Succession. Examples of ecosystem succession
succession
Types of successions
Secondary succession
Types of successional changes
Duration of succession
Examples of ecosystem succession
Communities are constantly changing. Their species composition, the number of certain organisms, the trophic structure and other indicators of the community change.
Communities change over time.
Succession is a consistent, natural replacement of some communities by others in a certain area of the territory, caused by internal factors of ecosystem development.
In order to understand the nature of ecological succession, imagine an IDEAL community (that is, the total production of autotrophs in energy terms exactly corresponds to the energy costs used to ensure the vital activity of its constituent organisms).
In ecology, the total energy consumption is called - the common breath of the community.
It is clear that in such an ideal case, the production processes are balanced by the respiration processes.
Consequently, the biomass of organisms in such a system remains constant, and the system itself remains unchanged or in equilibrium.
If the “total respiration” is less than the gross primary production, an accumulation of organic matter will occur in the ecosystem;
If it is more, it will be reduced.
In both the first and second cases, community changes will occur
If there is an excess of a resource, there will always be species that can master it, and if there is a shortage, some species will go extinct.
This change is the essence of ecological succession.
The main feature of this process is that changes in the community always occur in the direction of an equilibrium state.
1.1 Types of succession
Succession that begins in a place devoid of life (such as a newly formed sand dune) is called primary succession.
In nature, primary successions are relatively rare and last much longer than secondary ones - up to several centuries.
Primary succession- this is the overgrowing of a place that was not previously occupied by vegetation: bare rocks or frozen volcanic lava.
Example:
Formation of a community on an exposed area of rock, an area of solidified volcanic lava, on a newly formed sand dune, or after the retreat of a glacier.
Only a few plants are capable of living on such soil; they are called pioneers of succession. Typical pioneers are mosses and lichens. They change the soil, releasing acid that breaks down and loosens rocks. Dying mosses and lichens decompose under the influence of decomposer bacteria, and their remains are mixed with a loose rocky substrate (sand).
This forms the first soil on which other plants can grow. The need to destroy the parent rock is the main reason for the slow progress of primary successions; note the increase in soil layer thickness as succession progresses.
On soil poor in nutrients, grasses settle, which are more specifically capable of displacing lichens and mosses. The roots of grasses penetrate into the cracks of the rock, push these cracks apart and destroy the stone more and more.
Grasses are being replaced by perennial plants and shrubs, such as alder and willow. On the roots of alder there are nodules - special organs containing symbiotic bacteria that fix atmospheric nitrogen and contribute to the accumulation of large reserves in the soil, due to which the soil becomes more and more fertile.
Now trees can grow on it, such as pine, birch and spruce.
Thus, the driving force of succession is that plants change the soil beneath them, affecting its physical properties and chemical composition, so that it becomes suitable for competing species, which displace the original inhabitants, causing a change in community - succession, due to plant competition They do not always live where conditions are better for them.
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.
1.2 Secondary succession
The term "secondary succession" refers to communities that develop in place of a pre-existing, previously formed 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.
VEINIK- a genus of perennial herbaceous plants of the Poaceae or Poa family
Rice. 8.7. Secondary succession of the Siberian dark coniferous forest (fir-cedar taiga) after a devastating forest fire.
1.4 Duration of succession
The duration of succession is largely determined by the structure of the community. Studies of primary succession in places such as sand dunes indicate that, under these conditions, climax takes many hundreds of years to develop. Secondary successions, for example in clearings, proceed much faster. Still, it takes at least 200 years for the forest to recover in a moderate, humid climate.
If the climate is particularly harsh (as in the desert, tundra or steppe), the duration of the episodes is shorter, since the community cannot significantly change the unfavorable physical environment. Secondary succession in the steppe, for example, lasts about 50 years.
The main stages of secondary succession in temperate climates:
· the first stage of herbaceous vegetation lasts about 10 years;
· second stage of bushes? from 10 to 25 years;
· third stage of deciduous trees? from 25 to 100 years;
· fourth stage of coniferous trees? more than 100 years.
