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 3. The LEVELS of ECOSYSTEM STABILITY to PLANT PESTS and the CATEGORIES of BIOMASS of DOMINANTS

There are great many factors, which act destructively on ESPPs by means of breach of efficacy of CESPPs suppressing both FESPPs and prerequisites of them. Therefore, one may find ecosystems at diverse degrees of ESPPs - from absolute ones, where outbreaks and epiphytoties of PPs are unknown, to destroying ones, where dominants undergo complete mortality. In the latter situations, ecosystems change their character.

The biocenoses, which are protected from PPs by CESPPs of the Extrinsic (2.1., 2.2., 2.3., and 2.4.) and Intrinsic (2.5.) classes, are classified into three categories as to efficacy of CESPPs operating in them, namely:

3.1. The upper level. This category will be referred to as "Proper control."

3.2. The moderate level - "Lag control."

3.3. The lowest level - "Late control."

In articenoses, the protective role of the above classes of CESPPs is less, and in addition here it operates CESPPs of the Antropic class 2.6. "Human control measures." The levels of ESPPs in articenoses are analogous to those in biocenoses. To differ them, the levels in articenoses are indexed by the letter "A."

The characteristics of the levels of ESPPs in biocenoses and articenoses are set forth below.

Biocenoses
3.1. Proper control

The efficacy of CESPPs is so high that abundance of PPs inhabiting these ecosystems year after year does not exceeds the threshold of damage for dominants. This is so because prerequisites of CESPPs are very potent. Therefore, any stressor with exception of destructive human impacts or natural hazards are unable to decrease operation of FESPPs. At invasion into these ecosystems numerous individuals of PPs, their abundance decreases so soon that they unable to inflict a serious damage. The damage is possible, if amount of invaders is very large. The further risks of any ecosystem are penetration of exotic taxa of PPs, and sudden increase of aggressiveness of resident species of PPs. Nevertheless, such disturbances are rare events.

The level ESPPs 3.1."Proper control" is characteristic for the W.C. Cook's zones (b), (c), and (d), where climatic conditions allow CESPPs 2.3. "Routine weather suppression" in cooperation with other categories of CESPPs to keep a given taxon of PPs on the level below threshold of damage for dominants.

On the other hand, at the level ESPPs 3.1."Proper control", there exist obstaclesfor extinction of a taxon in such ecosystems. This is characteristic, however, for the W.C. Cook's zones (a), (b), and (c), where a taxon of PPs, it exists year after year. Contrary, in the W.C. Cook's zone (d), the effect of CESPPs 2.3. is so potent, that a resident population of a taxon cannot to exist continually. A taxon occurs in the zone (d) due to migration from other W.C. Cook's zones.

At the level ESPPs 3.1. "Proper control" the abundance of PPs is maintained on the level, at which they play exclusively an useful role in an ecosystem serving as a means of turnover substances, stimulation of nitrogen fixation, promoting of natural enemies of PPs by providing them with a food.

The useful role of PPs is possible in a result of natural distribution of the whole biomass of dominants on three categories according to the intensity of their self-protection against PPs, namely: the first (D - I), the second (D - II), the third (D - III).

The levels of ESPPs and distribution of biomass of dominants on the categories D - I, D - II, and D - III are pertinent for ecosystems of all the types. The best expression of their characteristics takes place in woody (forest) ecosystems, where differentiation of plant tissues is greatest.

In the Table 6, it is shown the structure of levels of ESPPs in forest biocenoses of evergreen coniferous and deciduous species.

D - I embraces the vitally important plant parts, whose loss due to consumption by PPs leads to mortality of plant organisms. These are conductive tissues in stems and crowns of woody species, buds, matured needles of ever-green coniferous species, excluding ones, which stay in heavy shadow, matured roots (of the first and the second order), seeds in environmental conditions with high activity of seed-consuming herbivores. All these plant parts possess a weak ability to regenerate after damage by PPs. The self-protection of D - I is realized by well-expressed operation of CESPPs of the categories 2.1.1.1. "Nonpreference" and 2.1.1.2. "Antibiosis." These CESPPs ensure a negligible consumption of D - I by arthropod herbivores with chewing mouthparts and phytopathogens independently on activity of other CESPPs.

