But these growing roots are doing far more than just binding together the rock particles that form the soil. They are taking the first step toward creating an entirely new kind of soil.
A Flemish physician who lived in the 17th century gave an interesting picture of this when he tried growing a willow spout in a tub of earth. For five years nothing was added except rainwater,and the willow grew into a small tree. At the end of the five years the tree was weighed. It had gained more than 164 pounds in weight, while the soil in the tub had lost only two ounces. Actually the soil weight must by now have included millions of microscopic root hairs from the tree, but the figures are accurate enough to show that those 164 pounds of tree must have come from somewhere outside the soil.
If we divide up the plant into the various elements that form its substance, we find that only five percent of its weight comes originally from the soil. The elements in a mature corn plant include carbon, 44.58 percent; oxygen, 43.79 percent; hydrogen, 6.26 percent; nitrogen, 1.43 percent. These all come originally from the air and water, and together they form more than 95 percent of the plant. A good proportion of them comes to the plant through the roots, by way of the soil, after earlier plants have fitted the soil to receive them. the rest of the plant, that which comes from the soil itself, includes potassium, 1.62 percent; calcium, .59%; silicon, 54%; magnesium, .44%; phosphorous, .25 percent; chlorine, .20 percent; sodium, .15 percent; iron, .10 percent, and sulfur, .05 percent. Some plants contain very small amounts of other elements, such as copper, boron, and cobalt. These we call trace elements.
All these elements are built together into a living plant through the agency of the chlorophyll, the green coloring matter that is carried in the leaves. This building process is the essentialfirst step that prepares the way for all the life that exists on earth. Chlorophyll has not yet yielded to man all the secrets, either of its composition or of the magic by which it transforms inert building blocks into living material, but we do know certain basic facts. The essential first step consists of building sugar out of sunlight, carbon dioxide and water. This is called photosynthesis (putting together). To make one molecule of sugar the chlorophyll produces the union of six molecules of water and six of carbon dioxide. With them it binds the energy from the sunlight, and in the process six molecules of free oxygen taken from the water and the carbon dioxide are released into the air.
As the roots spread through the soil, they fill it with this new living substance built from air, sunlight, and water. But this has not yet become a part of the soil. It is not until the plant itself dies that the dramatic change takes place in the soil. For now the dead plant's roots and leaves offer food to the small organisms that are among the most important factors in the whole cycle of life, the bacteria, the molds, and the rest, most of them too small for the eye to see. Their most important function lies in decomposing the remains of the higher plants and animals, changing them into new chemical combinations that can be used again by succeeding plant generations for food.
The decomposing plant attracts a host of small creatures that help to break it up. Earthworms eat it, mixing it with the soil particles that pass through their bodies, digesting the whole, and casting it up on the surface, a revitalized and richer soil. The number of earthworms in the soil depends largely on its chemistry and on the amount of plant material they find in the earth.
Myriads of small creatures spend parts of their lives in the soil: ants, beetles, wasps, spiders, and many others. Some of these come to eat the plants, and many meat eaters come to eat the plant eaters. Among these the shrews and moles play a very important part. In favorable locations there may be as many as 100 shrews to the acre, and each shrew may eat the equivalent of its own weight in other living things each day. All this activity combines to carry on the work of plowing, mixing, and fertilizing as the creatures add their remains to the land.
This hive of living things in the soil, the eaters and the eaten adds up to incredible numbers. The bacteria alone may range from comparatively few up to three or four billion in a single gram of dry soil. At the Rothamsted Experiment Station in England it has been estimated that in good soil the bacterial matter, living and dead, may weigh as much as 5,600 pounds per acre. At the rate of even one billion to a gram of soil the total body surfaceof the bacteria in an acre, if spread out flat, would equal 460 acres.
The fungi may add up to a million in a gram of dry soil, weighing over 1,000 pounds to the acre.
Each of these small living things adds its tiny bit to the building of the living earth until, in the average acre of good topsoil, with four-percent organic matter, there are stored about 80,000 pounds of such organic matter from plants and animals, containing energy from the sunlight equal to that in 20 to 25 tons of anthracite coal.
But while the soil lives, this stored-up energy is constantly being used for food by the teeming life it supports and, as we have seen, it must be constantly renewed by the plants in order to maintain this life. For good soil is actually a living thing, and its health is a matter of life and death to the plants and animals that live on its surface. We ourselves are as dependent on its health as the smallest of its creatures.