Successions can be of different scales. They can go slowly, over thousands of years, or quickly, over several days.
The duration of succession is largely determined by the structure of the community.
During primary succession, it takes many hundreds of years for the development of a stable community.
Pay attention!
The need to destroy the parent rock is the main reason for the slow progress of primary successions.
Secondary successions proceed much faster. This is explained by the fact that the primary community leaves behind a sufficient amount of nutrients and developed soil, which creates conditions for the accelerated growth and development of new settlers.
Example:
In Europe at the end Pliocene (3 million years ago) the Ice Age began. The glacier destroyed all life under its cover. He tore off and smoothed the soil cover, crushed rocks. With its retreat and climate warming, vast expanses of bare, lifeless land were exposed. Gradually it was populated by various plants and animals. Of course, these changes happened very slowly. Where the glacier destroyed tropical forests, their restoration continues to this day. These areas have not yet reached a steady state. So they didn’t have enough millions of years to complete succession.
The changes that led to broad-leaved forests also came slowly. Miocene (20 million years ago) to the current northern Central Asian deserts.
Succession occurs much faster after a forest fire, when one biocenosis is replaced by another in a certain sequence, which finally leads to the restoration of a stable community.
Fouling of exposed cliffs occurs relatively quickly: sections of rock as a result of erosion or landslide.
The fastest successions are observed in a temporary reservoir or when changing communities in the decomposing corpse of an animal, in a rotting tree trunk, in an infusion of hay.
General patterns of succession
In general, the phenomenon of ecological succession can be characterized by the following provisions:
Succession is a natural process, the course of which can be predicted.
Succession is the result of changes that the communities themselves make to the habitat, that is, the process is not set from the outside.
Succession ends with the formation of a climax biocenosis, which is characterized by the greatest diversity, and, consequently, the most numerous connections between organisms.
Thus, the climax biocenosis is maximally protected from possible disturbances from external factors and is in a state of equilibrium.
The main feature of ecological succession is that changes in the community always occur towards an equilibrium state.
When an ecosystem approaches its final stable state (climax state), in it, as in all equilibrium systems, all development processes slow down.
Observations of succession show that some certain properties of biocenoses change in one direction, whatever the type of succession.
Let's formulate them.
Species of plants and animals are constantly changing.
The species diversity of organisms increases.
The size of organisms increases during succession.
Linear food chains dominated by herbivores evolve into complex food webs. Detritivorous forms (consumers of dead organic matter) begin to play an increasingly important role in them.
Biological cycles are lengthening and becoming more complex, organisms are becoming more and more ecologically specialized.
The biomass of organic matter increases. There is a decrease in the net production of the community and an increase in respiration rate.
1.5 The meaning of succession
A mature community, with its greater diversity, richness of organisms, more developed trophic structure, and balanced energy flows, is able to withstand changes in physical factors (such as temperature, humidity) and even some types of chemical pollution to a much greater extent than a young community. However, a young community is capable of producing new biomass in much larger quantities than the old one. The remains of civilizations and deserts, the emergence of which is due to human activity, are excellent proof that man has never realized his close connection with nature, the need to adapt to natural processes, and not to command them. Nevertheless, even the knowledge that has been accumulated at present is sufficient to ensure that the transformation of our biosphere into one vast carpet of arable land is fraught with enormous danger. For our own protection, certain landscapes must be introduced to natural communities.
Thus, a person can reap a rich harvest in the form of pure products, artificially maintaining the community in the early stages of succession. Indeed, in a mature community at the climax stage, the net annual production is spent mainly on the respiration of plants and animals and may even be zero.
On the other hand, from a human point of view, the resilience of a community in the climax stage, its ability to withstand the effects of physical factors (and even manage them) is a very important and highly desirable property. A person is interested in both productivity and stability of the community. To support human life, a balanced set of both early and mature stages of succession, which are in a state of exchange of energy and matter, is necessary. The excess food created in young communities allows the maintenance of older stages that help withstand external influences.
