Existence and development
Part 3. Ecosystem diversity and dynamics
1. Evolution of ecosystems. Ecological successions.
2. Anthropogenic ecosystems.
2.1. Agroecosystems and their features.
2.2. Urban ecosystems.
Evolution of ecosystems. Ecological succession
The dynamics of an ecosystem are determined by a series of successive communities. The relatively long existence of a biocenosis in one place (pine forest, swamp) changes the biotope so that it becomes unsuitable for the existence of some species, but suitable for the 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.
Ecological succession(from lat. succession– sequence, change) - a gradual change in an ecosystem, the development of an ecosystem, in which within the same territory (biotope) there is a consistent replacement of one biocenosis by another in the direction of increasing the stability of the ecosystem.
The term “succession” was first used by the French botanist De Luc in 1806 to refer to changes in vegetation.
Succession is a process of self-development of ecosystems. Succession is based on the incompleteness of the biological cycle in a given biocenosis. It is known that living organisms, as a result of their vital activity, change the environment around them, removing some substances from it and saturating it with metabolic products. When a population exists for a relatively long time, they change their environment in an unfavorable direction and, as a result, find themselves displaced by populations of other species, for which the resulting environmental transformations turn out to be ecologically beneficial.
Succession processes take certain periods of time, most often years and tens of years. But there are also very rapid changes in communities, for example in temporary reservoirs, and very slow ones - secular changes in ecosystems associated with evolution on Earth.
The reason for the start of succession are changes in the fundamental properties of the habitat that arise under the influence of a complex of factors. Such factors are natural– glacier retreat, floods, earthquakes, volcanic eruptions, fires, and anthropogenic– clearing forest lands, plowing steppe areas, open-pit mining, creating ponds, reservoirs, fires.
Succession caused by external causes - exogenetic (allogeneic) and internal reasons – endogenetic (autogenous).
Exogenetic(allogeneic) successions - in this case, successional changes are caused by external, abiotic reasons; arise from various human impacts on biocenoses (reclamation drainage of swamps, pollution of water bodies, grazing)
Endoecogenetic(internal) successions are caused primarily changing the structure and system of connections in existing communities– overgrowing of rocks, overgrowing of lakes, roadsides, forest restoration after deforestation or fire.
By general character successions are divided into primary And secondary successions .
Primary successions begin on a substrate that is not modified by the activity of living organisms. For example, the formation of rock biocenoses or the formation of phytocenosis on glacial deposits.
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 (Fig...). A number of stages of overgrowth are observed here - first, a floating carpet is formed along the edges of the lake - a floating carpet of sedge, moss, etc.; then the lake is filled with dead plant remains - peat; the resulting swamp is overgrown with forest.
Rice. Succession during overgrowing of a small lake
Secondary successions have a restorative, demutational character. They develop on a substrate that was initially modified by the activity of living organisms that previously existed in a given place - before a fire, flood, deforestation, etc. In such places, the soil or bottom sediments are usually not destroyed, i.e. rich vital resources are preserved (the supply of “remains of life”) and succession is most often restorative (Fig.).
Rice. Phases of a typical terrestrial succession (according to N.F. Reimers) after a forest fire: A – meadow, B – overgrowing with shrubs, C – birch or aspen forest,
G – mixed forest, D – pine forest, E – pine-cedar forest,
F – cedar-fir forest
The change of succession phases occurs in accordance with certain rules. Each previous phase prepares the environment for the emergence of the next, and species diversity and layering gradually increase. Following the plants, representatives of the animal world are involved in succession, and the developing biocenosis becomes richer in species; The food chains in it become more complex, develop and turn into power networks. The activity of decomposers is activated, returning organic matter from the soil to the biomass.
Secondary, anthropogenic succession also manifests itself in eutrophication. The rapid “blooming” of water bodies is the result of their enrichment with nutrients caused by human activity.
Ecological succession is one of the most striking expressions of the mechanism for maintaining homeostasis at the ecosystem level.
