All of the ecosystems of the earth, together, form the biosphere. The biosphere is that portion of the earth within which life exists. It includes all oceans and freshwater, the lower layers of the atmosphere and the outer skin of the earth’s crush—the rocks and soil of the earth’s surface. Within the biosphere all of the functions and structure described for anyone ecosystem exist. Green plants capture sun energy and combine it with chemical raw materials from soil, water and air. The food they produce supports all animal life, including the decay organisms which return it to the soil for plant use once more.
Man is a part of the biosphere and depends on its continued functioning for his own existence. He can modify or even destroy any one ecosystem. He cannot, however, risk major modifications of the biosphere except at the risk of his own extermination. Thus, the continued production of plant materials, whether wild or cultivated, is the basis for the nutritional support of man as well as all other animals. The continued functioning of green plants is the source of atmospheric oxygen on which man and other animals depend. The continued functioning of the reducer organisms is the means by which the chemicals in human wastes or in the bodies of plants and animals are made available for further use by living things. A breakdown in any of these biospheric systems would imperil human survival.
Man has been able to take major risks in modifying local ecosystems. He cannot afford major risks in dealing with the functioning of the biosphere.