Formation of sustainable populations. Population is the form of existence of a species. Age structure of plant populations

Population is a collection of individuals of one species that exists for a long time in a certain territory (area) and is separated from other populations by some form of isolation. A population is the elementary structure of a species, in the form in which the species exists in nature.

The main property of populations, like other biological systems, is that they are in continuous movement and constantly changing. This is reflected in all parameters: productivity, stability, structure, distribution in space. Populations are characterized by specific genetic and environmental characteristics that reflect the ability of systems to maintain existence in constantly changing conditions: growth, development, stability. The science that combines genetic, ecological, and evolutionary approaches to the study of populations is known as population biology.

Types of populations. Populations may occupy areas of different sizes, and living conditions within the habitat of one population may also not be the same. Based on this characteristic, three types of populations are distinguished: elementary, ecological, and geographical.

1. An elementary (local) population is a collection of individuals of the same species occupying a small area of homogeneous area. There is a constant exchange of genetic information between them. For example, one of several schools of fish of the same species in a lake; clumps of trees of the same species (Mongolian oak, larch, etc.), separated by meadows, clumps of other trees or shrubs, or swamps.

2. Ecological population – a set of elementary populations, intraspecific groups, confined to specific biocenoses. Plants of the same species in a cenosis are called a cenopopulation. The exchange of genetic information between them occurs quite often. For example, fish of the same species in all schools of a common reservoir; tree stands in monodominant forests representing one group of forest types: grass, lichen or sphagnum larch (Magadan region, northern Khabarovsk Territory); forest stands in sedge (dry) and forb (wet) oak forests (Primorsky Territory, Amur Region); squirrel populations in pine, spruce-fir, and broadleaf forests in one area.

3. Geographic population– a set of ecological populations inhabiting geographically similar areas. Geographic populations exist autonomously, their habitats are relatively isolated, gene exchange occurs rarely - in animals and birds - during migration, in plants - during the spread of pollen, seeds and fruits. At this level, the formation of geographical races and varieties occurs, and subspecies are distinguished. For example, the geographical races of Dahurian larch are known: western and eastern. Zoologists distinguish tundra and steppe populations of the narrow-skulled vole. The common squirrel species has about 20 geographic populations, or subspecies.

The main characteristics of the population are: density, size, birth rate, mortality, age composition, distribution pattern within the territory and growth rate.

Density A population is determined by the number of individuals per unit area or volume. The territory occupied by different populations of the same species varies and depends on the degree of mobility of individuals. Each species is characterized by a certain population density, deviations from which in both directions negatively affect the rate of reproduction and vital activity of individuals.

Number is the total number of individuals in the allocated territory. The size or number of individuals in a population varies among different species and largely depends on the stability of the ecological situation. The number cannot be below certain limits; a reduction in number beyond these limits can lead to extinction of the population. During sexual reproduction, the exchange of genes transforms the population into a relatively integral genetic system. If cross-fertilization is absent and vegetative propagation predominates, genetic connections are weaker and the population is a system of clones or pure lines sharing the environment. Such populations are united mainly by ecological connections.

The modern theory of population dynamics considers fluctuations in population size as an auto-regulatory process. There are two fundamentally different aspects of population dynamics: modification and regulation.

Any population of organisms under specific conditions is characterized by a certain average level of abundance around which fluctuations occur. Deviations from this average level have different ranges, but normally, after each deviation, the population size begins to change with the opposite sign.

Population dynamics and density is determined mainly by fertility, mortality and migration processes. These are indicators that characterize population changes during a certain period: month, season, year, etc. The study of these processes and the causes that determine them is very important for forecasting the state of populations.

Fertility is distinguished between absolute and specific. Absolute fertility is the number of new individuals appearing per unit of time, and specific- the same quantity, but assigned to a certain number of individuals. For example, an indicator of a person's fertility is the number of children born per 1000 people during the year. Fertility is determined by many factors: environmental conditions, the availability of food, the biology of the species (the rate of sexual maturation, the number of generations during the season, the ratio of males and females in the population).

According to the rule of maximum fertility (reproduction), under ideal conditions, the maximum possible number of new individuals appears in populations; Fertility is limited by the physiological characteristics of the species. For example, a dandelion can cover the entire globe in 10 years, provided that all its seeds germinate. Willows, poplars, birches, aspens, and most weeds produce exceptionally abundant seeds.

