Kell L.S., candidate of science, St. Petersburg, Northern Aeration Station
SUMMARY
Herein are considered basic mechanisms of evolution of ecosystems from the viewpoint intensification of homeostasis with environment. Herein distribution of the biomass by trophic levels and ways of passage of biogenic elements by ecological cycles for the basic types of ecosystems is also analyzed.
The property of the living matter is decrease of entropy, which becomes apparent through tendency of evolving ecosystems to intensification of homeostasis with environment, i.e. of the ability to maintain a stable state within the range of changing conditions of environment. The indicator of homeostasis intensification is to the maximum possible content of biomass and number of links in an ecosystem for available energy flow. /Odum, 1983; Pianka, 1978; Stiebayev and oth., 1993./
It is this fact that explains such regularities of evolution of populations, entering into ecosystems, as increase of depth of the substrate utilization and economic coefficient of its consumption. /Piechurkin, 1978; Piechurkin, 1981; Kell, 1989./.
Taking into account the aforementioned, it would seem, that all ecosystems must evolve on the way of detrital and symbiotic alimentary links with increase of dimensions of individuals and duration of their life cycles, i.e. with decrease of specific energy inputs for structural and sustaining metabolism.
However many actually existing climacteric ecosystems do not meet aforementioned requirements. For example, the ecosystem of the open ocean, which has for the most part grazing alimentary links and small dimensions of producers. Other example of the ecosystem with high degree of passage of the net production through the grazing alimentary chains is ecosystem of steppes. /Odum, 1983; Anderson, 1981; Whittaker, 1975; Begon and oth., 1986/.
Let us try to examine these at first glance apparent contradictions. It is known that the state of any ecosystem is controlled by a set of limiting factors - Libich law of minimum. These factors may include quantity of received energy, biogenic elements, humidity, oxygen, temperature, pH and so on. One of the main factors, which limit majority of known ecosystems, is content of biogenic elements.
Let us examine ecosystem of the open ocean. As a rule the main limiting component in this ecosystem is content of biogenic elements. At that, microorganisms by virtue of their ability to the maximum fully use dissolved substances are the main producers in this ecosystem. / Odum, 1983; Humitake Seki, 1982; Pianka, 1978/. Because of high specific expenses for sustaining vital activity, inherent in microorganisms, it is more advantageous energetically for the ecosystem to maintain main quantity of biomass on the second trophic level. To the maximum possible energy transfer from the first to the second trophic level is in this case provided by the maximal primary net productivity of the first trophic level. Thus we have a grazing alimentary chain with consumption of more than 80 percent of the primary net production by zooplankton and with subsequent return of biogenic elements to the ecological cycle in the form of excrements. Moreover in accordance with Heirstone's, Smith's and Slobodkin's theory of natural equilibrium basic alimentary chains of the open ocean ecosystem must contain even number of trophic levels /Odum, 1983; Anderson, 1981/.
Other example of the ecosystem with high degree of passage of the net production through the grazing alimentary chains is ecosystem of steppes. Because of low average annual amount of precipitations, turnover of biogenic elements through the detrital alimentary chains is slowed-down. This fact dictates expediency for the ecosystem to have high share of the grazing alimentary chains. The main primary consumers are ruminants, possessing a remarkable "micro ecosystem"- the rumen, where symbiotic anaerobic microorganisms decompose vegetable material. However, unlike the open ocean ecosystem with microscopic dimensions of producers, in the ecosystem of steppes difference between specific expenses for maintaining vital activity on the first and the second trophic level is not so high. That is why in accordance with law of Lindemann, stating that ecological efficiency of energy and substance transfer over a trophic level does not exceed 10 percent, it is more advantageous for an ecosystem energetically to maintain to the maximum possible amount of biomass on the first trophic level. Thus to the forefront is brought not the maximal primary net productivity as is in case of the open ocean ecosystem, but maintaining maximal biomass on the first trophic level. Hence is presence of major part of biomass underground as well as formation of biotic regulation mechanisms, preventing excessive eating-away of vegetation. As a result the extent of consumption of the primary net production through the grazing alimentary chain drops from 80 percent in the open ocean ecosystem to 20 percent in the ecosystem of steppes / Odum, 1983; Shvidchenko and oth., 2000; Stiebayev and oth., 1993; Begon and oth., 1986; Anderson, 1981/.
