4. Ecological succession
The relatively long existence of a biocenosis in one place (pine or spruce forest, lowland swamp) changes the biotope (the place where the biocenosis exists) so that it becomes unsuitable for the existence of some species, but suitable for the introduction or development of others. As a result, a different biocenosis, more adapted to new environmental conditions, gradually develops in this biotope. Such repeated replacement of some biocenoses by others is called succession.
succession (from Latin successio - continuity, inheritance) is a gradual, irreversible, directed replacement of one biocenosis by another in the same territory under the influence of natural factors or human influence.
The term “succession” was first used by the French botanist De Luc in 1806 to refer to changes in vegetation.
Examples of succession are the gradual overgrowing of loose sand, rocky placers, shallows, the colonization of abandoned agricultural lands (arable land), fallow lands, clearings, etc. by plant and animal organisms. Former fields are quickly covered with a variety of annual plants. This also includes seeds of tree species: pine, spruce, birch, aspen. They are easily carried over long distances by wind and animals. In lightly turfed soil, seeds begin to germinate. Light-loving small-leaved species (birch, aspen) find themselves in the most favorable position.
A classic example of succession is the overgrowing of a lake or river oxbow and its transformation first into a swamp, and then, after a long period of time, into a forest biocenosis. At first, the water surface becomes shallow, covered with raft on all sides, and dead parts of plants sink to the bottom. Gradually, the water surface is covered with grass. This process will last several decades, and then a high peat bog will form in place of the lake or oxbow lake. Even later, the swamp will gradually begin to be overgrown with woody vegetation, most likely pine. After a certain period of time, the processes of peat formation on the site of the former reservoir will lead to the creation of excess moisture and the death of the forest. Finally, a new swamp will appear, but different from what was before.
Along with the change in vegetation, the fauna of the territory subject to succession also changes. Typical for an oxbow or lake are aquatic invertebrates, fish, waterfowl, amphibians, and some mammals - muskrats, minks. The result of succession is a sphagnum pine forest. Now other birds and mammals live here - wood grouse, partridge, elk, bear, hare.
Any new habitat - an exposed sandy river bank, frozen lava of an extinct volcano, a puddle after rain - immediately turns out to be an arena for colonization by new species. The nature of developing vegetation depends on the properties of the substrate. Newly settled organisms gradually change their habitat, for example, by shading the surface or changing its humidity. The consequence of such environmental changes is the development of new, resistant species and the displacement of previous ones. Over time, a new biocenosis is formed with a species composition noticeably different from the original one.
In the beginning, changes happen quickly. Then the rate of succession decreases. Birch seedlings form dense growth that shades the soil, and even if spruce seeds germinate along with the birch, its seedlings, finding themselves in very unfavorable conditions, lag far behind the birch ones. Light-loving birch is a serious competitor for spruce. In addition, the specific biological characteristics of birch give it advantages in growth. Birch is called the “pioneer of the forest,” a pioneer species, since it is almost always the first to settle on disturbed lands and has a wide range of adaptability.
Birches at the age of 2 - 3 years can reach a height of 100 - 120 cm, while fir trees at the same age barely reach 10 cm. Gradually, by 8 - 10 years, birches form a stable birch stand up to 10 - 12 m high. Under the developing The spruce begins to grow along the canopy of the birch, forming undergrowth of varying degrees of density. Changes also occur in the lower, grass-shrub layer. Gradually, as the birch crowns close, light-loving species, characteristic of the initial stages of succession, begin to disappear and give way to shade-tolerant ones.
The changes also affect the animal component of the biocenosis. At the first stages, May beetles and birch moths settle in, then numerous birds - chaffinch, warbler, warbler, small mammals - shrew, mole, hedgehog. Changing lighting conditions begins to have a beneficial effect on young Christmas trees, which accelerate their growth. If at the early stages of succession the growth of fir trees was 1 - 3 cm per year, then after 10 - 15 years it already reaches 40 - 60 cm. Around 50 years, the spruce catches up with the birch in growth, and a mixed spruce-birch stand is formed. Animals include hares, forest voles, mice, and squirrels. Succession processes are also noticeable among the bird population: orioles that feed on caterpillars settle in such a forest.
The mixed spruce-birch forest is gradually replaced by spruce. The spruce outstrips the birch in growth, creates significant shade, and the birch, unable to withstand the competition, gradually falls out of the tree stand.
Thus, succession occurs, in which first a birch and then a mixed spruce-birch forest is replaced by a pure spruce forest. The natural process of replacing birch forest with spruce forest lasts more than 100 years. This is why the process of succession is sometimes called century-long change .
If the development of communities occurs in newly formed, previously uninhabited habitats (substrates), where there was no vegetation - on sand dunes, frozen lava flows, rocks exposed as a result of erosion or ice retreat, then such succession is called primary.