Bacteria divide every 20 minutes and within 36 hours can cover the entire planet in a continuous layer. Fertility is very high in most insect species and low in predators and large mammals.

Mortality, Just like birth rate, it can be absolute (the number of individuals who died in a certain time) or specific. It characterizes the rate of population decline from death due to disease, old age, predators, lack of food, and plays a major role in population dynamics.

Stable, growing and declining populations. The population adapts to changing environmental conditions by updating and replacing individuals, i.e., by the processes of birth (renewal) and decline (dying), supplemented by migration processes. In a stable population, the birth and death rates are close and balanced. They may be variable, but the population density differs slightly from some average value. The range of the species neither increases nor decreases.

In a growing population, the birth rate exceeds the death rate. Growing populations are characterized by outbreaks of mass reproduction, especially in small animals (locusts, 28-spotted potato beetle, Colorado potato beetle, rodents, crows, sparrows; among plants - ragweed, Sosnovsky's hogweed in the northern Komi Republic).

If the mortality rate exceeds the birth rate, then such a population is considered to be declining. In the natural environment, it decreases to a certain limit, and then the birth rate (fertility) increases again and the population goes from declining to growing. Most often, populations of undesirable species are growing uncontrollably, while populations of rare and valuable species are declining, both economically and aesthetically.

The dynamics, condition and reproduction of populations are consistent with their age and sex structure. The age structure reflects the rate of population renewal and the interaction of age groups with the external environment. It depends on the characteristics of the life cycle, which differ significantly among different species (for example, birds and mammalian predators), and external conditions.

In the life cycle of individuals, three age periods are usually distinguished: pre-reproductive, reproductive and post-reproductive. Plants are also characterized by a period of primary dormancy, which they go through in the stage of feeding seeds. Each period can be represented by one (simple structure) or several (complex structure) age stages. Annual plants and many insects have a simple age structure. A complex structure is typical for tree populations of different ages and for highly organized animals. The more complex the structure, the higher the adaptive capabilities of the population.

Thus, both the scale and course of fluctuations in the number of any species in natural communities are historically determined by natural selection, depending on the characteristics of biology, the nature of intraspecific connections and interspecific relationships to which the species is adapted in certain environmental conditions. For each biological species there is an optimum of environmental factors, which is characterized by the greatest degree of favorableness for the existence of the species.

Not a single living organism of any kind exists separately from others - they all form groups called populations. There are quite complex interactions within a population, but both in relations with other populations and with the environment, the population acts as some kind of integral structure. Therefore, the lowest level of organization of living matter considered in ecology is the population level.

The main characteristic of a population is its total number or density (number per unit of space occupied by the population). It is usually expressed either in the number of individuals or in their biomass. Abundance determines the size of the population. It is characteristic that in nature there are certain lower and upper limits for population sizes. The upper limit is determined by the flow of energy in the ecosystem that the population belongs to, the trophic level it occupies, and the physiological characteristics of the organisms that form the population (the size and intensity of metabolism). The lower limit is usually determined purely statistically - if the number is too small, the likelihood of fluctuations sharply increases, which can lead to the complete death of the population.

One of the basic ecological principles is that in an unconstrained, stationary, organism-friendly environment, population size increases exponentially. However, as already mentioned, this is never observed in nature - the population size is always limited from above. Light, food, space, other organisms, etc. can act as a limiting factor (or limiting factors).

The dynamics of changes in the total population size are determined by two processes - birth and death.

The birth process is characterized by fertility - the ability of a population to increase in size. Maximum (absolute, physiological) fertility is the maximum possible number of offspring produced by one individual in ideal environmental conditions in the absence of any limiting factors and determined only by the physiological capabilities of the organism. Ecological fertility (or simply fertility) is associated with an increase in population under actually existing environmental conditions. It depends both on the size and composition of the population and on the physical conditions of the habitat.

The process of population decline is characterized by mortality. By analogy with fertility, a distinction is made between minimal mortality, associated with physiological life expectancy, and environmental mortality, which characterizes the probability of death of an individual in real conditions. It is obvious that environmental mortality far exceeds physiological mortality.

Considering the dynamics of an isolated population, we can assume that fertility and mortality rates are generalized parameters characterizing the interaction of a population with the environment.

