Ebook "STABILITY OF TERRESTRIAL ECOSYSTEMS TO PLANT PESTS: AN AXIOMATIC APPROACH"
Gleb I.Vasechko,
Cand.Sc.(Biology)
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Created 20.03.2005
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Gleb Vasechko.
PART I. A LIST OF THE AXIOMS
AXIOM 1. DRIVING FORCES BEHIND PLANT PEST
ABUNDANCE DYNAMICS
According to Ch.Darwin, living beings have an innate ability to unlimited multiplication. Factors of the environmental resistance serve as barrier to realization of this trait. If the environmental resistance gets less, PPs partially and temporary realize their ability to unlimited multiplication. "Temporarily" means that growth of density of PPs and their total number in every ecosystem take place during as a rule less than ten generations in succession. "Partially" means that index of multiplication (a ratio of the brood number, which has reached reproductive maturity, to the number of parents) is always less than potential one, i.e. it is less than a ration of maximal fecundity to the number of parents. The growth of PPs density above a baseline level of a given species makes a possibility of negative effect on its host-plants, and referred to as an outbreak or an epiphytoty. The increase of effectiveness of environment resistance factors leads to decrease of PPs density up to complete extinction of a species in an ecosystem.
The term "environmental resistance" embraces the extremely wide diversity of phenomena. A classification of them and a cognition of them in operation are possible on condition that the usage of the concept of an ecosystem. An analysis of processes in ecosystems allows reveal regularities of the problem under the study. Therefore, we cease to use the term "environmental resistance" and begin to use as a synonym the more exact term "ecosystem stability to PPs" (ESPPs), which implies terrestrial ecosystems.
AXIOM 2. THE NATURE of ECOSYSTEM STABILITY
to PLANT PESTS and its COMPONENTS
ESPPs is proceeded by great many factors of divers character. In every ecosystem existing without human support, it arises and operates a complex of factors, which we define as factors of ecosystem stability to PPs (FESPPs). This complex is definite for a given type of ecosystems and specific climatic conditions. "A factor is one of a number of possible causes that can produce an effect or affect a changed" (Longman and Payne, 1979, p. 64, AJ049). Within a scope of the province under study, the "causes" are forces, which are directed against the capacity of PPs to unrestricted multiplication.
As a phenomenon, ESPPs is defined as the state of an ecosystem, at which PPs inhabiting in it and penetrating from outside do not cause damage to the main component of vegetation (dominants), and therefore do not effect on a character of an ecosystem. The term "damage" means an direct plant mortality or their weakening that contributes to their mortality under an effect of abiotic stressors (drought, severe frost at a lack of a snow cover, etc.), biotic, anthropic (bad cultural practices, air pollution, etc.) ones as well as a decrease of their possibility in competition with other plant species. An ecosystem should be considered as stable one, if it is able to perform its function (with the human position) without protective measures against PPs on the part of people.
As a mechanism, ESPPs is defined as a protective subsystem of an ecosystem, which is turned against hostile biotic agents. It is composed by FESPPs, which operate at presence of the obligatory conditions of their efficacy - prerequisites. A combination of FESPPs and their prerequisites composes a component of ecosystem stability to PPs (CESPPs). Stressing an importance of the prerequisites, we will operate in the following discourse by CESPPs.
CESPPs have the following properties:
- They exert a suppressive effect on PPs. The term "suppressive effect" = "suppression" is used in a broad sense.
- This is a capacity of plants to repel PPs, to kill them or to disturb their development, as well as a capacity to tolerate damage on the part of PPs or to evade the damage. As to the capacity to tolerate and to evade the damage, it implies that other CESPPs assist them to suppress PPs.
- Also, numerous environmental factors serve as CESPPs, which interrupt activity of PPs independently on traits of plants.
- In undisturbed ecosystems, the suppressive effect of CESPPs has a regular character, i.e. it operates obligatory, because their prerequisites are in a proper state.
In accordance to the property "iv" suppressive factors of a force-major character, which are able to cause heavy, but episodic suppression of PPs cannot be CESPPs.