An example of primary succession is the process of colonization of newly formed sand dunes where there was previously no vegetation. Perennial plants that can tolerate dry conditions, such as creeping wheatgrass, first settle here. It takes root and reproduces on quicksand, strengthening the surface of the dune and enriching the sand with organic matter. The physical conditions of the environment in close proximity to perennial grasses change. Following the perennials, annuals appear. Their growth and development often contribute to the enrichment of the substrate with organic material, so that conditions suitable for the growth of plants such as willow, bearberry, and thyme are gradually created. These plants precede the appearance of pine seedlings, which establish themselves here and, growing up, after many generations form pine forests on sand dunes.
If vegetation previously existed in a certain area, but for some reason it was destroyed, then its natural restoration is called secondary succession . Such successions can result, for example, from partial destruction of the forest by disease, hurricane, volcanic eruption, earthquake or fire. The restoration of forest biocenosis after such catastrophic impacts takes a long time.
An example of secondary succession is the formation of a peat bog when a lake becomes overgrown. The change in vegetation in a swamp begins with the edges of the reservoir becoming overgrown with aquatic plants. Moisture-loving plant species (reeds, reeds, sedges) begin to grow in a continuous carpet near the banks. Gradually, a more or less dense layer of vegetation is created on the surface of the water. Dead plant remains accumulate at the bottom of the reservoir. Due to the low amount of oxygen in stagnant waters, plants slowly decompose and gradually turn into peat. The formation of a swamp biocenosis begins. Sphagnum mosses appear, on a continuous carpet of which cranberries, wild rosemary, and blueberries grow. Pines can also settle here, forming sparse growth. Over time, a raised bog ecosystem is formed.
Most of the successions currently observed anthropogenic , those. they occur as a result of human impact on natural ecosystems. This is grazing of livestock, cutting down forests, the occurrence of fires, plowing of land, flooding of soils, desertification, etc.
Ecological succession
One of the main achievements of ecology was the discovery that not only organisms and species develop, but also ecosystems. Communities are constantly changing. Some organisms die, others come to replace them. Energy and nutrients flow through the community in an endless stream.
Concept and types, 2018.
The sequence of changes in communities (ecosystems, biocenoses) on the same territory called succession.
There are three points to consider when defining ecological succession.
Firstly, succession occurs under the influence of the community, i.e. biotic component of the ecosystem.
Secondly, succession is directed in a certain way and can be predicted (anticipated).
The third aspect is that the culmination of succession is the emergence of a stabilized ecosystem in which per unit energy flow there is a maximum biomass and a maximum number of interspecific interactions.
The final stage of succession is called menopause community.
Traditionally, the process of succession is illustrated by the example of the overgrowing of a small reservoir in a forest (Fig. 34). The above-water parts of plants of coastal herbaceous vegetation die off annually, due to which the area of the clean water surface of the pond decreases.
Gradually, conditions favorable for the development of more powerful coastal plant species, such as willow, are formed near the shores. Taking root, the willow begins to pump water out of the pond, drying up the area of its existence. As a result, willow is replaced by small-leaved tree species: birch, hazel.
Concept and types, 2018.
The surface area of the pond continues to decrease, soil moisture decreases, and forest soil begins to form. Small-leaved trees are replaced by broad-leaved ones, oaks and lindens gradually appear, and various shrubs and herbaceous plants develop under their crowns. Conditions are gradually being created for the introduction of coniferous trees into communities. As a result of the excessive intake of biogenic chemical elements, mainly nitrogen and phosphorus, into the reservoir along with organic matter, a “blooming” of water occurs: unicellular algae multiply in huge quantities. There is an “aging” of lake ecosystems – their eutrophication.
Dying algae, along with foraminifera, fall “as rain” to the bottom, which leads to a decrease in the depth of the pond. As a result, a forest is formed in place of the reservoir, which is virtually no different from the one that surrounded the reservoir several decades ago. Under certain external conditions, the lake turns into a peat bog, which is a stable climax-type ecosystem.
There are a very large number of classifications of successions.
Depending on the reasons for succession, they distinguish
· exodynamically e (from the Greek word exo - outside) successions caused by factors external to a given ecosystem,
· endodynamic(from the Greek word endon - within) succession caused by internal mechanisms of the ecosystem
Exodynamic successions can be caused by climate changes, lowering groundwater levels, rising sea levels, etc. Such changes can last for centuries and millennia. They are associated mainly with the action of mechanisms of adaptation of the ecosystem to environmental factors, which in turn are based on the mechanisms of adaptation of living organisms in the ecosystem.
Endodynamic Succession is driven by special laws, the mechanisms of which are still largely unclear. It is known that on any, even absolutely lifeless, substrate such as sand dunes or hardened lava, sooner or later life blossoms. Moreover, forms of life, or more precisely, types of communities, successively replace each other in a given space, gradually becoming more complex and increasing species diversity, forming a so-called successional series, consisting of successive stages marking the replacement of one community by another.
The succession series ends at the maturity stage, at which the ecosystem changes very little. Ecosystems at this stage are called menopausal(from the Greek word klimax - ladder).
The duration of succession from the origin of an ecosystem to the climax stage can be up to hundreds and even thousands of years. Such a long duration is mainly due to the need to accumulate nutrients in the substrate.
There is another type of classification of successions.
It is necessary to distinguish autotrophic and heterotrophic successions. All autotrophic successions occur in ecosystems where the central link is vegetation (phytocenosis).