According to Clements(founder of the theory of succession, 1916), succession goes through phases:
· exposures (appearance of uninhabited space);
· migration (settlement by pioneer life forms);
Ecesis (colonization and adaptation to specific environmental conditions);
· competitions (competition with displacement of a number of primary invaders);
· reactions (reverse impact of the community on the biotope and living conditions);
· stabilization (formation of climax biocenosis).
An important stabilization mechanism is competitive relations.
Relationships between organisms in successional series can be of three categories:
Facilitation or stimulation model – corresponds to the phase of endoecogenetic succession.
Early settlers, through their activities, change the environment, making it accessible to the next wave of colonists.
Model of tolerance– competitive relations, selection of more tolerant and competitive species occurs. The change of species is based on their differences in resource consumption strategies. Later stages are more stable.
Inhibition model– all species of the community are able to simultaneously colonize the opened habitat and are resistant to the invasion of competitors, but later invaders are able to gain a foothold only after the loss of their predecessors.
The first plants are pioneer communities. A community is a set of interacting populations occupying a certain territory, a living component of an ecosystem.
Climax ecosystem. The community structure is being created gradually. Bare rock (volcanic island): algae, lichens enter and form pioneer communities ® soil ® mosses and ferns ® grasses ® shrubs (trees, shrubs) ® seed plants. 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 is called menopause , and the ecosystem is menopause .
As we approach menopause, the cycles of nutrients become increasingly closed and slow.
The climax community has one dominant or several codominant species. Oak forest - oak, forest - pine, steppe - feather grass, etc., but it is not always possible to identify dominants (tropical forest, ocean, savanna). Dominant species(from lat. dominantis- dominant) is a species that is dominant in numbers, biomass and development. Edifier species(from lat. edificatory- builder) - species that, through their vital activity, shape the habitat to the greatest extent, predetermining the existence of other organisms.
That. succession ends with the formation of a community that is most adapted to the complex of existing climatic conditions. Such a community was named by F. Clements climax formation or simply menopause (from Greek klimax- ladder). The concept of “climax” implies that within a region with a more or less homogeneous climate, phytocenoses that have completed the succession process form a mature community.
The theory of succession was developed in 1916 by Clements. He created the concept of monoclimax (in given climatic conditions, only one climax community can exist). According to Clements, the main factor determining the composition of the climax community is climate. In hot and humid climates it is a tropical rainforest, in dry and hot climates it is a desert. The Earth's major biomes are the climax ecosystems of their respective geographic areas.
The modern concept is polyclimax: menopause is formed under the influence of all physical factors, one or more may dominate (drainage, soil, temperature, topography, fires).
Anthropogenic ecosystems
In the biosphere, in addition to natural biogeocenoses and ecosystems, there are communities artificially created by human economic activity - agroecosystems, urban ecosystems.
Related information.
An ecological system (ecosystem) is a spatially defined set of living organisms and their habitat, united by material, energy and information interactions.
The term “Ecosystem” was introduced into ecology by the English botanist A. Tansley.
In natural ecosystems, constant changes in the state of populations of organisms occur. They are caused by various reasons.
Ecological succession proceeds through a series of stages, with biotic communities replacing each other. The replacement of species in succession is caused by the fact that populations, seeking to modify the environment, create conditions favorable for other populations. This continues until an equilibrium is reached between the biotic and abiotic components. The sequence of communities replacing each other in a particular area is called a series; only a few species persist from the initial stages of succession to the mature state of the ecosystem.
The succession process includes several stages: the emergence of an area not occupied by life; immigration, as well as the introduction of various organisms and their rudiments onto it; settlement of the site; competition and displacement of certain species; transformation of habitat by organisms, gradual stabilization of conditions and relationships.
