Questions and tasks for self-control. Causes of population fluctuations Periodic fluctuations in the number of individuals in a population

The study and discovery of the causes that determine fluctuations in the number of animals in nature is one of the most important tasks of modern zoology. These changes determine the dynamics of the biomass of the species, and, consequently, the degree of participation of the species in the work of biogeocenoses. Primarily quantitative changes in the number of individuals in populations are the result of exposure, factors of a biotic and abiotic nature. At the same time, it is necessary to distinguish between true changes in numbers and temporary ones, which can be caused by animal migrations from data, biotopes due to adverse conditions (drought, flood) or differences in periods of activity of different age and sex groups. Observations show that both biotic and abiotic factors have a particularly massive impact on the abundance of terrestrial vertebrates if they are associated with anthropogenic impact.

Periodic population fluctuations

Periodic fluctuations in the population of lizards are mainly associated with reproduction and age-related mortality, that is, they are seasonal. In some cases, quantitative fluctuations in abundance in populations can be caused by periodic outbreaks of prey species and periodic changes in the "pressure" of predators.

It is known that any population has its own specific rhythm of numbers (both by sex and by age). Unfortunately, there are only fragmentary data on the rhythm of the number of individuals of different sexes of the agile lizard by season. V. K. Zharkova (1973a), based on the fact that the quantitative ratios of different age groups in the population are different, on the basis of the material available to her, determines the difference in age-related mortality of males and females (Table 57).

As can be seen from Table. 57, during the first and second years of life among the lizards of the Meshcherskaya lowland, males die almost twice as many as females. Only in the third year of life, the percentage of death of females sharply increases. Such a course of population dynamics leads to the predominance of young females in the population. In older age groups, on the contrary, intensive mortality itself occurs, and the sex ratio evens out. Perhaps it is this differentiated mortality that explains the slightly higher number of females in some natural populations.

The rhythm of fluctuations in the number of individuals of different sexes is subject to the same general patterns that operate with age-related fluctuations, with some corrections for different sexes. At present, it is possible to build a model of the population dynamics of the lizard, determined by age-related mortality (Fig. 92). As shown in ch. XV, the rate of mortality of animals during their life turns out to be different. Let us trace the dynamics of the number of one generation. Taking the data table. 62 (Ch. XV) for the initial ones, we find that the largest number in the population is achieved at the time of oviposition (a laid fertilized egg is already an individual) and the release of young ones. Given the high mortality among newborns (50%), who die before wintering and after it, by next spring only 25% of those born in this generation will remain alive. In the second year, about 33% of those who have reached the age of one year will die. In the third year of life, mortality apparently decreases somewhat, but in the fourth and fifth years of life it reaches 50 and 100%, respectively, of the number of individuals that have survived to this age.

Table 57

Age-related mortality of males and females(difference in the number of individuals of the previous and subsequent generation *) quick lizard in the Meshcherskaya lowland[according to V.K. Zharkova (1973a) for 1965 - 1969]

* (In this case, generation refers to offspring that were born in one breeding season.)

The overall picture of seasonal fluctuations in the population of the agile lizard will be much more complicated, since at each moment in the population there will be individuals of five generations, each of which has its own mortality rate. As a result, the model of the size of the population of the agile lizard will look like this (Fig. 93). It should be emphasized that this model takes into account only the seasonal cycle; aperiodic cyclicity is almost always superimposed on it in nature.

Table 58

Population density dynamics (ind./1000 m2) of the lizard for 4 years in different biotopes(Zharkova, 1973b)

Of course, in different parts of the range, the dynamics of changes in the seasonal population size should be different. Long-term population dynamics is the result of seasonal dynamics. But this is not a simple sum, but, as it were, a superposition of various often oppositely directed phenomena, caused by various factors, on top of each other. Speaking of a combination of factors, we mean both biotic and abiotic factors acting on natural populations. Let us illustrate fluctuations in numbers in different years by the example of five groups of lizards in the northern forest-steppe of the European part of the USSR (Table 58).

