What is equilibrium in a community? Ecosystem Balance

1. What is the trophic structure of a community?

Answer. The trophic structure of a community is an ecological indicator of food relationships in it. Any community can be represented as a food network, that is, a diagram of all food, or trophic (from the Greek tropho - nutrition), relationships between the species of this community. A food web (its weaves can be very complex) usually consists of several food chains, each of which is a separate channel through which both matter and energy are transferred. A simple example of a food chain is given by the following sequence: vegetation - insect feeding on vegetation - population of predatory insect - insectivorous bird - bird of prey. In this chain, there is a unidirectional flow of matter and energy from one group of organisms to another.

2. What environmental factors do you know?

Answer. Ecological factors are individual elements of the environment that interact with organisms.

There are abiotic, biotic and anthropogenic factors. Abiotic factors: light, temperature, humidity and other climate components, air composition, soil and others, that is. elements of inanimate nature.

Biotic factors: living bodies, or organisms, all kinds of interactions between them (phytogenic, zoogenic).

Anthropogenic factors: deforestation, drainage of swamps, construction of a dam, release of various chemicals into the atmosphere, etc. (i.e., human activity).

Questions after § 86

1. What is succession?

Answer. Communities are constantly changing. Their species composition, the number of certain groups of organisms, trophic structure, productivity and all other indicators change. Communities change over time.

The natural and consistent process of change of communities in a certain area, caused by the interaction of living organisms with each other and the abiotic environment surrounding them, is called succession (from the Latin successio - heritage, change of generations, sequence).

2. Is equilibrium possible in a community where the “common respiration” of organisms is not equal in value to the gross output?

Answer. In order to understand the nature of ecological succession, let us imagine an ideal community in which the gross, i.e., total, production of autotrophs in energy terms exactly corresponds to the energy costs used to ensure the life of its constituent organisms. In ecology, the total energy consumption is called the total respiration of a community.

It is clear that in such an ideal case, the production processes are balanced by the respiration processes. Consequently, the biomass of organisms in such a system remains constant, and the system itself remains unchanged, or equilibrium.

If the “total respiration” is less than the gross primary production, the accumulation of organic matter will occur in the ecosystem; if it is more, it will decrease. Both will lead to community change. If there is an excess of a resource, there will always be species that can master it; if there is a shortage of it, some species will go extinct. Such changes constitute the essence of ecological succession. The main feature of this process is that changes in the community always occur in the direction of an equilibrium state.

Each stage of succession is a community with a predominance of certain species and life forms. They replace each other until a state of stable equilibrium occurs.

3. What types of successions do you know?

Answer. There are primary and secondary successions.

Primary successions occur on substrates not affected by soil formation and are associated with the formation of not only phytocenosis, but also soil. An example of primary succession is the settlement of crustose and leaf lichens on stones. Under the influence of lichen secretions, the rocky substrate gradually turns into a kind of soil, where bushy lichens, green mosses, then grasses and other plants, etc., settle.

Secondary successions develop in place of formed biocenoses after their disturbance, for example, as a result of erosion, drought, fire, deforestation, etc.

4. What are the differences between young and mature communities?

Answer. The duration of succession is largely determined by the structure of the community. During primary succession, it takes many hundreds of years for the development of a stable community.

Secondary successions proceed much faster. This is explained by the fact that the primary community leaves behind a sufficient amount of nutrients and developed soil, which creates conditions for the accelerated growth and development of new settlers.

A mature community, with its greater diversity and abundance of organisms, developed trophic structure, and balanced energy flows, is able to withstand changes in physical factors (e.g., temperature, humidity) and even some types of chemical pollution to a much greater extent than a young community. However, a young community is capable of producing new biomass in much larger quantities than the old one.

Thus, a person can reap a rich harvest in the form of pure products, artificially maintaining the community in the early stages of succession. Indeed, in a mature community, which is at the stage of sustainability and stability, the net finished product is spent mainly on the “general respiration” of plants and animals and may even be equal to zero.

On the other hand, the stability of a mature community, its ability to withstand the effects of physical factors (and even manage them) is a very important and highly desirable property.

Have you ever observed successional changes in nature? Tell us about your observations.

Answer. When a community is destroyed by human activity or natural disasters such as floods or fires, it begins a slow process of restoration to its original state known as succession. Succession is a successive series of changes that ultimately lead to the formation of a climax community (provided that no further disturbance occurs).

An example of succession is the restoration of a climax deciduous forest in an abandoned field. When the field ceases to be cultivated, it is soon overgrown with annual herbaceous plants, forming a variegated carpet: black mustard, ragweed, dandelions. These “pioneers”, having come to a new habitat, grow rapidly and produce seeds adapted to spread over a relatively wide area with the help of wind or animals. Soon taller plants appear here, such as goldenrod and perennial grasses. Because these newcomers shade the ground and their extensive root systems suck up all the moisture from the soil, seedlings of the species that arrive first in the field find it difficult to grow. But just as these tall weeds choke out the sun-loving first species, they in turn are shaded and deprived of water by the seedlings of pioneer trees, such as bird cherry and aspen, which settle more slowly, but, having reached sufficient size, take the lion's share of all resources . Succession does not end there, since pioneer trees are not species that form a mature climax forest; slow-growing oak and hickory or beech and maple species that emerge last and displace the pioneers, shading out their young trees.