As the planet roots and fungi grow into the soil, tying its rock particles together into a firm mass, opening the way for other living things, and filling the earth with organic matter containing packaged energy from the sunlight, a subtle change occurs.
The root tips release carbon dioxide, the source of carbonic acid. This reinforces the action of the rain, helping to dissolve minerals from the soil particles, making them available to the plants for food. As the bacteria decompose the dead plant matter, they too release carbon dioxide and contribute their share toward the enrichment of the soil. The decomposition of animal and vegetable residues by microorganisms produces many other acids besides carbonic acid, including citric, tartaric, oxalic, and malic. Such acids are probably of even greater importance in making minerals available.
In raw, unprepared soil these dissolved minerals might be carried away by very heavy rain, and the supply left for the plants would be rather precarious. But here the bacteria provide another service, for in the decomposition process there are some parts of the dead plants and animals that are more resistant than others. These stay in the soil for a long time, forming a dark, spongy, very absorbent material called humus. Humus stores rainwater, with its dissolved minerals, holding both as a reservoir for plants to draw on. It also stores minerals drawn up from below by the deeper roots.
As the bacteria use up their food supply, billions of them die of hunger or become inactive, and the life process in the soil slows down until further stores of food are added by plants or animals. As in most of nature's activities, this whole life cycle in the soil becomes a self-regulating system--an organized community, adjusting its numbers to the food supply so long as it is undisturbed by outside forces.
On the surface this community may appear to be merely a blanket of dead leaves and litter from last season, but under these lie the decaying remains of their predecessors from earlier seasons. As we go deeper these become mixed with soil particles and with the roots of plants living and dead. Through this material run myriads of passageways left by insects, mammals, and decayed roots. The whole makes a perfect protection for the earth and a sponge to check the runoff of rain, which it absorbs and introduces slowly into the soil reservoir below.
This power of the organized topsoil to store water and minerals is the key to the next step for the developing plant community.
For, while minerals form a very small part of the whole plant-- only one twentieth--that small fraction, together with sunlight, is the key that makes the whole function. Several years ago this was proved in some interesting tests in Kansas. In the western part of that state where there is comparatively low rainfall, the good earth was able to store most of the rain and the dissolved minerals near the surface. But in the eastern part of the state, the heavier rainfall supplied more water than the soil could hold. Here a good share of the mineral solutions was leached away. In a comparative analysis of wheat raised in the eastern and western parts of the state, it was found that grain raised in thedry western part contained nearly 50 percent more protein than the same kind of wheat raised in the eastern part. An important cause of this difference appears to lie in the leaching of the eastern soil which, having lost a share of its dissolved minerals, was unable to give the wheat a proper supply of them.
Obviously the quality of the soil has a great effect on the quality of the plant. And since animals, including man, require roughly the same elements as plants, the health of man and the lower animals must depend on the ability of the soil to supply those needed elements in the right proportions.
From this experiment, it might seem that plants grow best in a land where there is little rain. Actually, most food plants need huge amounts of water. For example, a single corn plant uses about 50 gallons in 100 days of growth. In one test, an acre of corn containing 6,000 plants used 325,000 gallons of water in 100 growing days. This equaled the amount of water it would havetaken to cover the acre 11 inches deep--or 11 acre inches.
To demonstrate the value of water, in an experiment in Utah some corn was raised on dry land without irrigation. It produced 26 bushels per acre. On another plot the corn was irrigated with 15 acre inches of water. It produced 53 bushels to the acre, more than double the other crop.
Hence, to raise a good crop of plants, the land must carry an adequate supply of moisture. It must have the ability to store it without leaching. To do so it must be well supplied with humus built into it by the plants and small living creatures of the soil. It is commonly estimated that nature may normally take as long as 500 years to build an inch of topsoil.
This topsoil is one of the keys to man's existence on earth.
As the tree draws in its raw materials from the air, water, soil, and sunlight, these are carried to the leaves. Here, through the miracle of chlorophyll, they are woven together and transformed into sugar. And, from this sugar, into the vast number of chemical combinations that form the living substance of the tree--into roots and leaves and branches, into flowers with their male and female parts that together produce seeds. In the seeds are stored food and microscopic cells for the production of new life. The cells will later develop, each one producing its own assigned part of a new plant with vitamins, enzymes, and other chemical combinations that interact to make it function as a living thing.