The introduction of spores, seeds, and the penetration of animals into the vacated area occurs mostly accidentally and depends on what species are in the surrounding biotopes. Of the species that arrive at a new location, only those whose ecological valence corresponds to the abiotic conditions of the given habitat are established. New species gradually occupy the biotope, compete with each other and displace the species least adapted to these conditions. Thus, both the restructuring of the community and the transformation of the habitat by the community occur in parallel. The process ends with the formation of a more or less stable ecosystem, ensuring a cycle of substances in which the impact on the environment is minimal.
During the final stages of wood decay, the soft, moss-covered trunk provides shelter for many small animals such as molluscs, millipedes, ants and other invertebrate animals. These, in turn, attract predators, and a new community forms in the trunk for some time. Each stage of destruction of a fallen spruce trunk is characterized by its own set of species and lasts longer than the previous ones. Only at certain intervals is it possible to register representatives of both successive communities. Thanks to their joint activities, over 100-150 years, the wood of a fallen tree is completely recycled.
If the development of an ecosystem begins in an area that was not previously occupied by any community (recently exposed rock, sand, or lava flow), the process is called primary succession. If the development of an ecosystem occurs in an area from which the previous community has been removed (for example, an abandoned field or clearing), then this will be secondary succession. It usually proceeds faster than the primary one, since the territory that was previously occupied already contains some organisms necessary for the exchange of substances with an environment more favorable for the development of the community than the “sterile” zone.
An example of primary succession is the overgrowing of the sand dunes of the lake. Michigan. The community of early settlers on the dunes consists of grasses, willow, cherry, cottonwood, and animals such as jumping beetles, burrow spiders, and grasshoppers. The community of the first settlers is followed by forest communities, each of which has its own animal world. Despite the fact that development began in a very dry and barren place, eventually a beech-maple forest grows here, unlike the bare dunes, it is wet and cold. The thick, humus-rich soil with earthworms and shellfish contrasts with the dry sand on which it was formed.
As an example of secondary succession, we will cite the restoration of a spruce forest. After cutting down or a fire, the conditions at the site of the spruce forest change so much that the spruce cannot repopulate the vacated area. In open areas, spruce seedlings are damaged by late spring frosts, suffer from overheating and cannot compete with light-loving plants. In the first two years, herbaceous plants develop wildly in clearings and burnt areas: fireweed, reed grass, etc. Soon numerous shoots of birch, aspen, and sometimes pine appear, the seeds of which are easily dispersed by the wind. Trees displace herbaceous vegetation and gradually form small-leaved or pine forest. Only then do conditions favorable for spruce regeneration arise.
Shade-tolerant spruce seedlings successfully compete with the undergrowth of light-loving deciduous trees. When the spruce reaches the upper tier, it completely displaces deciduous trees. In principle, the succession of the fir-cedar taiga proceeds in the same way (Fig. 1).
Each subsequent stage of succession lasts longer than the previous one, is characterized by a higher ratio of biomass to unit of energy flow and its own dominant species. Dominant plant species have a particularly strong impact on the environment.
The great contribution of plants to the formation of a community is associated not only with their role as primary producers, but also with the fact that they slowly decompose. Plants form not only biomass, but also the main part of necromass, i.e. dead organic matter.
Rice. 1.
The numbers show the time (in years) of the onset of succession phases (their end dates are indicated in parentheses). Biomass and biological productivity are given on an arbitrary scale.
Despite the high activity of bacteria and detritivores, plant debris accumulates in the form of leaf litter or peat. The ability of shrubs and trees in moderately humid habitats to displace grass vegetation is largely related to the development of their crown and root system. In turn, succession in terrestrial habitats entails a regular change of plant forms.
Plants found in early and late stages of succession are characterized by different growth and reproduction strategies. Plants belonging to the early stages of succession, due to their high ability to disperse, quickly occupy newly formed or disturbed habitats. Late-successional species spread and grow more slowly, but the shade tolerance of their undergrowth and the large size of mature plants give them advantages in competition with species forming early stages of succession. Plants of terminal communities are adapted to grow and thrive in the environment that they themselves create, while species that appear in the early stages of succession have the ability to colonize environments that are not yet used.