Brownfield succession is an example of secondary succession that occurs relatively quickly because it occurs on soil left over from the original climax forest. If the soil is severely depleted as a result of its mismanagement or is completely absent (as on bare rocks exposed after the retreat of ice, or on solidified lava flows), then succession proceeds much more slowly, since the growth of most plants becomes possible only after the formation of soil.

Succession that begins on bare rock is called primary succession. Soil formation can occur as a result of the erosion of the surface of the parent rock by the action of acid released by lichens, or the freezing and thawing of water that accumulates in crevices, causing the destruction of the rock. Dying lichens, in addition, introduce organic matter into the newly formed soil, and mosses can gain a foothold even on a very thin layer of lichen remains and mineral dust. As the mosses increasingly break down the rock and add their own dead material to the accumulating soil, it becomes possible for small, rooted plants to germinate and grow, beginning a process essentially similar to brownfield succession.

Succession can be observed even on a city street. Mosses, lichens and weeds colonize cracks in sidewalks; Quite large plants can grow in some corner where fallen leaves and dirt accumulate, and mosses can grow on roofs that require repair. If you stop cleaning and repairing streets, even the center of a large city can turn into a wooded area strewn with stones over the course of one generation.

The existence of a variety of sites undergoing succession provides a constant source of “wandering” plants - fast-growing weeds that suddenly appear and just as suddenly disappear. The seeds of these species can be dispersed over quite significant distances by wind or animals. In addition, the seeds of many “wandering” plants are capable of remaining dormant for long periods, germinating after some disturbance in the environment creates the appropriate conditions, such as increased light.

Equilibrium- this is the state of the ecosystem in which the composition and productivity of its biotic part (biocenosis) at each moment of time most fully corresponds to abiotic conditions - soil and climate; and this balance ensures the long-term stable existence of this natural complex.

The state of equilibrium is characterized, first of all, by the following signs:

A) Constancy of the species composition of the biocenosis. Those. there is a well-balanced set of species of animals, plants, and microorganisms. Trophic connections are stable, ecological niches are densely filled. the environment created in such a biogeocenosis is so specific that it limits the introduction of new species into the community; and because of this, indigenous communities can remain stable for a significant number of years.

B) Constancy of nutrient cycles. All carbon and nitrogen absorbed by the ecosystem from the atmosphere returns to it as a result of the activity of decomposers. All elements of mineral nutrition (P, K, Ca, etc.) after the decomposition of dead organic matter are returned to the soil solution for reuse by plant roots. If part of the organic matter turns into detritus, then the rate of its accumulation corresponds to the rate of its decomposition by decomposers. Those. all substances absorbed by the biota are returned to the environment.

You can imagine different types of balance. The first of them is typical for a closed community : no additional products come here, and the community’s own products remain entirely within it. Equilibrium here means that the entire gross production of autotrophs corresponds to the costs spent on ensuring the vital activity of the organisms that make up this biocenosis. Let's call it "common breath". Such a community is like a factory in which the cost of production is exactly equal to the profit received. The production process is balanced by the processes of “general respiration”. The second type of equilibrium is characteristic For some flowing water ecosystems , organic matter in which arises not only as a result of the functioning of autotrophs, but also as a result of influx from the outside. Equilibrium here means that “total respiration” is equal to the gross production of the community itself supplemented by the supply of organic matter from the outside. For some ecosystems, the removal of matter beyond their boundaries is so great that their stability is maintained mainly due to the influx of the same amount of matter from the outside; whereas the internal circulation is ineffective. These include flowing reservoirs, rivers, streams, and areas on steep mountain slopes. In agricultural agroecosystems (third type of equilibrium), on the contrary, there is a constant withdrawal of part of the production, as already mentioned. Therefore, in such ecosystems equilibrium is achieved only when the “general respiration” is ensured by the amount of production that will have to remain in the system after the removal of part of it by Man (the harvest). This principle about the impossibility of completely withdrawing products from the system if there are no additional compensating supplies of resources from outside.

With all this, it must be said that in reality, not a single ecosystem, even the largest one on Earth, has a completely closed cycle of substances. Those ecosystems that have a more or less complete cycle of substances are still, in fact, only relatively autonomous (forests, meadows, lakes, etc.). Continents exchange matter with the ocean and lithosphere (with the participation of the atmosphere in these processes); and our entire planet receives part of its matter and energy from space, and releases part into space.

If “total respiration” is less than gross primary production, accumulation of organic matter will occur in the ecosystem. If more, it disappears. Both will lead to change in the community. When there is an abundance of a resource, there will always be species that can utilize it. If there is a lack of resources, some species will become extinct. Those. the ecosystem will return to an equilibrium state again, but it will be different - new. Thus, The main feature of ecological balance is its mobility. Such changes constitute the essence of the so-called ecological succession - the successive replacement of one community by another. The main feature of this process is that community changes always happen towards equilibrium .

C) Complete dissipation of energy entering the ecosystem. All energy absorbed by the ecosystem, after passing through trophic chains, is dissipated in the form of heat and “burned” by organisms in the process of life and work. The ecosystem maintains balance due to the fact that new solar or chemical energy constantly enters it.

Ecological balance in ecosystems is maintained by complex mechanisms of relationships between living organisms and environmental conditions; between individuals of the same species and individuals of different species with each other, which we have already discussed. Relationships between organisms of the same trophic level are called horizontal(competition) , and between organisms of different trophic levels – vertical(predation, symbiosis).