The population of any species is distributed in space extremely unevenly, in groups. For example, stinging nettle, within its range, is found in moist, shady places with fertile soils, forming thickets in the floodplains of rivers, streams, around lakes, and along the edges of swamps. We see cabbage whites where cabbage is grown - in vegetable gardens and fields.

European mole - settlements found on forest edges and meadows in the form of mounds of earth. Groups of individuals of the same species can be large or small, exist for a long time over centuries or only during the life of 2 - 3 generations.

As a result of the spring flooding of rivers, temporary reservoirs and puddles are formed, where fish fry can fall or frogs lay eggs, and dragonfly and mosquito larvae can develop. But these groups are doomed, as the reservoir will dry up and they will die.

For evolution, the fate of individuals that persist for a long time and sustainably throughout the lives of many generations is important. Such groups of individuals of the same species, existing for a long time in a certain part of its range, are called populations.

Population - (populatio - people, population) is a collection of individuals of the same species, possessing a gene pool and occupying a certain territory, existing relatively isolated from other populations.

This term was introduced by V. Johansen in 1903.

The gene pool is the totality of all the genes of all individuals in a given population.

Why is a population capable of long-term existence? This is not a chaotic accumulation of individuals, but a stable integral formation - a supraorganismal form of organization of life. Individuals in a population differ in age, sex, and genotype, but are closely related to each other. This connection is especially clear in animals.

At the same time, some connections ensure the existence of an individual: birds and mammals mark their areas, protecting their territory from relatives. But many connections are aimed at reproducing the population. These are connections between genders and age groups. Individuals of different sexes find each other by smell, sounds, enter into marital relations, and take care of their offspring. For example, the need to establish a strong connection with the baby forces many animals to leave the herd during breeding (antelope, bison, reindeer), so during the calving period, a female reindeer and her baby go to the edge of the herd. She screams for two hours, then falls silent. Being in a herd, a fawn will confidently distinguish its mother’s voice from the voice of other deer.

What determines the stability of a population?

1. A sustainable population includes all age groups from newborns to the elderly. There are too many old individuals in a declining population.

2. The number of individuals in the population is different. For example, the population of insects is hundreds of thousands of individuals, but the population of large mammals is only a few hundred individuals.

Conclusion: the number of individuals in the population is constantly fluctuating. It is affected by outbreaks of locust reproduction, the Colorado potato beetle, and pathogenic microorganisms (influenza pandemic). A population will not be able to exist for a long time if its number is below certain limits.

Why is a population considered the unit of evolution?

Features of the population:

1. Great genotypic similarity.

2. Random free crossing (“mixing” of individuals occurs more easily and more often than between populations of the same species) due to territorial isolation from each other.

Example: One oak forest is located several kilometers from another, and oak pollen is carried by the wind several hundred meters. But during strong storms, pollen is picked up by the wind and can be transported over long distances and reach neighboring populations.

Question: Can population isolation be permanent (based on this example)?

Conclusion: No, the isolation of a population is relative.

3. Isolation (albeit not absolute) or separateness.

4. Hereditary changes may occur in individuals in a population and, as a result of free crossing, spread throughout the population. This leads to genetic heterogeneity of its constituent individuals. The heterogeneity of individuals in a population creates conditions for the action of natural selection, the development of adaptations to living conditions in species as a whole.

Any species exists in the form of specific groups of individuals - populations. There are many approaches to defining a population. Populations are called:

− elementary forms of existence of a species (S.S. Schwartz);

− intraspecific natural-historical groupings of individuals, which represent integral genetic-ecological systems (A.V. Yablokov);

− elementary units of the evolutionary process (I.I. Shmalgauzen).

From an ecological point of view, a population is an integral intraspecific grouping that corresponds to a minimal realized ecological niche.

From the point of view of genetics, a population is a genetic system that has a historically established gene pool, all individuals of which are potentially capable of interbreeding with each other.

The most complete and comprehensive definition of a population is the following:

A population is a minimal self-reproducing group of individuals of the same species, more or less isolated from other similar groups, inhabiting a certain area for a long series of generations, forming its own genetic system and forming its own ecological niche.

A number of clarifications are usually added to this definition:

A population is a form of existence of a species. The population is the elementary unit of evolution. The population is the unit of biomonitoring. The population is a unit of management, that is, a unit of exploitation, protection and suppression.

