Population structure and characteristics. Population as a natural system Population dynamics

The term “population” is used today in various fields and areas of science. It has the greatest influence in biology, demography, ecology, medicine, psychometrics, and cytology. But what is a population, and how is it characterized?

Introduction. Definitions

To date, population studies have primarily been conducted to identify genetic or environmental sequences. This makes it possible to determine the survival environment of species and their heredity. At the moment, there is another concept - “cell population”. This is the isolated offspring of a specific number of group of cells. This area is studied by specialists within the framework of cytology.

From the point of view of genetics, a population is a heterogeneous hereditary collection of forms of one species, which is opposed to the so-called pure line. The fact is that each family of individuals meets specific characteristics and represents a certain phenotype and genotype.

Main characteristics

Before you begin to understand in more detail what a population is, you need to know and understand its main components. There are 5 main characteristics in total:

1. Distribution. It can be spatial and quantitative. The first type, in turn, is divided into random and uniform distribution. The quantitative indicator is responsible for the size of the population or its individual group. The distribution of individuals directly depends on climatic conditions, genome, food chain and degree of adaptation.

2. Number. This is a separate characteristic of a population and should not be confused with a subtype of distribution. Here, abundance represents the total number of organisms in a particular unit of space. Most often it is dynamic. Depends on the ratio of mortality and fertility of individuals.

3. Density. Determined by biomass or the number of organisms per unit area (volume).

4. Fertility. It is determined by the number of individuals that appeared as a result of reproduction per unit of time.

5. Mortality. Divided according to age criteria. Represents the number of life forms killed per unit of time.

Structural classification

At the moment, the following types of populations are distinguished: age, sex, genetic, environmental and spatial. Each of these variations has its own specific structure. Thus, the age population is determined by the ratio of individuals of different generations. Representatives of the same species can have both ancestors and offspring.

The sexual population depends on the type of reproduction of the family and the set of determined morphofunctional and anatomical characteristics of organisms. Genetic structure is determined by variations in alleles and the way they are exchanged. An ecological population is a division of a family into groups relative to environmental factors. Spatial structure depends on the distribution and placement of individual individuals of the species in the area.

Isolation of populations

In different families, this property depends on the environment and the form of coexistence. If representatives of one species move over large areas, then such a population can be called large. In the case of weak development of distribution abilities, the family is determined by small aggregates, which can reflect, for example, the mosaic nature of the landscape. The population of sedentary animals and plants depends on the heterogeneity of the environment.

The level of isolation of neighboring families of the same species varies. In this case, populations can be sharply distributed in space or be clearly localized in a certain area. There is also a complete colonization of a huge area by one species. In turn, the boundaries between populations can be blurred and distinguishable.

Population dynamics can be of 3 types:

Most individuals survive to the maximum age threshold (humans and mammals),

Death can occur at any time (reptiles and birds),

The mortality rate is high already in the early stages of development (fish, plants, invertebrates).

A population consists of a collection of individuals that are similar to each other in morphophysiological properties, area, type of crossing, and origin. Such a group of organisms is called a species. It is a unit of population structure.

Types depend on the following criteria: morphological, genetic, physiological, biochemical. According to an additional classification, the characteristics of influence are geographical and environmental.

Each species arises, then develops and adapts. With a sharp change in environmental conditions, it may disappear.

The simplest form of existence of a species in nature is a population. In this article we will understand what this concept includes and find out what the role of the population is in the evolutionary process.

Population structure

In biology, a population is the integrity of all existing individuals of the same species, living in the same territory and having a common gene pool with the ability to interbreed freely. One type of living organism can include several ecosystems, which are most often isolated from each other.

If individuals of the same species taken from different ecosystems are placed in the same conditions, one can observe the preservation of their differences. However, to obtain fertile offspring, such crossing gives the best results.

Rice. 1. Examples of populations.

Populations provide the process of microevolution and are divided into:

• sexual;
• age;
• environmental;
• genetic;
• spatial structure.

Rice. 2. Population structure.

Sexual structure

Indicates the percentage of individuals of different sexes. It is determined by the difference in chromosome sets. However, it often happens that some females give birth to only female or only male individuals. In this case, the sex ratio deviates from 1:1. The reason for this may be not only genetic disorders, but also environmental conditions.

Age structure of the population

Includes the ratio of individuals of different ages that represent offspring of the same or different generations. A generation may include representatives of one or more offspring. Age affects the intensity of the reproduction process, the speed of generation change, and the mortality rate.

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Genetic structure

Determined by the diversity and variability of genotypes. A property of ecosystems is the presence of a certain level of diversity of traits that depend on ecology and genetic predisposition. In other words, one genotype is capable of producing many variations of phenotypes. Diversity depends on the number of individuals and the ecological situation. Changing the frequency of genes can lead to the extinction of a species.