As can be seen, each biotope is characterized by a certain dynamics of population fluctuations. So, on the slopes of the river, the number of lizards has been constantly increasing over these four years, while (while in pine plantations there is no lizard, then a sharp "surge" in numbers, followed by a decrease in the population. In a mixed forest, in a forb meadow, changes in the number of a different order occur.

Undoubtedly, population fluctuations in populations of the agile lizard, associated with the action of biotic factors, can be caused either by periodic outbreaks of the abundance of the main food (see Chapter VI), or by the periodic pressure of predator pressure, or, finally, by sharp outbreaks of the abundance of competing species.

An increase in the number of prey species, of course, leads to a certain increase in the population size (provided there is no pressure from predators and competing species), while a decrease in the number of food species in some cases leads to the fact that adults begin to devour the young (cannibalism), thereby reducing the number populations. In some cases, there is a false reduction in the population as a result of migration. Such migrations can be caused by an increase in the populations of competing species or a sharp decline in the abundance of prey species. Sometimes populations migrate completely and pass into biotopes that are completely uncharacteristic of it, at the stage of experience (see Ch. IX). It is important to add that prey species and competing species, apparently, do not have the same significant significance in the regulation of the population size of the lizard (Lukina, 1966; Tertyshnikov, 1972a, b; etc.)

Predators play an important role in population fluctuations. A sharp increase or decrease in the number of predators inevitably leads to a decrease or increase in the number of populations of the agile lizard. At the same time, the importance of predators as population regulators of the agile lizard has been extremely insufficiently studied. According to the calculations of M. F. Tertyshnikov, in the Stavropol Territory, on a stationary site located in the upper reaches of the river. Tomuzlovki, 37.2% of the total biomass of the lizard population perishes during the season as a result of the impact on them of the considered background enemies from vertebrates. In this region, the pressure of predators is an additional reason that delays the growth of the population. The "pressure" of predators is undoubtedly different in different populations. This is indirectly evidenced by the data in Chap. XIII on the different proportion of individuals with regenerated tails (i.e., individuals that were attacked by predators and successfully avoided them). Let us only recall that in some populations more than half of the adults bear traces of such attacks.

In general, it can be said that periodic fluctuations apparently do not cause changes in the number of populations of the agile lizard by more than one order of magnitude.

Aperiodic population fluctuations

Factors promoting mass reproduction of lizards in the population, and factors determining the onset of the subsequent period of population depression following it, can be caused by aperiodic phenomena. Such phenomena can be catastrophic changes in biogeocenoses as a result of fires, floods, drought, severe frosts in winters with little snow, and other phenomena that directly or indirectly favor or, conversely, hinder population growth. Population fluctuations can also be associated with long-term changes in natural conditions caused by human activity (desertification, salinization of large areas, etc., or long-term climate changes such as ice ages. Sometimes such causes can lead to mass death of animals, especially since fast the lizard, being a poikilothermic animal, is highly dependent on climatic conditions.

According to many authors (Terentiev, 1946; Lukina, 1966b; Garanin, 1971; Tertyshnikov, 1972b; Zharkova, 1973a; and others), the main reduction in the number of lizards occurs mainly due to the death of eggs. If we take all the eggs laid in the population as 100%, then by the time the young are released, from 40 to 60% die. This percentage increases sharply, especially in rainy and cold summers (Garanin, 1971). Another critical moment associated with the physical factors of the environment in the life of a quick lizard is autumn with early frosts. It is during this period of time, when the animals are about to go into hibernation, that such phenomena are most dangerous (especially for the young of this year of birth, which later go to winter).

Finally, the last critical moment influencing the size of populations is wintering. The number falls especially sharply in frosty winters with little snow (Garanin, 1971; Tertyshnikov, 1972b). Usually adult lizards overwinter in their burrows or in old rodent burrows (see Chapter V). If in open biotopes the burrows of most rodents are deep and, as winter shelters, are reliable (since the temperature in them does not fall below 0 °), then this cannot usually be said about burrows dug by the lizards themselves, usually shallow, as well as about cracks and crevices where juveniles often overwinter in the soil. In cold winters with little snow, when the soil is not sufficiently covered with snow, and consequently the temperature regime in winter burrows is disturbed, many animals wintering in such burrows die. Therefore, forest biotopes, forest belts, roadside ditches, ravines, etc., are much more reliable for the survival of lizards in winter, since it is in these places that a significant amount of snow accumulates, warming the soil. Here, the mass death of animals in frosty winters can occur only in exceptional cases. VI Garanin (1971) gives such an example. The harsh winter of 1968 with little snow in the Volzhko-Kama Reserve led to a sharp decrease in the number of lizards. It is during wintering that a sharp decrease in the number of young animals in the population occurs. Thus, M. F. Tertyshnikov showed that the loss of eggs and mortality of underyearlings during the first wintering in the Stavropol Upland is 25.7%.

