Ecology. Stability of ecosystems.

Ebook "STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH"
Gleb I.Vasechko,
Cand.Sc.(Biology)

© Gleb Vasechko, 2005 All rights reserved.
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Created 20.03.2005
Last modification 15.06.2021

PART I. A LIST OF THE AXIOMS. Continuation.

AXIOM 7. EVOLVING of STRUCTURE of ECOSYSTEM STABILITY to PLANT PESTS and PREREQUISITES of its PERFECTION

There exist two types of ecosystems with quite different life strategies, namely: the natural ecosystems (biocenoses) and the under warship ecosystems (articenoses). The first ones arise in a result on natural selection, and their life strategy consists in survivorship and reproduction. To reach this aim is possible on condition that at the process of natural selection, the ecosystems gain the general stability, i.e. the ability to withstand to all the potentially destructive factors of the environment, including PPs. Obviously ESPPs is a part of the general stability of an ecosystem.

Thus, as all the biological phenomena, ESPPs is a result of natural selection. It evolves simultaneously with development an ecosystem beginning with initial events of this process. To reach the perfect SES, i.e. the level ESPPs 3.1. "Proper control" for all the taxa of PPs, which are able to consume its dominants and the rest of vegetation, an ecosystem should undergoes natural selection on the part of its diverse types. Unlimited operation of all the diversity makes up the prerequisites of perfection of ESPPs.

In this report, the term "natural selection" is used in the wider sense than that at the conventional usage. As an example of the later, it serves the following definition: "RA003 natural selection (n.) The principal mechanism of evolutionary change" (Goodman and Payne, 1979, p. 558). Let it be natural selection is a driving force behind evolving ESPPs, when an ecosystem exists without human interference. Knowledge of these regularities is vitally important for good management by ecosystems.

Thus, the prerequisites of perfection of SES are evolving an ecosystem under effect the following types of natural selection:

7.1. Darwinian or evolutionary.

7.2. Local (due to abiotic factors and interspecific competition).

7.3. Hereditary.

7.4. Physiological.

7.5. Through-generations selection.

7.6. Extrinsic (selection on the part of other ecosystems).

7.1. The Darwinian natural selection supplies ecosystems with plant taxa, which have the traits operating as CESPPs 2.1. "Plant resistance to PPs", and taxa of organisms, which adapted to consume PPs - their natural enemies. They are CESPPs 2.2. "Natural enemies."

7.2. The local (due to abiotic and interspecific competition) selection retains only those plant taxa, which are adapted to inhabit in given climatic and soil conditions. It implies that seeds of any plant taxon existing on the Earth can penetrate into any habitat. If a complex of local environmental conditions is unacceptable for a plant taxon, its seeds or plants die under impact of abiotic factors or due to competition on the part of plants of other taxa, for which local conditions are more favorable. These events take place not only in initial stages of evolving an ecosystem before modification of local environment under the effect of vegetation, but later on because seeds of diverse species continually penetrate into ecosystems.

7.3. The hereditary selection rises in a result of heterogeneity of hereditary traits within a plant taxon. Individuals being less adapted to environment both abiotic and biotic die out. This selection begins to operate, when plants enter into competition for vital resources (nutrients and moisture in the soil, light), and continues over all the stages of evolving of an ecosystem.

7.4. The physiological selection. Being identical as to their hereditary traits, plants of any taxon within a habitat have unequal physiological state. This type of selection begins to operate with a germination of seeds. This is so because seeds have had varied properties (store of nutrients, inner infection etc.) or seeds have got in conditions of unequal value. Further, because environment within an ecosystem offers diversity of conditions (competition on the part other plants, insolation, etc.), plants from earliest time of their life span are subjected by very different effects that determine the heterogeneity of their physiological state. This gives material for the selection of the type 7.4., which maintains in the main stock dominants physiological state as a prerequisite of efficacy of CESPPs 2.1. "Plant resistance to PPs" that is necessary for maintenance of the level ESPPs 3.1."Proper control."

In forest ecosystems, mortality of trees due to operation of the type 7.4. is called the annual stem fall. This term is used concerning the period from the age when the trees begin to compete within a stand (10 or 20 years depending on a tree species) to the end of their life span. Normal values of the annual stem fall were found out empirically. In undisturbed ecosystems, they are ranged from a few percents of trees in young stands to fractions of a percent in old stands. The value of the annual stem fall serves as an index of prosperity of forest ecosystems. If this value for dominants exceeds the normal level, this is an evidence of weakening of the main stock of dominants.

7.5. Through-generations selection.

This selection concerns a change of generations in dominants. It includes two subtypes:

7.5.1. Mild change of the generations,

7.5.2. Catastrophic change of the generations.

7.5.1. includes:

7.5.1.1. Change of taxa of dominants (the succession).

7.5.1.2. Change of an old generation of dominants by new one of the same taxon.

7.5.1. Operates in the habitats with the unlimited effective depth of the soil where soil conditions do not preclude to dominants to exist up to their natural longevity. Nevertheless, they can be forced out by other taxa according to regularities of the succession. The change takes place at a course of the succession until the process reaches the last stage - the climax. This is a situation of the subtype 7.5.1.1. On the stage of the climax, composition of the dominants stays continual over a row of their generations. This is a situation of the subtype 7.5.1.2.