Spatial structure

It is determined by the density of placement and distribution of ecosystem members in a certain area. All individuals have both individual and group space. In this way, flocks, colonies, and herds are formed. Depending on the method of placement in a group, random, uniform and crowded distribution are distinguished.

Each individual plays its own role in the group, and a social hierarchy is formed.
She may be:

• linear (subordination by rank, when the next one dominates the previous one);
• parallel (males and females have separate leaders).

This system of relationships allows for the coordination of behavior that will be beneficial for all members of the group.

Environmental component

The ecological unit is the species. This structure implies the distribution of members into groups depending on the interaction with surrounding natural factors.

An ecological niche includes food, breeding and shelter sites, and other environmental factors that are necessary for the existence of a species. When characterizing an ecological niche, two indicators are used: width and degree of overlap with other niches.

Population dynamics

The dynamics and growth of ecosystems depends on external and internal factors, such as the availability of food, enemies, and climate.

The founder of population genetics is S.S. Chetverikov, who called the growth in numbers “waves of life.”

It is possible to accurately determine the average number of individuals under the condition of artificial complete isolation of the group. In nature, this is possible when studying island ecosystems. The number can be determined by the ratio of birth rates and deaths.

“Waves of life” sometimes help bring forward rare genotypes, testing them by natural selection. For example, after a cold winter, stronger, cold-resistant organisms remain alive.

Rice. 3. Example of population dynamics.

Meaning

With the help of the functioning of populations, the conditions necessary to support life on our planet are created. Through their vital activity, living organisms influence the environment of their area. The circulation of substances in nature depends on ecosystems, certain conditions are created, and interchange between living and inanimate nature occurs. The joint work of populations determines the characteristics of biotic conglomerations and environmental conditions.

What have we learned?

The concept of “population” in biology implies the number of all individuals of one species living in the same area and capable of interbreeding freely. The components of this concept are gender, age, environmental, genetic and spatial structures. All of them are closely interconnected, influence the environment and ensure the circulation of substances in nature.

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In evolutionary theory

A population is a group of individuals capable of more or less stable self-reproduction (both sexual and asexual), relatively isolated (usually geographically) from other groups, with representatives of which (during sexual reproduction) genetic exchange is potentially possible. From the point of view of population genetics, a population is a group of individuals within which the probability of interbreeding is many times greater than the probability of interbreeding with representatives of other similar groups. Populations are usually spoken of as groups within a species or subspecies.

In modern evolutionary theories (for example, in the Synthetic Theory of Evolution), a population is considered the elementary unit of the evolutionary process.

In medical research

Population is a collection of individuals from which a sample is selected and to which the results obtained for this sample can be generalized. The population may be the entire population (usually the population in epidemiological studies of the causes of disease) or it may consist of patients admitted to a particular clinic or patients with a specific disease (more often the case in clinical studies). Thus, we can talk about the general population or the population of patients with a specific disease. The epidemiological definition of a population differs from the biological (ecological) definition.

In ecology

Population is a collection of individuals of the same species occupying a certain area, freely interbreeding with each other, having a common origin, genetic basis and, to one degree or another, isolated from other populations of this species.

The study of populations, their interactions and dynamics is one of the main tasks of ecology. In particular, one of the simplest models of population dynamics is the logistic equation.

Notes

see also

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See what “Population” is in other dictionaries:

- (cf. lat. populatio, from lat. populus people, population), a collection of individuals of the same species that have a common gene pool and occupy a certain territory. Contacts between individuals within one P. are more frequent (which manifests itself, for example, in a higher level ... Biological encyclopedic dictionary

Population- (from Latin populus people, population), a collection of individuals of a species with general conditions necessary to maintain its numbers at a certain level over a long period, and with known properties that determine the unity of individuals (for example ... Ecological dictionary

Modern encyclopedia

- (cf. century lat. pupulatio from lat. populus people, population), in biology, a collection of individuals of the same species, occupying a certain space for a long time and reproducing itself over a large number of generations. In modern biology, population... ... Big Encyclopedic Dictionary

- [fr. population population] 1) mat., stat. set (general set), a collection of objects (elements, individuals (INDIVID), units) on which statistical conclusions are based; 2) biol. n. Mendel, a collection of individuals of plants and animals of the same... ... Dictionary of foreign words of the Russian language

POPULATION- means the population itself, but in genetics the concept of population is associated with a number of special concepts and patterns. In this special, genetic sense, the term P. was introduced by Johansen (Jo riansen) in his study “On ... ... Great Medical Encyclopedia