As to sap-sucking arthropods, which are able to evade from factors of Antibiosis in host-plants, in the conditions of high activity of CESPPs 2.2.1. "Natural enemies of invertebrate herbivores", it is possible an operation of CESPPs 2.1.1.3.1. "Tolerance to herbivores", although with certain limitations. In particular, buds are well protected against sap-sucking arthropods.

D - II is represented by the part of biomass of dominants, which can be lost in large degree without danger of mortality or weakening of plant organisms. These plant parts are foliage of deciduous woody plants, young needle and twigs, tillers of grassy plants, reproductive structures of plants (staminate "flowers", ovaries of fruits), seeds in seasons of abundant yielding, young roots (rootlets). Plant saps are easy to be regenerated. Therefore, plants are able to tolerate high numbers of sap-sucking arthropods per unit of tissue surface on condition that the plants have trait of detoxication of arthropod saliva.

The self-protection of such parts is realized by CESPPs of the categories 2.1.1.3.1. "Tolerance to herbivores", 2.1.1.4.1. "Evasion from herbivores", 2.1.2.1.1. "Superevasion from herbivores", and 2.1.2.3.1. "Supertolerance to herbivores." They are effective on condition that a cooperation with divers CESPPs of the categories 2.2., 2.3., 2.4. Combination of the above CESPPs depends on traits of guilds of the defoliators.

As to phytopathogens, on biomass of the category D - II, it operates widely CESPPs 2.1.1.3.2. "Tolerance to phytopathogens" and CESPPs 2.1.1.4.2. "Evasion from phytopathogens."

D - III includes worthless (waste) parts of biomass of dominants, which can be lost (consumed by PPs) without any damage for prosperity of dominants. Moreover, the soon consumption of D - III is favorable for dominants, because this leads to a release of nutrients from this biomass and ensures recycling of them. D - III is the annual stem fall in forest ecosystems (a low percentage of a stock of dominants, which annually dies due to old age and competition), the stem fall in grassy ecosystems (a decline of stems due to competition), seeds in seasons of low yielding, needles in shadow and senescent needles of evergreen coniferous plants. On D - III, it dominates the category of CESPPs - 2.5. "Effects of crowding." Therefore D - III is exposed to complete consumption by PPs and saprophagous organisms.

Density of herbivores characterizing by terms "Zero", "Negligible", "Low", "Intermediate", "High" is distributed by definite patterns along the levels of ESPPs and the categories of biomass of dominants.

On the level ESPPs 3.1. "Proper control", there exists density of PPs optimal for general stability of ecosystems, as follows.

On D - I, density of PPs is "Zero" or "Negligible." On D - II, density of PPs can be "Low" or "Intermediate." On D - III, density of PPs is often "High." Effect of phytopathogens is "Zero" or "Negligible" on D - I, "Low" or "Intermediate" on D - II, and "High" on D - III.

The patterns of distribution of density of PPs are determined by CESPPs operating on the categories of biomass. The well-expressed CESPPs 2.1.1.1. "Nonpreference" and 2.1.1.2. "Antibiosis" on D - I determine to lowest values of the density. On stems and young needles, it is "Zero" as to chewing herbivores and "Negligible" as to sap-sucking ones.

Advanced CESPPs 2.1.1.3.1."Tolerance to herbivores" operating on D - II, particularly on foliage of deciduous trees and the larch, Larix spp. allows density of defoliators to reach the levels "Low" or "Intermediate." Here, a further growth of the density is impeded by 2.2.1. "Natural enemies of invertebrate herbivores" and 2.1.1.4.1. "Evasion from herbivores."