The most significant changes in the habitat, which are catastrophic in nature, lead to a sharp decrease in its abundance. Typical phenomena of this order include fires, floods and droughts. According to V.K. Zharkova (1973a), in 1967, after heavy rains in the area of ​​the Oksky Reserve, the rivers overflowed their banks and flooded the biotopes occupied by the lizard. So, in the floodplain meadow and in the floodplain plantings of pine on the river. Higher in this year, there was a sharp decrease in the number of the lizard: in the floodplain meadow, the number of lizards on average decreased by 4 times compared with 1966, and in the floodplain plantings by 2.5 times. It should be noted that this may completely disappear individual demos or their entire groups. But populations in most cases are preserved, although they can at the same time reach minimum numbers, which are nevertheless sufficient for further existence.

The most significant changes in the environment associated with long-term changes in natural conditions are now more often associated with anthropogenic impacts. For example, one of the reasons for the death of agile lizards is the creation of new reservoirs on floodplain lands. On the banks of the Kuibyshev reservoir, Garanin (1971) notes a sharp decrease in the number of reptiles, including the lizard. At the same time, on some islands of the reservoir, the populations, which first decreased in number, then increased noticeably.

A more serious and widespread factor negatively affecting the number of lizards is the excessive use of pesticides in agriculture and forestry. There is a strict correlation between the treatment of Meshchera sites with pesticides and their population with lizards (Zharkova, 1973b). In this area, biotopes not inhabited by the lizard, but suitable for them, make up from 32 to 49% of the surveyed territory. There is every reason to agree with the warning of E. Rene (Rene, 1969) and K. Corbett (Corbett, 1969) about the danger of extinction of the lizard in developed industrial areas due to the complete anthropogenic destruction of natural biotopes.

There is another side of the anthropogenic influence on the population of the agile lizard. Observations made in 1970 - 1974 in different parts of the range of the agile lizard, show that in some areas it becomes an "anthropogenic species". The wide ability of this species to adapt to anthropogenic biotopes (see Table 5) will undoubtedly allow this species not only "not to reduce its numbers during contacts with civilization, but, possibly, even increase it in some parts of its range. For now, as a result of active anthropogenic impact, the number of nimble lizards is sharply reduced on the Stavropol Upland (Tertyshnikov, 1972c), in the Kaluga region (Streltsov), in many regions of Siberia (Baranov et al., reports), Latvian (Bakharev, 1971) and Estonian SSR (Veldre, personal comm.), and in the vicinity of the city of Makhachkala (Khonyakina, Kutuzova, pers. comm.), the agile lizard disappeared altogether.

Thus, at present, only with very sharp changes in the environment, usually associated with the destruction of biocenoses under human influence, the population size is reduced below a critical level, followed by the extinction of the population.

Periodic (seasonal and annual) fluctuations in the abundance of the lizard apparently do not exceed one order of magnitude, while aperiodic fluctuations are often more significant.

When a population stops growing, its density tends to fluctuate around the upper asymptotic level of growth. Such fluctuations can arise either as a result of changes in the physical environment, as a result of which the upper limit of abundance increases or decreases, or due to intrapopulation interactions, or, finally, as a result of interaction with neighboring populations. After the upper limit of the population size ( TO) is reached, the density may remain at this level for some time or immediately fall sharply (Fig. 8.7, curve 1 ). This drop will be even sharper if the resistance of the environment does not increase gradually, as the population grows, but appears suddenly (Fig. 8.7, curve 2). In this case, the population will realize the biotic potential.