In the ecosystems, which are able to exist without human assistance (biocenoses), it is observed five categories of CESPPs of the first order, namely:
2.1. Plant resistance to PPs.
2.2. Natural enemies.
2.3. Routine weather suppression.
2.4. Periodic (bottle-neck) suppression.
2.5. Effects of crowding - CESPPs entering into operation at crowding of a herbivore or phytopathogen population.
There is a principal difference between the CESPPs 2.1., 2.2., 2.3., 2.4. and, on the other hand, CESPPs 2.5. The former operate in an ecosystem independently on a presence in it a given PP and its density, i.e. they are density-uninduced. The latter (CESPPs 2.5.) enters into operation, when density of a PP reaches a definite level, i.e. they are density-induced. On the base of this difference, 2.1., 2.2., 2,3., and 2.4. are united in an "Extrinsic" class of CESPPs, i.e. outer as to a species of PPs. The category 2.5. belongs to an "Intrinsic" class of CESPPs.
In the ecosystems build up by people or in the ecosystems close to ones of the natural character, which, however, are used by people for his own aims (they will be called "under warship ecosystems" or "articenoses"), it operates in addition the sixth category of CESPPs, - 2.6. "Human control measures." The category 2.6. constitutes a special class of CESPPs -an "Anthropic" class. The category 2.6. includes CESPPs of the Extrinsic class often modified, and the measures invented by people. It includes the following:
2.6.1. is CESPPs of the categories 2.1., 2.2., 2.3., 2.4. adapted by people for own usage. To discriminate 2.6.1. from CESPPs analogous them and operating in biocenoses, their codes will be supplied with the letter "A" that means "artificial, operating in man-made ecosystems - articenoses."
2.6.2. Artificial measures of long-lasting character. These measures have been invented by men. They enter into operation by people, when it is need to protect plants from PPs.
2.6.3. Temporary suppressive measures. Again, they have been invented by people. They can be used for protection against PPs also in biocenoses at a decrease of their CESPPs. Usually, they are used in response on increased density of PPs. Depending on circumstances, they are put into operation at Low, Intermediate densities of PPs with the aim to preclude its growth to the value High. They might be used as a prophylaxy, i. e. without taking into account density of PPs., if an increase of the density to threshold of damage, i.e. to the values High, is probable.
Usually, great many plant species are present in ecosystems. The concept of ESPPs concerns the main species - dominants. The term dominant "Describes the major plant species in a community which, by its population and extent, determines the type and abundance of other species in the same community" (Longman and Payne, 1979, p. 333, GA176). The participation of plant species, which are not dominants, effect significantly on state of ESPPs. Therefore, these (satellite) species will be taken into account.
In the European literatures on phytocenology, in particular in Russian one, for the above definition is applied for the term "edificator" (èdificateur) proposed by J. Braun-Blanquet and J. Pavillard (1922, 1928, cited in E.M. Lavrenko, 1947). The term "dominant" is used for the plants, which dominate in every stage of a phytocenosis.
For definition of a totality of individuals of a given PP inhabiting an ecosystem, it will be used the term "a population" in the simplest definition, namely: a population is "The number of plants or animals living in a given area" (Longman and Payne, 1979, p. 325, GA011). "A given area" is an ecosystem.
In the Russian literature, this term has the wider sense, namely: a population is "a totality of organisms within a species with common gene pool, which inhabit an area over the large number of generations" (Reimers, 1991, p. 343). It is difficult to operate with this meaning in ecology, because there are no criteria for judgement that in an aggregate of organisms, they indeed have a common gene pool, rather that over the large number of generations.
Nevertheless, the idea to take into account the differences of populations within a species on hereditary level is valuable for understanding of ESPPs. Therefore, beside with the term "species", it will be used the term "taxon" ("taxa" in plural). The usage of the term "taxon" implies that in a given situation interrelations between PPs and host-plants are considered on the base not only a species, but also of such systematic categories in PPs as subspecies, ecotype, races (biotypes), pathotypes, and in host-plants - species, subspecies, phenological forms, varieties, clones, hybrids, breeding lines. Thus, in the following text, the term "taxon" will imply that in the considered situation, it can be important both a species and subspecies taxa. Recognition of the role of a taxon is extremely important for understanding of the interrelations among organisms, which embraced by the term "plant resistance to PPs", because closely relative taxa within a species might be very diverse in their ability to damage their host-plants.