The dynamics of heterotrophs are entirely subordinate to the dynamics of autotrophs - the change of animal communities depends on the change of plant communities. Autotrophic successions can theoretically last forever, since they are constantly fed by the energy of the Sun.
IN heterotrophic successions Only animals (heterotrophs, consumers) participate. Dead plants can also be involved in this process, for example, fallen trees, stumps, etc., which are, as a rule, a source of energy for heterotrophic succession.
Heterotrophic succession presupposes the obligatory presence of a certain supply of energy accumulated in organic matter. It ends when the energy resource is exhausted, that is, after complete decomposition of the original substrate. After this, the ecosystem ceases to exist. Thus, the concept of menopause is not defined for her. Unlike biogeocenoses, such ecosystems are mortal.
Examples of heterotrophic succession are changes in communities on the corpse of an animal (changes occur approximately in this order: bacteria - ants - carrion beetles, carpet beetles, lice beetles); on a pile of manure (or droppings); on the fruit left lying on the ground - an apple, for example.
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The longest heterotrophic succession is observed on the trunk of a large fallen tree.
Thus, in In heterotrophic succession there is no climax stage.
Heterotrophic succession is well associated with fossil fuel-dependent societies. The dynamics of heterotrophic succession are described by a curve with a rapid increase in the number of organisms until a certain maximum is reached, then the number of organisms gradually decreases as the energy resource is exhausted. It is not possible to achieve any stable state (climax). Such a society is rapidly progressing, but nevertheless it is initially doomed to extinction.
We have already “skimmed the cream” from most of the deposits. Their further operation will require increasingly greater energy investments over time. Therefore, the efficiency of mining will steadily decline. Along with this, the viability of a civilization built on heterotrophic succession will also decline, unless, of course, catastrophic changes occur even earlier. That is why we are devoting a huge amount of effort to finding new sources of energy. But even if we learn to control thermonuclear fusion, it will not change our destructive nature.
Depending on the initial conditions, succession is divided into
- primary(when organisms colonize empty areas that have never been inhabited before) and
- secondary(the process takes place in places that were already inhabited, but lost their inhabitants as a result, for example, of glaciation or human activity).
Primary succession- the process of development and change of ecosystems in previously uninhabited areas, beginning with their colonization.
A classic example of primary succession is the development of a community on cooled lava or ash in the zone of action of a volcano, on rocks and stones. Initially, lichens appear, enriching the surface with nitrogen.
Concept and types, 2018.
After some time, mosses begin to develop in the biotope. After this, grass grows along with mosses, then small-leaved trees. It is not difficult to notice that all this time the soil is developing in the ecosystem, making it possible for the growth of increasingly complex organisms.
Secondary succession occurs where a biocenosis previously existed, but it was destroyed as a result of natural or anthropogenic factors.
For example, secondary succession begins in places of deforestation, on abandoned arable land, in abandoned villages, after natural disasters: floods, tsunamis, forest windfalls, earthquakes. The study of pyrogenic (arising as a result of fires) successions is of particular importance, since with the development of human society, the proportion of fires caused by humans increases.
Secondary succession ends with a stable community stage in 150–250 years, and primary lasts about 1000 years.
4.2.1 Climax ecosystem.
Succession ends with a stage when all species of the ecosystem, while reproducing, maintain a relatively constant number and no further change in its composition occurs. This equilibrium state is called climax, and the ecosystem is called climax. Under different abiotic conditions, different climax ecosystems are formed. In a hot and humid climate it will be a tropical rain forest, in a dry and hot climate it will be a desert. The main biomes of the earth are the climax ecosystems of the corresponding geographical areas.
The spruce forest is the last climax stage of ecosystem development in the climatic conditions of the North, i.e., it is already an indigenous biocenosis (Fig. 33).
Rice. 33. Successive successions during the formation of a spruce forest.
Initially, so-called pioneer species, such as lichens and encrusting algae, settle on the lifeless substrate). Over 5-10 years, they somewhat enrich the substrate with nutrients, forming the beginnings of soil. Then grasses settle on these still very poor soils, further enriching the soil. About 15 years from the beginning of succession, the first shrubs settle in the once lifeless space, which are gradually replaced by deciduous light-loving trees, most often birch and aspen, which are characterized by rapid growth.
Concept and types, 2018.
By the age of 50, the strongest trees stand out in the young deciduous forest, which shade the weaker shoots, which die, making it possible for spruce to settle under the canopy of the deciduous forest. Spruce is more shade-tolerant; under the protection of deciduous trees, it gradually catches up with them in growth, winning their living space. Around the age of 70, the ecosystem reaches the stage of mixed spruce-deciduous forest. By that time, deciduous trees have time to grow old, and gradually the spruce reaches the first tier, shading and thinning out all deciduous vegetation. By the age of 90, this ecosystem reaches the climax stage, which is characterized by the almost complete absence of deciduous trees; spruce becomes the dominant edificatory species, forming in a special way the entire life of the community inhabiting this ecosystem.
The law of thermodynamics is called the law of conservation of the structure of the biosphere).