Rice.

However, exponential growth cannot last long. When the exponent reaches the “paradoxical point” of striving for infinity, as a rule, a qualitative leap occurs - a rapid increase in the number is replaced by a mass death of individuals. An example of such fluctuations is an outbreak of insect reproduction, followed by their mass death, as well as the reproduction and death of algae cells (blooming of water bodies).

A situation is also possible in which the population size jumps over the limit level (Fig. 8.7, curves 3 , 4). This, in particular, is observed when animals are introduced to places where they did not exist before (for example, stocking new ponds with fish). In this case, nutrients and other factors necessary for development have been accumulated even before the start of population growth, and the mechanisms of population regulation are not yet in operation.

There are two main types of population fluctuations (Figure 8.8).

Rice. 8.8.

In the first type, periodic environmental disturbances such as fires, floods, hurricanes, and droughts often result in catastrophic, density-independent mortality. Thus, the population of annual plants and insects usually grows rapidly in spring and summer, and sharply decreases with the onset of cold weather. Populations whose growth gives regular or random bursts are called opportunistic(Fig. 8.8, graph /). Other populations, the so-called equilibrium(characteristic of many vertebrates) are usually in a state close to equilibrium with resources, and their density values ​​are much more stable (Fig. 8.8, graph 2).

The two distinguished types of populations represent only the extreme points of the continuum, however, when comparing different populations, such a division is often useful. The significance of contrasting opportunistic populations with equilibrium ones lies in the fact that the density-dependent and density-independent factors acting on them, as well as the events taking place in this case, affect natural selection and the populations themselves in different ways. R. MacArthur and E. Wilson (1967) called these opposite types of selection r-selection and K-selection according to the two parameters of the logistic equation. Some characteristic features of r-selection and /r-selection are given in Table. 8.1.

Of course, the world is not painted only in black and white. None of the species is subject to only r-selection or only AG-selection; everyone must reach a certain compromise between these two extremes. Indeed, one can speak of each specific organism as an “r-strategist” or “/^-strategist” only when compared with other organisms, and therefore all statements about the two selected types of selection are relative. However,

The main features of / -selection and A "-selection

Table 8.1

deny that there are two opposite breeding strategies that populations resort to depending on fluctuations in the capacity of the environment. Figure 8.9 shows how the mechanism of m-selection or A "-selection could be fixed in evolution: in A"-selective environments, selection contributes to the formation of mechanisms that compensate for environmental fluctuations, and in /*-selective environments, the population "improves" in the ability to quickly populate Wednesday at the right time of the year.

In temporal terms, fluctuations in population size are non-periodic and periodic. The latter can be divided into fluctuations with a period of several years and seasonal fluctuations. Non-periodic fluctuations are unforeseen.

Rice. 8.9.

In the Pacific Ocean, especially in the area of ​​the Great Barrier Reef northeast of Australia, since 1966 there has been an increase in the number of starfish crown of thorns (Acanthaster planci). This species, being previously small in number (less than one individual per 1 m 2), reached by the beginning of the 1970s. density 1 individual per 1 m 2. The starfish causes great harm to coral reefs, as it feeds on the polyps that make up their living part. She cleared a 40-kilometer strip of reef off Guam in less than three years. None of the hypotheses proposed to explain the sudden increase in the abundance of the starfish (the disappearance of one of its enemies - the gastropod mollusk tritonium horn (Charonia triton is), which is mined because of shells containing mother-of-pearl; an increase in the content of DDT in sea water and, in connection with this, a violation of the natural balance; effect of radioactive fallout) cannot be considered satisfactory.

An example of periodic population fluctuations with a period of several years is given by the populations of some arctic mammals and birds. In the hare and lynx, the period of population fluctuation is 9.6 years (Fig. 8.10).

As can be seen from the figure, the maximum abundance of the hare, as compared to the abundance of the lynx, is usually shifted back by one or two years. This is quite understandable: the lynx feeds on hares, and therefore fluctuations in its numbers must be associated with fluctuations in the number of its prey.