In addition to the equilibrium state, any ecosystem is also characterized by a degree of stability, stability and endurance. Sustainability ecosystems are the ability to withstand any external and internal disturbances, including anthropogenic ones, and each time return to the original equilibrium state. Those. the ability to recover every time. Stability ecosystems - the ability to remain unchanged, to maintain their structure and functional characteristics, indefinitely, due to their internal regulatory mechanisms. If an unstable ecosystem can deviate in one direction or another in its development even without any special disturbances from the outside, then a stable one always remains unchanged. Endurance – the ability to endure long-term external and internal disturbances and adapt to them, maintaining a dynamic (mobile) balance. Those. the ability to adapt to survive unfavorable conditions.

History knows many cases when a person’s shortsightedness caused the most serious ecosystem disturbances. What reasons this could be caused by:

A)extermination from the ecosystem of a predator(at the same time, the populations of prey and the resources on which they feed suffer - all the grass is eaten by ungulates);

b)inclusion into the ecosystem of a species for which there is no decomposer– as a consequence, the accumulation of undecomposed detritus (in Australia, cows and dung beetles);

V)inclusion into the ecosystem of plant species that do not have phytophages here– as a consequence – excessive reproduction and suppression of local species (in Australia, the prickly pear and the fire butterfly);

G)similar consequences also have the opposite effect - the destruction of a phytophage in the ecosystem;

and)changes in climatic, soil, hydrological conditions includes changes in the flora and fauna of the ecosystem.

Balance (stability) and diversity are connected by very complex interdependencies. Great diversity does not always provide the biocenosis with proper stability and sustainability. After all, systems that are too large and complex with many highly specialized and carefully adjusted elements are often easy to destroy.

DYNAMICS OF ECOSYSTEMS

In living nature there are no frozen, static processes. Naturally, ecosystems are constantly undergoing some kind of change. They return the ecosystem to an equilibrium state again and again. As we have already said, the main feature of ecological balance is its mobility - dynamism. At their core these ecosystem changes can be attributed to two main types: a) reversible cyclic (periodic) and b) irreversible progressive.

A) Cyclic processes are connected with the frequency of external conditions. Circadian rhythms changes in temperature, humidity, and illumination also determine a corresponding change in the activity of various organisms inhabiting the ecosystem. Seasonal changes in ecosystems occur throughout the year from spring to spring and already affect quantitative indicators characterizing the state of the biocenosis. This is especially pronounced in climatic zones with contrasting winter and summer conditions. The ratio of different species of organisms at different times of the year can vary significantly depending on reproduction cycles, seasonal movements and migrations. In addition, a number of species may completely disappear from the life of the community, falling into torpor or hibernation. In plants, depending on the seasons of the year, some tiers may be completely absent (annual grasses in winter). Long-term periodic variability depends primarily on changes in climate indicators over the years.

B) In addition to the noted cyclical changes in ecosystems, directed progressive processes, leading to irreversible changes and irreversible consequences. As a result, a biocenosis may appear that is qualitatively different from the previous one. Sometimes in nature, a change in biocenoses occurs suddenly, as a result of sudden and even catastrophic events in the environment(fire, large oil spill, deforestation, flooding or lowering of groundwater levels, vehicle wheels passing through the tundra). This can occur either under the influence of natural forces or as a result of human activity. Moreover, the factor causing the change is in no way connected with the previous course of development of the ecosystem and sets a different direction for its development, which can lead to a disruption of biocenotic connections, ecological balance and even the death of the biocenosis. But with the cessation of the factor, a new line of development of living cover may begin in this territory, which over time, step by step, will lead to the emergence of a new community here, possibly similar to the original one that was destroyed. But more often in nature, the replacement of one biocenosis by another occurs gradually, as if stepwise, sequentially, in parallel with changes in environmental conditions– with an increase (or decrease) in soil moisture or richness, with climate change, etc. In this case, the composition of living organisms, the type of substance cycle and the level of productivity of the ecosystem changes. Gradually, the role of some species decreases, while others increase; different species drop out of the ecosystem or, conversely, replenish it. This phenomenon of sequential gradual replacement of one biogeocenosis with another is called succession. Such changes in the community lead, as a rule, from a less stable state to a more stable one. Those. any biogeocenosis strives in its natural development towards balance, as has already been mentioned.

The total community respiration (solid lines) In situ at 15 °C was about 16 ml Oz per 1 m2 in 1 hour, and the total benthos biomass (dashed lines) was 4.2 mg/m2.[...]

The content of chlorophyll per 1 m2 in different communities is approximately the same, i.e. in entire communities the content of green pigment is distributed more evenly than in individual plants or parts thereof. The ratio between green and yellow pigments can be used as an indicator of the ratio of heterotrophic to autotrophic metabolism. When photosynthesis exceeds respiration in a community, green pigments dominate, and when community respiration decreases, the content of yellow pigments increases.[...]