Some soil fungi form a direct partnership with trees and other plants. Beeches and pines apparently cannot make healthy growth unless there exists an active association between their roots and certain kinds of fungi. This partnership--or symbiotic association--is known as mycorrhiza. Its exact function is something of a mystery, but it evidently plays an important part in the transfer of food from the soil to the root system.
One group of plants, the legumes--clovers, beans, locust trees, and other pod bearers--joins forces with bacteria to form a sort of chemical laboratory in the earth. When nature has built up the soil's chemistry to a condition that will support them, these legumes take their place in the plant community. They offer a home in the soil to nitrogen-fixing bacteria that enter their roots and cause them to swell into lumps called nodules, where the bacteria live in colonies of many millions. Taking their energy from the sugar in the plant roots, the bacteria gather nitrogen from the air to form nitrogen compounds, which they store in the nodules.
When the roots die, the nitrogen is left in the soil, and with this enrichment the plant community bursts into full life.
Near Athens, Ohio, a plantation of cedar trees was set out on an area of very poor soil by the U.S. Forest Service, who wanted to test different trees in an effort to start a successful forest. In one part of the cedar plantation they set out among the cedars a number of locust trees (legumes), which carried the nitrogen-fixing bacteria on their roots. Eleven years later the cedar trees, planted alone, stood on the average about 30 inches high, while those among the locusts averaged perhaps seven feet. Between the small cedars the ground carried a thin, sickly cover of low grass, an occasional white poplar, and very few other plants. The soil was dry and offered little food or shelter for living creatures. Here nature might need a century or more to establish a healthy forest.
Under the locust-and-cedar combinations, however, one can step across from a semi-desert into a rich young embryo natural forest. The ground is covered with a lush growth of grasses, weeds, and vines. On its surface a litter of dead leaves and stems has begun to collect from plants of past seasons. Beneath the litter the soil is cool and damp with moisture stored from the rains. Shaded from the strong sunlight, it is a good natural bed for the seeds of forest trees to make their start in. And here are growing seedlings of tulip trees, red oak, red maple, white ash, and others.
Both groups of trees were given an equal start on the same kind of soil, and the entire area has been bombarded every year by millions of seeds of many kinds, brought in by wind and bird and mammal.
Why are the white poplars the only new trees to start among the cedars? Why are there so few poplars under the taller cedar-and- locust combination, while many other trees have started here whose seeds failed to survive out in the open? The answer is that each plant is a specialist, adapted by its own habit of growth and its own special requirements for light and moisture to grow best n its own preferred environment.
Among the cedars only the seedlings of the grasses and the poplars have been able to withstand the drying heat of the strong sunlight. Under the taller, richer, cedar-and-locust growth the white poplars have had to meet the competition of red oak, red maple, white ash, and tulip, whose seedlings thrive in partial shade.
These trees are growing here, not because the ground received any more of their seed, but because the seeds that fell found the added moisture, soil quality, and just the amount of protection from sunlight that they must have to get a secure start in life.
This young forest will now follow the normal steps of forest development; for the seedlings will eventually grow up to overtop the earlier trees that nursed them and finally crowd them out by robbing them of sunlight and the free space they need for growth. And with the early trees will go many of the small plants of the ground surface that also need sunlight.
For many years this slow development will continue, until the crowding newcomers start to battle among themselves. At length towering crowns will cut off the sunlight that their own seedlings must have in order to live. Then they will have destroyed their own power to reproduce. But among the many kinds of seedlings now struggling in the forest shadows, there are some that do thrive under these new conditions. Chief among them are the hemlock, beech, and sugar maple. These will now outgrow the others in their race to reach upward for the sunlight, finally touching their tops against the forest canopy. Here they will await their turn until a windstorm strikes some giant oak or tulip to the ground, making room for one of the newcomers. And at last these will take over to form the enduring climax forest. They are now the dominant trees. Nothing else can compete with them under the conditions they have established.
In some forests, where the soil is shallow, as the great trees fall, one by one, tearing their spreading root systems out of the shallow soil, the roots lift huge balls of earth with them. In this process the ground is plowed up into a series of small ridges and depressions that fill with water after rains or melting snow. The water is held until it sinks slowly in the ground, and thus has reinforced the absorption system already established by humus and insects.