Rice. 8.10. Periodic fluctuations of hare populations (graph 1) and lynxes (chart 2), determined by the number of skins harvested by the Hudson Strait Company

Cyclic changes in numbers with an average period of four years are typical for the inhabitants of the tundra: snowy owl, arctic fox, and lemming. According to many scientists, the periodicity of 9.6-year cycles in the hare and lynx is determined by phenomena occurring in space, and is somehow connected with solar cycles. A similar dependence is observed, for example, in the Atlantic Canadian salmon, the maximum number of which is observed every 9-10 years.

The causes of other periodic population fluctuations are well known. Off the coast of Peru, there is a transgression of warm waters to the south, known as El Nino. Approximately once every seven years, warm waters displace cold waters from the surface. The water temperature quickly rises by 5 ° C, salinity changes, plankton dies, saturating the water with decay products. As a result, fish die, followed by seabirds.

Cases of seasonal changes in the number of populations are well known to all. Clouds of mosquitoes, a large number of birds inhabiting the forests are usually observed in a certain period of the year. In other seasons, the populations of these species may practically disappear.

Invasions of voles, mice, locusts have been known to mankind since biblical times. Even Aristotle left a description of the "rise and fall" of the mouse population. He noted that the invasion of rodents was a disaster comparable to the plague. They multiplied monstrously, destroyed the crops and their own "food base" and eventually disappeared, as if they had fallen through the ground. In ancient Russia, the years of mass reproduction of rodents were called the years of "mouse misfortune", they are even mentioned in the annals. Population fluctuations, which are especially noticeable in mouse-like rodents and other species with a short life cycle and a rapid change of generations, are characteristic of all plant and animal populations (Fig. 23).

In rapidly reproducing species, there is a periodic alternation of ups and downs in numbers - population cycles. So, the cycles of voles, lemmings and other mouse-like rodents usually last 4 years. During this period, the number of animals increases from negligible to maximum, then drops to almost zero and a new cycle begins. What are the reasons for this periodicity? It is difficult to give an exhaustive explanation. Apparently, a significant role in this process is played by predators, whose numbers fluctuate in proportion to the growth and decline of the rodent population. For example, the more voles, the more owls hatch. Kestrels, harriers, buzzards and other birds constantly living in one place during the heyday of the mouse population feed all the hatched chicks, and in a famine year many chicks die. However, predation is only one of many causes of population fluctuations. The predator moves no more than what he needs, and is powerless to cope with the hordes of animals during their mass reproduction. Sharp fluctuations in the number of rodents can also be associated with outbreaks of epidemics.

The causes of population waves in some cases are less known, in others they are more studied and explainable. Thus, it is well known that the yield of spruce cones increases after a warm, dry summer, and this, in turn, has a positive effect on the growth of the squirrel population.

Sharp non-periodic declines in numbers occur as a result of drought, fire, floods and other natural disasters. In this case, exceptionally favorable conditions are always inevitably created for the development of some organisms, unfavorable for others. For example, at the site of forest fires, Ivan-tea grows wildly. Its number increases over several years, then this plant is gradually replaced by other herbs, shrubs, trees.

Sharp outbreaks of the number of species are observed when they enter new conditions suitable for life. It is enough to give an example of the consequences of the settlement of the muskrat in Europe and the USSR, the conquest of Australia by rabbits. However, after several generations, a species new to a given biogeocenosis becomes a victim of new predators for it, new diseases to which immunity has not been developed. As a result, after an unprecedented rise in numbers, a period of decline inevitably sets in. So it was in the USSR with muskrat in the 50-60s, so it was in 1987-1988. on Lake Sevan with whitefish acclimatized here.

In nature, populations fluctuate. Thus, the number of individual populations of insects and small plants can reach hundreds of thousands and a million individuals. In contrast, animal and plant populations can be relatively small in number.

The actuation of regulatory mechanisms can cause fluctuations in the number of populations. Three main types of population dynamics can be distinguished: stable, cyclical, and spasmodic (explosive).

Any population cannot consist of a smaller number of individuals than is necessary to ensure the stable implementation of this environment and the stability of the population to environmental factors - the principle of the minimum population size.

