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Regularities of population dynamics. Types of population dynamics. Factors affecting population dynamics. Among the most important properties of populations is the dynamics of the number of individuals characteristic of them and the mechanisms of its regulation.

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PAGE \* MERGEFORMAT 3

MINISTRY OF AGRICULTURE OF THE RUSSIAN FEDERATION

FSBEI HPE "Kuban State Agrarian University"

Department of Phytopathology, Entomology and Plant Protection

ESSAY

in the discipline "Ecology of insects"

Theme: " Population Dynamics and Biotic Potential of Insects».

Performed:

1st year student of the Faculty of Defense

Plants Kaloeva D.B.

Checked:

Professor, d.b.s. A. S. Zamotailov

KRASNODAR

2015

INTRODUCTION ………………………………………………………………………3

1. POPULATIONS AND THEIR PROPERTIES………………………………………..4
2. POPULATION DYNAMICS…………………………6
   1. Patterns of population dynamics………………….7
   2. Types of population dynamics………………………………8
   3. Factors affecting population dynamics………...11
3. BIOTIC POTENTIAL OF INSECTS………………………14

CONCLUSION………………………………………………………………18

LIST OF USED SOURCES………………………...19

INTRODUCTION

Ecology is always based on the life of an individual, its relationship with the environment. Populations are made up of individuals. In the simplest cases, the response of a population to an external influence is determined by the statistical distribution of the properties of its constituent individuals, and more complex relationships often take place. Finally, the totality of populations of animals and plants of different species living in the same territory and / or biologically related to each other is an even more complex system called a biocenosis or ecosystem.

Accordingly, it is possible to consider the ecology of insects sequentially in three levels of complexity: individual population biocenosis (ecosystem). Naturally, the principles of insect ecology are the same as those of general ecology.

Among the most important properties of populations is the dynamics of the number of individuals characteristic of them and the mechanisms of 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 adaptive mechanisms that contribute both to a decrease in abundance, if it significantly exceeds the optimal value, and to its restoration, if it decreases below optimal values.

Populations, as a rule, have adaptive mechanisms that contribute both to a decrease in abundance if it significantly exceeds the optimal value, and to its restoration if it decreases below normal values. For any population and species as a whole, the so-called biotic potential is characteristic, which is understood as the possible offspring from one pair of individuals in the exercise of the ability of organisms to biologically determined reproduction.

Purpose of the work: to study the population dynamics and biotic potential of insects.

1. POPULATIONS AND THEIR PROPERTIES

Due to the heterogeneity of conditions, the species is never evenly distributed over its range. In favorable places, groups of individuals appear that are more closely related to each other. Such more or less isolated groupings are called populations.

So, a population is a collection of individuals of the same species inhabiting a certain territory. Under constant and sufficiently favorable conditions, the population is able to persist indefinitely due to self-reproduction. The population has genetic variability and can adapt to new conditions. In the most common case of bisexual reproduction within a population, there is a constant exchange of genetic information, i.e. common gene pool. This exchange may be hindered to some extent by mating selectivity or for other reasons.

Thus, the population has more or less defined spatial boundaries and usually a common gene pool. Individuals included in the population are located in a certain way on the ground. The most important characteristics of a population are its abundance and, accordingly, density, i.e. the number of individuals per unit area (or volume of the substrate). The population at any given moment has a certain age composition and sex ratio.

Fertility, mortality, emigration and immigration are among the so-called dynamic characteristics. Their unstable balance leads to more or less abrupt changes in the number and, accordingly, in the density of the population. These changes over time are called population dynamics. As a rule, changes in abundance are accompanied by changes in the spatial distribution of individuals.

Thus, the population has properties that repeat the properties of the individual at a new level. Like a separate organism, a population arises, grows, differentiates, and has a certain resistance to external influences. A population, unlike an organism, can be a creature for an indefinitely long time, although it may die under unfavorable conditions.

The properties of a population are determined by the properties of its constituent individuals and their gene pool. Knowing the percentage of individuals of certain ages, the sex of the physiological state, we can build a multidimensional characteristic of the population - a population portrait. However, the properties of a population depend not only on the properties of individual individuals, but also on the spatial and temporal distribution of these individuals and their interactions with each other. Therefore, when considering ecological relationships, a population usually acts as a single whole.

1. POPULATION DYNAMICS

Population dynamics is the change in population size over time. These changes may be associated with processes occurring spontaneously within the population itself, caused by the impact of abiotic environmental factors, or by interactions between populations of different species within the biocenosis.

When studying the dynamics of the number of insects, it is necessary to take into account the number (density of populations) simultaneously of all insects of a given species at all stages of their development, or only at one stage. When taking into account the abundance, especially one stage, its seasonal changes will be very clearly expressed. Thus, an insect usually experiences an unfavorable season at any one, most often dormant stage of development (egg, pupa). At this time, the number of individuals at other stages of development, as a rule, is equal to zero.