Animal bodies decompose much more quickly, but sometimes their remains, like plant remains, determine the structure of the community and the course of succession. This occurs, for example, when calcified skeletons accumulate during coral growth. More often, animals react passively to vegetation succession. It is possible, of course, that seed-eating birds also influence the change of vegetation.
Communities that replace each other in the process of ecosystem development are characterized by different characteristics. Thus, immature ecosystems in the early stages of ecological succession are characterized by low species diversity and simple nutritional patterns: many producers, herbivores and few decomposers. Plants, mostly annual grasses, spend most of their energy producing small seeds for reproduction rather than on their root systems, stems and leaves. They receive nutritional material, as a rule, with runoff from other ecosystems, since they themselves cannot retain and accumulate nutrients.
Mature ecosystems are characterized by species diversity, stable populations, and complex feeding patterns. The system is dominated by decomposers that decompose large amounts of dead organic matter. The plants are represented by large perennial herbs and trees that produce large seeds. They spend the bulk of energy and nutritional materials on maintaining the root system, trunk, leaves, and not on the production of new plants. Such ecosystems themselves extract, retain and process part of the nutrients they need.
During the development of the community, the total biomass increases, while the maximum productivity occurs in one of the intermediate phases of succession. Typically, during development, the number of species increases, since with increasing plant diversity, niches appear for an increasing number of species of insects and other animals. However, the community that forms at the final stage of development is inferior in species richness to communities of earlier stages. In climax communities, factors other than those leading to species diversity are more important. These factors include an increase in the size of organisms, which allows them to store nutrients and water to survive during periods when they are scarce. This and other factors lead to increased competition between species and a reduction in their number in later stages of development.
The terminal, or stable, community of a developing series is the climax community. In the climax community, in contrast to communities of developing and other unstable stages, the annual net production of organic matter is minimal or completely absent. For each natural zone it is convenient to distinguish between a single climatic climax and a different number of edaphic climaxes. Climatic climax is a theoretical community towards which the entire development of an ecosystem in a given area is aimed, being in balance with general climatic conditions.
The theoretical community is implemented where the physical conditions of the environment are not so extreme as to alter the effects of the prevailing climate.
Rice. 2.
Where the terrain, soil, water bodies, swamping and other factors prevent the development of climatic climax, succession ends with the formation of an edaphic climax. Thus, depending on the topography and soil characteristics, different communities develop on adjacent sea terraces with the same parent rock (Fig. 13.4). Since the main modifying factor of an ecosystem is the biotic community, the more extreme the physical conditions of the environment, the greater the likelihood that the development of the ecosystem will stop without reaching equilibrium with general climatic conditions.
Humans often influence the development of an ecosystem, preventing it from reaching a climax state. When a community that does not represent the climatic or edaphic climax for a given area is maintained by humans or domestic animals, it is called disclimax, or anthropogenic subclimax. For example, overgrazing can create a desert community where the regional climate could have preserved steppe. The desert community in this case is a disclimax, and the steppe is a climatic climax.
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.
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).
Thirdly, the culmination of succession is the emergence of a stabilized ecosystem in which per unit of 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, as a result of 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 of this, willow is replaced by small-leaved tree species: birch, hazel. 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 excess supply of biogenic chemical elements, mainly nitrogen and phosphorus, to the reservoir along with organic matter, a “blooming” of water occurs: unicellular algae multiply in huge quantities. Lake ecosystems are “aging”—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. At the same time, life forms, 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 succession classification.
It is necessary to distinguish autotrophic And heterotrophic succession . 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. That is, 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. 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. Therefore, we are devoting a huge amount of effort to finding new sources of energy. But even if we learn to control thermonuclear fusion, this 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. 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 rainforest, in a dry and hot climate it will be a desert. The main biomes of the earth are the climax ecosystems of their respective geographic areas.
The spruce forest is the last climax stage of ecosystem development in the climatic conditions of the North, that is, it is already a native 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. 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, shaping 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).
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.
Question 30. Succession. Primary and secondary succession.