Minimum population size specific to different species. Going beyond the minimum leads the population to death. Thus, further crossing of the tiger in the Far East will inevitably lead to extinction due to the fact that the remaining units, not finding breeding partners with sufficient frequency, will die out over a few generations. The same threatens rare plants (orchid "Venus slipper", etc.).

There is also a population maximum. 1975, Odum, - population maximum rule:

Population density regulation occurs when energy and space resources are fully utilized. A further increase in population density leads to a decrease in food supply and, consequently, to a decrease in fertility.

There are non-periodic (rarely observed) and periodic (permanent) fluctuations in the number of natural populations.

The stable type is distinguished by a small range of fluctuations (sometimes the number increases several times). It is characteristic of species with well-defined mechanisms of population homeostasis, high survival rate, low fecundity, long life span, complex age structure, and developed care for offspring. A whole complex of efficiently operating regulatory mechanisms keeps such populations within certain density limits.

Periodic (cyclic) fluctuations in the number of populations. They are usually performed within one season or several years. Cyclic changes with an increase in numbers after an average of 4 years were registered in animals living in the tundra - lemmings, snowy owls, arctic foxes. Seasonal fluctuations in abundance are also characteristic of many insects, mouse-like rodents, birds, and small aquatic organisms.

In ecosystems with simple structure (agrobiogeocenoses, desert, semi-desert and tundra ecosystems), the community of organisms is subject to strong physical stresses. In such biogeocenoses, the number of populations is largely influenced by the characteristics of the weather, water and air currents, the chemistry of the environment, and the degree of its pollution. In natural biogeocenoses with a complex structure and rich species diversity, consisting of a large number of populations, population fluctuations are mainly controlled by biotic factors. Therefore, when studying the causes that cause fluctuations in the number of a particular population, it is necessary to have a clear idea of ​​how independent , so about density dependent factors.

The first includes factors that act on the population constantly. These are abiotic and, above all, climatic factors of mortality. Unfavorable weather can cause the death of individuals in a population that have not yet reached a stable phase of development. The influence of temperature, illumination, and humidity on life span, fertility, mortality, and other properties of organisms is well known. Moreover, climatic factors have a direct and stronger effect on poikilothermic animals than on homoiothermic ones. The latter, having perfect physiological mechanisms, become relatively independent of the external environment. The decrease in population size during sharp drops in temperature is more noticeable in insects than in birds and especially in mammals.

The action of climatic factors is not always manifested immediately, immediately. For example, in the taiga, favorable weather conditions lead to a high seed yield in a year, and an increase in the animal population with abundant food is observed only after two years. At the same time, weather conditions act regardless of population density.

Regardless of density, other factors also come into play. Thus, the number of hollows in trees in a particular forest determines the number of hollow nesters. It goes without saying that the number of hollows does not depend in any way on the density of hollow-nesting populations. On the other hand, living space can limit population size. For example, the number of white partridge, a number of mammals (muskrat, etc.) is sharply reduced if they do not find suitable habitats even with a favorable combination of other factors.

Density dependent factors generally affect the rate of population growth. However, it can change in three directions.

In species with strong population fluctuations (mouse-like rodents, insects), population growth rates usually stabilize at high population density, i.e. almost do not change until the population reaches the maximum size. At maximum density, the growth rate drops sharply.

The third direction, due to the influence of density-dependent factors, is that population growth rates can be maximum even at average density values. But even in this case, the population density, having reached a maximum, begins to decrease. This is especially true for some birds and insects.

7 Intrapopulation regulation of population size

Population density usually has a certain optimum. At any deviation of the number from this optimum, the mechanisms of its intrapopulation regulation begin to work. One of the main mechanisms contributing to the establishment of stable stability in a population is the action of density-dependent factors. Abiotic factors also affect the mortality of a population, but do not independently create its stable stability.

The regulation of populations in different species of animals and plants is carried out in different ways. Nevertheless, in each of them, in a certain way, the optimum density is established.

An increase in the population density of many insects is accompanied by a decrease in the size of individuals, a decrease in their fecundity, an increase in the mortality of larvae and pupae, a change in the rate of development and sex ratio, and an increase in the number of diapausing individuals, which sharply reduces the active part of the population.