Population- (in medieval Latin populatio, from Latin populus people, population) (biological), a collection of individuals of the same species, occupying a certain space for a long time and reproducing itself over a large number of generations. In modern... ... Illustrated Encyclopedic Dictionary

population- and, f. population f. Collections of individuals of flora or fauna of a certain area, belonging to one or another species; form of existence of a species. SIS 1954. It is almost impossible for the city police to monitor the character and behavior of a mobile... Historical Dictionary of Gallicisms of the Russian Language

POPULATION- (population). In biology, all individuals of one region and one taxon or other group, considered as a biological or statistical unity. For example, a hybrid population consisting of various dissimilar hybrid forms of the same thing... Terms of botanical nomenclature

POPULATION, and, women. (specialist.). A long-lived group of individuals of the same species. P. cats. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

A collection of individuals of the same species that occupies a certain space for a long time and reproduces itself over a large number of generations. The term applies to essentially any culture of microorganisms. (

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Introduction

1. Concept of population

2. Properties of populations

3. Factors in population dynamics

4. Spatial distribution of populations

Conclusion

Bibliography

Introduction

Everything is interconnected with everything - says the first environmental law.

Already in the last century, human concern arose for the fate of the planet, and in the current century it has reached a crisis in the global ecological system due to increasing stress on the natural environment. Environmental pollution, depletion of natural resources and disruption of ecological connections in ecosystems have become global problems

Despite the measures taken by the Russian state to improve the environment, environmental relations continue to develop in a direction unfavorable for nature and society:

a) the departmental approach still prevails, as a result of which each environmental user exploits natural resources based on their departmental interests;

6) the so-called resource approach to environmental use is applied, as a result of which many environmental connections and natural objects that do not have resource value remain outside legal protection.

Currently, during a period of impending environmental crisis on the entire planet, all living people need to solve the problem of transition from exploitation and conquest of Nature to its conservation and cooperation with it. population size placement

By intensively consuming natural resources with the help of colossally increasing technological means, humanity has progressively improved the conditions for the development of its civilization and its growth as the biological species Homo sapiens. Ecology, like any science, has two aspects. One is the desire for knowledge for the sake of knowledge itself, and in this regard, the first place is given to the search for patterns of development of nature, as well as their explanation; the other is the application of collected knowledge to solve environmental problems. The rapid increase in the importance of ecology is explained by the fact that not a single issue of enormous practical importance can currently be solved without taking into account the connections between the living and non-living components of nature.

The practical solution of ecology can be seen, first of all, in solving environmental management issues; it is precisely this that must create the scientific basis for the exploitation of natural resources.

1. Population concept

Individuals of most species are distributed unevenly within their species range.

There are several options for defining a population. A population is a collection of individuals of the same species inhabiting a certain territory or water area for a long time, connected by varying degrees of free interbreeding and sufficiently isolated from other similar populations. As follows from the above definition of a population, it includes the following features inherent to it:

· Existence over a large number of generations, which distinguishes a population from short-term unstable associations of individuals.

· The presence of a certain degree of free crossing of individuals. It is this feature of the population that ensures its unity as an evolutionary structure.

· The degree of free crossing within a population is higher than between different (even neighboring) populations.

· A certain degree of isolation of populations from each other.

The reasons that force individuals of a population to group within limited areas are extremely numerous and varied, but the main one is the uneven distribution of environmental conditions in geographic space and the similarity of requirements for these conditions among organisms of the same species.

Depending on the size of the occupied territory, three types of populations are distinguished: elementary, ecological and geographical.

1 - species range; 2-4 - geographical, ecological and elementary populations, respectively.

· Elementary, or micropopulation, is a collection of individuals of a species occupying a small area of ​​homogeneous area. They usually include genetically homogeneous individuals. The number of elementary populations into which a species is divided depends on the heterogeneity of environmental conditions: the more uniform they are, the fewer elementary populations there are, and vice versa.

· Ecological population is formed as a set of elementary populations. Basically, these are intraspecific groups, weakly isolated from other ecological populations of the species, so the exchange of genetic information between them occurs relatively often, but less often than between elementary populations. An ecological population has its own special features that distinguish it in some way from another neighboring population. Thus, squirrels inhabit various types of forests, and “pine”, “spruce”, “fir”, “spruce-fir” and their other ecological populations can be clearly distinguished.

· Geographic population covers a group of individuals inhabiting an area with geographically homogeneous living conditions. Geographic populations occupy a relatively large area, are quite thoroughly demarcated and relatively isolated. They differ in fertility, size of individuals, and a number of ecological, physiological, behavioral and other features. Geographic populations are characterized by genetic exchange, and although it may be rare, it is still possible.