The balance between gross primary production and the respiration of the community contains the meaning of the contradiction between human economic aspirations and the development strategy of nature. A person is interested in increasing the annual yield of net products of the community, and the development strategy of any ecosystem is aimed at not only producing as much as possible during the annual cycle, but also consuming everything produced during the same time. However, equality between income and expenditure is a rather rare phenomenon; it is observed in the most stable communities, in particular in the tropical zone, and creates objective difficulties for the development of agriculture there. Man, by burning down a lush tropical forest, hopes to get high yields from the vacated territory. However, it soon turns out that the soils in the exposed area are absolutely barren - all the annual production of the forest growing in this place was consumed by various consumers, and nothing was deposited in the soils.[...]

To the left of the diagonal, the gross production P exceeds the respiration of the community I (PR greater than one = autotrophy). On the right is the opposite picture (P/I less than one = heterotrophy). In the latter case, the community lives at the expense of organic matter received from the outside, or through its preliminary storage or accumulation. The directions of autotrophic and heterotrophic succession are indicated by arrows. Communities located along the diagonal consume on average approximately as much per year as they created, and they can be considered metabolically climax.[...]

In most cases, there is a predominance of gross primary production over community respiration, resulting in the accumulation of unconsumed organic matter, for example, in the form of coal, oil shale, dry leaves, etc. Imbalance in energy supply and consumption has serious consequences for the ecosystem. [...]

This approach, used in studying the activity of plankton in the euphotic zone of the Krasnoyarsk reservoir, showed that a significant proportion (about half) of the total respiration rate of the community is phytoplankton.[...]

Biomass accumulation is an increase in the amount of organic matter equal to the difference between gross primary production and the total expenditure of organic matter on community respiration. This difference is also called net ecosystem production, but this term is completely different from the concept of net primary production. In climax communities, such as the forest described in Table. 5-1, net primary production can be high (1390 g/m2-year), but the net production of the ecosystem in this case is equal to or approaches zero, since gross primary production and the total costs of the community for respiration are approximately the same.[... ]

Of particular interest is the possibility now being explored of using the ratio between yellow pigments, carotenoids, and green pigments, chlorophylls, as an indicator of the ratio of heterotrophic to autotrophic metabolism in the whole community. When photosynthesis exceeds respiration in a community, chlorophylls dominate, and when community respiration increases, the content of carotenoids increases. You immediately notice this when looking at the landscape from an airplane: fast-growing young crops or forests appear bright green in comparison with the yellow-green color of older forests or ripe crops. Margalef (1961, 1967) found that the ratio of the optical density of acetone extracts of pigments at a wavelength of 430 nm to that at a wavelength of 665 nm gives a simple ratio of yellow to green pigments, which is inversely proportional to the P/R ratio in cultures and planktonic communities. Thus, this ratio is usually low (for example, from 1 to 2) for young crops or during the spring “blooming” of water bodies, when respiration is low, and high (3-5) in aging crops or in planktonic communities at the end of summer, I when breathing is relatively intense.[...]

In any complex system of the real world, maintaining processes that run against the temperature gradient is of paramount importance. As Schrödinger showed, to maintain internal order in a system located at a temperature above absolute zero, when there is thermal movement of atoms and molecules, constant work is required to pump out disorder. In an ecosystem, the ratio of the total respiration of a community to its total biomass (R/B) can be considered as the ratio of energy expenditure to maintain life activity to the energy contained in the structure, or as a measure of thermodynamic order. If we express R and B in calories (units of energy) and divide them by absolute temperature, the ratio RIB becomes the ratio of the increase in entropy (and associated work) associated with maintaining the structure to the entropy of the ordered part. The larger the biomass, the higher the maintenance costs; but if the size of the units into which the biomass is divided (individual organisms, for example) is large enough (say, these are trees), then the costs of maintaining processes that go against the temperature gradient, in terms of the structural unit of biomass, will be lower. One of the currently intensely debated theoretical questions is whether nature seeks to maximize the ratio of “structural” to “maintenance” metabolism (see Margalef, 1968; Morowitz, 1968) or whether this refers to the flow of energy itself.[...]

Self-purification is associated with the chemical composition of water and bottom sediments of the river. The term “purification” means the removal of a dissolved or undissolved substance that has “polluting” properties. Chemical substances present in water and bottom sediments are relevant environmental factors for aquatic organisms, and therefore self-purification causes certain secondary effects. The main reason for this is the dynamic relationship between photosynthetic oxygen production and community respiration.[...]

Liman is a good example of an associated system in which a reliable balance has been achieved between physical and biotic components and, as a consequence, a high level of biological productivity. It consists of several main subsystems, connected to each other by tides and water flow, which is driven by the hydrological cycle (river flow) and the tidal cycle; both cycles provide additional sources of energy for the system as a whole. The main subsystems include: 1) shallow-water production zone, in which the intensity of primary production exceeds the intensity of community respiration; this includes reefs, shoals, seagrass or seagrass beds, seagrass beds and salt marshes; this subsystem exports energy and nutrients to the deeper waters of the estuary and the waters of the adjacent coastal shelf; 2) a subsystem of sediments in deeper channels, straits and lagoons, in which respiration exceeds production and which use formed and dissolved organic matter from the production zone; here nutrients are regenerated, circulated and stored, and vitamins and growth regulators are also formed; 3) plankton and nekton, which move freely between two fixed subsystems, producing, converting and transporting nutrients and energy at times corresponding to daily, tidal and seasonal periods. This subsystem is able to quickly respond to the local abundance or poverty of available resources.