The trees have even made a new climate for all the lives that exist under them. Near Cleveland, Ohio, measurements were taken to compare the climate in a climax forest of this type with that in an open field nearby. It was found that on a bright day there was 750 times more light in the open field than in the forest. The trees slowed down the speed of the wind until, 100 feet inside the forest border, its velocity in summer was only one tenth as much as in the open field. In winter, when the leaves that slowed it had fallen, wind speed increased to one quarter that in the field.
These differences influence the moisture in the forest; for the wind and the sun's heat have a great effect on evaporation. It was found in the Cleveland tests that average moisture evaporation was 55 percent less in the forest than in the field during the summer, and 38 percent less in the winter.
With its blanket of humus and its protection from chilling winds, the forest soil is guarded against freezing in the winter, while the branches shade the snow against rapid melting. The soil is always ready to receive the water from the slowly melting snow, while the surface of the hard-frozen field nearby sheds the snow water like a roof, as it floods off under the warmth of the spring sun.
So, in addition to building timber and providing environment for many kinds of plants and animals, the forest also builds a reservoir to catch and store the huge amounts of water it must have for growth. From this reservoir, springs break out and brooks slowly cut their channels and bring life-giving water to land outside the forest.
Our growing community of plants now provides three essentials needed by other forms of life: shelter, water and a dependable supply of food. Many forms of plant eaters now come to use it: insects, mammals, birds and other creatures.
Here, for example, is a caterpillar busily transforming the organic substance of a leaf into the juices and organs that make up the parts of a caterpillar. Eventually this substance will go through a series of further changes until at last it turns into a flying insect. Then it will repay with interest the damage it has done to the plant, for it becomes a partner in the plant's life process, carrying pollen to fertilize the blossoms. Insects make possible the continued existence of many plants.
Nearly all the fruits and vegetables used by man are directly dependent on this partnership with insects. But this partnership requires the most exacting regulation to meet two fundamental laws of nature on which all life is based.
First, insects, like all living things, must have the power to multiply faster than their normal death rate to insure against the catastrophes of disease and weather. Without this insurance no species could survive.
But this power carries with it tremendous danger; for insects, if allowed to multiply unchecked, would soon destroy all the leaves and kill all the plants that support them. We can see that danger most clearly in the life of soil bacteria, the smallest and simplest of all living things. One of these invisible cells may seem like a very insignificant part of the living community. But watch its numbers grow.
Each individual multiplies by dividing into two complete new ones. Under favorable conditions this may happen about twice each hour. Even if it happened but once each hour, and if each one lived, the offspring from a single individual would number 17,000,000 in 24 hours. By the end of six days the 17,000,000 would have increased to a bulk larger than the earth, and every living thing on earth would have become engaged in a suffocating struggle for food and air and life.
The useful bacteria, when kept in their place make possible the higher forms of life, would have turned into irresistible destroyers. The same principle, of course, applies to all forms of life. The insects that pollinate the blossoms are no exception.
To control them, nature uses a highly organized police force of flesh eaters: bacteria, insects, mammals and birds, each one a specialist designed for a particular role to which it is best adapted.
For example, in this small community we are watching, the larvae of many insects spend parts of their lives in the upper soil. Shrews hunt them under the leaf mold. Other insects and some of the molds and bacteria also feed on them. On the surface many ground-nesting birds, such as the towhee, turn up the leaves to find them. The brown thrasher hunts here too and continues the search among the bushes where it makes its nest. It is joined by the warblers and vireos, which extend theirsearch up to the tree tops.
As the trees grow larger and their lower branches die, the fungi may decompose and soften the wood in the knotholes, offering favorable nesting sites for woodpeckers and nuthatches, whose feet are adapted for hunting on the bark oftree trunks. The woodpeckers go one step further, drilling holes through the bark to catch the insects hidden within.
Some of the insect pupae that survive this search emerge to fly over the tree tops, but here they are met by the swifts and swallows by day, by the nighthawks at dusk, and later by the bats that are equipped with nature's radar systems to hunt in the darkness. Each one of these controlling predators must in turn hold its own place against others that are larger or stronger or more active: hawks, owls, foxes, weasels.
It may seem a haphazard collection of predators and prey. Actually it has evolved into a very highly organized, beautifully controlled form of community government, with its own automatic system of farming, pedigree breeding, sanitation, policing and insurance.
There may be many niches in the forest that give shelter for a nest or a den, but their inhabitants can survive only if the surrounding territory can offer enough food to support life. They can rear their families and multiply onlyif they can find the added food to feed their young, and many young birds and animals require the equivalent of their own weight in meat or insects each day.