A population has only its own characteristics: numbers, density, spatial distribution of individuals. There are age, sex, and size structure of the population. The ratio of groups of different age and gender in a population determines its main functions. The ratio of different age groups depends on two reasons: on the characteristics of the life cycle of the species and on external conditions.

Compound. Conventionally, three ecological age groups can be distinguished in the population:

· pre-reproductive - a group of individuals whose age has not reached the ability to reproduce;

· reproductive - a group that reproduces new individuals;

· post-reproductive - individuals who have lost the ability to participate in the reproduction of new generations. The duration of these ages in relation to the total lifespan varies greatly among different organisms.

Population density is the size of a population per unit of space: the number of individuals, or biomass, of a population per unit area or volume. Density depends on the trophic level at which the population is located. The lower the trophic level, the higher the density.

There are three types of distribution or settlement of individuals within a population: uniform, random and group

A - uniform distribution; B - random distribution; B - group distribution.

· Uniform distribution in nature is often associated with intense competition between different individuals. This type of distribution is observed in predatory fish and sticklebacks with their territorial instinct and purely individual character.

· Random distribution occurs only in a homogeneous environment. This is how aphids are initially distributed across the field. As it multiplies, the distribution acquires a group or spotted (congregational) character.

· Group distribution is the most common. Thus, in a pine forest, trees initially settle in groups, and later their distribution becomes uniform. In a population, group distribution provides higher resistance to unfavorable conditions compared to an individual.

Number and density express the quantitative characteristics of the population as a whole. The size of a population is expressed by the number of individuals of a given species living per unit area occupied by it. The dynamics of population numbers develops through the interaction of the main population-dynamic processes: 1) birth rate, 2) mortality, 3) growth rate, 4) immigration of new individuals from other populations, 5) emigration of some individuals outside the range of a given population.

Fertility characterizes the frequency of appearance of new individuals. Fertility refers to the number of individuals (eggs, seeds, embryos) produced per unit of time per female.

· Maximum fertility is the formation of the theoretically maximum possible number of new individuals under ideal conditions, when there are no limiting factors and reproduction is limited only by physiological factors.

· Realizable fertility is an increase in the population due to the birth of new individuals under actual, real environmental conditions.

The mortality rate characterizes the death of individuals in populations. By definition, mortality is the number of individuals dying per unit of time per individual in a population. All dead individuals are taken into account, regardless of the cause of mortality (old age, elimination by predators, diseases, etc.).

· Maximum mortality is a constant value that characterizes the death of individuals under ideal conditions, when the population is not exposed to limiting factors.

· Realizable (ecological) mortality, i.e. a value that, like ecological fertility, depends on the actual conditions of the biotic and abiotic environment.

The amount of population growth per unit of time per individual represents the rate of population growth. As the population grows, the environmental resources available to each individual decrease. When resources are depleted, population growth slows down and eventually stops. Populations of different species have an amazing ability to grow rapidly. This issue was considered by Aristotle (4th century BC), Machiavelli (around 1525), and later Buffon (1751). Charles Darwin drew attention to numerous cases of amazingly rapid reproduction of some animals in their natural state, when conditions were especially favorable. He extended the idea of ​​geometric growth, when the population size grows exponentially, to all species of animals and plants, placing the postulate of the high reproductive potential of species at the basis of his theory of natural selection.

Migration is a special case of movement of individuals when almost the entire population leaves a certain area for a time. Seasonal or daily migrations allow organisms to take advantage of optimal environmental conditions in places where they could not live permanently. Moving from place to place following the movement of optimal conditions, such species can remain highly active, maintain a high population density even during those periods when non-migratory species become inactive (into a state of diapause or

In nature, there are other factors that influence population dynamics. This is due to the following reasons. For some species, physical factors are critical. The number of individuals in populations can be limited by factors such as a lack of natural resources (for example, food or places suitable for reproduction), the inaccessibility of these resources and a lack of time for reproduction (short wet season, short days, for example in the Arctic).

“Waves of life” dramatically complicate planning for the exploitation of a given population, since the annual removal (shooting, fishing) of the same number of individuals may mean that in one year, say, only 5% of individuals will be removed, and in another year, when the population size will fall 10 times - 50% of individuals from the existing population composition. In addition, population fluctuations encourage humans to increase the minimum theoretically permissible population size.