High gross productivity have ecosystems with return of biogenic elements to ecological cycle through detrital and symbiotic alimentary chains, e.g. ecosystems of pluvial tropical forest. These ecosystems are distinguished by high value of biomass, being maintained by the unit of the energy flow. This is achieved by low ratio of the structural metabolism on the first trophic level to maintaining metabolism with at the same time low specific expenses for maintaining vital activity. The major part of biomass is amassed on the first trophic level, and transfer of energy to subsequent trophic levels is carried out through detrital and symbiotic alimentary chains as well as through fruit, which provide reproduction of producers. Accordingly, total biomass of consumers is much lower than in the ecosystem of steppes - there are many termites that are detritus consumers of the tropical forests / Odum, 1983; Anderson, 1981; Whittaker, 1975/.
It is necessary to mention ecosystems where a great part plays abiotic factor of return of biogenic elements to the turnover - pyrogenic climacterics. These ecosystems include Californian chapparral and also some ecosystems of forest, steppe and savanna. Sufficiently high productivity of these ecosystems indicates efficiency of such mechanisms of biogenic elements return.
Not in all ecosystems biogenic elements are one of the main limiting factors. For instance, in ecosystems of deserts and polar areas of Earth, where the main limiting factors are correspondingly water and temperature, biogenic elements often do not play limiting part. At that low microbe decay rate in deserts is compensated by rodents, thus providing the grazing path of return of biogenic elements to the ecological cycle / Odum, 1983; Pianka, 1978; Begon and oth., 1986/.
In conclusion of the presented review it should be noted that majority of climacteric ecosystems, in particular aquatic ones, in spite of apparently high degree of limitation by biogenic elements, are not able to maintain their higher concentration over a long period of time at their volley inflow from the outside. In particular, lakes may evolve towards oligotrophic conditions in case of slowing-down of influx of biogenic substances towards the stable state, which is determined by functional characteristics of the ecosystem. At that a part of phosphorus is lost in bottom sediments / Odum, 1983; Whittaker, 1975; Anderson, 1981 /. It is unlikely that this loss of phosphorus is an indication of imperfection of existing ecosystems. More likely that it is manifestation of protective mechanisms with purpose of preservation from action of more destructive limiting factors - e.g. content of dissolved oxygen for aqueous ecosystems.
CONCLUSION
An ecosystem evolves as a single whole in the effort to intensification of homeostasis with environment, at the same time changing environment itself. Homeostasis is achieved owing to evolution in the course of natural selection and functioning of community of organisms as a single whole, consisting of coevolving species of organisms unitized by mutualistic links. The indicator of homeostasis is amount of biomass and number of links in an ecosystem under available energy flow.
Depending on conditions of environment, where ecosystems function, the share of grazing, detrital or symbiotic path of passage of biogenic elements along the ecological cycle, as well as distribution of biomass by trophic levels greatly varies. This fact may serve as a basis when classifying ecosystems.
LIST OF LITERATURE
Anderson G. M., 1981.- Ecology for Environmental Sciences: Biosphere, Ecosystems and Man. Edvard Arnold Ltd.
Begon M., Harper J. L., Townsend C.R., 1986.- Ecology: Individuals, Populations and Communities. Blackwell Scientific Publications, Oxford.
Whittaker R.H., 1975.- Communities and Ecosystems. Macmillan Publishing Co., Inc., New York.
Shvidchenko A.Z., Nilson S., Stolbovoy V.S., Gluck M., Shepachenko D.G., Rojkov V.A. 1990 - Experience of aggregated evaluation of indicators of basic bioproductive process and carbonic budget of ground ecosystems of Russia. Reserves of organic mass. Ecology #6. p.403-410