Ecological succession - concept and types. Classification and features of the category "Ecological succession" 2017-2018.
Ecological succession is a change of biocenoses. If a biocenosis is stable, then it exists indefinitely. But we often have to observe how one biocenosis (ecosystem) turns into another: a lake becomes a swamp, and a meadow becomes a forest.
Types of successions
There are two types of succession: primary and secondary.
During primary succession, a new biocenosis is formed in an initially lifeless biotope. In this case, the colonization of rocky or sandy surfaces occurs.
The starting substrates can be:
- volcanic lava;
- sands;
- rocks;
- ravines;
- river sediments, etc.
Of particular importance in the colonization of such substrates is the accumulation of substances available to plants for root nutrition.
Rice. 1. Primary succession.
The first plants and bacteria to colonize lifeless surfaces change their chemical composition due to their metabolism, as well as when they die.
Any succession is long-lasting. Although every year during primary succession an enrichment of the species composition is observed, it will reach a state of stability after tens of years.
Secondary succession is the replacement of one biocenosis by another.
Its most common causes:
- changes in climatic conditions;
- establishing more stable relationships between species;
- human impact;
- change in geological conditions.
Every plant has limiting environmental factors. When the hydrological, soil or weather regime changes, some plants can leave the ecosystem, others can populate, changing its appearance.
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Rice. 2. Secondary succession.
Human activity contributes to the change of biocenoses. For example, in Africa and Asia, due to soil degradation due to livestock grazing, the desert ecosystem is replacing the savanna.
Modern steppes differ significantly in the species composition of plants from the pristine steppes. Therefore, areas of the surviving reference steppes are recognized as protected areas and are protected by law.
Features of succession
Let us consider what the main feature of ecological succession is: only such relationships between organisms and the organisms themselves are preserved over time that cannot be replaced by others under given conditions.
The leading role in the change of biocenoses belongs to plants.
Succession occurs by changing stages.
Stages of succession
Correctly composed ecological succession has the form of successively replacing each other stages.
Succession (from Latin succesio - continuity, inheritance) is a consistent irreversible and natural change of one biocenosis (phytocenosis, microbial community, etc.) to another in a certain area of the environment over time as a result of the influence of natural factors (including internal forces) or human impact.
There are many classifications of successions. They usually distinguish between primary and secondary successions.
Primary succession. Primary succession is usually understood as succession, the development of which begins on an initially lifeless substrate. Let us consider the course of primary succession using the example of terrestrial ecosystems. If we take areas of the earth's surface, for example abandoned sand pits, in various geographical areas (in forest, steppe zones or among tropical forests, etc.), then all these objects will be characterized by such patterns as: settlement by living organisms, an increase in their species diversity, gradual enrichment of soils with organic matter, an increase in their fertility, strengthening connections between different species or trophic groups of organisms, a decrease in the number of free, ecological niches, the gradual formation of increasingly complex biocenoses and ecosystems, increasing their productivity. Smaller species of organisms, especially plants, in this case, as a rule, they are replaced by larger ones, the processes of substance circulation are intensified, etc. In each case, successive stages of succession can be distinguished, by which we mean the replacement of some ecosystems by others, and successional series end with relatively little changing ecosystems. They are called climax (Greek climax - ladder), radical or nodal
Secondary successions. Secondary successions differ from primary ones in that they usually do not start from zero values, but arise in the place of disturbed or destroyed ecosystems. For example, after deforestation, forest fires, when areas that were under agricultural land are overgrown. The main difference between these successions is that they proceed incomparably faster than the primary ones, since they begin with intermediate stages (grasses, shrubs or woody pioneer plants) and against the background of richer soils
For any succession, especially primary, the following general patterns of the process are characteristic: .
1. At the initial stages, species diversity is insignificant, productivity and biomass are low. As succession develops, these indicators increase.
2. With the development of the succession series, the relationships between organisms increase. The number and role of symbiotic relationships is especially increasing. The habitat is becoming more fully developed, and power supply chains and networks are becoming more complex.
3. The number of free ecological niches decreases, and in the climax community they are either absent or at a minimum. In this regard, as succession develops, the likelihood of outbreaks in the numbers of individual species decreases.
4. The processes of circulation of substances, energy flow and respiration of ecosystems are intensified.
5. The speed of the successional process largely depends on the life expectancy of organisms that play the main role in the composition and functioning of ecosystems. 6. The immutability of the final (climax) stages of succession is relative. Dynamic processes do not stop, but only slow down. Dynamic processes continue due to changes in the environment, changes in generations of organisms and other phenomena. A relatively large share is occupied by dynamic processes of a cyclic (fluctuation) plan.