Populations of animals, plants, fungi and microorganisms have the ability to naturally regulate their numbers, that is, with more or less significant fluctuations, they remain in a state of dynamic equilibrium, at some level between the upper and lower limits. This is ensured by the action of specific adaptive mechanisms based on the fact that the supply of energy necessary for the survival of the population does not exceed a certain level and thus ensures the size of the given population. The ability of a population to maintain stability due to the ability to self-regulate through its own regulatory mechanisms is called population homeostasis. Thus, an increase in population size leads to a depletion of food supplies, followed by a decrease in the birth rate of organisms, an increase in their mortality (negative relationships), and, consequently, a decrease in numbers. The latter, in turn, increases food supplies, which causes an increase in the birth rate and population size (positive connections). The equilibrium state of the population (state of dynamic equilibrium) is short-term and is achieved through rapid alternation of positive and negative feedbacks. To optimize human relations with nature, it is important to take into account the population size, take into account the fact that the population size can be affected by the depletion of the resources it needs due to a reduction in the food supply, competition from domestic animals, trampling of the soil and deterioration of its aeration, and a decrease in oxygen in the water during pollution and eutrophication. A person can artificially regulate the number of populations, for example, of animals by prohibiting hunting or limiting its duration for certain species, or introducing licenses. This has already yielded positive results - it has prevented a number of species from extermination, in particular, elk, beaver, and bison. By fighting agricultural and forestry pests and life-threatening species, people limit the size of their populations.

In general, the population size, its growth rate (in a more general sense, the rate of its change, population dynamics) are very labile parameters, highly sensitive to the influence of abiotic, biotic, and anthropic factors. Therefore, a person must have a good understanding of all the features of the population that is being exploited in order to ensure reproduction and its stable long-term existence. The complexity of this task increases due to the numerous connections between populations of different species inhabiting the same

2. WITHproperties of populations

The values ​​of the birth and death rates depend on many factors acting on the population from the outside, as well as on its own properties. An objective indicator of the ability of organisms to increase their numbers is the maximum instantaneous rate of population growth. This parameter is inversely proportional to the life expectancy of organisms. This is easy to verify by referring to the hyperbolic relationship between the innate rate of increase in population size and the average generation time, expressed in days. Small organisms have higher values ​​of rmax than large ones, which is explained by a shorter generation time. The reason for this correlation is understandable, since it takes a lot of time for an organism to reach large sizes. Delaying the breeding season also inevitably leads to a decrease in rmax.

Nevertheless, the advantages provided by large body sizes must exceed the disadvantages associated with a decrease in r max, since otherwise large organisms would never have evolved. The tendency for body size to increase over geological time, traced from fossil remains, served as the basis for the introduction of the concept of increase in phyletic size.

Large body size provides very obvious advantages: a larger organism should attract fewer potential predators and, therefore, it has a greater chance of not becoming prey and should have better survival; small organisms are closely dependent on the physical environment, even very slight changes in which can be destructive for them. Larger organisms more easily tolerate the effects of such changes and, accordingly, are better protected from them. However, larger organisms require more matter and energy per individual per unit of time than small ones. In addition, there are much fewer shelters and safe places for them.

In the life of all organisms in a population, three main periods can be distinguished: pre-reproductive, reproductive and post-reproductive. The relative duration of each of them varies greatly among different species. For many animals, the first period is the longest. A striking example is mayflies, in which the pre-reproductive period reaches 3 years, and the reproductive period takes only 2-3 hours to 1 day. The American cicada has a pre-reproductive period of 17 years. But there are species whose individuals, as soon as they are born, begin to multiply intensively (most bacteria).

The reproductive capabilities of a population depend on its age composition. The lifespan of individuals in a population can be estimated using survival curves. There are three types of survival curves.

The first type (curves 1) corresponds to a situation where a larger number of individuals have the same life expectancy and die within a very short period of time. The curves are characterized by a highly convex shape. Such survival curves are characteristic of humans, and the survival curve for men is less convex compared to a similar curve for women, therefore an insurance policy for men in most Western countries is 1.5 times more expensive than for women. For most ungulates, survival curves are also convex, although to varying degrees among species and also depending on sex. The second type is characteristic of species whose mortality rate remains constant throughout their life. Therefore, the survival curve transforms into a straight line. This shape of the survival curve is characteristic of freshwater hydra. The third type is strongly concave curves, reflecting the high mortality of individuals at an early age. This characterizes the lifespan of some birds, fish, and many invertebrates.

Knowing the type of survival curve makes it possible to construct an age pyramid. Three types of such pyramids should be distinguished. A pyramid with a wide base, which corresponds to a high percentage of young animals, is characteristic of a population with a high birth rate. The average type of pyramid corresponds to a uniform distribution of individuals by age in a population with balanced birth and death rates - a leveled pyramid. A pyramid with a narrow base (inverted), corresponding to a population with a numerical predominance of old individuals over young animals, is characteristic of declining populations. In such populations, the mortality rate exceeds the birth rate.

