52 MAN ON THE LANDSCAPE 



poses become at least temporarily extinct on limited areas where 

 man's inept hand has been at work. For instance, erosion induced 

 by man has so sorely affected millions of acres of lands that they 

 will not support domestic crops at all; and in many places where 

 hardwood forests formerly grew in grandeur, only lichens, mosses, 

 weeds, wild blackberries, etc., are found today. Once-rich grass- 

 lands have given way to weeds and scrub, or have been turned into 

 deserts of bedrock, subsoil, or sand dunes. 



Sea and Land Forms of Life. While it is possible that life may 

 have originated on land, evidence favors the seas as its birthplace, cer- 

 tainly as its early home. Water under natural conditions offers a 

 more stable environment and forces less adjustment on its inhabitants. 

 Water temperature changes with weather at about one-fourth the rate 

 at which land heats or cools. The dispersion of mineral salts (washed 

 from the land) in water is more uniform than the occurrence of good 

 topsoil. Sunlight penetrates to a greater depth and with a uniform 

 gradation of intensity. The seas reached a condition early in geologic 

 time which would support life, and have not changed greatly since. 

 Thus life tends to remain at a comparatively low level of organization, 

 though expressed in a great variety of forms. The species which have 

 developed have found their places according to light, pressure, tem- 

 perature, oxygen, carbon dioxide, and food supply variations. (These 

 are ecological factors which largely determine the biotic complex.) 



Fundamentally, life in water is not different from life on land. 

 Chlorophyll and photosynthesis are its basis. Mineral, vitamin, and 

 protein requirements of life demand the presence of soil elements in 

 the water. Plants do not necessarily need roots, only a method of 

 absorbing water and minerals. Aquatic plants must have carbon 

 dioxide the same as their land cousins, and aquatic animals must have 

 oxygen. These gases are not only released by plants and animals 

 themselves, but the overturning action of waves is constantly trapping 

 air. Cool water absorbs oxygen from air, while warm water loses it. 

 Largely for this reason, our greatest fisheries are found in the cooler 

 waters of the earth. 



The upper layer of water, both salt and fresh, receives the greatest 

 force of sun energy and thus sustains and activates the greatest 

 amount of chlorophyll, which may be in broad-leaved floating or 

 anchored plants, or in microscopic green forms such as certain of the 

 algae. On microscopic plants feed microscopic animals; the combina- 

 tion of these two is called plankton and is the watery range on which 

 larger aquatic animal forms graze. Commercially and recreationally 

 valuable fish, mollusca, crustaceans, and sponges are thus dependent 

 en plants and sunlight. Whatever the food needs of these animals, 

 however complex the food chain, the terminal link with the earth is 

 plants. 



In passing, we should mention that man cuts his own throat by 

 interfering with the fundamental factors. His erosional debris mud- 

 dies the waters, shutting off the sunlight. His industrial, mining, and 

 municipal wastes either poison aquatic life directly 3 make the waters 