Flow of energy and cycles of substances in the biosphere
The cycle of substances is understood as a repeating process of transformation and movement of substances in nature, which has a more or less pronounced cyclic nature. All living organisms take part in the cycle of substances, absorbing some substances from the external environment and releasing others into it. Thus, plants consume carbon dioxide, water and mineral salts from the external environment and release oxygen into it. Animals inhale the oxygen released by plants, and by eating them, they assimilate organic substances synthesized from water and carbon dioxide and release carbon dioxide, water and substances from the undigested part of the food. When bacteria and fungi decompose dead plants and animals, additional amounts of carbon dioxide are formed, and organic substances are converted into minerals, which enter the soil and are again absorbed by plants. The continuous circulation of chemical elements in the biosphere along more or less closed paths is called the biogeochemical cycle. The need for such circulation is explained by the limited supply of them on the planet. To ensure the infinity of life, chemical elements must move in a circle. The cycle of substances is manifested in the repeated participation of substances in processes occurring in the atmosphere, hydrosphere, lithosphere, including those layers that make up the biosphere There are two main cycles of substances: large (geological) and small (biological, or biotic), which cover the entire planet. Geological (great) cycle - this is the exchange of substances between land and the World Ocean. First, global circulation of water occurs, i.e. first precipitation, then surface and underground runoff, infiltration, evaporation and finally condensation, then precipitation again The water cycle consumes almost a third of all solar energy reaching the Earth The large cycle of substances is not closed: a certain amount of substances is removed from the cycle and is preserved in sedimentary rocks in the form of limestone, peat, oil and other rocks and minerals. This will ensure the progressive development of the earth’s crust and biosphere. Geological and biological cycles are closely interconnected, interact with each other, sometimes merging together. But still, structurally and functionally they are significantly different biological cycle It has the following characteristic features in comparison with the geological one: its action occurs, as a rule, within the biogeocenosis, while the geological one occurs over large territories - continents and adjacent parts of the ocean;
The main cause and driving force of the biological cycle is the different nutritional patterns of producers, consumers and decomposers, and the geological cycle is the water cycle between the ocean and land; the small cycle involves only biogenic elements, while the large cycle involves all the chemical elements that are in the earth’s crust
The duration of cycles of chemical elements in the biological cycle is short-term (a year, several years, tens and hundreds of years), and the duration of the cycle in the geological cycle is tens and even hundreds of thousands of rocks. The cycle of substances, like all processes occurring in nature, requires a constant flow of energy. The basis of the biogenic cycle that ensures the existence of life is solar energy. The energy bound in organic substances at the stages of the food chain decreases, because most of it enters the environment in the form of heat or is spent on processes occurring in organisms. Therefore, a flow of energy and its transformation is observed in the biosphere. Thus, the biosphere can be stable only if there is a constant cycle of substances and an influx of solar energy.
Substances of the biosphere
While developing the doctrine of the biosphere, V.I. Vernadsky paid great attention to the planetary geochemical role of living matter that makes up the biosphere. As we have already noted, V.I. Vernadsky believed that the substance of the biosphere consists of several heterogeneous natural parts. Living matter - the entire set of bodies of living organisms inhabiting the Earth is physical and chemically united, regardless of their systematic affiliation. The mass of living matter is relatively small and is estimated at 2.4...3.6 1012 tons (in dry weight) and constitutes less than one millionth of the entire biosphere (approx. 3 1018 tons), which, in turn, represents less than one thousandth of the mass Earth. But this is “one of the most powerful geochemical forces on our planet,” since living organisms do not just inhabit the earth’s crust, but transform the appearance of the Earth. Living organisms inhabit the earth's surface very unevenly. Their distribution depends on geographic latitude.
Nutrient - a substance created and processed by a living organism. During organic evolution, living organisms passed through their organs, tissues, cells, and blood a thousand times over most of the atmosphere, the entire volume of the world's oceans, and a huge mass of minerals. This geological role of living matter can be imagined from deposits of coal, oil, carbonate rocks, etc.
Inert substance - products formed without the participation of living organisms.
Bioinert substance - a substance that is created simultaneously by living organisms and inert processes, representing dynamically equilibrium systems of both. These are soil, silt, weathering crust, etc. Organisms play a leading role in them.
A substance undergoing radioactive decay.
Scattered atoms , continuously created from all kinds of terrestrial matter under the influence of cosmic radiation.
Substance of cosmic origin .
Introduction
succession
1 Types of succession
2 Mechanisms of action of succession
3 Succession process
Change of ecosystems
1 Change of ecosystems under the influence of the vital activity of organisms
2 Change of ecosystems under the influence of human activities
3 Change of ecosystems under the influence of abiotic factors
Examples of ecosystem succession
Conclusion
Bibliography
Introduction
Ecological succession is a process of gradual change in the composition, structure and function of ecosystems under the influence of external or internal factors.
The restoration of the disturbed balance by the ecosystem goes through clearly defined stages.
An ecosystem can be thrown out of equilibrium in many ways. This usually happens due to fire, flood or drought. After such an imbalance, the new ecosystem restores itself, and this process is regular and repeats itself in a variety of situations. What happens in a disturbed ecosystem? At the site of a disturbance, certain species and the entire ecosystem develop in such a way that the order of appearance of these species is the same for similar disturbances and similar habitats. This sequential replacement of some species by others is the essence of ecological succession.
However, there is another model that explains the mechanism of succession as follows: the species of each previous community are displaced only by consistent competition, inhibiting and “resisting” the introduction of subsequent species.