Natural ecological systems (biogeocenoses) exist for a long time - tens and hundreds of years, preserving their structure and functional properties. those. have a certain stability. To maintain the stability of an ecosystem, it is necessary to balance the flows of matter and energy, metabolic processes between organisms and the environment.

The state of the ecosystem, in which the composition and productivity of the biotic component at any given time corresponds to abiotic conditions, is characterized as ecological balance.

Let's imagine a community in which the gross (that is, total) production of autotrophs in energy terms exactly corresponds to the energy costs used to ensure the vital activity of its constituent organisms. The total energy expenditure is called the total respiration of the community.

It is clear that in this ideal case there can be neither accumulation of biomass nor its loss. Therefore, the biomass of organisms in such a system remains constant, and the system itself remains unchanged, or equilibrium (production processes are balanced by respiration processes).

One can imagine different types of equilibrium.

First of which is characteristic of a closed community: no additional products come here, and the community’s own products remain entirely within it.

Second type of balance characteristic of some ecosystems of flowing water, the organic matter of which arises not only as a result of the functioning of autotrophs, but also as an influx from the outside. Equilibrium here means that “total respiration” is equal to the gross production of the community itself, supplemented by the supply of organic matter from the outside.

Third type of balance(in agricultural ecosystems), on the contrary, involves the constant withdrawal of part of the product. Therefore, in such ecosystems, equilibrium is achieved only when the “total respiration” is equal to the amount of production that remains in the system after part of it is removed.

If the “total respiration” is less than the gross primary production, the accumulation of organic matter will occur in the ecosystem; if it is more, its disappearance will occur. Both will lead to community change. When there is an abundance of a resource, there will always be species that can utilize it. If there is a lack of resources, some species will become extinct. The main feature of this process is that changes in the community always occur in the direction of the equilibrium state.

Stability An ecosystem over a long period of time presupposes the relative stability of the populations of its species. It can be long-lasting only if changes in the environment caused by some organisms are accurately compensated by the activities of others with opposite ecological requirements. This condition is violated when the circulation of substances is disrupted, and then some of the populations that cannot withstand competition are replaced by others for which these conditions are favorable, and homeostasis is restored.

The position of a species' population in an ecosystem is determined, on the one hand, by a set of requirements for abiotic conditions, and on the other, by a set of connections with populations of other species and the form of participation in the general functions of the biocenosis. The place of an organism in nature and the entire pattern of its life activity (the so-called life status), including its relationship to environmental factors, types of food, time and methods of feeding, places of reproduction and shelter, etc. defined as ecological niche. Long-term coexistence as part of a single multi-species community led to the evolutionary formation of such a system of relationships in ecosystems, in which each species spatially and functionally occupies a certain position in the biocenosis, i.e. occupies its ecological niche.

The set of species, the composition and complexity of trophic networks, and the most stable forms of interaction between populations in an ecosystem reflect adaptability to the characteristics of the environment and are aimed at maintaining the circulation in these conditions.

An important characteristic of ecosystems is their sustainability, which refers to the ability of an ecosystem to return to its original (or close to it) state after exposure to factors that throw it out of balance. The greatest stability, due to the reasons discussed above, is characterized by ecological systems with many relatively small species.

§4 Dynamics of ecosystems (succession)

The main feature of the ecological balance of an ecosystem is its mobility. Any ecosystem, adapting to changes in the external environment, is in a state of dynamics. A distinction is made between cyclic and directional dynamics. An example of cyclic dynamics is a seasonal change in the vital activity of organisms, or a periodic change in the number of individual species in a long-term series. Directional dynamics represents the progressive development of ecosystems. This type of dynamics is characterized by either the introduction of new species into ecosystems, or the replacement of some species by others, which ultimately leads to a change in biocenoses and ecosystems as a whole. Changes in species structure and biocenotic processes in an ecosystem are called ecosystem succession. Thus, succession is a process of sequential change of ecosystems occurring over time with a gradual directed change in environmental conditions.

Large-scale changes in the geographical situation or type of landscape under the influence of natural disasters or human activities lead to certain changes in the state of biogeocenoses of the area and to the gradual replacement of some communities by others. Such changes are called ecological succession(from Lat. . successio– continuity, consistency).

Yu. Odum (1986) understands ecological succession as the entire process of ecosystem development. A more specific definition of this phenomenon is given by N. F. Reimers (1990): "Succession- a successive change of biocenoses, successively arising in the same territory (biotope) under the influence of natural factors (including internal contradictions of the biocenoses themselves) or human influence.”

Ecological succession occurs over a certain period of time, during which the species structure of the community and the abiotic environment of its existence change until the culmination of its development - the emergence of a stabilized system. In a narrower sense, it is a sequence of communities that replace each other in a given area.

If autotrophic organisms appear at the first moment, succession is called autotrophic. For example, forest development on an abandoned field. The species composition of organisms changes from year to year, and organic matter accumulates in the community.

Heterotrophic succession characterized by the initial predominance of heterotrophic organisms and occurs in cases where the environment is oversaturated with organic matter. Energy reserves here are initially maximum and decrease with succession, unless, of course, additional organic matter is introduced.

The flow of energy passing through the community decreases during heterotrophic succession. It ends after the excess organic matter has been exhausted. In contrast, with the autotrophic type of succession, the energy flow may even increase.

For succession to occur, free space is necessary, and depending on the initial state of the substrate, there are primary And secondary succession.