The differences among populations of insect herbivores in behavior traits are important also from the view of ESPPs. To describe these differences, it will be used the term an "ecotype." "An ecotype consists of organisms that have physiological or morphological characteristics adapting them to their habitats" (Longman and Payne, 1979, p. 143, CA025). It this text, an ecotype is a population, which differs from the rest of them within a species by some traits, particularly of the behavior character having no, however, the status of a subspecies.
When it is insignificant to take into account the taxonomic nicety, it will be used the term "a population", which means a part of a taxon inhabiting a given ecosystem.
It will be used the term "a guild." "A guild is a group of species (regardless of taxonomic relationship) having similar resource requirements and foraging strategies and, therefore, having similar roles in the community (Lincoln et al. 1982)" (cited in R.N. Coulson et al., 1986). It is known the diverse guilds within leaf-eating defoliators, needle-eating defoliators, stem borers, sap-sucking arthropods, etc. The criterion of belonging of species to a definite guild is a presence of them on the same tissues within host-plants and a competition for food at an increase of their density. The members of a guild occupy the same ecological niche.
An activity of CESPPs determines density of PPs. The density is considered as a probability of damage of host-plant tissues taking into account an amount of a PPs species on these tissues. The density is characterized by five categories namely: Zero, Insignificant, Low, Intermediate, and High. In insect herbivores
- Zero. Complete absence of a PPs species; the damage is out of question.
- Insignificant. Signs of affection by a PPs species are occasional; the damage is negligible.
- Low. Signs of affection are well visible, but it is below of threshold of damage.
- Intermediate. It is probable decrease of increment, yield or quality of production, and, if physiological state of host-plants before the affection has been weakened, it is possible mortality of them.
- High. Loss of vitality of affected tissues; host-plants are endangered by mortality or loss on increment, serious decrease of yield and quality of production.
The categories Zero and Insignificant densities seems to be obvious. As to the rest, the suggestions should be done concerning of concrete PPs species. As an example of rather clear issue, it might be used defoliators of deciduous trees. In them, Insignificant density is a damage of a few percents of leaf surface, Low - up to one third, Intermediate - up to two thirds, High - above two thirds.
In sap-sucking arthropods, the equal number of individuals per unit of host-plant tissue exerts a very different negative effect depending on Tolerance of a host-plant taxon. Therefore, judgement as to category of their density should be done on the base of the effect on vitality of a taxon of host-plants.
If the density gets High, beside with the danger for dominants, it enters into operation the special class of CESPPs - 2.5. "Effects of crowding." In so doing, PPs themselves are able to determine ESPPs.
In interactions of phytopathogens with their host-plant, there exist the phenomena of advanced Tolerance up to symbiosis, so that values of signs of infection on host-plant tissues do not characterize negative effect these PPs. Viruses often affect plants without any external sings of a disease. Affection of plants by phytopathogens can be subdivided in five categories - Zero, Insignificant, Low, Intermediate and High on condition that study on their effect on vitality of host-plants.
The above categories of CESPPs are subdivided on a number of subcategories of subsequent orders. Thus, the classification of CESPPs is built on the hierarchical principle. This classification is shown in the Tables 1, 2, 3, 4, and 5.
THE EXTRINSIC CLASS.
COMPONENTS of ECOSYSTEM STABILITY to PLANT PESTS of the CATEGORY
2.1. PLANT RESISTANCE to PLANT PESTS
Plant resistance to PPs is concerned to host-plants of a given taxon of PPs, rather than nonhost-plants. Host-plants of a taxon of PPs are those, which have been recorded as being consumed by this PP in natural or experimental conditions having a possibility to make a choice. Taking into account a choice within various plant species needs, because, when it is absent, herbivores are forced to consume plant species outside of the range of their host-plants. Nevertheless, the term "Plant resistance to PPs" should spread on some plant taxa, which are avoided by a given species of PPs. This is relevant, when these species are close relatives of preferred ones, and there are grounds to suppose that the trait of complete resistance has been developed as a response on potent selective pressure on the part of this PP.
CESPPs 2.1. includes the subcategories 2.1.1. and 2.1.2. (Table 1). CESPPs 2.1.1. operates on the level of individual plants. According to R.H.Painter (1951), there are four subcategories:
2.1.1.1. Nonpreference (antixenosis).
2.1.1.2. Antibiosis.
2.1.1.3. Tolerance.
2.1.1.4. Evasion.
Prerequisites of CESPPs 2.1.1. operating on the level of a individual plant (they are marked by the letter "P") are the following:
2.1.1.P.1.(A.2.1.1.P.1.) The hereditary ability of a host-plant taxon (species, subspecies, phenological forms, varieties, strains, hybrids, lines) to resist against a given taxon of PPs.