As the growing families move out to forage for themselves, they find the best hiding places occupied, the best feeding areas already in use. They are then more exposed to their enemies. The first to tire or weaken in the search for food becomes the easiest prey. In this way nature removes the unlucky and the weaklings, saving in the long average the best, hardiest, or quickest to learn, to live and to carry on their kind. Thustheir population is adjusted to the number that the land can safely support.
Sometimes a predator species may be wiped out for a time by weather or disease. But nature carries insurance against such catastrophes, for the area patrolled by each species will slightly overlap that of its neighbors. For example, if the ground-dwelling towhee disappears, the thrasher, the shrew, and the white-footed mouse all are ready to step in to reap the more abundant food supply and, with more plentiful food, to rear larger families.
This local police organization is backed up by a more mobile patrol force; for the larger predators, requiring more food, must manage to cover more ground than their smaller prey, taking off the season's increase wherever it is easiest to catch. They concentrate on areas of heaviest production.
In this hierarchy of life some creatures may fall prey to smaller predators, the disease germs that attack them from within or the parasites from without: the worms that live in their bodies, for example, or the fleas and lice living on their skins. As a rule, these take only a small share of the daily product of energy from their host, for the body of a healthy animal can usually control such parasites. But when the host is weakened by hunger or age, the parasites may prove fatal. Then the sanitary agents of the community step in to do their share. Vultures eat the dead flesh, bacteria and the larvae of flies feed in it and decompose it. Burying beetles carry its remains underground to enrich the soil, laying the foundation for a new cycle of life on a cleansed surface.
So the living community in the forest grows, the plants drawing in elements from air and water, and energy from the sun, building them into life--the plant eaters staying near their stationary food supply, turning its energy intomeat, passing this on to the larger, more mobile flesh eaters. These in turn pass on their energy to the hierarchy of larger forms, each group in its turn becoming fewer in number as the larger creatures require more small bodies to feed them, with each successive individual forced to cover the larger area of ground needed to raise these many smaller lives. But in the end even the largest succumb to the bacteria and beetles that complete the circle inthe earth.
All these creatures live and flourish completely unaware that they owe their existence to a few hundred million invisible bacteria living in the roots of some locust trees that pioneered the way for them many years ago.
Throughout the ages of its existence the fortunes of the great forest community are constantly fluctuating under the influence of four fundamental forces and a host of lesser ones. The long- range cycles of weather, the cycles of disease and prosperity among member groups, and the resulting changes in the influence of these groups on each other, all play a major part in the existence of a whole community. But the changing seasons of the year have the most obvious effect.
In winter the plants of the northern climates rest from their function of producing food, and all the creatures of the forest must adjust their lives to a reduced food supply. Most of the birds go south. Many animals such as the chipmunks, woodchucks, skunks, snakes, and frogs, and many insects, retire for a long sleep during the winter. Those that remain active must depend on food stored up from the summer's harvest.
The ruffed grouse and cardinal will find it stored in the seeds, fruits, and buds of trees and bushes. The nuthatch finds the eggs of the insects hidden in the bark of trees. Woodpeckers drill through the bark to find the insect larvae that winter beneath it. The red squirrel, flying squirrel, and gray squirrel live on stored seeds and nuts. Perhaps the most active and numerous animals of the forest are the white-footed mice and the shrews, and in them we see a good example of interdependence. The mice live on their stores of seeds and on what insects they can find hidden in winter retreats, while the shrews hunt day and night, digging tunnels through soil and humus and through rotting logs to capture the mice as well as great numbers of insects.
These mammals and birds in their turn offer food to the fox, the weasel, and the barred owl, which of course can exist only in smaller numbers, since for their support they require so many of the lesser creatures.
As the snow melts and the suns of March and April warm the ground, a change comes over the forest. The low plants on the earth's surface come to life. The early insects come out of hiding and many more hatch to take advantage of the new supply of food. The skunk, the chipmunk, and several kinds of snakes, toads, and frogs wake from their winter sleep, and all include in their varied diets great numbers of insects. Now the ground grows bright with spring flowers-spring beauties, yellow adder's-tongue, hepatica, trillium, and many others. The flower buds of the elms and red maples offer food for gray squirrels. Frogs add their music to the notes of birds returning from their winter in the south. For now nature's food factory has begun again to build new life from sunlight, air, and water. The green algae in the pools, the forest plants, shrubs, and trees are all doing their share, offering their stores of energy to the creatures that come to feed on them. And the creatures respond. Insects of different kinds attack every part of every tree and plant--buds and blossoms, leaves and bark and wood. Spiders and predatory insects feed on these plant eaters, and all are in turn preyed on by the larger animals, including snakes and frogs, and by waves of migrating birds that spread up from the South to find nesting sites and hunting grounds to support their hungry young.