Succession is an irreversible change in one biocenosis, the emergence of another. It can be caused by any natural phenomena or occur under human influence. Ecological succession was initially studied by representatives of a science such as geobotany. Subsequently, this phenomenon became a subject of interest to other ecologists. The pioneers who revealed the importance of succession were F. Clements, V. N. Sukachev, S. M. Razumovsky.
Primary and secondary succession. What do these concepts mean? Let's look further. Primary succession is characterized by the fact that it takes place in a lifeless area. This could be bare rock without vegetation, sandy areas, solidified lava, and the like. When organisms begin to inhabit such areas, their metabolism affects and changes the environment. Then more complex development begins. And then the species begin to replace each other. An example of succession is the formation of the original soil cover, the colonization of an initially lifeless sandy area, first of all by microorganisms, plants, and then fungi and animals. A special role here is played by plant remains and substances resulting from the decomposition of organic matter. Thus, the soil begins to form and change, and the microclimate changes under the influence of microorganisms, plants and fungi. As a result, the community of organisms expands. This succession is an ecogenetic change. It is called that because it changes the very territory on which it exists. And the initial appearance of soil in a lifeless area is called syngenetic change.
Secondary successions. These processes lead to the colonization of the territory by species after some damage. For example, a forest partially destroyed by fire. The territory where it was previously located retained the soil and seeds. A grass community will be formed literally next year. And then deciduous trees appear. Under the cover of aspen or birch forests, spruce trees begin to grow, subsequently displacing deciduous trees. The restoration of dark coniferous trees occurs within approximately 100 years. But the forest in some areas is being cut down again. Due to this, recovery does not occur in such areas.
Question 31. Name the successive stages of primary succession. Climax.
A.G. Voronov (1940, 1973) distinguishes two phases in the primary succession of vegetation on bare soils or ground:
Colonization of bare territory and the formation of a phytocenosis from plants settling in the bare area.
Replacement of one formed phytocenosis by another.
a) factors determining the development of vegetation in the first phase of succession - in bare areas
Plants penetrate into the vacated territory by transferring diaspores (seeds, spores, plant pieces) with the help of wind, water, animals or humans, or through the gradual vegetative growth of plants located near the boundaries of the bare territory. The composition of a new phytocenosis is often dominated by plants with diaspores that are easily carried by the wind, and near water - with diaspores that adhere well to the water. Often the action that caused the bareness of the soil (deposition of sediment by water, blowing of sand by the wind) also contributes to the appearance of diasporas in this territory, i.e. human activity. That is why weeds and ruderal plants grow so quickly in these areas.
Cases of plant introduction into a new territory from the edges only through the formation of vegetative underground or above-ground shoots without the formation of generative organs are observed many times less often than introduction through the introduction of seeds.
The settlement of a new territory depends on a number of factors that are random in relation to the characteristics of the territory itself:
Depending on what plants and how far they grow near the disturbed area,
From their quantity,
From the direction of the prevailing wind,
From the height and strength of the flood,
From the quality of the substrate of the plots,
On the nature of hydration, etc.
It should be noted that the lightness of seeds, which facilitates their transfer by wind, is achieved by reducing nutrient reserves, and this negatively affects the development of seedlings, reducing the chances of their preservation.
b) echesis and its features
After the plant has penetrated the bare area, it begins to adapt to the new conditions. The process of adaptation of plant individuals to new conditions is called ecesis. It ends when the plant has produced fruits and seeds.
Not all diasporas that enter a bare area germinate immediately. The seeds of most species remain viable for a long time, often for tens or even hundreds of years. Moreover, they do not germinate in one year, but under a favorable combination of circumstances. This provides conditions for better preservation of seedlings
EXAMPLE. Lespedetsa in the oak trees around the village. Gornotaezheoe (Ussuriysk region) resumed in the first year after the fire, forming a continuous cover. There had been no fire for more than 20 years. Only a few species (horse chestnut, choicenia, willow, etc.) have seeds that lose their viability within a few days or a few weeks.