Primary succession– this is if the formation of communities begins on an initially free substrate, and the secondary succession– this is the consistent replacement of one community that existed on a given substrate by another, more perfect for given abiotic conditions.

Primary succession allows you to trace the formation of communities from the very beginning. It can occur on a slope after a landslide or landslide, on a sandbank formed during the retreat of the sea and a change in the river bed, on exposed aeolian sands of the desert, not to mention anthropogenic disturbances: fresh cutting, alluvial strip of the sea coast, artificial reservoirs.

A typical example is the settlement of rock outcrops. First, lichens and algae appear on the rocks; a complex of species of microscopic algae, protozoa, nematodes, some insects and mites is formed, which contributes to the formation of primary soil. Later, other forms of lichens and specialized species of mosses appear, then vascular plants settle and the fauna becomes enriched.

Figure 4 shows a diagram of the development of a typical terrestrial primary succession.

Fig.4.

Diagram of the development of a typical terrestrial primary succession.

The spruce forest is the last climax stage of ecosystem development in the climatic conditions of the North, i.e. already indigenous biocenosis. At first, birch, alder, and aspen forests develop here, under the canopy of which spruce trees grow. Gradually they outgrow the birch and displace it, taking over the space (Fig. 5). The seeds of both tree species are easily carried by the wind, but even if they germinate at the same time, birch grows much faster - by 6-10 years, spruce barely reaches 50-60 cm, and birch - 8-10 m.

Rice. 5.

Replacement of a birch forest by a spruce forest (according to I. N. Ponomareva, 1978)

Under the already closed crowns of birch trees, their own microclimate arises, the abundance of leaf litter contributes to the formation of special soils, many animals settle, a diverse herbaceous cover appears, and consortiums of birch with the environment are created. And the spruce continues to grow in such a favorable environment and, finally, the birch cannot compete with it for space and light and is replaced by spruce.

A classic example of natural succession is the aging of lake ecosystems - eutrophication. It is expressed in the overgrowing of lakes with plants from the shores to the center. There are a number of overgrowth stages– from initial – farthest from the shore - to the reached near the shore. These stages are shown and described in Fig. 6.

Rice. 6.

Overgrowth of a eutrophic reservoir with stagnant or low-flow water(Soloviev, 1983)

(the dotted line shows the lower water level)

Zones: 0 - free-floating plants 1 – low (bottom) submerged plants, 2 – tall submerged plants, 3 – plants with floating leaves, 4 – tall emergent plants, 5 – low and medium-tall emergent plants, 6 – black alder swamp.

Sediments: 1 – sapropelite, 2- 3 – sapropelite peat, 4 – reed and reed peat, 5 – sedge peat, 6 – forest peat

Ultimately, the lake turns into a peat bog, representing a stable climax-type ecosystem. But it is not eternal either - a forest ecosystem can gradually emerge in its place thanks to terrestrial succession in accordance with the climatic conditions of the area,

Eutrophication of a reservoir is largely determined by the introduction of nutrients from outside. Under natural conditions, nutrients are carried away from the catchment area. This eutrophication has the features of primary progressive succession.

Secondary succession is, as a rule, a consequence of human activity. In particular, the change in vegetation described above during the formation of a spruce forest most often occurs as a result of secondary succession that occurs in the clearing of a previously existing forest (spruce forest). Secondary succession ends with a stable community stage after 150-250 years, and primary succession lasts 1000 years.

Secondary, anthropogenic succession also manifests itself in eutrophication. The rapid “blooming” of water bodies, especially artificial reservoirs, is the result of their enrichment with nutrients caused by human activity. The “trigger mechanism” of the process is usually an abundant supply of phosphorus, less often nitrogen, and sometimes carbon and silicon. Phosphorus usually plays a key role.

The biocenosis is almost completely degenerated. Massive fish kills are observed. In especially severe cases, the water acquires the color and consistency of pea soup and an unpleasant putrefactive odor: the life of aerobic organisms is excluded.

A successive series of communities gradually and naturally replacing each other in succession is called successional series. It is observed in nature not only in forests, swamps and lakes, but also on the trunks of dying trees and in stumps, where there is a natural change of saprophytes and saprophages, in puddles and ponds, etc. In other words, successions are of different scales and hierarchical, as well as are the ecosystems themselves.

Each stage of succession represents a specific community with a predominance of certain species and life forms. The individual stages of succession development are called serial stages and the state of final equilibrium is menopause.

Successions caused by the action of external factors are called exogenetic. Such successions can be caused, for example, by climate change in one direction (cooling or warming) and other changes in abiotic conditions. Such changes can occur over centuries and millennia and are called secular successions. If, as a result of changes in environmental conditions, some species become extinct, while others change under the influence of natural selection, this process is considered as evolutionary succession.

If succession occurs due to internal interactions, it is called endogenetic. Endogenetic succession is observed in nature when, in the process of its development, a community changes the environment so that it becomes more favorable for another community. The emerging new community, in turn, makes the environment even more unfavorable for the old community. There is a process of changing ecosystems, going through several stages, until the final population equilibrium is reached. Succession ends with the formation of a community adapted to climatic conditions, capable of maintaining itself indefinitely, the internal components of which are balanced with each other and with the environment. The community completing succession - stable, self-renewing and in balance with the environment - is called climax community.