7.5.1.1. At evolving of an ecosystem, its environment undergoes changes under effect of vegetation. This is a result of ever-increasing richness of the soil and establishing of new microclimate. In turn, it changes composition and structure of dominants and the rest of vegetation. This is a content of the phenomenon of ecosystem succession. In so doing, it changes a composition of CESPPs and taxa of PPs, as well as it appears new prerequisites of activity of natural enemies of PPs. For example, it appears new alternative hosts, sources of imaginal feeding, shelters, such as forest litter, and specific microclimate. Therefore, a complex of natural enemies gets different as well as their activity. Thus, all the SESes gets new ones.

Because the subtype 7.5.1.1. operates due to gradual enrichment of environment within an ecosystem in a course of its evolving, the level ESPPs 3.1. "Proper control" is not disturbed, and a change of dominants proceeds over decades, when the number of annually dying trees does not exceeds the normal annual stem fall. When the subtype 7.5.1.1.enters into operation, it appears a new driving force (the change of the environment within an ecosystem) behind the development of an ecosystem. Then, a benefit in the inevitable competition gains other tree taxa. At operation of natural selection of the subtype 7.5.1., in the annual stem fall, it prevails the coming off dominants.

The subtype 7.5.1.2. This category embraces the cases, where ecosystems are subverted by impacts, which counteract to processes pertinent to the subtype 7.5.1.1. "Change of taxa of dominants (the succession)", so that the succession does not take place. The impacts are of diverse character depending on local environmental conditions.

The subtype 7.5.2. The aged crash of forest is defined as a mortality of dominating trees (4.1.a. "The main stock dominants") at exhausting of vital resources in the soil. The mortality spreads on all the dominants or most part of them, at last on a significant part of them. The aged crash of forest should be considered as a sudden stopping of the ecosystem succession in specific environmental conditions after that it begins again. The conditions are characterized by the shallow effective depth of the soil. This means a presence closely to its surface a layer, which is impenetrable for root systems of dominants. They are the habitats with the following features:

1. Rocky or permafrost subsoil common in mountain and boreal areas.
2. The soils of the podzol type where hardpan layer lies on the depth one-two feet.
3. The sandy soils with hard streaks.
4. The poorly drained soils where the water table stays on the depth less than a meter.
5. Presence in the soil the continually dry stratum impenetrable for roots of dominants.

When the dominants growing in the above conditions reach a definite age, the vital resources in the upper soil layer, confined their roots, occur on the limit. An onset of a stressor in this time exerts potent destructive effect on physiologically state of the dominants. Among such stressors, there are droughts, severe winters (low air temperatures with poor snow cover), potent winds especially at heavy rains. On the same site, the aged crash of forest returns over a number of decades. The duration of intervals between recurrent the crash is diverse, and determined by local conditions. Approximately, the intervals are more 20 years and less than 100 years.

Independently to soil conditions the aged crash of forest takes place in oak stands of the coppice origin beginning the age of the trees 60 years. This is a result of mortality of the main deep penetrating root due to cutting of stems.

The aged crash of forest always is accompanied by affection of such trees by insect herbivores (needle-eating and/or stem borers), which consume the weakened or killed trees. At a presence of 4b."Regrowth of dominants", it takes place a change of dominants by moving into 4a. of trees of the same species. In others cases, a composition of dominants is changed.

In the period between the aged crashes of forest, it takes place the natural selection of the subtype 7.5.1., i.e. competition among the stem stock results in the annual stem fall. When the subtype 7.5.1. operates, ESPPs is maintained on the level 3.1. "Proper control". If an ecosystem undergoes the aged crash of forest, its ESPPs drops to the level 3.3. "Late control."

7.6. Extrinsic selection. It occurred to be situations when external environmental conditions are changed to such extend that an ecosystem becomes inappropriate for new circumstances, and it suffers a radical modification. Physiological state of its dominants becomes weakened, i.e. their self-protection by CESPPs 2.1. "Plant resistance to PPs" has been lost. Former dominants undergo a decline, then it rises new ones. Causes of the changes concern climatic events, perturbations of water regime of a terrain. In a result, it takes place a change of character of an ecosystem in the direction of better adaptation to new-appeared environmental conditions.

Thus, in nature, that means the lack of human interference, the diversity of natural selection takes part in processes in ecosystems and provides them with all the CESPPs, which need for ESPPs to stay on the level 3.1. "Proper control." Quite another situation arises in the under warship ecosystems (agricultural, silvicultural, recreational, etc.), and in ecosystems of natural origin, but used by people for own aims. They get out of impact of natural selection. In a result, their SESes are defective, and the dominants suffer on the part of PPs. To protect the dominants, people use diverse control measures or reconstruct such ecosystems. Both these measures are related to CESPPs 2.6. "Human control measures." Therefore, it is the grounds to point out on the principal difference between the ecosystems under operation of natural selection, on the one hand, and those under human impact, on the other hand. Further, to stress the difference, it should give them the special names - ecosystems with perfect SES - "biocenoses", and "articenoses", i.e. ecosystems, which are able to exist on condition that it takes place the assistance by people.

The best forest articenoses become close as to their general stability to biocenoses, so that the formers need not the human assistance.

The post-Darwinian kinds of natural selection (7.2 - 7.6) allow us to point out the driving forces behind conversion of Chaos in Order. Providing the ecological succession, they produce a chain, every link of which is a result of a change in participation of organisms under effect of the selection, so that more and more ones adequate for progressive conditions dominate in ecosystems. It takes place increasing of richness of ecosystems and decreasing of entropy in them.