Of great importance for increasing the population size are the costs of offspring, expressed in certain breeding tactics. Not all offspring are created equal: those produced late in the growing season are typically less likely to survive to adulthood than offspring born earlier. How much effort should parents expend on each offspring? At a constant value of reproductive effort, the average fitness of an individual offspring is inversely related to their number. One extreme version of the breeding tactic is to invest everything in a single very large and well-adapted offspring, the other is to maximize the total number of offspring produced, investing as little as possible in each individual. However, the best breeding tactics are a compromise between producing as many offspring as possible and producing offspring with the highest possible fitness.

This relationship between the quantity and quality of offspring is illustrated by a simple graphical model.

In the unlikely event that the fitness of offspring is linearly dependent on the costs of the parents, the fitness of each individual offspring decreases with increasing litter or clutch size. Since the fitness of the parents or, what is the same thing, the overall fitness of all offspring is a constant value, from the point of view of the parents there is no optimal clutch size. However, since initial expenses on offspring make a greater contribution to the fitness of offspring than subsequent ones (there is a 5-shaped dependence of the fitness of offspring as the contribution of parents increases), it is obvious that there is some optimal clutch size. In this hypothetical case, parents spending only 20% of their reproductive effort on each of their five offspring would receive a greater return on their investment than at any other clutch size. Such tactics, while optimal for the parents, are not the best for each individual offspring, whose maximum fitness is achieved if it is the only offspring to receive the full contribution of effort from its parents. Therefore, in this case there is a “conflict between parents and children.”

The competitive environment has a particularly strong influence on the shape of the S-curve. In a highly rarefied environment (competitive vacuum), the best reproductive strategy should be considered to be the maximum contribution of matter and energy to reproduction in order to produce as many offspring as possible in the shortest possible time. Because there is little competition, offspring can survive even if they are very small in size and have low fitness. However, in a saturated environment where mass effects are prominent and competition is intense, the optimal strategy is to expend large amounts of energy to overcome competition, increase one's own survival, and produce more competitive offspring. With such a strategy, it is better to have large offspring, and since they are energetically more expensive, fewer of them can be produced.

So, the properties of a population can be assessed by indicators such as fertility, mortality, age structure, sex ratio, gene frequency, genetic diversity, rate and shape of the growth curve, etc.

The density of a population is determined by its internal properties and also depends on factors acting on the population from the outside.

3. Factors in population dynamics

There are three types of dependence of population size on its density. In the first type (curve 1), the population growth rate decreases as density increases. This widespread phenomenon helps explain why some animal populations are relatively resilient. First, as population density increases, the birth rate decreases. Thus, in the great tit population, with a density of less than one pair per 1 hectare, there are 14 chicks per nest; when the density reaches 18 pairs per 1 hectare, the brood is less than 8 chicks. Secondly, as population density increases, the age at sexual maturity changes. For example, the African elephant, depending on population density, can reach sexual maturity between the ages of 12 and 18 years. In addition, this species, at low density, gives birth to 1 elephant calf in 4 years, while at high density, the birth rate is 1 elephant calf in 7 years.

With the second type of dependence (curve 2), the population growth rate is maximum at medium, rather than at low, density values. Thus, in some species of birds (for example, gulls), the number of chicks in the brood increases with increasing population density, and then, having reached its greatest value, begins to decrease. This type of influence of population density on the rate of reproduction of individuals is characteristic of species in which a group effect is noted. In the third type (curve 3), the growth rate of the population does not change until it reaches a high density, then drops sharply.

A similar picture is observed, for example, in lemmings. When the population peaks, the density of lemmings becomes excessive and they begin to migrate. Elton described the migration of lemmings in Norway: the animals passed through villages in such numbers that the dogs and cats that initially attacked them simply stopped noticing them. Having reached the sea, the exhausted lemmings drowned.

Regulation of the number of equilibrium populations is determined primarily by biotic factors. Among them, the main factor is often intraspecific competition. An example is the struggle of birds for nesting revenge.

Intraspecific competition may be responsible for the physiological effect known as shock sickness. It is noted, in particular, in rodents. When population density becomes too great, shock disease causes decreased fertility and increased mortality, which returns population density to normal levels.

In some animal species, adults feed on their own offspring. This phenomenon, known as cannibalism, reduces the population size. Cannibalism is characteristic, for example, of perches: in the lakes of Western Siberia, 80% of the food of large individuals consists of juveniles of the same species. The young, in turn, feed on plankton. Thus, when there are no other fish species, adults live off plankton.

Predation as a limiting factor in itself is of great importance. Moreover, if the influence of the prey on the population size of the predator is beyond doubt, then the opposite effect, that is, on the prey population, does not always happen. Firstly, the predator destroys sick animals, thereby improving the average qualitative composition of the prey population. Secondly, the role of a predator is noticeable only when both species have approximately the same biotic potential. Otherwise, due to the low reproduction rate, the predator is not able to limit the number of its prey. For example, insectivorous birds alone cannot stop the massive reproduction of insects. In other words, if the biotic potential of the predator is much lower than the biotic potential of the prey, the action of the predator becomes constant, independent of its population density.