Arable lands, for example, should be considered young successional stages. They are maintained in this condition thanks to the continuous labor of the farmer. Forests, on the other hand, are older, more diverse and more stable communities with low net production. It is extremely important that people give equal attention to both types of ecosystems. If a forest is destroyed in pursuit of temporary income from timber, water supplies will decrease and soil will be swept away from the slopes. This will reduce the productivity of the areas. Forests are valuable to humans not only as suppliers of wood or a source of additional areas that can be occupied by cultivated plants.
Unfortunately, people have little awareness of the consequences of environmental violations that occur in the pursuit of economic gain. This is partly due to the fact that even environmental specialists cannot yet make accurate predictions of the consequences that various disturbances of mature ecosystems lead to. The remains of civilizations and deserts, the emergence of which is due to human activity, are excellent proof that man has never realized his close connection with nature, the need to adapt to natural processes, and not to command them.
Nevertheless, even the knowledge that has been accumulated at present is sufficient to ensure that the transformation of our biosphere into one vast carpet of arable land is fraught with enormous danger. For our own protection, certain landscapes must be represented by natural communities
ATTACHMENT:
Indicate the stages of overgrowing of a reservoir from the proposed vegetation: sphagnum, sedge, marsh pine, mixed forest, wild rosemary (sedge, sphagnum, wild rosemary, marsh pine, mixed forest).
Distribute the stages of succession in the correct order: annual plants, shrubs, deciduous trees, perennials, coniferous trees (annuals, perennials, shrubs, deciduous trees, coniferous trees)
Arrange the ongoing stages of succession in time: colonization of the territory by mosses. germination of herbaceous plant seeds, colonization by shrubs, formation of a stable community, colonization of bare rocks by lichens
1. colonization of bare rocks by lichens
2. colonization of the territory with mosses
3. germination of herbaceous plant seeds
4. colonization by shrubs
5. building a sustainable community
The course of evolution (development) of a community cannot be predicted.
The most general patterns of evolution of biocenoses:
1.The types of plants and animals during the development of a community can be predicted
2.Lowers diversity of species of organisms.
3. Sizes of organisms during succession are decreasing.
4, Food chains shortened and simplified. They are beginning to play an increasingly important role detritivores.
5.Biological cycles become more complicated , organisms become increasingly ecologically specialized.
6. Biomass of organic matter during community development increases. Happening height clean community products and slowdown breathing intensity.
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 grow 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 carried 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, 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 in 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 give rise to 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.
Ecological succession
What is succession called?
Give examples of primary and secondary succession.
What communities are called pioneer and climax?
Explain the phenomenon of layering using the example of a typical deciduous forest.*The community structure is created gradually over time. An example that can be used as a model for community development is the colonization of lifeless substrate by organisms ( Substrate (from lat. substratus- litter, base) - a supporting ecological element (base), in some cases also serving as a nutrient medium; for example, for terrestrial organisms the substrate is soil). Thus, on an initially lifeless substrate, they first appear (pioneer communities), e.g. crust algae, crustose lichens.