How quickly ecosystems change depends on the degree to which their equilibrium shifts. Succession is a natural process of ecosystem development. During succession, changes occur slowly and gradually. At all stages of the process of replacing some species with others, the system is quite balanced. In the process of succession, increasingly complex biocenoses and ecosystems are formed and their productivity increases.

When there are sudden, dramatic changes that cause a “population explosion” of some species at the expense of the death of most other species, they speak of an ecological disturbance.

Disturbances can occur due to the invasion of introduced species or due to thoughtless human influence on nature. In modern conditions, the constant increase in anthropogenic load on natural ecosystems (drainage of swamps, excessive loads on forests, for example, as a result of recreation, fires, increased grazing, chemical pollution of the environment) often leads to a relatively rapid change in their structure. Anthropogenic impacts often lead to the simplification of ecosystems. Such phenomena are usually called digressions (for example, pasture, recreational and other digressions). When the disturbances are so great that virtually no component of the ecosystem is preserved, they speak of its destruction. After the death of an ecosystem, a new succession may begin in the vacated area.

Many ecosystems exist for tens or even hundreds of years.

The stability of ecosystems is their ability to withstand fluctuations in external factors and maintain their structure and functional characteristics. To maintain such sustainability of ecosystems, it is necessary to balance the flows of matter and energy, exchange processes between organisms and their environment. A sustainable ecosystem must receive substances from the environment in the required quantities and get rid of waste. Depending on the method of maintaining sustainability, ecosystems are divided into open and closed.

Open ecosystems continuously receive energy and matter from the environment. In such ecosystems, processes of accumulation and decomposition of matter constantly occur. This type includes natural ecosystems; equilibrium in them is maintained spontaneously.

In closed ecosystems there is no constant exchange of matter and energy with the environment. The system is unable to get rid of unnecessary products. In this case, equilibrium can be maintained artificially. Without outside intervention, closed systems are unstable and quickly lose stability.

Ecological balance is the relative constancy of the species composition of living organisms, their numbers, productivity, distribution in space, as well as seasonal changes, biological circulation of substances and other biological processes in an ecosystem, leading to the long-term existence of this ecosystem. Of course, no ecosystem is absolutely stable or motionless: the number of some species periodically increases, while the number of others decreases. Such processes occur periodically and generally do not throw the system out of balance. Therefore, the main feature of ecological balance is its mobility. There are two types of equilibrium mobility:

Reversible changes in an ecosystem are changes that occur from spring to spring in the ecosystem while maintaining the species composition;

Ecological succession (Latin succesio - continuity, inheritance) is a sequential change of ecosystems that arise in the same territory or water area (biotope) under the influence of natural or anthropogenic factors. Depending on the initial state of the substrate on which succession develops, primary and secondary succession are distinguished.

Primary succession is the process of development of ecosystems in previously uninhabited areas, beginning with their colonization. For example, the settlement of islands resulting from volcanic activity. Or the overgrowth of bare rock, first with lichens, then with mosses, grasses and other plants. In the process of primary succession, not only phytocenoses are formed (Greek phyton - plant, koinos - general), i.e. plant biocenoses, but also soil.

Secondary succession is the restoration of ecosystems that once existed in a given biotope. Such places usually preserve rich life resources. Therefore, secondary successions lead to the formation of biocenoses much faster than primary ones.

There are autotrophic and heterotrophic successions.

Autotrophic succession occurs under the influence of solar energy, while autotrophic organisms predominate in the community. During autotrophic succession, the community becomes more complex and the stability of ecosystems increases.

An example of autotrophic succession is an ecosystem that begins to recover after a forest fire (Fig. 5). After 1-2 years, the area of ​​the fire is overgrown with grass, after a few years the first shrubs appear, after 20-25 years the area is covered with forest: first deciduous, and then shade-loving conifers grow.

Fig.5. Autotrophic succession in a forest fire

Heterotrophic succession is typical for those cases when there is an excess of organic matter in the ecosystem (polluted water bodies, rotting plant remains, etc.) In this case, heterotrophic organisms predominate in the community, so the energy supply does not increase, but decreases. The result of heterotrophic succession is either the death of all organisms or a significant simplification of the community.

An example of heterotrophic succession is the “aging” of water bodies. Two main stages of succession can be distinguished: developing and mature. These stages are characterized by different properties and varying degrees of stability.

Mature systems have a greater ability to retain substances in the exchange fund. The gyres in them are mostly closed. Increasing the stability of ecosystems during succession leads to the fact that each subsequent stage lasts longer than the previous one.

Mature systems are the most stable over time. But they can also be destroyed during natural and man-made disasters. If we consider large periods of time, it turns out that any succession is a cyclical process. The cycles may not completely repeat each other and may take different periods of time, but the cyclicity remains.

Depending on the reasons that caused the change in ecosystems, successions are divided into autogenous and allogenic.

Autogenic successions (self-generating) arise due to internal causes (changes in the environment under the influence of the community), allogenic (generated from the outside) are caused by external causes (for example, climate change).

At the last stages of succession, stable communities are usually formed, which are called climax.

The climax community, klimax (Greek klimax - ladder) is characterized by the highest productivity and the greatest diversity.

The ability of a population or ecosystem to maintain a mobile-stable equilibrium when environmental conditions change is called ecosystem homeostasis.

