As in the case with short term development, as described earlier in this post, the long term evolution of ecosystems is shaped by the interaction of allogenic geological and climatic changes and autogenic processes resulting from the activities of the living components of the ecosystem. In a broad sense the ‘’strategy’’ of long term evolutionary development is the same as that of short term ecological succession, namely, increased control of, or homeostasis with, the physical environment in the sense of achieving maximum protection from its perturbations.

     Although we may never know exactly how life began on earth, the generally accepted theory is that the first living things were tiny anaerobic (living without free oxygen) heterotrophs that lived on organic matter synthesized by abiotic processes. The first successional development then, may been more like the hay infusion culture model (see that post) than the autotrophic culture model. The atmo phere at the time of the origin of life 3 billion years ago containe nitrogen, hydrogen, carbon dioxide, water vapor, but little or oxygen. It also contained carbon monoxide, chlorine, and hydrogen sulfide in quantities that would be poisonous to much of present day life. The composition of the atmosphere in those early days was largely determined by the gaseous stuff that comes out of volcanos. The geologist would speak of this as ‘’atmospheric formation by crusts outgassing.’’ The earth’s early reducing atmosphere (a term to contral with oxygenic atmosphere) may have been similar to that now found on venus or jupiter. Because of the lack of gaseous oxygen there was no ozone laver, as there is now. Molecular oxygen o₂ acted on b short – wave untraviolet radiation produces ozone, or 0₃ which in turshields out the deadly radiation. Thus, at first, life could existonly inshielded by water or other barrers, but straange to say it was the short waved radiation that is thought to have created a chemical evolution leading to complex organic molecules such as amino acide that became the building blocks of life. This synthesis also provided food for the first organisms.

      For millions of years life apparently remained as only a tiny foot hold, limited in habitat and energy source, in a violent physical world. The big change began with the appearance of the first photosynthetion algae which were able to make food from simple inorganic substances and which released gaseous oxygen as a by product. As the oxygen diffused into the atmosphere, the ozone shield developed and life could then spread to all parts of the globe, and there followed an almost explosive evolution of increasingly complex aerobic organisms. The broad pattern of the evolution of organisms of organisms and the oxygenic atmosphere that make the biosphere absolutely uniqe in our solar system is shown in this post. Over long stretches of time production exceeded respiration (P/R > 1) so oxygen increased and CO₂ decreased. Our fossil fuels were also formed during periods when P exceeded R by a wide margin.

      Incidentally, I can think of no batter way to dramatize man’s dependence on his environment and to become a wise custodian of this frail earth than to recount how our atmosphere came into being, emphasizing, of course, that it was built by microorganisms, not by men. I think the story of our air should be told to every school child every citizen. It is a fascinating drama of living history with enough mystery and potential tragedy to intrigue teacher and pupil alike. It is a subject lends itself to student participation in learning since the possibilities for study projects, artwork, plays, and the like are unlimited. Berkner and marshall have writter both a population account (1966) and a more technical treatise (1964) that provide good reference.

     The  aciples  of ecological ecosystem development are of the greatest importance to mankind. Man must have early succcessional stages as a continuous source of food and other organic products, since he must have a large net primary production to harvest; in the climax community, because production is mostly consumed by respiration (plant and animal), net communitu production in an annual cycle may be zero. On the other hand, the stability of the climax and its ability to buffer and control physical forces (such as water temperature) are desirable characteristics from the viewpoint of the human population. The only way man can have both a productive and a stable environment is to consure that a good mixture of early and mature successional stages are maintained, with interchanges of energy and materials. Excess food produced in young communities helps feed older stages that in return supply regenerated nutrients and help buffer the extreme of weather (storms, floods and so on).

   In the most stable and productive natural situation there is usually such a combination of successional stages for example, in areas such as the inland sea of japan or long island sound, the uoung communities of plankton  feed lder, more stable communities on the rocks and on the bottom (benthic communities) the large biomass structure and diversity of the benthic communities provide not only habitat and shelter for life hisory stages of pelagic forms but also regenrtated nutrients necessary for continued productivity of the plankton. A similar, favorable situation exists in many terrestrial landscapes where productive croplands on the plains are intermingled with diverse forests and orchards on the hills and mountains. The  crop fields are, ecologically speaking, ‘’young nature’’ in that they are maintained as such by the constant labor of the farmer and his machines. The forests represent older, more diverse, and selfsustaining communities that have lower rates of net production but do not require the contant attention of man. It is important that both types of ecosystems be considered together in proper relation. If the forests are destroyed merely for the temporary gain in wood production, water and soul may wash down from the slopes and reduce the peoductivity of the plains. Ruins of civilizations and man made deserts in various parts of the world stand as evidence that man has not been fully aware of his need for protective as well as productive environments. Mature systems have other values to mankind in addition to products; they should not be considered as crops in the sense of wheat or corn. The conservationist speaks of a policy of balancing contradictory needs as ‘’multiple use,’’ but in the past he has found it difficult to translate long term values into monetary units. Consequently, too often the possibilities for immediate economic gain in harvest overrides what later turns out to be a more important value.