They somewhat enrich the substrate with organic substances that can be absorbed by plants. Then individual herbaceous plants appear, capable of developing the poor substrate. This stage is replaced by subshrubs and shrubs, and is replaced by deciduous tree species (most often birch, aspen, and willow). The latter are characterized by rapid growth, but being distinguished by their great love of light, they quickly thin out (by the age of 40-50). As a result, favorable conditions are created under their canopy for the settlement of shade-tolerant spruce, which gradually catches up in growth with aging deciduous tree species and enters the first tier. At this stage, a climax mixed spruce-deciduous community or spruce forest with its characteristic set of other plant and animal species is formed. Substrate Such a replacement of one biocenosis by another on a certain area of the earth's surface over a certain period of time is called succession succesio . - continuity, inheritance, sequence, change). The term "succession" was proposed by G. Coulson in 1898. The final community - stable, self-renewing and in balance with the environment - is called menopause Climax(from Greek
klimax- ladder) - a stable, final state of development of an ecosystem under the conditions of a given environment. The term "menopause" was introduced by F. Clements in 1916. The type of succession that begins with the settlement of an initially lifeless space (substrate) (for example, sand dunes, a former glacier bed, rock dumps, products of volcanic eruptions) is called primary succession . Unlike her
A classic example of succession with the formation of a stable biocenosis is the overgrowing of a lake and the emergence of a peat bog in its place or the formation of a spruce forest on abandoned lands. The spruce forest goes through several stages in its development. The first tree species to appear on abandoned arable land are birch, aspen and alder, since the seeds of these trees are easily carried by the wind. Once on lightly turfed soil, they germinate. The most persistent of them populate abandoned or plowed territory, establish themselves there and gradually change the environment, creating new conditions to which they themselves eventually turn out to be unadapted. However, these conditions turn out to be suitable for “invader” plants, displacing the pioneers and beginning to dominate the community until, as a result of their activity, the conditions change again and they begin to be replaced by more adapted forms. Conditions favorable for spruce are created only after the crowns of birch trees close, i.e. after about 30-50 years. A mixed forest is gradually forming. It exists for a relatively short time, since light-loving birches cannot tolerate darkening and their renewal does not occur under the canopy of spruce trees. A stable spruce forest on abandoned arable land is formed approximately 80-120 years after the first birch shoots.
The theory of succession was first developed in detail in 1916 by F. Clements. He studied communities in North America and concluded that the main factor determining the composition of the climax community was climate. According to Clements, in given climatic conditions, only one climax community can exist, which is called climax (monoclimax concept). More modern is polyclimax concept , according to which menopause is formed under the influence of all physical factors, and one or more of them may predominate (for example, drainage, soil, fires, etc.)
**Typical terrestrial climax communities include deciduous forests. Most primary production is produced in the tree canopy, with most decomposition occurring at ground level. an example of the division of ecological niches by different organisms, although some of them may use different tiers (for example, squirrels live mainly in trees, but also descend into bushes, and sometimes onto the ground). Within the same tier, different animals find different food.
Community structure of a typical broadleaf forest
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, 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.
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Introduction
Changes in ecosystems under stress impacts
Natural, anthropogenic, autogenic and allogenic successions
Classification
Succession of development (primary)
Regenerative successions (secondary)
Anthropogenic successions
Mechanisms of action of succession
Conclusion
Bibliography
Introduction
ecosystem stress biocenosis succession
Ecological succession is the restoration of a disturbed balance by an ecosystem; it passes through clearly defined stages.
Succession is a sequential change of biocenoses (ecosystems), expressed in changes in species composition and community structure.
A successive series of communities replacing each other in succession is called a successional series. Successions include desertification of steppes, overgrowing of lakes and formation of swamps, etc.
An ecosystem can be thrown out of equilibrium in many ways. This is usually 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. The restoration of the disturbed balance by the ecosystem goes through clearly defined stages.
Changes in ecosystems under stress impacts
Change in communities can be cyclical and incremental.
Cyclic changes -- periodic changes in the biocenosis (daily, seasonal, long-term), during which the biocenosis returns to its original state.
Diurnal cycles are associated with changes in illumination, temperature, humidity and other environmental factors during the day and are most pronounced in continental climates. Circadian rhythms manifest themselves in changes in the state and activity of living organisms.
Seasonal cyclicity is associated with changes in environmental factors throughout the year and is most pronounced in high latitudes, where the contrast between winter and summer is great. Seasonal variability manifests itself not only in changes in state and activity, but also in the quantitative ratio of individual species. For a certain period, many species are excluded from the life of the community, hibernating, torpor, migrating or flying to other areas.
Long-term variability is associated with climate fluctuations or other external factors (the degree of river flooding), or with internal reasons (features of the life cycle of edificatory plants, repetition of mass reproduction of animals).
Progressive changes - changes in the biocenosis, ultimately leading to the replacement of this community by another, with a different set of dominant species. The reasons for such changes may be factors external to the biocenosis that act for a long time in one direction, for example, increasing pollution of water bodies, increasing drying out of swamp soils as a result of reclamation, increased grazing, etc. These changes from one biocenosis to another are called exogenetic. In the case when the increasing influence of a factor leads to a gradual simplification of the structure of the biocenosis, depletion of their composition, and a decrease in productivity, such shifts are called digressive or digressive.
