Desert biomes occur in regions with less than 10 in, of annual rainfall, or sometimes in hot regions where there is more rainfall, but unevenly distributed in the annual cycle. Lack of rain in the mid – latitudes is ofter due to stable high pressure zones; deserts in temperate regions often lie in ‘’rain shadows,’’ that is, where high mountains block off moisture from the seas. Two types of north american deserts are a ‘’cool’’ desert in washingtom with sage brush, and a ‘’hot’’ desert in arizona, where creosote bushes and cacti are conspicuous. The characteristic spacing of deesert vegetaiton and the pssibility of birth control mechanisms were discussed in this post. North american deserts are not as extreme as those in other continents, such as the african sahara or the asian gobi. Some seasonal rain can be expected every year in U.S. deserts, but rainless periods in extreme deserts may span years.

      Four very distinctive life forms of plants are adapted to the desert ecosystem: (1) the annuals (such as cheat grass) which avoid drought by growing only when there is adequate moisture. (2) the desert shrub with numerous branches arising from a short basal trunk, and small, thick leaves that may be shed during dry periods; the desert shrub survies by its ability to become dormant before wilting occurs. In the coller deserts, the shrubs develop very deep root systems that tap moisture that remains. Available after the surface completely dries out. In such the leaves and stems may remain green and active throughout the summer. (3) the succulents, such as the cacti of the new world or the euphorbias of the old world, which store water in their tissues. (4) microflora, sich as mosses, lichens, and blue green algae that dormant in the soil but are able to respond quickly to cool or wet periods.

    Animals such as reptioles and some insects are ‘’preadapted’’ to deserts, for their impervious integuments and dry excretions enable them to get along on the small amount of water. Mammals as a group arre poorly adapted to deserts but some few species have become secondarily adapted. A few species of nocturnal rodents, for exampe, that excrete very concentrated urine and do not use ater for temperature regulation, can live in the desert without drinking water. Other animals such as camels must drink periodically but are physiologically adapte to withstand tissue dehydration for periods of time. For more on adaptations of desert animals.

      In the past mankind has developed remarkable culturess, includeing adapted domestic plants and animals for life in or along the edges of deserts. In fact, life in dry regions requires ingenuity and a conservation ethic, two attributes badly needed in more benign regions. Because watr is the dominant limiting factor, the productivity of a given desert region is almost a linear function of rainfall. In the california mohave desert a 100 mm annual rainfalp will result in about 600 kg dry matter/ha while 200 mm will increase net production to about 1000 kg/ha. Where evaporative losses are less in the cooler great basin deserts, a 200 mm rain produces 1500 – 2000 kg/ha.

   Where soils are suitable, irrigation can convert deserts into some of our most productive agricultural land. Whether productivity continues or is only a temporary ‘’bloom’’ depends on how well man is able to stabilize biogeochemical cycles and energy flow at the new increased rates. As the large volume of water passes through the irrigation system, salts may be left behind that will gradually accumulate over the years until they become limiting, unless means of avoiding this difficulty are devised. The water supply itself can fail if the watershed from which it comes is abused. The ruins of old irrigation systems, and civilizations they supported, in the deserts of the old world warn that the desert does not continue to bloom for man unless he understands the laws of the ecosystem and acts accordingly.

    Large, easily recognized terrestrial community units are known as biomes. In a given biome the life form of the climax vegetation is uniform, and is the key to recognition. Thus, the dominant chmax vegetation in the grassland biome is grass. Although the species of dominant grasses will vary in different geographical regions where the grassland biome occurs. Other types of vegetation will be included in the biome, as for example, ‘’weedy’’ seral stages in succession, forest subclimaxes related to local soil and ater condition crops, and other vegetation introduced by man.

    The distribution of six major biomes in relation to temperature and rainfall. If you will check the mean annual temperature and rainfall of your locality you can determine from which biomes you live in, even if you are now sitting in the middle of a city with no climax vegetation anywhere around. Several other biomes, such as chaparral, tropical savanna, thorn shrub, and tropical monsoon forests are related to seasonal distribution of rainfall rather than annual means.

   For the past 6 year most nations of the world have taken part in what is known as the ‘’international biological program’’ involving govermental grant support for interdisciplinary team research and systems modelling of major biomes. Both the ‘’modelling up’’ and the ‘’modelling down’’ approaches, as mentioned, are bing tested in fefforts to develop realistic working model that will help man better inderstand his impact on the matiral biome matrisx in which his civilization is embedded. For a brief rebiw of the inited states program and some of its accomplishments, see hammond (1974). Other natons have, in general, taken a less holistic approach with varying emphasis on such aspects as natural area inventory, impact of grazing animals, secondary production, detailed descriptive analysis of vegetation, environmental health, and preservation of genetic stocks of endangered wild and domestic organisms.