Often, with an excessive increase in population density, cannibalism is stimulated. A striking example is the phenomenon of eating their own eggs by flour beetles. Cannibalism is observed in some species of fish, amphibians and other animals.

One of the important mechanisms of intrapopulation regulation of abundance is emigration, the intensity of which is stimulated by an increase in population density. This is quite typical for many insects, in which, at a certain population density, some of the individuals, sometimes significant, are evicted to their less preferred habitats of the same range. In some species of aphids, an increase in population density is accompanied by the appearance of winged individuals capable of settling. When the population is overcrowded, emigration occurs in a number of mammals (especially in mouse-like rodents) and birds.

The regulatory role of intraspecific competition for limited resources has been sufficiently studied. In carrion flies, so many larvae come out of the huge number of eggs laid on the corpse that there is not enough food for everyone. As a result, their mortality rate at early ages increases catastrophically. A similar phenomenon has been found in bark beetles), lasius ants, some dragonflies and other insects.

In the simplest cases, intrapopulation regulatory mechanisms of abundance manifest themselves in the form of direct competition for the resources necessary for life, the amount of which is insufficient to meet the needs of all individuals. It is known that the population density of the codling moth and cabbage moth is regulated by competition for food and pupation sites. Intraspecific competition in some flies in the case of an increase in population density to a certain level leads to a decrease in the mass of pupae, which is accompanied by increased mortality.

The problem is important "minimum viable population" , the essence of which is to determine the minimum population size that would guarantee its existence for some sufficiently long period. At the same time, a drop in population density below the optimal level, for example, with increased extermination of rats, causes an increase in fertility and stimulates their earlier puberty.

Some mechanisms of population size regulation can simultaneously act as mechanisms that prevent intraspecific competition. So, if a bird marks its nesting area by singing, then another pair of the same species nests outside it. Marks left by many mammals limit their hunting area and prevent the introduction of other individuals. All this reduces intraspecific competition and prevents excessive population compaction.

In plants, the regulatory mechanisms of population size are, first of all, intraspecific competition. It is usually associated with increased density of growth. In over-compacted crops, for example, there is a decrease in the amount of seed production, which is of great importance for agriculture and forestry. Most often, plants of the same species compete for light and moisture. In dense crops, they shade each other, with a limited amount of water, they lack it. As a result, some of them die. This phenomenon is most typical for many garden crops and forest plants. There are always significantly more young plants in the forest than old ones. Intraspecific competition for moisture explains the often occurring regular distribution of desert plants. It seems as if someone seated them at a strictly defined distance from each other. In lowlands, in oases, this uniform sparsity of plant populations immediately disappears. Light-loving and relatively moisture-loving baobabs are distributed in a similar way in the African savannas.

However, it should be borne in mind that the population is usually part of the community and that the sustainable existence of biocenoses is possible only with certain quantitative ratios of all components. This is the reason for the need to regulate the abundance, which ensures a stable state of both individual populations and biocenoses as a whole.

8 Population as a self-regulating system

Populations of animals, plants and microorganisms have the ability to naturally regulate density, i.e. the density, with more or less significant fluctuations, remains in a steady state between its upper and lower limits. This is ensured by the action of certain adaptive mechanisms. It is based on the fact that the supply of energy necessary for the survival of a particular population does not exceed a certain level and thus preserves the size of this population.

The tendency of living systems, including populations, to maintain internal stability through their own regulatory mechanisms is called I am homeostasis and fluctuations in the number of populations within some average value - their dynamic balance.

Biological regulation (dynamic equilibrium, homeostasis) of a population, or its automatic self-regulation, cannot be caused by abiotic factors independent of population density if they act in isolation from biotic factors. Only factors dependent on the density of the population are able to regulate the population and ensure its balance.

All biological systems are characterized by a greater or lesser ability to self-regulate, i.e. to homeostasis. With the help of self-regulation, the existence of each system as a whole is supported - its composition and structure, characteristic internal connections and transformations in space and time. Such homeostatic systems are, first of all, each individual, and then the population. Since self-regulating systems are not closed, they actively interact with the external environment and therefore are subject to change. Changes are not only cyclic with a return to the original state, but also historically irreversible. However, both are regulated in the direction of preserving the system, in this case, the population.