However, this theory only considers the competitive relationships between species, without describing the whole picture of the ecosystem as a whole. Of course, such processes are taking place, but competitive displacement of previous species is possible precisely because they transform the biotope.
Thus, both models describe different aspects of the process and are valid at the same time. As we move along the succession series, there is an increasing involvement of nutrients in the cycle in ecosystems; a relative closure of the flows of such nutrients as nitrogen and calcium (one of the most mobile nutrients) within the ecosystem is possible.
1. Succession
1.1Types of successions Any ecosystem, adapting to changes in the external environment, is in a state of dynamics. This dynamics can concern both individual parts of ecosystems (organisms, populations, trophic groups) and the system as a whole. In this case, the dynamics can be associated, on the one hand, with adaptations to factors that are external to the ecosystem, and on the other, with factors that are created and changed by the ecosystem itself. These changes in some cases can be repeated to some extent, but in others they are unidirectional, progressive in nature and determine the development of the ecosystem in a certain direction. Primary successions. Primary usually refers to succession, the development of which begins on an initially lifeless substrate. Let us consider the course of primary succession using the example of terrestrial ecosystems. If we take areas of the earth's surface, for example, abandoned sand pits, in various geographical areas (in forest, steppe zones or among tropical forests, etc.), then all these objects will be characterized by such patterns as: · colonization by living organisms · increase in their species diversity · gradual enrichment of soils with organic matter · increase in their fertility · strengthening connections between different species or trophic groups of organisms · reduction in the number of free ecological niches · gradual formation of increasingly complex biocenoses and ecosystems · increasing their productivity. Smaller species of organisms, especially plants, are usually replaced by larger ones, the processes of substance circulation are intensified, etc. In each case, it is possible to distinguish successive stages of succession, by which we mean the replacement of some ecosystems by others, and the succession series end with relatively little changing ecosystems. They are called menopause (Greek menopause - stairs), radical or nodal Primary succession occurs in several stages. For example, in a forest zone: dry lifeless substrate - lichens - mosses - annual forbs - cereals and perennial grasses - shrubs - trees of the 1st generation - trees of the 2nd generation; in the steppe zone, succession ends at the grass stage, etc. Secondary successions. The term “secondary succession” refers to communities that develop in place of a previously existing community. In places where human economic activity does not interfere with the relationships between organisms, a climax community develops, which can exist for an indefinitely long time - until any external influence (plowing, logging, fire, volcanic eruption, flood) disrupts its natural structure. If a community is destroyed, succession begins in it - a slow process of restoring its original state. Examples of secondary successions: overgrowing of an abandoned field, meadow, burnt area or clearing. Secondary succession lasts several decades. It begins with the appearance of annual herbaceous plants in the cleared area of soil. These are typical weeds: dandelion, sow thistle, coltsfoot and others. Their advantage is that they grow quickly and produce seeds adapted to dispersal over long distances by wind or animals. However, after two or three years they are replaced by competitors - perennial grasses, and then by shrubs and trees, primarily aspen. These rocks shade the ground, and their extensive root systems take all the moisture from the soil, so that the seedlings of the species that first hit the field find it difficult to grow. However, succession does not stop there; a pine tree appears behind the aspen; and the last ones are slow-growing shade-tolerant species, such as spruce or oak. A hundred years later, the community that was on the site of the field before the foresting and plowing of the land is being restored on this site. Assistance. Pioneer species that appear in a new ecosystem make it easier for other species to subsequently colonize. For example, after a glacier retreats, the first to appear are lichens and some shallow-rooted plants—that is, species that can survive in barren, nutrient-poor soil. As these plants die, a layer of soil builds up, allowing late successional species to take root. Likewise, early successional trees provide shade and shelter for the growth of late successional trees. Containment. Sometimes pioneer species create conditions that complicate or even make it impossible for later successional plants to emerge. When new surfaces appear near the ocean (for example, as a result of the construction of concrete piers or breakwaters), they quickly become overgrown with pioneer species of algae, and other plant species are simply crowded out. This displacement occurs very easily, since the pioneer species reproduces extremely quickly and soon covers all available surfaces, leaving no room for subsequent species. An example of active containment is the emergence of bitterweed, an Asian plant that has spread throughout the American West. Gorchak significantly alkalizes the soil in which it grows, making it unsuitable for many wild herbs. Coexistence. Finally, pioneer species may not have any effect on subsequent plants at all - neither beneficial nor harmful. In particular, this occurs if different species use different resources and grow independently of each other (see Differential resource use). 