       Fresh water is very very important in our life. Much of what was said about estuaries also applied to freshwater marshes, they tend to be naturally fertile ecosystems. Tidal action, of course, is absent, but periodic fluctuation in water levels resulting from seasonal and annual rainfall variations often accomplishes the same thing in terms of maintaining longe range stability and fertility fires during dry periods consume accumulated organic matter thereby deepening the water holding basins and aiding subsequent aerobic decomposition and release of soluble nutrients, thus increasing the rate of production. In fact, if such events as drawdown and fire do not occur, the build up of sediments and peat (undecayed organic matter), tends to lead to the invasion of terrestrial woody vegetation. Where man controls water levels by dikes in marshes he generally finds that chemical herbicides or mechanical methods have to be used if the area is to continue to exist as a true freshwater marsh ecosystem suif able for ducks and other semiaquatic organisms.

    The general public prejudice against marshes is understandable, since they are sometimes the home of mosquitoes and other disease carriers and pests. Before much was known about the life history and ecology of the arthropods and anails as disease carriers, destroying their habitat (that is, draining the marsh) was about the only solution. Our present knoledge now makes it unnecessary to destroy the ecosystem in order to control undesirable species.

     In addition to producing ducks and fur bearers, marshes are valuable in maintaining water tables in adjacent ecosystems. The florida everglades are an exceptionally large and interesting stretch of freshwater marshes characterized by naturally fluctuating water levels. Complete drainage (even if possible or otherwise desirable) would not only ruin the area as a wildlife paradise but would also be  risky in that salt water might then intrude into the underground water supply needed by the large coastal cities. Likewise, complete stabilization of water levels would also destroy the unique features of the everglades, for reasons given at the beginning of this section.

     Finally, it is significant that rice culture, one of the most productive and dependable of agricultural systems yet devised by man, is actually a type of freshwater marsh ecosystem. The flooding, dranining, and careful rebuilding of the rice paddy each year has much to do with the maintenance of continuous of continuous fertility and high production of the rice plant, which, itself, is a kind of cultivated marsh grass. 

          Riverand Stream Ecosystems is very important in the world , The history of man has often been shaped by the rivers that provide water, transportation, and a means of waste disposal. Although the total surface area of rivers and streams is small compared to that of oceans and land mass, rivers are among the most intensely used by man of natural ecosystems. As in the case of estuaries, the need fo ‘’multiple use’’ (as contrasted to a ‘’single use’’ approach to such ecosystems as cropland) demands that the various areas (water supply, waste disposal, fish production, flood control, and so on) be considered together and not as entirely separate problems.

         From the energetic standpoint rivers and streams are incomplete ecosystems; that is, some portion, often a large protion, of the biological energy flow is based on organic matter imported from adjacent terrestrial ecosystems, or sometimes from adjacent lakes. Although streams are naturally adapted waste treatment systems for degradable wastes (recall our frequent comment about ‘’free sewers’’) almost all of the world’s great rivers are severely over loaded with the residues of man’s civilization. As geographer M. G, Wolman (1971) has concluded, ‘’demands on water resources are increasing at a rate that exceeds the rate of installation of waste treatment facilities.’’ This is another one of those ‘’mismatched rates’’ that are at the heart of man’s troubles with his environment. In all parts of the world man has so extensively dammed, diked, and channelized rivers that it is getting hard to find a truly wild river of any size. It is turning out that some of these manipulations bring only temporary or local benefits at great cost, and create additional problems costing still more money to correct (as in the case of some flood control projects). Accordingly, flood damages that used to be considerednatural disaster’s (and therefore, unavoidable) are more and more proving to be man – made disasters (and, therefore, avoidable). In the future, proposed alterations will have to be subjected to a more thorough cost – benefit analysis than was the case in the past. More about this in the next post.

         The stream ecologist finds it convenient to consider flowing water ecosystems under two subdivisions: (1) streams in which the basin is eroding and the bottom, therefore, is generally firm; and (2) streams in which material is being deposited and, therefore, the bottom is generally composed of soft sediments. In many cases these situations alternate in the same stream, as may be seen in the ‘’rapids’’ and ‘’pools’’ of small streams. Aquatic communities are quite different in the two situations owing to the rather different conditions of existence. The communities of pools resemble those of ponds in that a considerable development of phytoplankton may occur and the species of fish and aquatic insects are the same or similar to those found in ponds and lakes. The life of the hard bottom rapids, however, is composed of more unique and specialized forms, such as the ner spinning caddies (larvae of insects called caddies flies or trichoptera), which constructs a fine silk net that removes food particles from the flowing waters.

        The load of sediment discharged into the oceans by the great rivers of the world tell us something about man’s treatment of the land. The rivers of Asia, the continent with the oldest civilizations and the most intense human pressure on the land, discharge 1500 tons of soil per square mile of land area annually.