Thus, populations have a number of properties, which are not inherent in a single individual or simply a group of individuals. Let's take a closer look at some characteristics of the population.

1. Number

There is a lower limit of size, below which a population cannot exist for a long time. In this case, it is necessary to take into account not all individuals, but only those that take part in reproduction - this is the effective population size.

Typically, the size of populations is measured in hundreds and thousands of individuals (such populations are called mesopopulations). In large terrestrial mammals, the population size can decrease to several tens of individuals (micropopulations). Plants and invertebrates also have megapopulations, reaching millions of individuals. In humans, the minimum population size is about 100 individuals.

In most cases, the absolute size of a population cannot be determined. Then a derived characteristic is used - population density. Density is defined as the average number of individuals per unit area or volume of space occupied by a population. In ecology, density is also defined as the mass (biomass) of members of a population per unit area or volume. Low population density reduces its chances of reproducing but increases its chances of survival. High density, on the contrary, increases the chances of reproduction, but reduces the chances of survival. Therefore, each specific population must have some optimal density.

Number and density are static characteristics of a population.

2. Population area (spatial structure of populations).

Population density is closely related to its spatial structure. In island-type populations (with a well-defined distribution boundary), the density of distribution of individuals can be uniform. However, in lowland populations the distribution boundary is always blurred. In an ideal population, one can distinguish its core (territory with maximum density, for example, a circle), subperiphery (territory with low density, for example, a ring) and periphery (territory with low density, which does not ensure reproduction of the population). In real populations there are many types of spatial structure and, accordingly, types of density distribution.

3. Self-reproduction

Populations reproduce themselves through the process of reproduction of individuals. Based on the method of reproduction, the following types of populations are distinguished:

amphimictic - the main method of reproduction is normal sexual reproduction;

amphimictic panmictic - when mating pairs are formed, panmixia (free crossing) is observed;

amphimitic inbreds - when mating pairs are formed, inbreeding is observed (inbreeding, inbreeding, incest); the extreme case of inbreeding is self-fertilization;

apomictic - various deviations from the normal sexual process are observed, for example, apomixis, parthenogenesis, gynogenesis, androgenesis; observed in agamic (asexual) forms;

clonal - in the absence of the sexual process and reproduction only by vegetative means or with the help of asexual reproduction spores (for example, conidia);

combined - for example, clonal-amphimictic with alternating asexual and sexual reproduction; A special case of cloning is polyembryony - the development of several embryos from one zygote.

Every community is forced to adapt to different types of stress created by a variety of environmental factors. It is the ecological interaction between the environment and the community that determines the population size. This value serves as an indicator of how successfully the community subjugates the environment (as a result of conscious activity or in some other way).

The great diversity of human ecosystems is reflected in the equally diverse numbers of its different populations. The data given in table. 4.1 may serve as some illustration of the relationship between the form (or stage) of economic development and population density.

Each economic group is affected by many factors. The population density under given environmental conditions depends on the following reasons:

on the nature of climate, soil and relief;
flora and fauna;
cultural level, ability to use natural resources (farm productivity).

In simple communities with a relatively homogeneous cultural level in a given territory, it is possible to establish a direct connection between population density and a certain value characterizing the influence of the environment.

Population Density (J. Harrison, 1979)

Economic stage	Area per person (sq. miles)	Number of inhabitants per 1 sq. a mile
Gatherers
Upper Paleolithic people (England)
Aborigines of Australia
Indians of Tierra del Fuego
Residents of the Andaman Islands
Hunters and fishermen Eskimos and North Indians
eastern territories of Canada
Eskimos (Alaska)
Mesolithic people (England)
Indians in the Pampas
British Columbia Indians
Ancient landowners
Neolithic people (England)
Pastoral and nomadic peoples
Late Neolithic farmers
Iron Age people
Middle Ages people
Swedish peasants

According to Birdsell (1953), in Australia this value can be the average annual rainfall, which clearly correlates with population density (correlation coefficient +0.8). For populations of about 500 individuals, the equation applies Y=l 12.8 A^- 1.5845, where Y- the size of the area occupied by a given population, and A is the average annual precipitation.