1.3Succession process. The process of succession, according to F. Clements, consists of several stages: ) the emergence of an area unoccupied by life; ) migration of various organisms or their rudiments to it; ) their establishment in this area; ) their competition with each other and displacement of certain species; ) transformation of habitats by living organisms, gradual stabilization of conditions and relationships. Currently, almost the entire land surface accessible to life is occupied by various communities, and therefore the emergence of areas free from living beings is local in nature. These are either places that were vacated as a result of the retreat of glaciers, retreat of the water edge in reservoirs, landslides, erosion, etc., or that arose as a result of human activity, for example, the removal of large masses of deep-seated rocks to the top during the development of mineral resources. The introduction of spores, plant seeds, and the penetration of animals into the vacated area are mostly random and depend on what species are in the surrounding biotopes. Of the species that find themselves in a new habitat, only those whose ecological valency corresponds to a given set of abiotic conditions take root. Established species gradually occupy the entire new biotope, entering into competition with each other. As a result, a restructuring of the species composition and quantitative relationships of different forms occurs. In parallel, there is a process of transformation of the habitat itself under the influence of the developing community. The process ends with the formation of a more or less stable system with a balanced type of biological cycle. Succession of any scale and rank is characterized by a number of general patterns, many of which are extremely important for practical human activity. succession ecosystem organism abiotic 2. Change of ecosystems Reasons for changing ecosystems. Observing the same biogeocenosis, you can see how its appearance noticeably changes throughout the year. The scorched steppe at the end of summer is not like the same steppe in spring, colorful with blooming tulips, irises, primroses, and crocuses. The winter forest, dressed in snow caps, is completely different from the autumn forest, painted in orange, yellow, and crimson colors. The appearance of the meadow changes as different grasses bloom on it in spring and summer. At the same time, in addition to seasonal changes, long-term changes also occur in ecosystems. Despite the fact that an ecosystem is a stable, self-regulating system, it is characterized by development. The development of any system is understood as an irreversible qualitative change, which is usually accompanied by quantitative changes. Thus, in the process of ecosystem development, simple communities in it are replaced by more complex ones, with a rich species composition, with complex spatial and trophic structures. That is, the development of an ecosystem is based on the change of communities (plant, animal, fungal, microbiological) that are part of the biocenosis of a given ecosystem. Changes in natural communities can occur under the influence of biotic factors and humans. 1 Change of ecosystems under the influence of the vital activity of organisms The change of communities under the influence of the vital activity of organisms lasts hundreds and thousands of years. Plants play the main role in these processes. An example of a change in community under the influence of the vital activity of organisms is the process of overgrowing of water bodies. Most lakes gradually become shallow and decrease in size. Over time, the remains of aquatic and coastal plants and animals, as well as soil particles washed away from the slopes, accumulate at the bottom of the reservoir. Gradually, a thick layer of silt forms at the bottom. As the lake becomes shallower, its shores become overgrown with reeds and reeds, then with sedges. Organic residues accumulate even faster and form peaty deposits. Many plants and animals are replaced by species whose representatives are more adapted to life in new conditions. Over time, a different community forms in place of the lake - a swamp. But the change of communities does not stop there. Shrubs and trees that are unpretentious to the soil may appear in the swamp, and ultimately the swamp may be replaced by a forest. Thus, a change in communities occurs because, as a result of changes in the species composition of communities of plants, animals, fungi, and microorganisms, the habitat gradually changes and conditions favorable for the habitat of other species are created. It has been noted that the process of changing communities tends to end with the stage of a mature community: with a rich species composition, branched food networks, and the ability to self-regulate. As a result, a stable ecosystem is formed - one that is in relative balance with the environment. 2 Change of ecosystems under the influence of human activities Change of communities under the influence of human activity. If the change of communities under the influence of the life activity of the organisms themselves is a gradual and long process covering a period of tens, hundreds and even thousands of years, then the change of communities (underlying the change of ecosystems) caused by human activity occurs quickly, over several years. The rapid (spasmodic) development of ecosystems is often accompanied by a reduction in their species diversity and a slowdown in the processes of self-regulation and sustainability. As a result, in such ecosystems communities of a simplified type are formed, with a poor species structure. For example, people turn mixed-grass steppes into arable land, and floodplain meadows turn out to be flooded with reservoirs. Thus, the plowing of virgin lands in the second half of the twentieth century led to the destruction of natural steppe ecosystems in Kazakhstan and southern Russia. As a result, many species of insects, mammals, and various types of grasses disappeared. Suburban forests are under great pressure due to the massive number of people visiting them. Due to trampling of grass, the ground organs of plants are injured, the soil is compacted, and the undergrowth is damaged. As a result, the forest is thinned out and lightened. Shade-loving and shade-tolerant grasses are replaced by light-loving ones, characteristic of meadow ecosystems. Overgrazing of livestock changes meadow and steppe ecosystems: those grasses that are not eaten by animals (wormwood, thistle) are widely distributed, and the abundance of forage grains decreases. Many plants do not have time to bloom and produce seeds. As a result, the species diversity of the ecosystem decreases, its structure and food webs become simplified. Reservoirs also experience anthropogenic impact. If wastewater, fertilizers from fields, or household waste get into them, then the oxygen dissolved in the water is spent on their oxidation. As a result, species diversity decreases, various aquatic plants (floating salvinia, amphibian knotweed) are replaced by duckweed, algae are replaced by blue-green algae, and “water blooms” occur. Valuable commercial fish are being replaced by low-value ones, shellfish and many