In D - III, where operation of CESPPs of the Extrinsic class is low of insignificant, it takes place the highest density of PPs.

Such a distribution of herbivore density and affection by phytopathogens along the categories of biomass of dominants ensures both the ESPPs of the level 3.1. "Proper control", and continual inhabiting of PPs in ecosystems. The latter is necessary for existence of ecosystems being a driving force of turnover of substances.

In forest ecosystems, PPs of the groups of stem insect borers and stem affecting phytopathogens serve as a means, which optimizes a structure of a stem stock. They attack trees that undergo weakening, firstly, due to competition within the stock for nutrients and moisture. The competition rises, because ever-increasing demand of trees for the soil resources with growth of a stand. Further, above-mentioned groups of PPs attack the trees weakened due to their senility. In the optimal conditions of growth of forest ecosystems, i.e. on the level 3.1. "Proper control", there exists a contrast character of tree weakening.

The stem-inhabiting PPs (stem borers) cull the trees of least vitality, so that the value of declined trees is uniform year after year, and the annual stem fall reaches no more a few percents of the whole stem stock. In a result of the culling, the rest of stem gets space for spreading of roots and crowns obtaining nutrients from decomposing biomass of declined trees and necessary moisture. The soon the decomposition runs, the better prospects for dominants. Thus, the stem-inhabiting PPs (stem borers) bring an important contribution in maintenance of ESPPs, and the general stability of ecosystems.

On the level ESPPs 3.1. "Proper control", dominants being well protected against PPs allow to survive a great many species of herbivores and phytopathogens. Here, there exists a wide diversity on niches, where these organisms find food and shelter over an unlimited period. In D - III, the annual stem fall is a place of existence (a special term - the "vitation") of numerous species of consumers and decomposers, an active zone of the biodiversity. In D - II, where operation of CESPPs 2.1.1.3.1. and 2.1.1.4.1. allow PPs to reach Low and Intermediate density, the abundant complex of PPs species stays annually. Even in D - I, it is common annually presence of sap-sucking arthropods - insects and mites. Because their host-plants have the traits of CESPPs 2.1.1.3.1. "Tolerance to herbivores" to these herbivores, they reach the noticeable number per unit of host tissue.

3.2. Lag control

On this level of ESPPs, it occurs a decrease of efficacy of CESPPs the categories 2.1., 2.2., 2.3., and 2.4. under impact of diverse stressors. These disturbances lead to growth of PPs density to the category "Intermediate" that exceeds threshold of damage for dominants. At this level, it is common a decrease of increment of biomass of dominants, and it is possible growth of the annual stem fall. The level 3.2. "Lag control" is common on D - II. Rarely, it occurs on D - I. At ceasing of action of stressors, activity of CESPPs are restored, and an ecosystem returns on the level of ESPPs of 3.1. "Proper control."

In the ecosystems with dominants of evergreen coniferous tree species, on the biomass of the category D - I (stems, branches, needle on the light), on the level ESPPs 3.2. "Lag control", it operates CESPPs 2.1.1.2.1.2.2. "Antibiosis to herbivores, Physiological (biochemical), Delayed."

In the ecosystems with dominants of deciduous tree species, on the biomass of the category D - I (stems, branches), it operates the same CESPPs - 2.1.1.2.1.2.2.

On the biomass of the category D - II (foliage of deciduous tree species), on the level of ESPPs 3.2. "Lag control" in the spring-summer guild, it takes place restoring of activity of CESPPs 2.2.1. "Natural enemies of invertebrate herbivores" especially in a cooperation with CESPPs 2.3. "Routine weather suppression." The role of CESPPs 2.1.1.3.1.2."Tolerance to herbivores, Repair or compensation of host-plant tissues" is significant.

In the early-spring guild of defoliators, it takes place restoring of activity of CESPPs 2.1.1.4.1. "Evasion from herbivores" in cooperation with CESPPs 2.1.1.3.1.2. Tolerance to herbivores, Repair or compensation of host-plant tissues."