During the year, peaks in abundance appear according to the number of generations, but if there are many of these generations, the development of insects of different generations, as a rule, overlaps. In a number of cases, the long life of an insect at any of the stages also smooths out the population peaks. Such, for example, are many ground beetles whose adults live for several years.

It is these changes that are commonly referred to as population dynamics. It should be borne in mind that, although there is a certain correlation between the number at successive stages, in the development cycle it is relative and is limited only by the fact that the number at each stage of development, starting from the egg, should not be greater than the previous one. Strictly speaking, this rule is not always followed, since the population size can increase due to migrants. Accordingly, the number of adults in this area may be much higher than the number of pupae that took place.

2.1 Patterns of population dynamics

Long-term observations of populations of different insect species show that the number of insects in nature varies from year to year, but these changes occur within certain limits. The upper limit, of course, is determined by the available resources for the existence of a given population, the capacity of its environment. The lower limit is the zero line, upon reaching which the population completely dies out. It is quite possible that the latter is a common case, but this does not mean that these insects will be completely absent in this biotope next year. Immigrants from neighboring surviving populations will re-establish the population.

In principle, the ability of insects, like other organisms, to increase populations through reproduction is unlimited. In nature, however, the upper limit of abundance is almost never reached for the following reasons.

First, under favorable conditions, spontaneous changes in the genetic structure of the population occur, leading to the fact that the ability of the population to grow gradually decreases (internal resistance). The fact is that it is under favorable conditions that genetically inferior individuals survive and give offspring. As a result, both the viability of the population as a whole and its ability to reproduce are reduced. Interestingly, under certain conditions, rhythmic changes in the average characteristics of the population spontaneously occur with a period of 1 2 or more generations. Apparently, genetic changes in the properties of a population play an important role in the population dynamics ("waves of life"). Unfortunately, this issue remains little studied. It should be added to this that the genetic structure of populations of other organisms interacting with a given species can change in a similar way over time: microorganisms, plants, other insects, etc.

Secondly, the external environment, which includes a lot of abiotic and biotic factors, prevents the unlimited growth of the population (medium resistance). Each of the factors has both specific and indirect effects.

In nature, insect populations can be observed that persist for tens and hundreds of years. Therefore, the literature often expresses the idea of ​​population fluctuations in a biocenosis as a self-regulating process. Figuratively speaking, the population is considered as an elastic stretched thread, which can be deflected by external factors up or down to certain limits, but when the impact is weakened, it returns to the previous level again.

1. Types of Population Dynamics

stable type is characterized by a small range of fluctuations (by several times, but not by several orders of magnitude). 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.

fluctuating typefluctuations occur in a significant range of densities that differ by onetwo orders of magnitude. At the same time, three phases of the oscillatory cycle are distinguished: increase, maximum, rarefaction of the population. The return to a stable state is fast. Regulatory mechanisms do not lose control over the number of populations, increasing their effectiveness following an increase in density. Slightly inertial inter and intraspecific interactions prevail. Such a course of numbers is widespread in different groups of animals.

A fluctuating type of population dynamics is characteristic of many xylophages (consumers of bark and wood): barbels, gold beetles, and bark beetles. They are characterized by joint colonization of food objects weakening of trees. This allows you to quickly reduce the stability of the tree, but at the same time, the cohabitation of xylophages aggravates the competitive relations between them, which acts as an inertialess mechanism for regulating the abundance.

For bark beetles, which are the first to settle on weakened trees, with an excess supply of nutrients (soluble carbohydrates and starch), development is limited by the influence of protective reactions of the tree, such as, for example, resin production in conifers. In addition, there are no symbiotic microorganisms yet. The resistance of still viable trees can be broken by a concentrated attack and an excessively high pest density. Another limit to the suitability of a tree for bark beetles is the complete death and destruction of the bast. Between the two indicated states of the tree for bark beetles, optimal feeding conditions are created, the maximum survival rate of all development phases and the highest reproduction rates are noted.

Explosive type with outbreaks of mass reproduction the termination of the action of modifying factors does not cause a rapid return of the population to a stable state. The population dynamics consists of cycles in which five obligatory phases are distinguished: population increase, maximum, rarefaction, depression, recovery. Populations are periodically characterized by extremely high and unusually low levels of abundance. According to the phases of the cycle, the indicators of reproduction, the age and sex structure of the population, the physiological state, behavior, and sometimes the morphological features of its constituent individuals also change greatly. Such a course of numbers is found most often in species with a short lifespan, high fecundity, and a rapid turnover of generations. It is characteristic, for example, of some insects (locusts, forest pests - barbels, bark beetles, a number of lepidoptera and sawflies, etc.).