The abundance of phytophagous insects is often determined by a combination of species-specific responses of insects and plants to the effects of pollutants. Pollution reduces the resistance of plants, as a result of which the number of insects increases. However, if there is too much pollution, the number of insects decreases, despite the decrease in plant resistance.

The above differentiation of population dynamics factors allows us to understand their real significance in the life and reproduction of populations. The modern concept of automatic regulation of population numbers is based on a combination of two fundamentally different phenomena: modifications, or random fluctuations in numbers, and regulations operating on the principle of cybernetic feedback and leveling fluctuations. In accordance with this, modifying (independent of population density) and regulating (depending on population density) ecological factors are distinguished, the first of which affect organisms either directly or through changes in other components of the biocenosis. Essentially, modifying factors are various abiotic factors. Regulatory factors are associated with the existence and activity of living organisms (biotic factors), since only living beings are able to respond to the density of their population and the populations of other species according to the principle of negative feedback.

If the influence of modifying factors leads only to transformations (modifications) of fluctuations in numbers without eliminating them, then regulating factors, leveling out random deviations, stabilize (regulate) numbers at a certain level. However, at different levels of population size, the regulatory factors are fundamentally different. For example, polyphagous predators, capable of weakening or strengthening their activity (functional reaction) when the number of prey changes, act at relatively low values ​​of the prey population size.

Oligophagous predators, which, unlike polyphages, are characterized by a numerical response to the state of the prey population, have a regulatory effect on it in a wider range than polyphages. When the prey population reaches an even higher number, conditions are created for the spread of diseases and, finally, the limiting factor of regulation is intraspecific competition, leading to the depletion of available resources and the development of stress reactions in the prey population. In a real situation, this parameter depends on a large number of factors, in particular those that do not have a regulatory effect on population density according to the feedback principle.

Individuals in a population interact with each other, ensuring their livelihoods and sustainable reproduction of the population.

In animals that lead a solitary lifestyle or create families, the regulating factor is territoriality, which affects the possession of certain food resources and is of great importance for reproduction. An individual protects the space from invasion and opens it to another individual only during reproduction. The most rational use of space is achieved if a real territory is formed - an area from which other individuals are expelled. Since the owner of the site psychologically dominates it, for expulsion most often it is enough only to demonstrate threats, persecution, or at most feigned attacks that stop at the borders of the site. In these animals, individual differences between individuals are of great importance - the most adapted have a large individual food range.

In animals that lead a group lifestyle and form flocks, herds, colonies, group protection from enemies and joint care of offspring increases the survival rate of individuals, which affects the population size and its survival. These animals are organized hierarchically. Hierarchical relationships of subordination are built on the fact that everyone's rank is known to everyone. As a rule, the highest rank belongs to the oldest male. The hierarchy controls all interactions within the population: mating, individuals of different ages, parents and offspring.

In animals, the mother-child relationship plays a special role. Parents pass on genetic and environmental information to their offspring.

4. Pspatial distribution of populations

At the population level, abiotic factors influence such parameters as fertility, mortality, average life expectancy of an individual, population growth rate and its size, often being the most important reasons that determine the nature of population dynamics and the spatial distribution of individuals in it. A population can adapt to changes in abiotic factors, firstly, by changing the nature of its spatial distribution and, secondly, through adaptive evolution.

The selective attitude of animals and plants to environmental factors gives rise to selectivity to habitats, that is, ecological specialization in relation to areas of the species' range that it is trying to occupy and populate. The area of ​​the range occupied by a population of a species and characterized by certain environmental conditions is called a station. The choice of station is usually determined by one factor; it could be acidity, salinity, humidity, etc.

Eurythermal species are characterized by a zonal change of stations, i.e., a strictly directed change in stations when a species moves from one natural zone to another: when moving to the north, drier, well-warmed open stations with sparse vegetation cover are selected, often located on light sandy or rocky soils. soils; when moving to the south, the same species inhabits more moist and shady areas with dense vegetation cover and clay soils. In the diagram below, according to the nature of the vegetation cover and microclimate, all stations are divided into three ecological groups - xerophytic, mesophytic and hygrophytic. The shift of species populations to wetter stations as they move south is shown by oblique arrows. At the same time, moisture-loving populations of the forest and partly forest-steppe zones are deprived of the opportunity to penetrate into the southern regions, since more humid stations than hygrophytic ones are physically and ecologically unthinkable.