From this, however, it does not follow that the population density must necessarily be the same in areas with the same environmental conditions and the same form of economy. Population density also depends on differences in the biosphere and food sources. The population density of the Kalahari Bushmen or the Shoshone Indians of Nevada differs from that of the Aborigines of Australia, despite approximately the same amount of rainfall in these areas. The farm productivity of the Indians of southern California is approximately the same as that of the aborigines of Australia, and the rainfall in California is even lower than in Australia. However, the population density of Indians is 50 times higher than that of Australians. This can be explained by the abundance of a special type of plant food (about a dozen different types of agave grow near the Gulf of California). The material culture of simple communities - gatherers and hunters - is very poor; in more developed societies, a dependence of population density on certain variables characterizing the social structure of the population and production technology arises, since highly organized communities are able to more efficiently use natural resources and overcome the limitations imposed by the environment.

For most of human history, production technology remained at an extremely low level: the main means of obtaining a livelihood were gathering and hunting, consumer gardening and market gardening. As a result, the population remained very low for a long time, and its growth was slow. Man began to explore colder areas only at the end of the Pleistocene, and Australia and America were inhabited only 12,000-15,000 years ago. Accelerated population growth began approximately 8,000 years ago with the development of agriculture, which made urban life possible. Humanity entered the current phase, characterized by colossal population growth and the development of ever new areas, only with the development of industrialization. It can be assumed that in the era when people lived by hunting and gathering (which corresponds to the Paleolithic and Mesolithic cultures), the population density was less than 1 person per 3 km 2. During the Neolithic era, when people began to cultivate the land, the density increased approximately 10 times; in the Bronze and Iron Ages - another 10 times. The total number of people in the Neolithic era is estimated at approximately 5 million, and during the period of the appearance of the first large cities - at 20-40 million.

Modern look Homo sapiens it took about 20 thousand years to reach a population of 200 million (during the Roman Empire). Over the next 1500 years (by 1600 AD), the world's population increased to 500 million, and after another 200 years it more than doubled (about 1 billion in 1800).

At any stage of development, the geographical distribution of population on the globe was uneven. Even today, half the world's population lives on just 5% of the inhabited land area, and 80% of this area has an average density of just over 3 people per square kilometer. Since the emergence of the first cities, along with large, high-density populations, small communities have existed in various places around the globe.

The dynamics of human populations can be very diverse: they can increase, decrease, or remain stable. Already in primitive human communities there was a tendency to maintain population size at a certain level. Here it is worth mentioning the natural regulation systems operating in the animal world. The basic principles of these systems:

1) approaching a stable population size actually means that population fluctuations are limited to certain limits;
2) a stable average level of density is determined by the capacity of the system (an example is the population of Australian aborigines);
3) processes that limit the range of fluctuations in an ecosystem of constant capacity are internal; they are called density-dependent processes;
4) external events can sharply upset the balance in the ecosystem, but do not participate in establishing stability;
5) any ecosystem has an “optimal” population size, and the processes regulating the population are aimed at establishing an optimum (which, however, is not always achieved under given conditions).

These principles also apply to human populations, but with certain limitations. Firstly, the size of the human population is regulated not only by biological, but also by cultural factors. The action of the latter may be purposeful, but in many communities it is so closely intertwined with the action of ingrained customs that it seems as faceless as the manifestation of the action of biological factors. Secondly, over time the system becomes increasingly capable of expansion. In the last period, it became possible to continuously increase the population size without approaching equilibrium.

Density-dependent regulatory processes can be divided into two groups - “competition-driven” and “inverse”.

first group changes with changes in population size: when the number increases, it increases, and when it decreases, it weakens. Thus, with an increase in population density, the struggle for the main sources of food, water, fuel and raw materials, and the development of territories and places suitable for habitation, intensifies. An increase in population density may be accompanied by environmental pollution or other deterioration of living conditions, which leads to more frequent outbreaks of epidemics and the rapid spread of diseases due to both direct contacts between people and pollution of air, food, water, and household utensils. The limiting effect of these processes weakens with a decrease in population size.

Limiting effect of regulatory processes second group changes in the direction opposite to the change in population size. The intensity of these processes decreases with increasing population density, and increases with its decrease. As the population increases in size, it becomes able to use natural resources more efficiently and more successfully obtain its means of subsistence, especially food. There are better opportunities for joint hunting and fishing, clearing land and carrying out irrigation work, building houses, etc. A larger community can organize more effective mutual defense of its members or provide offensive actions against other groups and animals. It should also be taken into account that with an increase in population size, the pool of genetic variability increases. As density decreases, all these advantages are lost: a small community scattered over a large area is less likely to successfully compete with other populations and is less capable of conducting collective activities.