In the Siberian taiga, among hidden-living species, an explosive type of population dynamics is characteristic of the Altai larch barbel, large black coniferous barbel, larch bud gall midge, and some others. Among open-living leaf-eating insects, the ability to give outbreaks of mass reproduction is characteristic only of certain species of Lepidoptera and Hymenoptera (sawflies, weavers). Distinctive features of the ecology of these species are: high survival in a highly variable environment due to special adaptations, high migratory activity, high and variable fecundity. In open-living species, the group effect and phase variability are often detected.

One of the most dangerous pests of conifers is the Siberian silkworm Dendrolimus sibiricus, distributed from the Urals to the Pacific Ocean. In the outbreaks of the Siberian silkworm, the sex ratio changes significantly in the phases of the outbreak. The proportion of females varies from 32 to 76%. When the outbreak increases, females dominate, while attenuation, males dominate. In overcrowded populations, the mortality of females increases at all phases of development, and their higher migration activity from breeding centers is also noted. In the phase of abundance maximum on the periphery of the outbreak, the proportion of females is up to 73%, and in the center 44%.

2.3 Factors affecting population dynamics

Insects acquire the importance of pests of agricultural crops only if their number exceeds the economic thresholds of harmfulness, since an individual, even the most voracious insect, is not able to cause any significant damage to the crop. Therefore, the planning of protective measures and the corresponding scientific research are aimed at reducing the number of individuals in populations to these thresholds.
The dynamics of the number of insect populations is manifested either in a seasonal change in their number throughout the year, or over a number of years, while acquiring, due to the exceptional energy of reproduction of many species, the nature of regularly alternating population waves.

There are two opposite points of view on the role of factors of different categories in the regulation of population size. Assuming that the level of abundance is determined by factors independent of population density, supporters of one point of view refer to the rarity of the combination of conditions necessary for the constant growth of populations. Examples of mass reproduction of insects, in their opinion, are rare exceptions to the rule and express the specific properties of a few species. The number of populations of the overwhelming majority of species is limited by the shortness of time when combinations of conditions ensure population growth. At the same time, limited resources, their relative inaccessibility with a weak development of migration and search abilities, as well as the transience of the period when the birth rate prevails over mortality, and the population growth rate is positive, can be considered the main factors limiting the population. However, random fluctuations in numbers in response to changes in conditions that are not related to population density will sooner or later lead populations to the lower limits of numbers and extinction.
Taking a different point of view, which favors factors that depend on population density, researchers adherents of the opposite direction formulated the concept of automatic population control. Until recently, the search for criteria for assessing the regulatory role of these factors was limited only by population density, which decreases if a certain average level is exceeded, or, conversely, increases if this level remains unattained.

1. BIOTIC POTENTIAL OF INSECTS

The fecundity of insects and their ability to reproduce is often unusually great. Often this ability to reproduce is denoted by the concept of reproductive potential, orbiotic potential. It is most rational to designate by it not the fecundity of a species in general, but the theoretical maximum of offspring obtained from one pair of individuals (in parthenogenesis, from one individual) for the whole year. For example, the codling moth lays an average of 100 eggs, so its biotic potential in two generations will be 50 per pair of individuals (with an equal number of males and females in the population). 2 , i.e. 2500. In aphids that give up to 15 or more parthenogenetic generations over the summer with the same fecundity, i.e. 50 individuals per female, the biotic potential reaches astronomical indicators in this example 50 15 , i.e., billions of billions of individuals.

Academician V. I. Vernadsky considered the reproduction of organisms as a manifestation of one of the properties of living matter - the ability to spread over the earth's surface as a result of chemical work and the creation of new quantities of living matter. He designated this ability by the concept of the rate of transmission of life, which is a constant value and characteristic of each type of organism; it is determined by the size and weight of the body, sexual productivity, the number of generations in a given period of time and the requirements for the habitat. In general, the life transfer rate characterizes the geochemical energy of species and is expressed as a number of cm/s.

For example, the rate of transmission of life in grasshoppers is approximately 1315 cm/s, and in the meadow borer 45 cm/s; this means that the distribution of these insects would be completed on Earth, given the length of the equator of about 40 thousand km, in the first case within about 9 years, and in the second about 3 years.

Biotic potential and the rate of life transmission are theoretical abstractions and in real nature the reproduction of organisms never corresponds to these values. However, both of these concepts are valuable in that they allow us to establish numerical indicators for species of their potential energy of reproduction.

The impossibility of full realization of the biotic potential of species in nature is a consequence of the limiting influence of the external environment: under its influence, either a decrease in fertility or the death of part of the offspring occurs. On the whole, the enormous reproductive ability of insects insures them against complete death and extinction in nature when unfavorable environmental conditions arise.