    To illustrate thesee difficulties, let us consider the controversy over national forest management policy that received considerable public attention in the early 1970s.for the most part national forests have been managed on a ‘’selective cut’’ basis; that is, selected trees, inclding a portion of mature trees that are no longer growing are removed periodically leaving the stand more or less intact to serve other uses (recreation, soil and water stabilization, and so on) and leaving room for younger trees to grow faster.  As the demand for paper and other wood products became acute threr was pressure for harvesting on a ‘’clear cut and replant’’ cycle, since the yield would then be greater and subsequent rate of net production incrrased. But right after the clear cut the system would be subject to various disorders such as soul erosion and nutrient loss; the cost of taking care of these problems could cancel out the value of extra wood yield. Thus, both plans have advantages and disadvantages. A sensible solution to the dilemma would be to vary the management according to the site capability. Where topography is steep and soil thin, or where the vegetation is botanically unique or of great scenic beauty, a selective cut plan would be best in the long run. Where topography is more level, the soil deep and stable, and the species caplable of repid regrowth, then a clear cut procedure could be a desirable choice.

In essence there are only two basic ways to meet the problem of youth and maturity in the landscape. One would be to maintain intermediate states as naturally occurs in pulse stabilized systems, and the other would be to compartmentalize or ‘’zome’’ the landcape so as to have separate areas primarily managed for production and for protedtion. Both requure that society adopts regional land use plans, an idea whose time is coming.

    Even without external perturbations the climax does not necessarily remain unchanged forever. Observations in very old forest suggest that self destructructive biological changes may be occuring, which, in the individual, we would call aging. Thus, young trees may not be quite replacing the old ones as they die, or regenetation of nitrients may be lagging and the whole metabolism thus slowing down. There is little data at present, but we wonder if communities may not suffer gradual aging after reaching maturity, just as do individual organisms. Storms and disease, of course, could hasten the aging and death of a climax and the start of a new cycle of developmental stage. In fact, a cyclic climax may be a common phenomenon. The californis chaparral vegetation mentioned. This dwarf woodland almost seems to ‘’program itself’’ for periodic destruction by fire. As the community matures, latter and dead wood pile up faster than they can be decomposed during the long, dry summers. Antibiotic chemicals produced by the shrubs also accumulate in the soils and inhibit growth of ground cover. As the communitu becomes more and more combustible, fire sooner or later sweeps through the woodland. Detritus is removed, antibiotics neutralized, and the shrubs and trees killed back down to ground level. A successional development then repeats itself as the woody vegetation resprouts and grows to maturity again. In this way the aging communitu becomes youthful again for a while.

   So far we have emphasized the destabilizing effect of allogenic physical surges. But acte perturabations can also be stabilizing if they come in the form of regular pulses that can be utilized by adapted species as an extra energy subsidy. If fact, a rhythmic, short term perturbation imposed from without can maintain an ecosystem in some intermediate point in the developmental sequence, resulting in, so to speak, a compromise between youth and maturity. What we called ‘’fluctuating water level ecosystems’’. Estuaries, intertidal shores, rice paddies, and florida everglades are held in ahighly productive carly seral stage by daily or seasonal fluctuations in water levels to which the biota are strongly adapted and coupled in terms of life cycles. These pulse stabilized subclimaxes are very important components of the general landcape because the surplus net production that is a property of young systems passes into and helps nourish neighboring systems. This is one reason why ecologists are generally united in recommending that estuaries be preserved and utilized in their more or less natural state.

      In ecological terminology the developmental stages are known as seral  stages, and the final steady state as the climax. The entire gradient of communities that is characteristic of a given site is called a sere. Succession that begins on a sterile area where conditions of existence are not at first favorable as, for example, a newly exposed sand dune or a recent lava flow is termed primary succession. The term secondary succession refers to community development on sites previously occupied by well developed communities, or succession on sites where nutrients and conditions of existence are already favorable, such as abandoned croplands, plowed grasslands, cut over forest, or new ponds. As would be expected, the rate of change is much more rapid, and the time required for the completion of the sere is much shorter, in secondary succession. Finally, it it important to distinguish between what may be called (for lack of better terms) autotrophic succession and heterotrophic succession. The former is the widespread type in nature that begins in a predominantly inorganic environment and is characterized by early and continued dominance by autotrophic organisms. Heterotrophic succession characterized by early dominance by heterotrophs occurs in the special case where the environment is predominantly organic as, for example, in a stream heavily polluted with sewage or, on asmaller scale, in a fallen log. Energy is maximum at the beginning and leclines as succesion occurs unless additional organic matter is importedor until an autotrophic regime takes over. In contrast, energy low does not necessarily decline in the autotrophic type but is usually maintained or increased during succession.