2.1.1.P.2. (A.2.1.1.P.2.). The proper level of physiological state of a host-plant.
The prerequisite 2.1.1.P.2. (A.2.1.1.P.2.) is necessary for normal expression of protective traits of a host-plant.
CESPPs 2.1.2. operate on the level of a plant community, i.e. they put into operation at presence of interactions within plants in a community. Here, there are five subcategories:
2.1.2.1. Superevasion.
2.1.2.2. Disappearance.
2.1.2.3. Supertolerance.
2.1.2.4. Heterogeneity of plant populations.
2.1.2.5. Antibiotical parasitism.
Prerequisites of CESPPs 2.1.2. operating on the level of a plant community (they are marked by the letter "P") are the following:
2.1.2.P.1. In biocenoses, forming of a dominant stock at the natural conditions.
A.2.1.2.P.2.
In articenoses, a resemblance of a dominant stock to that in biocenoses or special cultural practices with the aim of effective operation of CESPPs A.2.1.2.2.1.1. CESPPs operating on the level of an individual plant
2.1.1.1. Nonpreference to PPs embraces plant traits of physical and chemical nature, which provide divers stimuli (visual, tactile, olphactory, hustatory, and unknown nature) putting obstacles in a way of herbivores to start feeding by tissues of these plants or to oviposite on them; as to phytopathogens, the stimuli force their agents to avoid plants.
2.1.1.1.1. Nonpreference to herbivores is the presence of repellents or absence of attractive stimuli, and includes the following subcategories:
2.1.1.1.1.1. Visual or tactile.
2.1.1.1.1.2. Olphactoral or hustatoral.
2.1.1.1.1.3. Of unknown nature.
2.1.1.1.2. Nonpreference to phytopathogens is composed by unknown stimuli, which induce negative response of zoospores to some taxa of a host-plant species.
2.1.1.2.1. Antibiosis to herbivores. It is composed by diverse traits of host-plants directed to destruction, pushing out of attacking herbivores, decrease of their fecundity, retard of their development. The difference between Nonpreference and Antibiosis consists in the fact that the former forces herbivores to have no contact with a plant or leave it with no damage for both sides, whereas the latter leads to mortality or suppression of attacking herbivores, some damage is probable in attacked plants. CESPPs 2.1.1.1.2. includes the following subcategories:
2.1.1.2.1.1. Structural.
2.1.1.2.1.2. Physiological (biochemical).
2.1.1.2.1.3. Symbiosis with fungi toxic for herbivores.
2.1.1.2.1.4. Symbiosis with natural enemies of herbivore.
2.1.1.2.1.5. Nutrient inadequacy of host-plant tissues.
2.1.1.2.1.1. includes one subcategory of a permanent pattern (CESPPs 2.1.1.2.1.1.1.) and embraces such structures as a solid cover of a tissue, pubescence, thorns, etc. They provide protection against herbivores.
2.1.1.2.1.2. is subdivided on three subcategories:
2.1.1.2.1.2.1. Permanent.
2.1.1.2.1.2.2. Delayed.
2.1.1.2.1.2.3. Induced.
2.1.1.2.1.2.1. is a bulk of protective substances (toxic and/or sticky for herbivores), which present in special containers (cells, blisters, systems of ducts piercing all the tissues). They put into operation, when herbivores penetrate into a tissue and break up walls of these containers.
2.1.1.2.1.2.2. is a protective response of a host-plant with a delay. The cause of this delay consists in the fact that in a moment of attack by herbivores, this plant stays in weakened state. Restoration of plant healthy under effect of favorable weather allows such a plant to put in operation a conversion of its nutrient stock into protective substances. Isolation of attacking herbivores by the substances is able to kill pests or eject them.
2.1.1.2.1.2.3. is a necrosis of tissues surrounding of arthropod herbivores penetrated into plant tissues. In a result, such herbivores die due to starvation.
2.1.1.2.1.3. is a phenomenon of inhabiting on a plant surface fungi, which produce toxic media for herbivores.