If in communities the number remains at the same level for a long time, then it follows that mortality in them is not so high that in order to maintain a constant number, fertility is equal to 6-8, and, therefore, there is an effective limitation of the birth rate. A similar situation is typical for tribes of gatherers and primitive farmers. Numerous data indicate that out of every 4-5 children born, only two or three reach adulthood. The family size is therefore quite small. In high-density agricultural communities, mortality is usually much higher and the population can only be maintained if the total fertility rate is 6-8.

In societies with limited resources and conservative technology, fecundity is such that, after a long enough time, it is inevitable that the population size will exceed the level corresponding to the amount of resources. If the number remains constant, then, apparently, there are some regulatory processes that maintain stability - famine, epidemics, wars. According to Malthus, population growth obeys the law of geometric progression, while food resources increase according to the law of arithmetic progression. Malthus justified “moral” measures to limit births, such as later marriages, which would inevitably lead to smaller families.

Some theorists (R. Hawley, 1950), analyzing population dynamics in primitive communities, fully support Malthusian positions. In their opinion, population sizes tend to change, directly related to changes in the available sources of food and materials necessary for the life of the community. Under such conditions, the problem of population size is solved only in relation to a given area, since geographic isolation limits the possibility of receiving outside help. The low level of technology makes the population closely dependent on changes in the environment. Such a population is virtually defenseless against unfavorable climate changes (drought, floods), as well as against the destruction of food sources by pests or predators. Thus, local features of the physical and biotic environment are a very unstable factor for groups that maintain low and unchanged farming techniques.

Population size automatically adjusts to changes in natural resources, most often through changes in mortality. In unfavorable years, mortality increases sharply, and in periods of abundance it falls. Although our knowledge of the demographics of isolated groups is fragmentary, the average annual mortality rate for such populations appears to be about 40 per 1000 people. In some cases it reaches 100 per 1000, and can also drop to 25. In general, however, isolated populations are almost always in danger of disaster. This is indicated not only by the high average mortality rate, but also by the low average life expectancy (30 years or less). The “margin of safety” in such populations is sometimes so small that a decrease in annual precipitation of just a few centimeters or the loss of a few days necessary for the normal development of crops is enough to significantly increase mortality.

In such conditions, the birth and death rates are high, but the population renewal rate is also very high.

According to Hawley, a population's intensive use of natural resources is maintained by a high, constant rate of reproduction. In the isolated group, women are almost constantly pregnant, but fetal and infant mortality is so high that few children survive to adulthood. However, there is always some potential population ready to absorb any excess food that may appear in a high-yield year. A group can always quickly replace losses incurred during periods of acute food shortage, unless its numbers have fallen to excessively low numbers.

The concept that many demographers have arrived at from observations of simple communities and animal populations is that a population actually strives to maintain a constant size, and this goal is always more or less achieved.

Limiting population size by increasing mortality in a primitive community is a regulatory process that depends on population density and is in one way or another associated with a tendency toward overpopulation. Unsanitary living conditions and other consequences of overcrowding particularly affect infants and young children. The primitive community is constantly in need of labor, and the resulting lack of child care becomes a contributing factor to population decline. Diseases impair food-procuring activities. Elderly and sick people cease to participate in obtaining food, but continue to lay claim to it, while experiencing great deprivation.

Population growth should be considered as a process associated with the balance of three demographic factors - fertility, mortality and migration. Well-documented evidence supports migration on a small scale, such as the division of simple agricultural communities into local groups; There is also evidence of large population movements (both migrations should be distinguished from movements of groups associated with cutting down and burning vegetation for crops).

The "optimum" theory emphasizes the role of "natural" regulatory processes in human ecosystems associated with the action of density-dependent regulatory factors. On the one hand, there is a tendency for population growth, which would provide a real increase in income from various types of services and the number of goods per capita. On the other hand, there is a desire to reduce profits, since for a given volume of natural resources and (capital) equipment and the existing level of technology, any increase in the number of workers in the population leads to a decrease in other quantities on which the level of production depends (for example, land), and therefore, to a decrease in output per employee. The action of these opposing forces ultimately leads to the fact that, given a certain level of knowledge for a given time and other constant conditions, a state is reached that corresponds to obtaining maximum profit: the amount of labor expended is such that both an increase and a decrease in it leads to a decrease in profit.