Many of these birds stop merely for a rest and a meal and hurry on to find summer homes farther north. Others stay to fill every available niche in the forest and, as the leaves unfold, the branches that yesterday were bare and inhospitable, now offer shelter from weather and predators.
By summer's end, conditions began to change again for our forest community. Much of the tree and plant growth had stopped, providing less appetizing food for insects. Their own time of greatest multiplication had passed. The birds begin to move south again.
By the beginning of October the last of the summer birds have gone, and now the forest is filled by great waves of southward- bound migrant birds coming down from the north, stopping to feed onwild grapes and on the fruits and berries of many trees and bushes. They vary this diet with beetles, grubs, and other insects, which they dig out from under dead leaves.
By early November most of the leaves have fallen and the bare tree tops again admit light to the forest floor. The robins and thrushes continue southward. Bobwhites may come in from the fields to gather beechnuts in the woods.Tracks in the early snows of late November tell of the search for food by squirrels, cottontail rabbits, red fox, white-footed mouse, and short- tailed shrew.
Thus, in every forest the living creatures that make up the community are actually selected by the dominant trees and the lesser plants that determine the environment in which they must live. From this we see that the forest is a great organization made up of many separate and indispensable parts. Some of these parts may appear to be harmful to its life. But in most cases the degree of harm or value will depend on the perfection of the control or balance that the different members achieve among themselves.
Owing to the hazards of climate and disease, this balance is never quite achieved, and its fluctuations play an important part in forest life. And on the degree of its attainment will depend on the amount of life that the land can support, in other words its carrying capacity. It is interesting to note how this principle is applied by nesting birds in the forest where each air selects and defends enough territory to support its family. But this defense is exerted only against members of the same species, while nests of other, non-competing species might be tolerated in the same tree.
There is much still unknown about the proper management of watersheds, and since the conditions on each will vary with soil and slope, climate and vegetation, the management required on them may differ widely. But in every case the first requirement is to protect the soil, which is nature's reservoir for storing water where it falls.
So, we see the river and the lands of its watershed as a great living organism, with its heart in the mountains that supply its life blood. This blood flows out through the streams that form the arteries above ground and below, coming down from a hundred thousand hidden sources--the mountain springs and meadows, the patches or moist woodland with the porous soil beneath them, the shaded snow banks and the afternoon thunderstorms, the flow of every raindrop held back by the delaying stems of grass and flowers, absorbed by bits of rotting wood, filtering into the soil through a million root tunnels and worm holes, delayed, but slowly moving down the hillside through the silt, tobring a steady, even flow of life to the great functioning body of civilization in the valley.
Every action that affects the lands of the watershed has its direct influence on the functioning of the whole organism. The growing leaf that shades the snow to delay its melting is doing its microscopic share to give an even flow of water through the summer. Combining its influence with a hundred billion other leaves, it may determine the success or failure of the harvest in the valley.
We may see and appreciate the devastation caused on the watershed by a carelessly dropped cigarette. But what of the sheep whose hoof compresses a worm hole or destroys the protecting grass rootson the hillside? Nature may repair these tiny bits of damage if they are kept within limits with which she can cope. But when too many sheep combine to lay a hillside bare, they set the stage for rivers filled with siltand for next season's floods and droughts over huge areas of distant lands.
The people who use the watershed hold in their hands the lives and well-being of the millions who depend on it. Every action that affects the health of the watershed becomes a matter of vital concern to those millions and to all the communities of plants and animals that together make up the whole.
And so we see that every community may be divided into four parts: first, its members with their immediate environment; second, the distant and unknown lands that send out their influence by stream and wind, by wing and padded foot to affect the local environment; third, the actions of men whose influence spreads out to affect in some way nearly every community living on the earth; and last, but most important, those influences that mold the minds of men, giving them the incentives to wise or unwise actions; for in the end these lie at the very center of earth's great web of life. To support this web, the soil must be maintained alive and functioning.