The vertical change of stations is similar to the zonal one, but manifests itself in mountainous conditions. Its most common form is the transition of populations to more xerophytic stations as the level of their habitats increases. For example, the gray grasshopper in the forest belt of the Caucasus lives on meso- and hygrophytic stations, and in the alpine zone - on xero- and mesophytic ones.

As can be seen from the pattern of station changes, an important ecological factor that determines the choice of habitats for terrestrial animals and plants is air humidity.

The special occurrence of woodlice is associated with the content of water vapor in the air. They are numerous along the shores of the seas, where the air is saturated with moisture, and live there openly. In high mountain areas with dry air, woodlice spend most of their time under stones and tree bark.

Woodlice Lygia oceanica lives along the coasts of the seas. Woodlice spend the daytime in shelter. But when the air temperature rises to 20 °C outside and to 30 °C under the pebbles, they leave their shelters and crawl out onto the rocks facing the sun. The reason for this movement is that this species, which is very poorly adapted to terrestrial habitats, has an easily permeable cuticle. When air humidity is low, woodlice lose a lot of water through evaporation, which is what happens on rocks exposed to the sun. Intense evaporation reduces the animal’s body temperature, which is 26 °C when it is on a rock. If, however, the woodlice continues to hide under pebbles, where the relative humidity is close to 100% and evaporation is zero, the body temperature reaches 30 ° C.

The distribution of stations in the aquatic environment is determined by other factors, in particular acidity. The acidic waters of peat bogs promote the development of sphagnum mosses, but there are absolutely no bivalves in them. Other types of bivalve mollusks are also extremely rare in them, which is due to the lack of lime in the water. Fish tolerate acidity of water within the pH range from 5 to 9. At a pH below 5, their mass death can be observed, although some species adapt to an environment whose pH value reaches 3.7. The productivity of fresh waters with acidity less than 5 is sharply reduced, which entails a significant reduction in fish catches.

Another important factor limiting the distribution of aquatic animals and plants is the salinity of the water. Many large taxonomic groups such as echinoderms, coelenterates, bryozoans, sponges, annelids, etc.) are all or almost all marine.

Often only minor shifts in water salt concentrations affect the distribution of closely related species. The number of inhabitants of brackish waters is very large, but their species composition is poor, since only euryhaline species of both freshwater and marine origin can live here. For example, a lake with a salinity varying from 2 to 7% is inhabited by freshwater fish, such as carp, tench, pike, pike perch, which tolerate low salinity, and marine fish, such as mullet, which tolerate insufficient salinity.

Abiotic factors have a significant impact on the population density of animals and plants. A drop in temperature often has a catastrophic effect on animal populations: in areas adjacent to the northern boundaries of its range, a species can become rare and even disappear completely. In addition, frosts in some cases also have an indirect effect, since food hidden under a thick layer of ice or snow becomes completely inaccessible to animals. In areas exposed to strong winds, plant growth is stunted and fauna may be partially or completely destroyed.

Zconclusion

The evolution of living organisms is based on natural selection, operating at species or lower levels. But natural selection also plays an important role at the ecosystem level. It can be divided into mutual selection of autotrophs and heterotrophs dependent on each other (coevolution) and group selection, which leads to the preservation of traits that are favorable for the ecosystem as a whole, even if they are unfavorable for specific carriers of these traits.

In its broadest sense, coevolution refers to the coevolution of two (or more) taxa that share close ecological connections but that do not exchange genes. Natural selection operating in a population of predators will constantly increase the efficiency of searching, catching and eating prey. But in response to this, the prey population develops adaptations that allow individuals to avoid capture and destruction. Consequently, in the process of evolution of the predator-prey relationship, the prey acts in such a way as to free itself from the interaction, and the predator acts in such a way as to constantly maintain it.

Predators and other “exploiters” have no less sophisticated ways of overtaking their prey. Consider, for example, the social hunting behavior of lions and wolves, the curved poisonous teeth of snakes, the long sticky tongues of frogs, toads and lizards, as well as spiders and their webs, deep-sea angler fish or boa constrictors that strangle their victims.

A feature of the animal world is that this object is renewable, but this requires compliance with certain conditions directly related to the protection of animals. If exterminated or the conditions of their existence are violated, certain species of animals may completely disappear, and their renewal will be impossible.

Bibliography

1. Voronkov N.A. Fundamentals of general ecology. M.: Agar, 1997

2. Our nature [Electronic edition] // Wikipedia. - Access mode: Free encyclopedia - https://ru.m.wikipedia.org/wiki

3. Populations [Electronic edition] // - Access mode: http://ours-nature.ru/

4. Stadnitsky G.V., Rodionov A.I. Ecology. St. Petersburg: Chemistry, 1997

5. Structure and properties of populations [Electronic edition] // - Access mode: http://sbio.info/

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