Technological progress makes it possible to reach a new “optimal” level, but effective performance may involve different paths taken and perhaps a period of time during which overcrowding cannot be avoided. Rice production is a good illustration. Expansion and improvement of the existing irrigation system may be more profitable at this stage than the creation of new irrigation canals. However, choosing the first path does not guarantee progress in the distant future, and ultimately new problems will arise. The idea is that in primitive societies the population size is close to optimal, although this “optimum” is threatened by “fertility pressure”, the presence of public measures to limit the birth rate. Moreover, the “optimal” situation is fundamentally different from the passive Malthusian conditions. An example of such social regulation is the Australian aborigines and other communities leading a gathering economy: nomadic life in search of water and food involves long journeys, and it is difficult for mothers to carry more than one child with them. As a result, the tribe, moving to a new site, often kills or abandons newborns. The population size in primitive agricultural communities is determined by the maximum amount of labor needed to sow crops and harvest crops. The society must have a sufficient number of workers to carry out labor-intensive agricultural work in a short time. During the remainder of the year, unemployment and overcrowding often occur due to the gradual depletion of food supplies. However, the population density as a whole corresponds to the average equilibrium density within a year.

In many other communities, less serious measures are often taken, such as postponing marriage, legalized by various traditions. Interestingly, getting married before the age of 17 ultimately leads to a decrease rather than an increase in overall fertility (statistics for India). There is also evidence that the ability to bear children does not always appear simultaneously with the onset of menstruation. Prolonging the breastfeeding period can delay conception by up to a year. Some communities practice abstinence from sexual intercourse during the postpartum period. In developed countries, abstinence and a certain rhythm of sexual intercourse were used to reduce fertility; nowadays, they began to resort mainly to mechanical and chemical contraceptives.

It is often assumed that population growth is a normal feature of developing human societies, but in fact, throughout most of human history, population numbers have remained stable. According to the “optimum” theory, the population can change with a change in the nature of social organization and with the growth of productive forces; An example is the modern period of human development.

The standard of living of a human community depends on the manner in which that community achieves equilibrium under certain environmental conditions. This balance may be achieved through higher overall mortality or high morbidity, as well as through strenuous work, poor health, or lack of material wealth. To characterize equilibrium, you can use various criteria (including those that determine the level of consumption, efficiency, energy or monetary income per capita), as well as various demographic indicators. Two indicators deserve special mention - infant mortality and average life expectancy.

The most important indicator of the effectiveness of environmental control can be infant mortality(mortality in the first year of life for every thousand newborns), since it reflects the negative aspects of the social structure - malnutrition, overcrowding, lack of necessary sanitary conditions. This is a particularly sensitive indicator because the reduction in child mortality is not directly related to the reduction in mortality in older age groups. In Great Britain at the end of the 19th century. The mortality rate of children in the first year of life was still very high (150 per 1000 newborns), but the mortality rate of children older than one year was steadily declining. Infant mortality rates reflect socio-economic living conditions, including nutritional conditions. In all primitive communities, infant mortality is invariably high, and, as already mentioned, serves as one of the factors regulating numbers in ecosystems with limited capacity.

Index average life expectancy for human society of the Stone Age and later prehistory is determined from skeletal remains.

The studied skeletal material is distributed into 4-5 age groups, delimiting them as follows: children (from 0 to 12-13 years), adolescents and young men (from 12-13 years to 21 years), adults (from 21 to 40 years) , middle-aged people (from 40 to 59 years old), elderly (from 60 years old and older). Based on available data, it can be roughly established that throughout almost the entire history of mankind, people have died at a much earlier age than in our time. No more than 10% of the population survived the age of 40, and only 50% reached the age of 20.

There are also some exceptions: 20% of Guanche skulls belonged to people who died over the age of 50; in Melanesia (New Ireland) over 75% of people lived to marriageable age.

Life expectancy in ancient times averaged 20-30 years; The life expectancy of the English in the 13th century was 35 years.