The mechanism for maintaining homeostasis is based on two principles:

The principle of cycling is the repeated use of nutrients in the ecosystem. This makes the reserves of minerals in the ecosystem practically inexhaustible.

The principle of “feedback” is that the deviation of an ecosystem from a state of equilibrium sets in motion forces that return it to an equilibrium state.

BIOSPHERE

4.1. BIOSPHERE – THE LIVING ENVIRONMENT OF THE EARTH

The biosphere, as defined by V.I. Vernadsky as a “zone of life,” covers the lower part of the atmosphere (troposphere), the entire hydrosphere and the upper part of the lithosphere (soil). In other words, the biosphere is a global biotope inhabited by all living organisms, including humans.

The biosphere is a set of parts of the geospheres (litho-, hydro- and atmosphere), which is populated by living organisms, is under their influence and is occupied by the products of their vital activity. It does not form a dense layer with clear boundaries, but rather “permeates” other geospheres of the planet. The upper boundary of the biosphere extends from the Earth's surface to the ozone layer, the maximum density of which is at an altitude of 20-25 km. Organisms cannot live above this limit: they are detrimentally affected by ultraviolet radiation from the Sun and very low temperatures (-56 ° C).

Almost the entire hydrosphere, including the deepest trench (Mariana) of the World Ocean (11022 m), is occupied by life.

The lower boundary of the biosphere runs along the ocean floor in the hydrosphere and at a depth of 3.0-3.5 km in the earth's crust of the continental zone, where the temperature of the interior reaches 100 °C and above. This temperature is also destructive for all living things.

The land most densely populated by organisms is the surface waters of the ocean and its bottom at shallow depths (up to 250 m), where the sun's rays penetrate. There are especially favorable living conditions here.

The average thickness of the biosphere is slightly more than 20 km. Compared to the diameter of the globe (13,000 km), the biosphere is a thin film. However, in mountain glaciers, at altitudes up to 6 km, communities of ticks live; among birds, the condor can rise to a height of 7 km; in the depths of the ocean (up to 11 km) there are communities of animals and microorganisms; in the underground oil waters of land at depths of up to 15 km, communities of bacteria (chemoautotrophs) can be found.

The mass of the biosphere is estimated to be about 1.5 10 21 kg.

The biosphere has a system of properties that ensure its functioning, self-regulation, stability and other parameters. The main properties are as follows.

1. The biosphere is a centralized system. Its central element is living organisms (living matter).

2. The biosphere is an open system. To maintain life on Earth, it is necessary to receive energy from outside, and part of this energy is reflected and goes into outer space.

3. The biosphere is a self-regulating system , which, as V.I. Vernadsky noted, is characterized by organization. Currently, this property is called homeostasis.

4. The biosphere is a system characterized by great diversity. The biosphere, as a global ecosystem, is characterized by the greatest diversity among other systems. For any natural system, diversity is one of its most important properties. Associated with it is the possibility of duplication, backup, replacement of some links with others, the degree of complexity and strength of food and other connections. Therefore, diversity is considered as the main condition for the sustainability of any ecosystem and the biosphere as a whole.

5. The presence in the biosphere of mechanisms that ensure the circulation of substances and the associated inexhaustibility of individual chemical elements and their compounds.

The biosphere is a complex natural system. It includes:

Living matter is the totality of bodies of living organisms living on planet Earth;

A biogenic substance is a substance created and processed by living organisms (coals, limestones, bitumens);

Inert matter is a substance in the formation of which life does not participate (rocks, gases);

Bioinert substance is a substance that is created simultaneously by living organisms and inert processes (natural water, soil, salt sea water, weathering crust, troposphere);

Radioactive elements have a complex isotopic composition, coming from the depths, dispersed and dispersed, creating and changing the energy of the biosphere;

Scattered atoms;

Substance of cosmic origin (meteorites, cosmic dust).

Living organisms have significantly changed the appearance of our planet, transforming the earth's crust, hydrosphere and lower layers of the atmosphere. And currently they are involved in the destruction of rocks, in the formation of soil, minerals, for example, peat, and regulate the content of oxygen and carbon dioxide in the atmosphere.

Even in the early stages of evolution, living matter spread across the lifeless spaces of the planet, occupying all places potentially accessible to life, changing them and turning them into habitats. V.I. Vernadsky called this ability to distribute living matter “the ubiquity of life.”

V.I. Vernadsky considered living matter to be the most powerful geochemical and energetic factor, the leading force of planetary development. The pinnacle of the evolution of living matter on Earth was man, who acquired not only consciousness (the perfect form of reflection of the surrounding world), but also the ability to make and use tools in his life. Through tools, humanity began to create an artificial environment, its habitat, and the evolution of the biosphere entered a new phase - the phase of the noosphere. The noosphere (Greek noos - mind, sphaira - ball) is the sphere of the mind, the highest stage of development of the biosphere, when intelligent human activity becomes the main determining factor in its global development. The term “noosphere” was first introduced in 1927 by the French philosopher E. Leroy to designate the shell of the Earth, including human society with its industry, language and other types of intelligent activity. V.I. Vernadsky wrote: “The noosphere is a new geological phenomenon on our planet. In it, for the first time, man becomes the largest geological force. He can and must rebuild the area of ​​his life with his work and thought, rebuild it radically in comparison with what was before.”