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.

Ecosystem development as an autogenic process may be defined in terms of the following three parameters (1) it is the orderly process of community changes; these are directional and, therefore, predictable. (2) it results from the modification of the physical environment and population structure by the community. (3) it culminates the establishment of as stablean ecosystem as is biologically possible on the site in question. It is important to emphasize that this kind of ecological change is community controlled; each set of organisms changes the physical substrate and the microclimate ( local conditions of temperature, light, and so on), and species composition and diversity is altered as a result of competitive and other population interactions described in this post. When the site and the community has been modified as much as it can be by biological processes, a steady – state develops at least in theory. Also, in theory energy utilization is optimized in that maximum biomass (or information content) is maintained per init of available energy flow. The species involved, time required, and degree of stability achieved depend on geography, climate, substrate, and other physical factors, but the process of development itself is biological, not physical. That is, the physical environment determines the pattern of change but does not eause it.

In summary, increasing the efficiency of energy atilization so that each unit of structure is maintained with the least possible work can be considered to be ‘’the strategy of ecosystem development.’’ Strong physical forces or surges, as well as large harvests or pollution input from man’s fuel – powered systems, will modify, halt, or abort this developmental course. As we shall see, understanding man’s impact on the developmental process is one of the most important considerations in achieving a resonable working balance between man and nature.

     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.