Over the next five centuries, average life expectancy changed little. The table compiled by the astronomer Halley based on records made in Breslau in 1687-1691 shows that the average life expectancy was 33.5 years. The value of 35.5 was obtained by Wigglesworth by studying mortality statistics in Massachusetts and New Hampshire before 1789.

Gradual increases in living standards over the past two centuries have led to an increase in average life expectancy. From the tables compiled by the statistician W. Farr for England and Wales and relating to the years 1838-1854, it follows that the average life expectancy at that time was 40.9 years. With the development of medicine and hygiene, average life expectancy increased to 49.2 years (1900-1902). In the USA in 1945, average life expectancy reached 65.8, i.e. increased by about 16 years over five decades.

A population is the human, animal or plant population of an area.

There are gender, age, territorial and other types of structure. In theoretical and applied terms, the most important data is on the age structure, which is understood as the ratio of individuals (often combined into groups) of different ages. Animals are divided into the following age groups:

juvenile group (children)

senile group (senile, not involved in reproduction)

adult group (individuals engaged in reproduction)

The population is also characterized by a certain sex ratio, and, as a rule, the number of males and females is different (the sex ratio is not 1:1). There are known cases of a sharp predominance of one sex or another, alternation of generations with the absence of males. Each population can also have a complex spatial structure, being subdivided into more or less large hierarchical groups - from geographical to elementary (micropopulations).

Typically, the most viable populations are those in which all ages are represented relatively evenly. Such populations are called normal. If senile individuals predominate in a population, this clearly indicates the presence of negative factors in its existence that disrupt reproductive functions. Such populations are considered regressive or dying out. Urgent measures are required to identify the causes of this condition and eliminate them. Populations represented mainly by young individuals are considered to be invading or invasive. Their vitality usually does not cause concern, but there is a high probability of outbreaks of excessively high numbers of individuals, since trophic and other connections have not been formed in such populations. It is especially dangerous if such populations are represented by species that were previously absent here. In this case, populations usually find and occupy a free ecological niche and realize their reproductive potential, intensively increasing their numbers. If the population is in a normal or close to normal state, a person can remove from it the number of individuals or biomass (the latter indicator is usually used in relation to plant communities) that grows over the period of time between removals. It is clear that individuals of post-productive age (who have completed reproduction) should be seized first of all. If the goal is to obtain a certain product, then the age, gender or other characteristics of the populations are adjusted taking into account the goal.

The most important properties of populations include the dynamics of their inherent numbers of individuals and the mechanisms for its regulation. Any significant deviation in the number of individuals in populations from the optimal is associated with negative consequences for its existence. In this regard, populations usually have adaptation mechanisms that contribute to both a decrease in numbers, if it significantly exceeds the optimal value, and its restoration, if it decreases below the optimal values. Each population is characterized by the so-called biotic potential, which is understood as the theoretically possible offspring from one pair of individuals when the ability of organisms to increase their numbers exponentially is realized. Typically, the lower the level of organization of organisms, the higher the biotic potential

However, the biotic potential is realized by organisms to a significant degree of completeness only in individual cases and for short periods of time.

For most populations and species, survival is characterized by a curve of the second type, which reflects the high mortality of young individuals or their rudiments (eggs, eggs, spores, seeds, etc.). With this type of survival (mortality) the population size is usually expressed as an S-shaped curve. This curve is called logistic. But even in this case, periodic fluctuations in the number of individuals are significant. Such deviations from the average abundance are seasonal (as in many insects), explosive (as in some rodents - lemmings, squirrels) or gradual (as in large mammals) in nature.

One of the most important conditions for sustainability (by the way, this is the answer to one of the tasks, if anyone still remembers it) is internal diversity. Although the debate among scientists about how structural and functional diversity relates to the stability of a system does not subside, there is no doubt that the more diverse a system is, the more stable it is. For example, the more diverse the individuals of a population are in their genetic makeup, the greater the chance that when conditions change in the population there will be individuals capable of existing in these conditions.

One of the most important factors in maintaining population numbers is intraspecific competition. It can manifest itself in various forms: from fighting for nesting sites to cannibalism.

However, it is necessary to take into account that population stability is not limited to density regulation. Optimal density is extremely important for optimal use of resources (as density increases, resources may become scarce), but this does not guarantee a sustainable population.