Natural,anthropogenic,autogenousAndallogeneicsuccession
Depending on the reasons that caused the change in biocenosis, successions are divided into natural and anthropogenic, autogenic and allogenic.
Natural successions occur under the influence of natural causes not related to human activity.
Anthropogenic successions are caused by human activities. They are caused either by a constantly acting external factor (livestock grazing, trampling, pollution), or represent the process of restoration of ecosystems after they have been disturbed by humans (overgrowth of fallow lands, restoration of pastures after the cessation of intensive grazing, restoration of forests after clearing, swamping of drained lands, etc. ).
Autogenic successions (self-generating) arise due to internal causes (changes in the environment under the influence of the community).
Allogeneic successions (generated from the outside) are caused by external factors (for example, climate change).
In its development, the ecosystem strives for a stable state. Successional changes occur until a stable ecosystem is formed that produces maximum biomass per unit of energy flow. A community that is in equilibrium with the environment is called climax.
Classification
There are many classifications of successions, according to indicators that can change during succession or for reasons of change:
· by time scale (fast, medium, slow, very slow),
by reversibility (reversible and irreversible),
· according to the degree of process constancy (constant and non-constant),
· by origin (primary and secondary),
· according to trends in productivity changes (progressive and regressive),
· according to the trend of changes in species richness (progressive and regressive),
· according to anthropogenicity (anthropogenic and natural),
· by the nature of the changes occurring during succession (autotrophic and heterotrophic).
If we classify successions on the basis of ongoing processes, then we can distinguish two main groups: endogenous, occurring as a result of the functioning of communities, and exogenous, occurring as a result of external influences. The driving force behind endogenous successions is the unbalanced exchange of communities.
Distinguish endogenous And exogenous succession.
The first are determined by intracoenotic reasons. They are divided into two categories:
1) succession development, primary, the beginning of which is the development by biota of a substrate that has not yet been occupied (on rocks, cliffs, loose sand, in new reservoirs, etc.).
2) succession restorative, secondary. The category of exogenous successions caused by external factors includes both long-term, for example climatogenic, and catastrophic ones - the result of spontaneous natural phenomena (as a result of deforestation, fire, plowing, volcanic eruption, etc.).
successiondevelopment(primary)
Primary autogenous successions are a consequence of the development of territories in which there was no life. At the same time, as a result of the development of new habitats under the influence of the vital activity of plants and heterotrophic organisms, soil is formed from the dead substrate and the species composition of the ecosystem is enriched.
Succession during overgrowth of rocks: the process of formation of ecosystems on rocks has been described in various areas of the planet.
During succession, a number of stages are distinguished.
1. Settlement of crustose lichens (they may be preceded by a stage of cyanobacteria, which combine the functions of photosynthesis and nitrogen fixation and therefore most effectively colonize new “soilless” habitats). In lichen communities, heterotrophic components, in addition to the fungal component of the lichen, are represented by protozoa, rotifers, and nematodes. Rock crevices can harbor mites and small insects. In such extreme habitats, life “pulsates”; all organisms become active after rain and sharply reduce their vital activity during dry times.
2. Stages of leaf lichens, the settlements of which are prepared by the activity of crustose lichens. These organisms more actively transform the environment, and the acids they secrete destroy the surface of the rock, on which a thin layer of detritus appears. New conditions allow a much larger number of heterotrophs to live - springtails, oribatid mites, hay eaters, larvae of pusher mosquitoes, etc. The diversity of the microflora of decomposers that process animal excrement and their dead remains also increases.
3. After the thickness of the “soil” reaches several millimeters, mosses replace leaf lichens. Their rhizoids penetrate a layer of fine earth, the thickness of which gradually increases to 3 cm. Mosses reduce temperature fluctuations on the surface of the substrate, which increases the diversity and activity of heterotrophic biota.