Due to activity of the above CESPPs, ESPPs returns from the level 3.2. "Lag control" to the level ESPPs 3.1. "Proper control."

3.3. Late control

This level of ESPPs takes place, when the activity of CESPPs 2.1., 2.2., 2.3., and 2.4. has been too suppressed to be restored. In so doing, PPs density reaches the category "High", when CESPPs 2.5. "Effects of crowding" enters into operation. The damage of dominants inflicted by PPs reaches the higher values.

The affection of D - I, when PPs consume tree stems or needles of coniferous tree species, particularly if the damage of needles is repeated over several seasons in succession, results in significant mortality of dominants, and change of a character of an ecosystem. A degree of the damage of dominants depends on an intensity of stressors.

The affection of D - II, when PPs consume foliage of deciduous tree species, continued over several seasons in succession results in decrease of increment of biomass of dominants and mortality of them, which can be heavy. The degree of the damage is depended on expression of CESPPs 2.1.1.3.1.2. "Tolerance to herbivores, Repair or compensation of losses of host-plant tissues", and duration of affection by PPs. In turn, the expression of CESPPs 2.1.1.3.1.2. is determined by activity of the prerequisites of this CESPPs. Heavy mortality of dominants is possible, when physiological state of dominants is very bad, i.e. the prerequisite 2.1.1.P.2 (A.2.1.1.P.2.) are weak.

On the level ESPPs 3.3."Late control", it occurs the after-effect of High density of herbivores, when they being in a great abundance obtain an ability to overcome CESPPs 2.1.1.1.1.3., 2.1.1.2.1.2.1., 2.1.1.2.2., and 2.1.1.3.1.2. of healthy plants. This is particularly peculiar for stem borers and phytopathogens. Nevertheless, the affection of healthy host-plants provokes an operation of potent CESPPs 2.5."Effects of crowding" that results in suppression of density of PPs.

After that, if the level ESPPs 3.3. is limited by biomass of the category D - II, the affected ecosystems return on the level ESPPs 3.1."Proper control", although it is possible mortality of a part of their dominants. If the level 3.3. spreads on biomass of the category D - I, it is possible complete mortality of the dominants.

Articenoses
A.3.1. Proper control

On this level, it operates the same CESPPs of the Extrinsic class (2.1., 2.2., 2.3., and 2.4.) as those in biocenoses or the CESPPs practicing by people. The latter are embraced by CESPPs 2.6.1. "Promotion to ecosystem stability by means of exploiting of the ecosystem stability components of the categories 2.1., 2.2., 2.3., 2.4. per se or with modifications", or 2.6.2. "Artificial measures of long-lasting character."

A.3.2. Lag control

On this level, the categories of CESPPs of the Extrinsic class are not possible to keep articenoses to the level A.3.1."Proper control", and it is a forecast to turn ESPPs to the level A.3.3."Late control." Therefore, it needs to apply CESPPs 2.6.3. "Temporary suppressive measures."

In articenoses similar to biocenoses (in particular forest ones) with advanced CESPPs 2.1.1.3.1.2."Tolerance to herbivores, Repair or compensation of host-plant tissues", it is possible to go without CESPPs 2.6.3. "Temporary suppressive measures."

A. 3.3. Late control

On this level, CESPPs 2.5. "Effects of crowding" enter into operation. Because they operate, when density of herbivores or affection by phytopathogens reach the value "High", damage of dominants exceeds the threshold. Therefore, it takes place decrease of yield or loss of it, and loss or decrease of production and quality in crops, so that CESPPs 2.6.3. "Temporary suppressive measures" are necessary to preclude a decrease of ESPPs on the level A.3.3. "Late control."

Table 6. Structure of the levels of ecosystem stability to plant pests (ESPPs) of some forest biocenoses