The seeds that form the soil reserve often belong to plants of different life forms and therefore ensure the development of plants under various environmental conditions (seeds of some species germinate at higher temperatures, others at lower temperatures, some at higher soil moisture, others at lower, etc.). d).
Plants that have invaded the bare area begin to bear fruit and themselves become a source of diaspores. Now diasporas enter the populated area not only from the outside, but also from those plants that are already growing and bearing fruit here.
Depending on the living conditions, the bare territory is inhabited by one or several species. The harsher the conditions, the fewer plant species can begin to develop here. The poorest composition of seedlings is typical for highly saline soils, rock outcrops, etc.
When a plant moves from the seedling stage to later stages of development, its needs for water and food increase, and the reserves of nutrients in the seed or fruit are exhausted by this time, and the plant is entirely dependent on the food resources of the external environment. Therefore, as plants grow, competition intensifies. The more severe the environmental conditions, the greater the role for plants entering a given territory, played by the direct influence of external conditions, and the less important is competition. The less severe the environmental conditions, the less role external conditions play and the greater the importance of competition.
c) stages of development of phytocenosis of primary succession (according to A.G. Voronov)
A pioneer group is a random combination of plants. Phytocenoses that form in bare areas are characterized at the first stage of development by:
Random composition of plants,
The absence of a closed plant carpet,
Low impact on the environment and
Almost complete absence of mutual influence between individuals.
Pioneer group May be clean(single-species, Fig. 6), both in the lower part of the slope with sea buckthorn thickets, and mixed(multi-species) - on the same slope, in other areas. If environmental conditions quickly change in the direction of increasing severity (for example, the soil dries out, becomes salinized, etc.), then the number of species settling in the technogenic area decreases and the mixed pioneer group becomes impoverished and, in the end, can turn into a pure pioneer group.
Simple grouping– the next stage of development of the phytocenosis after the pioneer group. In this grouping the vegetation cover is:
In the above-ground part they are not closed, but the plants are located much closer together than in the pioneer group.
The mutual influence of plants is clearly visible.
The group distribution of plants is common: around the individual that produced the seeds, its offspring develop.
Simple groups, like pioneer groups, can be pure (single-species) or mixed (multi-species), formed by several species, and the plants in them, unlike mixed pioneer groups, always belong to one life form. Simple groups are usually formed by some of the species that were part of the pioneer groups.
Simple mixed groups that have existed for a very long time - communities of the same type (for example, crustose) lichens on stones. Simple groups usually represent the weedy stage of the deposit.
Complex grouping– the stage of development of phytocenosis following simple grouping. It is characterized by the following features:
The species composition is not completely constant,
The community is not closed - new species can easily penetrate into it;
Species are not yet diffusely distributed, although individuals of other species may penetrate into clusters of individuals of one species;
Tiers are outlined;
The mutual influence of plants becomes even more noticeable;
Usually formed by several species of different life forms.
EXAMPLE. An overgrown valley in the construction area of the Ussuri Thermal Power Plant. Complex groups here form cattails (in hollows), sweet clover, sedges of various sizes, and small grasses. The vegetation cover is sparse, but tiers have already emerged: - cattail - up to 1 m, sweet clover, quinoa, wormwood, aspen grass - 0.7-0.8 m, sedges - 0.4-0.5 m, small cereals and grasses not tall more than 10 cm.
Stage of closed phytocenosis - the next stage of development of phytocenosis is characterized by:
It is extremely difficult for new species to penetrate into it.
Uniform, not too dense distribution of individuals of individual species.
Group growth is the exception.
It is represented by two forms of plant combinations - phytocenoses of thickets and 2 or more tiered phytocenoses.
Thickets develop under conditions in which a community of a large number of species cannot exist: high salinity, extreme dryness, waterlogging, high competition, etc. Single-tiered. A layer is formed either by one species (pure thickets) or by several species (mixed thickets).