4. The stage of formation of communities of mosses and vascular petrophyte plants from the genera thyme, alyssum, rockweed, etc. The soil layer becomes thicker, and the participation of large invertebrate saprophages - enchytraeids, earthworms, insect larvae, etc., increases in the composition of heterotrophic animals. .
5. Subsequently, the diversity of vascular plants increases more and more, and shrubs and then trees, primarily pine, are added to the grasses. This creates conditions for the appearance of birds and small mammals in the biota.
Succession during overgrowing of sands. Changes similar to succession on rocks occur when loose sands become overgrown. For example, in the Karakum Desert, succession begins with the settlement of the perennial grass aristida, which is capable of living in shifting sand conditions. The roots of this plant are cord-like and enclosed in a cover of cemented grains of sand. This protects the roots from drying out and mechanical damage if they end up on the surface. At the expense of the aristida, some insects can already exist, and therefore lizards begin to run into the dunes in search of food. Following the aristida, the rhizomatous sand sedge settles, which secures the moving surface of the sand. Following the sedge, juzgun and white saxaul shrubs, as well as numerous ephemerals, settle in.
The enrichment of the species composition of vegetation allows the presence of the slender-toed ground squirrel, the woolly-footed jerboa, and the midday gerbil. The variety of insects that serve as food for lizards is increasing. Birds appear - the saxaul jay and the bustard, snakes and small-feeding predators.
Recently, interesting data have been obtained on primary successions in new sandy habitats of the Aralkum desert, which was formed as a result of a decrease in the level of the Aral Sea. The area of the new desert has already exceeded 40 thousand km, since due to high water intake from the Amu Darya and Syr Darya rivers, the sea level dropped by 20 m, and the process of drying out of the Aral Sea could not be stopped. The nature of primary successions depends on how saline the areas of the exposed seabed are. However, in all cases the following sequence can be traced: annual explerents - aristida - species-rich communities with the participation of shrubs and saxaul. The settlement of shrubs and trees begins after 30 years.
Succession during overgrowing of lakes. The overgrowing of shallow lakes with their gradual transformation into grass swamps is considered as primary succession. At the first stage, the water column is populated by pondweed, and a film of duckweed that covers the water surface makes its contribution to succession. Due to the deposition of sapropel, the bottom level gradually rises, and coastal aquatic amphibian plants - reeds, cattails, horsetail - rush to the center of the lake, and then they are replaced by sedges - slender, coastal, vesicular. When the lake completely turns into a grass swamp, woody plants - black alder and ash willow - settle on it.
Succession on frozen lava flows. These successions are a favorite subject of ecologists and therefore have been studied very well. As a rule, succession begins with the settlement of leguminous plants, most often from the genus lupine. Legumes enrich the substrate with nitrogen. After this, cereal grasses, shrubs and trees settle. Succession proceeds tens of times faster than the succession of overgrowing rocks or substrates formed after the melting of glaciers. The reason for this is the warm climate in the areas where most volcanoes occur. In addition, the substrate of lava flows is quite rich in mineral nutrition elements.
Similar to the described options for primary autogenous succession, the process of overgrowing of overburden dumps at the mining site occurs. Depending on the favorable climate and the presence of nutrients in the overburden rocks, succession proceeds at different rates. In the Southern Urals, after only 30 years, birch trees grow on the dumps and a closed ground cover of grasses forms. In Yakutia, quarries formed during gold mining are overgrown extremely slowly, and the first trees appear no earlier than 100 years later.
successionrestorative(secondarye)
Secondary succession develop on a substrate initially modified by the activity of a complex of living organisms. Such successions most often have a restorative (demutational) character.
Secondary succession goes everywhere. Widespread examples of this include the overgrowing of pastures with shrubs, the development of fallow lands in abandoned fields, and the regeneration of forests after clearing. If human intervention has led to the formation of a stable community different from the climax, then it is called plagioclimax, and succession is pressured.