species of insects are disappearing. A rich aquatic ecosystem turns into an ecosystem of a decaying reservoir. There are many cases where humans have disrupted the species structure of an ecosystem as a result of introducing new species into it. So, at the beginning of the 19th century. The prickly pear cactus was brought to Australia from America to create thorny fences in pastures. It multiplied so much that it began to shape the appearance of many communities, displacing familiar plant species, and led to a change in a number of ecosystems. By the middle of the twentieth century. Australia could have turned into a continent of continuous thorny thickets, but this did not happen thanks to the cactus moth butterfly brought to the mainland, whose caterpillars eat prickly pear. After the cactus population was regulated with the help of caterpillars, the disturbed ecosystems gradually recovered. If the human impact that caused the change in communities stops, then, as a rule, the natural process of self-healing of the ecosystem begins. Plants continue to play a leading role in it. Thus, after the cessation of grazing, tall grasses appear on the pastures, typical forest plants appear in the forest, the lake is cleared of the dominance of unicellular algae and blue-greens, and fish, mollusks, and crustaceans reappear in it. If the species and trophic structure of the ecosystem are simplified so much that the process of its self-healing can no longer occur, then man is again forced to intervene in this natural community, but now for good purposes: grass is sown in pastures, new trees are planted in the forest, reservoirs are cleaned and watered. there are young fish there. An interesting experience is being used in the Stavropol Territory: hay is brought to already unproductive pastures, scattering it over the surface. Hay contains seeds of the entire complex of plant species in the steppe ecosystem. After three to four years, this area becomes close to the natural steppe. The ecosystem is capable of self-healing only with partial disturbances. Therefore, the influence of human economic activity should not exceed the threshold after which the ecosystem cannot carry out self-regulation processes. To do this, the human impact on ecosystems is normalized: they determine how many livestock can be kept per 1 hectare of pasture, how many vacationers can visit a suburban forest park, and compare the total amount of wastewater with what the aquatic ecosystem itself can neutralize. 3 Change of ecosystems under the influence of abiotic factors Change of ecosystems under the influence of abiotic factors. The development and change of ecosystems were and are greatly influenced by sudden climate changes, fluctuations in solar activity, mountain-building processes, and volcanic eruptions. These factors are called abiotic - factors of inanimate nature. They disrupt the stability of the habitat of living organisms. Let's look at examples of ecosystem changes under the influence of climate change. In the history of the development of life on Earth, the climate has changed several times. During warm periods, when there was a large amount of precipitation, ecosystems were dominated by species with increased requirements for heat and moisture. Tropical rainforests spread across the planet. Significant uplift of the land as a result of tectonic processes led to the development of an arid climate. As a result, a change in ecosystems occurred across a large part of the Earth: forests were replaced by savannas, steppes, and deserts arose. New ecosystems were characterized by a different complex of species and had a different species, spatial, and trophic structure.
Conclusion Ecosystems are dynamic; they constantly change in accordance with the daily and seasonal rhythms of changes in abiotic factors, with fluctuations in precipitation and temperature regimes in different years, under the influence of animal activity or the death of large plants. All these changes are reversible, and any state of the ecosystem that can be observed during such changes will sooner or later repeat itself. Ecosystems also recover “by themselves” after disturbances, when as a result of fire, deforestation, plowing of the steppe, etc. the entire ecosystem or a significant part of its species is destroyed. In a fire or clearing, a forest gradually grows, in an abandoned arable land the steppe is restored, etc. However, the restoration process can take a long time if there are no nearby areas from which the reviving ecosystem can be populated by plant and animal species. In each ecosystem, several types of changes, reversible and irreversible, occur simultaneously. Therefore, studying their dynamics can be quite difficult. Bibliography 1.Smirnova O.V., Toropova N.A. Succession and climax as an ecosystem process // Modern advances. biology. T. 128. - 2008, No. 2.- P. 129-144. 2.Akimova T.V. Ecology. Human-Economy-Biota-Environment: Textbook for university students / T.A. Akimova, V.V. Haskin; 2nd ed., revised. and additional - M.: UNITY, 2009. - 556 p. Recommended by the Ministry of Education. RF as a textbook for university students. .Odum Yu. Ecology vol. 1.2. World, 2006. 4. General ecology. Lazutkina Yu.S., Somin V.A. (AltSTU; 2007, 134 p.) Khandogina E.K., Gerasimova N.A., Khandogina A.V.. Ecological foundations of environmental management, M., “Forum”, 2007.
However, in addition to cyclical (circular) changes, irreversible changes in ecosystems occur, during which the composition of species and (or) its productivity and biomass changes. Such changes are called ecological successions. Succession of ecosystems is also very diverse and occurs under the influence of the vital activity of their biota (for example, when rocks or lakes become overgrown) or under the influence of external factors, usually associated with human activity (grazing, trampling, the entry of runoff rich in nutrients into a reservoir, etc.). d.).
If the influence of the external factor stops (excess livestock leaves the pasture, new portions of pollutants do not enter the reservoir, suburban forests are landscaped, and vacationers walk along special paths), then the succession caused by this factor will stop. And then another, restorative succession will begin instead. Moreover, the process will proceed on its own, in much the same way as the succession of overgrowing rocks. The grass will be restored in the pasture, the water in the reservoir will be cleared and green and diatom algae will again replace cyanobacteria, the composition of macrophyte plants, fish, amphibians, etc. will be restored.