Self-regulation of the population is carried out by two mutually balanced buffer forces acting in nature. This, on the one hand, is the ability of organisms to reproduce, on the other hand, reactions that depend on the density of the population and limit reproduction.

Self-regulation is a necessary adaptation of organisms to maintain life in constantly changing conditions.

In the evolutionary development of organisms, changes do not concern an individual, but their totality - a population. These changes are also regulatory in nature. That is why the population as an elementary evolving unit has not only a specific structure, but also the ability to self-regulate. At the same time, its number is regulated by the rate of reproduction, phenotypic diversity - by natural selection, and genetic - by mutation, crossing, natural selection.

Populations are open systems. There are many channels through which information enters the population. These input channels connecting the population with the external environment are specialized and controlled by the population itself. Therefore, all regulatory processes are always carried out due to the forces acting within the population. Therefore, biological regulation is self-regulation. However, despite the fact that the population has an internal mechanism of self-regulation, the action of which is aimed at maintaining the constancy of the structure, the latter does not remain unchanged in the new environment, i.e., with a change in the conditions of existence, the population also changes.

Since, when considering issues related to fertility, mortality, migrations of individuals, with the influence of density-dependent and independent factors on the number of intraspecific groups, with intraspecific competition, the group effect, phase variability, and other phenomena, the processes of self-regulation of the number of populations have already been illustrated, we restrict ourselves to the following examples. It is well known that changes in environmental conditions can lead to a sharp increase in mortality. As a result, a signal appears in the population informing about a catastrophic reduction in numbers. This affects the physiology of all members of the population, which is manifested in the mobilization of its resources to minimize energy costs, to maintain normal life, to increase the resistance of individuals to adverse factors. As a result, the rate of aging of individuals decreases, the relative number of females increases, and their fecundity increases. This phenomenon has been studied in populations of many animals, especially insects, amphibians, and mouse-like rodents.

Diametrically opposite character is self-regulation with a sharp increase in population density. The overcrowded population receives a corresponding signal, and its individual individuals, becoming cannibals, intensively exterminate their fellows. In addition, the fertility of females sharply decreases, and the mortality of the weakest individuals increases. As a result, after a relatively short period of time, the population size returns to normal.

An important mechanism for the regulation of abundance, which manifests itself in an overcrowded population, is stress response (from English stress - stress). If the population is affected by some strong stimulus, it responds to it with a non-specific reaction, which is called stress. In wildlife, there are many forms of stress: anthropic (occurs in animals under the influence of human activity); neuropsychic (manifested when individuals in a group are incompatible or as a result of population overcrowding); thermal; noise and others. For example, as a result of population overcrowding, individual individuals experience such physiological changes that lead to a sharp reduction in the birth rate and an increase in mortality. In mammals, this phenomenon is called stress syndrome . In this case, the animals become so aggressive (violent fights, intolerance of the presence of a neighbor, etc.) that they almost completely stop breeding. In a stressful state, the adrenal cortex increases and the concentration of corticosteroid hormones increases. In females, ovulation is disturbed, resorption of embryos occurs, instincts for caring for offspring do not appear, etc.

The nature of the signals perceived by the population as an “order” to act is very diverse, and the signaling system works flawlessly. Therefore, even extremely high density or mortality does not cause sharp disturbances in the structure of the population. This guarantees the restoration of the population size within the optimum within a relatively short period of time. So ended, for example, numerous outbreaks of mass reproduction of insect pests.

Consequently, any population of plants, animals and microorganisms is a perfect living system capable of self-regulation. At the same time, we must not forget that the population is the smallest evolving unit. It does not exist in isolation, but in connection with populations of other species. Therefore, non-population mechanisms of automatic regulation, more precisely, inter-population mechanisms, are also widespread in nature. At the same time, the population is a regulated object, and the biogeocenosis, which consists of many populations of different species, acts as a regulator. The biogeocenosis as a whole and the populations of other species included in it most significantly affect this particular population, and each population, for its part, affects the biogeocenosis of which it is a part.