Example secondary succession under the influence of internal factors the process of overgrowing of the lake may occur. Under the influence of the vital activity of the organisms inhabiting it, the lake is slowly filled with dead organic matter. In addition, sedimentary materials may enter the lake. Gradually, the depth of the lake decreases, and eventually it turns into a swamp (upstream or downstream, depending on the location), and then into dry land.
TO secondary successions There are also those in which the initial force causing a change in communities turns out to be disruptions of stable interactions in the biocenosis.
As you can see, secondary succession develop faster than primary ones. In the third year after the cessation of the technogenic impact of the group secondary succession cover 40 - 60% of the damaged surface. Within 10 - 15 years, the natural appearance of the disturbed swamps is almost completely restored.
For primary and secondary succession, a source of seeds, plant spores, and animals capable of colonizing habitats is required. For secondary succession An important factor is the presence of a fertile layer of soil. If the fertile layer of the earth is destroyed, then succession can proceed like the primary one. The succession process ends at a stage when all species forming an ecosystem maintain relatively constant numbers at all trophic levels. This equilibrium state is called climax, and the ecosystem is called climax.
Aanthropogenicsuccession
Nowadays they are especially widespread anthropogenic succession, arising as a result of human economic activity. They occur under the influence of fires, livestock grazing, recreation, etc. deep transformation of soil and vegetation cover is caused by construction work, mining, etc. Plant cover and fauna change under the influence of air, water and soil pollution.
Along with the negative impact on biota on the farm, human activity can be constructive. Natural systems in which reclamation measures are carried out aimed at increasing their productivity: forest, meadow, fish. Hunting and other works are transferred to the semi-natural category.
Finally, anthropogenic ecological complexes are created: agricultural, gardening, water management and other costs for managing these complexes are fully borne by humans.
The fundamental difference between these systems is that in natural ecosystems the reproduction of living matter and its environment-forming functions are performed by themselves, while natural economic systems cannot reproduce themselves. To maintain their sustainable existence, costs are necessary, and the more unnatural the natural economic systems are, the higher the price a person must pay. Left to their own devices, they strive to return to their natural state through a series of restorative successions.
Due to constant disturbances, the modern biogeocenotic land cover is practically devoid of ecosystems that have reached climax in their natural development - the final stage when ecosystems are in the most complete unity with environmental factors. However, this should not serve as a basis for concluding that the biosphere is undergoing destruction.
According to modern concepts, mature climax communities are less resistant to external factors. This is a consequence of the narrow specialization of climax communities and the high degree of balance of their characteristic functional processes. Subclimax communities, which are on the way to the terminal stages, are less specialized and therefore have a greater ability to restore their structure.
At present, when the influence of anthropogenic factors is becoming almost ubiquitous, it is this category of ecosystems that, due to its adaptability, is most widespread.
Among the diverse forms of dynamics, fundamentally different dynamic categories are distinguished: fluctuations, succession and transformation of ecosystems by humans.
Under fluctuations phytocenoses are understood as undirected changes from year to year, ending with the return of the phytocenosis to the original or, more precisely, close to the original state. With some degree of convention, fluctuations in plant communities include changes caused by economic activities - haymaking, grazing, and forestry activities.
Successional processes, as opposed to fluctuations, develop in a certain direction. They never have the character of oscillations around some average state.
Mechanismsactionssuccession
Studying succession in ecosystems, ecologists have identified three mechanisms of its action:
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
Example
Succession cannot be observed directly until the equilibrium state of the community is disturbed in some way. If a forest is cut down for agricultural purposes, it is usually restored again after agricultural work ceases. The surface of exposed rocks or abandoned roads is covered first with colonies of mosses and lichens, then with grasses and shrubs, and later, under favorable conditions, with perennial woody plants. Changing the course of a river can cause increased erosion in one place and deposition of silt in another. The silt is anchored by salt-tolerant marsh vegetation, and then as the soil layer becomes thicker, the salt is leached out, allowing grasses and shrubs to grow in the area. All these examples indicate that the community structure is changing and evolving towards a more mature stage, menopause, characteristic (and therefore predictable) of certain environmental conditions.
