One of the most dramatic and important consequences of biological regulation in the community as a whole is the phenomenon generally known as ecological succession, but better described by the pharse, ecosystem development. When a cultivated field is abandoned in the eastern part of north s, america, for example, the forest that originally occupied the site returns only after a series of temporary communities have preceded it. The successive stages may be entirely different in structure and function from the forest that eventually develops on the site. In fact, we may think of such temporary communities as developmental stages analogous to the life history stages through which many organisms pass before reaching adulthood. Capacity for self development constitutes an important property that distinguishes systems with major biological components fro systems that are primarily physical. Models of ecological systems that fail to include short term developmental and longer term ecolutionary processes will fall short of the mark. In other words, when dealing with ecosystems we must include developmental parameters in addition to parameters derived from physical laws (such as laws of thermodynamics).

     To look at the situation in another way we can say that change with time in ecosystem structure and function results from an interaction of physical farces impinging from without (recall the discussion of the concept of ‘’forcing function’’) and developmental processes generated within the system. For convenience we may speak of a sequence of changes primarily due to the former as allogenic syccession (allo = outside, genic = relating to) and internally generated sequences as outogenic succession (auto = self – propelling ) or outogenic development. As we shall see, allogenic processes dominate some ecosystem and autogenic processes others. But first let us consider autogenic development as a uniqe feature of most ecosystems.

    Between the forests to the south and the arctic ocean and polan icecaps to the north lies a circumpolar band of about 5 million acres of treeless country called the arctic tundra. Smaller, but ecologically similar, regions found above the tree limit on high mountains are called alpine tundras. As is deserts, a master physical factor rules these lands, but it is heat rather than water that is in short supply in terms of biological function. Precipitation is also low, but water as such is not limiting because of the very low evaporaton rate. Thus, we might think of the tundra as an arctic desert, but it can best be described as a wet arctic grassland or a cold marsh that is frozen for a portion of the year. 

      Although the tundras are often known as the ‘’barren grounds’’ and may be expected to have a relatively low biological productivity, a surprisingly large number of species have evolved remarkable adaptations to survive the cold. The thin vegetation mantle is composed of lichens, grasses, and sedges, which are among the hardiest of land plants. During the long daylight (long photoperiod) of the brief summer the primary production rate is remarkably good where topographic conditions are favorable (as in low lying areas). The thousands of shallw ponds, as well as the adjacent arctic ocean, provide additional food to tundra food chanis. There is enough combined aquatic and terrestrial net production, in fact, to support not only thousands of breeding migratory birds and emerging insects during the summer, but also permanent resident mammals that remain active throughout the year. The latter range from large animals such as musk ox, caribou, reindeer, polar bears, wolves, and marine mammals, to lemmings that tunnel about in the vegetation mantle. The large land herbivors are highly migratory, since there is not enough net production in any one local area to support them. Where man tries to ‘’fence in’’ these animals or select for domestication nonmigratory strains, such as domestic reindeer, overgrazing is almost inevitable unless judicious ‘’rotating the pastures’’ is employed to offset the absence of migratory behavior. The dramatic ups and downs in the density of lemmings was discussed on this post. the difference in response to light by plants in the arctic and alpine tundras was mentioned in thispost. Man’s impact on the tundra will increase as he strives to exploit oil and mineral resources from polar regions. Its special fragility need to be recognized when roads and pipelines are built.

      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.