A more general and complete summary of important changes in community structure and function in the sere, as revealed by the study of the large, open systems of nature, is showing this post....

Expected trends in the gradient from youth to maturity are grouped under several headings. Although ecologists have studied succession in many parts of the world most of the emphasis to date has been on the descriptive aspects such as the qualitative changes in species structure. Only recently have the functional aspects of succession also been considered. Consequently, some of the items listed in this post must be considered hypothetical in the sense that they are based on good experimental or theoretical evidence, but have not been verified by adequate data from the field. Five aspects seem most significant require a bit more explanation as follows.

The kinds of plants and animals that change continuously with succession. Those species that are important in the pioneer stages are not likely to be important in the climax. When the density of species in a sere is plotted against time, a characteristic stair – step graph is obtained, as illustrated in this post. Such a pattern usually is apparent whether we are considering a specific taxonomic group, such as birds, or a trophic group, such as herbivores or producers. Typically, some species in the gradient have wider tolerances or niche preferences than others and, therefore, persist over a longer period of time. Thus, in the terrestrial succession pictured in thispost pine trees and cardinals persist throush longer periods of time than do most of the other species. In general, the more species in the group (whether taxonomic or ecological) that are geographically available for colonization, the more restricted will be the occurrence of each species in the time sequence. This kind of regulatory adjustment is the result of competition coexistence interactions discussed in the next post.

Biomass and the standing crop of organic matter increase with succession. In both aquatic and terrestrial environments the total amount of living matter (biomass) and decomposing organic materials tend to increase with time. Also, many soluble substances accumulate; these include sugars, amino acids, and many organic products of microbial decomposition. These liquid products that leak out from the bodies of organisms are often collectively known as extra metabolites. Some of these substances provide food for microorganisms, and perhaps also for microorganisms. Other   substances are equally important in that they may act as inhibitors (antibiotics) or as growth promoters (as, for example, vitamins), sub stances produced by one organism may inhibit the further growth of that species (thus providing population self regulation, or they may act on completely different species. This was dramatically drought to our attention by the discovery of penicillin and other bacterial antibiotics produced by fungi. In other cases, increasing organic matter stimulates the growth of bacteria that manufacture vitamin B_12, a necessary growth promoter for many animals (many are unable to manufacture this and other vitamins themselves). Where extra metabolites do prove to be regulatory, we would be justified in calling these substances environmental hormones since by definition a hormone is a ‘’chemical regulator.’’ Chemical regulation is one way of achieving community stability as the climax is approached, because the physical as well as the chemical perturbations (as, for example, light and water relations) are buffered by a large organic structure. There is no question that the increase in amount of and the change in organic structure are two of the main factors bringing about the change in species during ecological development.

The diversity of species tends to increase with succession. Initially this is the case, although it is not clear from the present data that  the change in variety of taxa follows the same pattern in all ecosystems. Increase in diversity of heterotrophs is especially striking; the variety of microorganisms and heterotrophic plants and animals is likely to be much greater in the later stages of succession than in the early stages. Maximum diversity of autotrophs in many ecosystems seems to be reached earlier in succession. The interplay of opposite trend makes it difficult to generalize in regard to diversity. The increase in size of individual organisms and the increase in competition tend to reduce diversity, while the increase in organic structure and variety of niches tends to increase it. As we have already pointed in the discussion of diversity in this post there may be an optimum level of diversity for a given energy flow pattern. We can state that, in general, rapid growth seral stages  will tend to have a low diversity on the order of 0.1 or 0.2 on the scale used in this post while mature stages will tend to have a higher level on the order of 0.7 or 0.8 unless there is a large energy subsidy that counteracts this patter.

A decrease in net community production and a corresponding increase in community respiration are two of the most striking and important trends in succession. These changes in community metabolisms are shown graphically in this post which compares ecosystem development in a small laboratory microcosm and in a large natural forest. Total production (P_G ) increases faster than energy expenditure (P) at first, so a large net production (P_N ) results in a rapid increase in biomass (B). gradually, equilibrium is established, in about 100 days in the microcosm and 100 or more years in the forest. Perhaps the best way to picture this overall trend is as follows; species, biomass, and the P/R ratio continue to change long after the maximum gross primary production possible for the site has been achieved. As one evidence for this we may cite the situation in regard to leaves in a terrestrial broadleaved succession. Agricultural scientists have repeatedly found that maximum productivity of broad leaved crops occurs when the leaf surface area exposed to the incoming light from above is about 4 or 5 times the surface area of the ground. Any increase in leaves beyond this level does not increase the photosynthetic rate per square mater, since increased shading cancels any advantage that might accrue from increased photosynthetic tissue. In fact, the increased respiration of the extra leaves that do not receive adequate light may reduce the net production of the crop. In a forest the leaf area apparently continues to increase far beyond that limit experimentally shown to increase gross production, since leaf area per ground is often 10 or more in an old forest. Since forests are among, the most successful of ecosystems with a long geological history of survival, we may well consider the possibility that the extra leaves have other important functions in the ecosystem in addition to production of food. The undoubtedly help moderature and moisture and provide reserves that are important during periods of climatic stress or insect of climatic stress or insect or disease attack.  

     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.

     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.