2.1.1.2.1.4. embraces host-plant structures, which are used by natural enemies of herbivores for feeding, paring or nesting. In turn, the natural enemies protect these plants against herbivores.
2.1.1.2.1.4.1. is a providing of parasites of herbivores by feeding and paring.
2.1.1.2.1.4.2. is a producing of structures for nesting of predators or promotion to them at building of their nests.
2.1.1.2.1.5. is a low content of nutrients or a lack of necessary components of nutrients. This CESPPs impedes growth of density of herbivores due to retarding of their development and reducing of their fecundity. CESPPs 2.1.1.2.1.5. is subdivided on the following subcategories:
2.1.1.2.1.5.1. Permanent nutrient inadequacy.
2.1.1.2.1.5.2. Induced nutrient inadequacy.
2.1.1.2.2. Antibiosis to phytopathogens. It is composed by traits of host-plant directed on destruction of pathogen's agents at inoculation. It includes the following subcategories:
2.1.1.2.2.1. Structural.
2.1.1.2.2.2. Physiological (biochemical).
2.1.1.2.2.1. with the only subcategory - permanent (CESPPs 2.1.1.2.2.1.1.) is provided by structural barriers, which do not allow pathogen agents to penetrate into host-plant tissues.
2.1.1.2.2.2. is composed by toxic for pathogens substances, which present in plants continually (CESPPs 2.1.1.2.2.1.1. Permanent) or are produced in a response of an inoculation (CESPPs 2.1.1.2.2.2.2. Induced).
2.1.1.3. Tolerance to PPs. This is a diversity of traits, which allow to plants to coexist with herbivores and phytopathogens.
2.1.1.3.1. Tolerance to herbivores includes the following subcategories:
2.1.1.3.1.1. Deactivation of arthropod digestive juices.
2.1.1.3.1.2. Repair or compensation of losses of host-plant tissues.
2.1.1.3.1.3. Indifference to host-plant tissue losses.
2.1.1.3.1.4. Tolerance to gall-forming arthropods.
2.1.1.3.1.1. is an ability of plants to neutralize digestive ferments injected by sap-sucking arthropods, which have a digestion of an outer type (aphids, psyllids, mites). Therefore, the excretions are not toxic for them in contrast to the plants, which have no such a trait.
2.1.1.3.1.2. is a possessing of a store of nutritive substances and an ability to produce structures, which need for restoring of the tissues consumed by herbivores.
2.1.1.3.1.3. is a capacity of plants to keep normal vital activity and productivity in spite of a part of their tissues has been consumed by herbivores. This is possible, when the tissues under a threat are not of vitally importance, and a mass of these tissues is redundant. The latter means that a shortage of environmental resources (nutrients, moisture, light) limits a possibility to use these tissues for further plant development - growth, yielding.
2.1.1.3.1.4. is an ability to keep vitality in spite of presence of galls - degenerative tissues induced by activity of herbivores.
2.1.1.3.2. Tolerance to phytopathogens includes the following subcategories:
2.1.1.3.2.1. Limited compatibility.
2.1.1.3.2.2. Symbiosis with former parasites.
2.1.1.3.2.3. Demarcation of affected tissues.
2.1.1.3.2.4. Indifference to losses of host-plant tissues.
2.1.1.3.2.5. Permissible susceptibility.
2.1.1.3.2.2. is subdivided on the three subcategories as follow:
2.1.1.3.2.2.1 Compatibility.
2.1.1.3.2.2.2. Mutualism.
2.1.1.3.2.3.3. Compatibility and cooperation with natural enemies of parasites.
2.1.1.3.2.1. Limited compatibility is capacity of host-plants to minimize a negative effect of phytopathogens.
2.1.1.3.2.2.1. is a co-existence of plants with avirulent pathotypes of pathogens, which protect such plants against virulent pathotypes of the same species and against the wide range of other phytopathogen species.
2.1.1.3.2.2.2. is a subcategory, which differs from CESPPs 2.1.1.3.2.2.1. by so close co-existence that both partners are impossible to exist separately.
2.1.1.3.2.2.3. is the situation of CESPPs 2.1.1.3.2.2.1. enforced by activity of natural enemies of phytopathogens, often with nitrogen fixating species.
2.1.1.3.2.3. is a building of barriers - lines of demarcation, which protect vital tissues from spreading of a pathogen.