There is a definite relationship between the organisms in a community and the physical and chemical properties of the habitat. Under favorable conditions, the community will develop; otherwise it will simply disappear or degrade. Silt deposition stimulates the change of communities in marsh vegetation, while erosion influences this process in the opposite direction. Eutrophication of lakes in the presence of an external source of mineral elements eventually leads to complete swamping of the lake, and leaching of these substances from the soil can limit the development of the plant community, for example in heather thickets. Such changes are called allogeneic succession.
Autogenous succession occurs when there is a positive feedback within the community (endogenous changes), such as nitrogen fixation, increased organic matter content in the lake (lake swamping) or drainage of the soil as a result of transpiration.
Autogenic succession is a very long process. There is a well-known example of postglacial succession that was observed in Glacier Bay in Alaska. Since 1750, in Glacier Bay, glaciers have retreated more than 100 km and left moraines devoid of vegetation. The boulders were covered with mosses and two or three types of grasses with an undeveloped root system. More than 15 years later, willows began to grow there - first creeping forms, and then shrubby ones. After 50 years, alder appeared and formed thickets up to 10 meters high. The alder was replaced by the spruce, which after 150 years formed a dense forest that continued to develop and reached maturity. After 200 years, sphagnum mosses appeared in areas with excess moisture, retaining water and causing waterlogging, which led to the death of trees and the formation of swamps. Thus, the climax of this area is the swamp.
One of the main factors contributing to the acceleration of succession and development of the communities described above is the accumulation of large amounts of nitrogen. Alder oxidizes the soil to such a state that it becomes suitable for the growth of spruce, which replaces alder using the accumulated supply of nitrogen. During mature stages of succession, soil nitrogen content decreases as nitrogen enters the tree biomass.
Changes in topography and soil type or hermitage grazing, as well as fires, can lead to the formation of a highly diverse plant community within a given region .
Variability in the distribution of plant species and their numbers is characteristic of all plant communities in climax states. It is caused by landslides, deforestation or cyclical changes in the vegetation itself. Description of changes in the heather community ( Calluna vulgaris), native to heathland in northern Europe, is the most complete example. If heather communities are not destroyed by fire or grazing, or if they are not replaced by trees, then the heather begins to slowly degenerate. The plants die, forming a gradually expanding wasteland in the center of the area, inhabited, in addition to heather, by other plants. A heterogeneous heather community is gradually formed at different stages of development. In this case, variability is due to plant growth characteristics, but cyclical changes on larger spatial and temporal scales can be caused by climatic factors.
Conclusion
Studying communities, we come to the conclusion that thoughtless human activity can destroy them. For example, changes in trophic links. But knowledge of elementary processes in communities allows one to avoid a number of such environmental disasters.
Mastering environmental knowledge contributes to a careful attitude towards nature, its conservation and fewer retaliatory attacks on humanity from it.
The mechanism of succession is that the processes of creating and maintaining a specific bioenvironment, the gradual accumulation of conditions for its degradation, and the formation of a more complex community or one more consistent with the conditions of the abiotic environment occur sequentially in the community. Depending on the initial conditions, it is customary to distinguish between primary successions, which begin on completely lifeless substrates, for example, on dunes, and secondary successions, which begin with more favorable starting conditions, for example, after a fire, deforestation or in an abandoned field.
Recently, the biosphere has been characterized by secondary successions, which is mainly associated with human activity.
Listusedliterature:
1. Reimers N.F. Nature management: Dictionary-reference book. -M.: Mysl, 1990. pp. 485-486.
2. Kormilitsyn M. S. Fundamentals of ecology. M.: MPU, 1997. P.24.
3. J. M. Anderson Ecology and Environmental Sciences. L.: Gidrometeoizdat, 1985. P.96-101.
4. Marichenko A.V. Ecology. 2nd ed. M.: 2008-328 p.
5. Stepanovskikh A.S. General ecology. 2nd ed. M.: 2005-687 p.
6. Peredelsky L.V., Korokin V.I., Prikhodchenko O.E. Ecology. M.: 2007-512 p.
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