Multi-tiered phytocenosis(simple of 2 tiers, complex - of more than 2 tiers), developing not in such harsh conditions as thickets. These are all types of meadows (floodplain, upland, fallow), all forest communities. One should not think that the stage of development of a phytocenosis ends its dynamics. It enters the second phase of the succession process: the replacement of one formed phytocenosis by another.
Not in all cases, a phytocenosis necessarily goes through all of the listed stages in sequence - pioneer group ® simple group ® complex group ® thickets or complex phytocenosis. This path can be both easier and more difficult.
EXAMPLE. On the rocks: often a pioneer group of blue-green algae ® a pioneer group of lichens ® a mixed or simple group of lichens ® a mixed long-existing thicket of lichens ® a complex phytocenosis involving lichens, mosses ® a complex phytocenosis ® flowering plants.
On the bottom of a fresh lake freed from under the water: mixed pioneer (hygro) group ® pure pioneer group (xerophyte) ® pure simple group ® mixed simple group ® complex group ® complex phytocenosis. In other cases, a pure grouping is replaced by a pure thicket, which exists in this area for an indefinitely long time.
Thus, the path of development of a phytocenosis is varied: longer and shorter, including some stages or others. But in all cases, its development proceeds from a separate-group composition to a diffuse one, from an open cover to a closed one, from an open cover to a closed one.
d) stages in the development of vegetation according to V.N. Sukachev
V.N. Sukachev (1938, 1964, etc.) identified the following stages of phytocenosis formation:
1. Absence of phytocenosis (corresponds to a pioneer group in the initial stage of its existence).
2. Open phytocenosis (corresponds to a pioneer group in a significant part of the period of its existence and a simple group).
3. Closed, undeveloped phytocenosis (corresponds to a complex grouping).
4. Evolved phytocenosis.
e) the essence of the concepts of syngenesis, endoecogenesis and hologenesis
At the very first stages of community development, the process that V.N. Sukachev (1942) called it syngenesis. This is the process of the initial formation of vegetation cover, associated with the invasion of plants into a given territory, their establishment (ecesis), and then competition between them for means of life. Then another process begins, called endoecogenesis by V.N. Sukachev. This is the process of changing a phytocenosis under the influence of an environment changed by itself. Endoecogenesis gradually intensifies and, in the end, becomes the main process that determines the course of changes in the phytocenosis.
A third process, called hologenesis by V.N. Sukachev (1954), is superimposed on these two processes. This is “the process of changing vegetation cover under the influence of the entire geographical environment or its individual parts: atmosphere, lithosphere, etc., i.e. changes in a larger unity that includes a given biogeocenosis.
All three processes occur simultaneously, but at different stages of development one of them acquires predominant importance. Undoubtedly, syngenesis dominates only at the initial stages of development of the phytocenosis, and then the dominant role passes to endoecogenesis. The hologenetic process occurs constantly, but, obviously, at turning points in the geological history of the Earth, its role intensifies.
This course of development of the phytocenosis continues for more or less time until some external forces, random in relation to the course of development of the phytocenosis, sharply disrupt it. Then the change caused by the internal development of the phytocenosis itself (endodynamic) is interrupted, and the change caused by an external push (exodynamic) begins.
Based on the above, stand out two main types of changes in phytocenoses(Sukachev, 1928):
1. endodynamic, occurring as a result of the gradual development of the phytocenosis itself, changing the environment and at the same time changing; The main role is played by the internal characteristics of the community.
2. exodynamic(Sukachev, 1928; Lavrenko, 1940), or spontaneous (Yaroshenko, 1953), or sudden (Yaroshenko, 1961), arising under the unforeseen influence of external factors.
The reasons for the occurrence of successions (changes) of vegetation cover are very diverse.
In the process of succession, biogeocenoses arise that best correspond to environmental conditions, both climatic and edaphic, and also consist of species “adapted” to cohabitation with the phytoclimate and hydrological regime characteristic of this cenosis. The habitat within such a cenosis was transformed by him. This final stage of succession is called climax. menopause.