2.1.1.3.2.4. is the lack of response of a phytopathogen in the situations, when host-plant tissues are insignificant for vital processes.
2.1.1.3.2.5. is capacity of plant to keep vitality in spite of parasitization of a phytopathogen; this type interrelation is characteristic for the early stage of interrelations of plants and their parasites.
2.1.1.4. Evasion from PPs. It consists in a lack of coincide of attacking stage of herbivores or active spores of phytopathogens with a vulnerable stage of host-plants. If PPs get into activity, when a vulnerable stage of their host-plants have not appeared, they suffer mortality due to starvation. In such cases, Antibiosis of Structural (Permanent) subcategory (CESPPs 2.1.1.2.1.1.1.) operates as a cooperator of CESPPs 2.1.1.4. When the getting into activity is too late, PPs undergoes a risk to die due to inadequate state of food or impossibility to find shelter from their natural enemies. Hence, in such cases, the cooperators of CESPPs 2.1.1.4. are Antibiosis again CESPPs 2.1.1.2.1.1.1. and of physiological (biochemical ) subcategory (CESPPs 2.1.1.2.1.1.2.), as well as the category of activity of natural enemies (CESPPs 2.2.1.). It is important an effect of CESPPs 2.1.1.4., which consists in a decrease of the period of damage. This helps to operate CESPPs 2.1.1.3.1.2. "Tolerance to herbivores, Repair or compensation of losses of host-plant tissues." The obligatory cooperator of CESPPs 2.1.1.4. is CESPPs 2.4. Periodic (bottle-neck) suppression of PPs.
2.1.1.4. includes the following subcategories:
2.1.1.4.1. Evasion from herbivores.
2.1.1.4.2. Evasion from phytopathogens.
2.1.2. CESPPs operating on the level of a plant community
This category includes the following subcategories:
2.1.2.1. Superevasion.
2.1.2.1.1. Superevasion from herbivores.
2.1.2.1.2. Superevasion from phytopathogens.
2.1.2.2. Disappearance.
2.1.2.2.1. Disappearance from herbivores.
2.1.2.3. Supertolerance.
2.1.2.3.1. Supertolerance to herbivores.
2.1.2.3.2. Supertolerance to phytopathogens.
2.1.2.4. Heterogeneity of dominants in an ecosystem.
2.1.1.5. Parasitic Nonpreference and Antibiosis.
2.1.2.1. is the same as CESPPs 2.1.1.4., but it is effective as a means of self-protection of plants on condition that nearly all the dominants of an ecosystem have this trait.
2.1.2.2. is a periodicity of an amount of seed yield. In years of low seed yielding, the volume of food resource gets very small, so that density of herbivores decreases due to starvation or heavy competition for scarce food. In a result, in years with high seed yielding, herbivores occurred to be in Low density and consume a less part of a seed stock.
2.1.2.3. is a necessity for existence of ecosystems a mortality of part of their plant stock on diverse stages of plant development. An amount of seed production is redundant in every plant species. This amount exceeds greatly possibilities of a brood to survive, because resources of habitats are limited. Therefore, mortality even an overwhelming part of a seed stock and young plants (in seedling stage) due to consumption by PPs does not exert a negative effect on existing of a plant species. In further stages of plant development, when competition among them for vital resources takes place, a part of plant stock is condemned to die, because demands for the resources grow with increase of age of plant stock. This part composed by weakened plants is selected and consumed by PPs that provides the rest of plant stock with the resources due to recycling of nutrients.
2.1.2.4. consists in diverse subcategories of Nonpreference (CESPPs 2.1.1.1.) and Antibiosis (CESPPs 2.1.1.2.) operating in individual plants within a stock of dominants. Such a diversity increases the rate of mortality of PPs, when they search for food. CESPPs 2.1.2.4. includes two subcategories:
2.1.2.4.1. Hereditary heterogeneity of a host-plant species.
2.1.2.4.2. Heterogeneity of a species composition of a stock of dominants.
2.1.2.5. is a phenomenon, when a plant species gets shelter in a sphere of operation of Nonpreference (CESPPs 2.1.1.1.) and Antibiosis (CESPPs 2.1.1.2.) pertinent to other plant species and having well-expressed self-protection on the part these CESPPs.