Assume that a female of a given species lays an average of 200 eggs (the fecundity F is 200) and that mortality throughout development is zero. If the sex ratio in the offspring, as is most often 1:1 (the proportion of females q = 0.5), this means that in the first generation there will be Fq those. 200 0.5 = 100 females. Each of these females will give birth to another hundred females in the next generation, resulting in 10,000 females in the second generation. It is obvious that in n In the first generation, the number of females can be calculated using the following formula:

If initially we have not one female, but N females, then through n their generations will be:

(1)

Obviously, under such conditions, the population size will increase steeply exponentially (power function). Generational change still takes some time. Then the rate of population change with a large number of generations or their rapid change can be represented as the result of dividing the increase in population by a time interval (absolute population growth rate), or based on the initial number of individuals

With a successive decrease in the time interval (0), we obtain the instantaneous population growth rate r( biotic potential):

(2)

Returning to the population growth formula (1), we can now write it as follows:

(3)

where population size over time t , N initial population size, e base of natural logarithms, r biotic potential, t time interval. The graph of this exponential (exponential) function is shown in Fig.27. If we take the logarithm of formula 3, we get the following expression:

(4)

The graph of this function is a straight line. The biotic potential on this graph can be represented as the tangent of the slope of the graph to the x-axis. Obviously, biotic potential is not a purely speculative category. Knowing the size of the population N at the time t , and the subsequent number N at the moment t , you can determine the biotic potential by the formula:

(5)

In the beginning, we assumed that the mortality of insects during development is zero. In such a situation, the biotic potential will be the maximum possible under the given conditions. In nature, this condition is almost never fulfilled, and the determined biotic potential will be determined by the difference between fertility and mortality. Due to the desire to reproduce, insects could increase their numbers indefinitely, if it were not for factors that inhibit population growth, reduce fertility or lead some insects to death. Suchmedium resistancecan be defined as the difference between the maximum possible and actually observed biotic potential.

CONCLUSION

The number of populations does not remain constant, as the conditions for their existence change. The range of fluctuations in the number of populations depends on the degree of variability of abiotic and biotic factors, as well as on the biological characteristics of a particular species (fertility, the rate of generation change, the age at which individuals reach sexual maturity, etc.). The largest ranges of population fluctuations are characteristic of small, rapidly multiplying organisms, including insects.

Insects, being small creatures, have an exceptionally high biotic potential. The high value of the biotic potential means the possibility of sudden outbreaks of numbers that are dangerous for human economic activity. In addition, the ability to quickly increase their numbers is the basis for the use of insects as a source of animal protein.

LIST OF USED SOURCES

1. Bay Bienko G. Ya. General entomology. Textbook for universities and agricultural universities. 3rd ed. M.: Higher school, 1980. 416 p.
2. Zakhvatkin Yu. A. Course of general entomology. M.: Kolos, 2001, 376 p.
3. Chernyshev V.B. Ecology of insects. Textbook. M.: Publishing House of Moscow State University, 1996. 304 p.
4. Yakhontov VV Ecology of insects. Moscow: Higher school, 1964. 460 p.
5. http://www.entomologa.ru/book/35.htm
6. http://www.plam.ru/ekolog/obshaja_yekologija/p9.php#metkadoc12
7. http://biofile.ru/bio/6684.html
8. http://www.vitadez.ru/katalog/populyatsiyanasekomich/dinamikachislennostipopulyatsii
9. http://slovo.ws/urok/biology/11/01/txt/04.html

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8.5.1. Biotic Potential

Any population is theoretically capable of unlimited growth in numbers, if it is not limited by environmental factors. In such a hypothetical case, the population growth rate will depend only on the magnitude biotic potential, characteristic of the species. The concept of biotic potential was introduced into ecology in 1928 by R. Chapman. This indicator reflects the theoretical maximum of offspring from one pair (or one individual) per unit of time, for example, for a year or for the entire life cycle.

In calculations, it is most often expressed by the coefficient r and is calculated as the maximum possible increase in the population ΔN over a period of time Δ t, assigned to one individual, at the initial population size N0:

The magnitude of the biotic potential is extremely different in different species. For example, a female roe deer can produce 10–15 kids in a lifetime, a trichina (Trichinella spiralis) can lay 1,800 larvae, a female honey bee can produce 50,000 eggs, and a moonfish can produce up to 3 billion eggs. If all embryos were preserved and all offspring survived, the size of any population at certain intervals would increase exponentially.

The curve reflecting such a population growth on the graph quickly increases the steepness and goes to infinity (Fig. 122). Such a curve is called exponential. On a logarithmic scale, such a dependence of the population size on time will be represented by a straight line, and the biotic potential r will be reflected by its slope with respect to the horizontal axis, which is steeper, the larger the value r.

Rice. 122. Real (1) and theoretical (2) paramecia population growth curve

In nature, the biotic potential of a population is never fully realized. Its value is usually added up as the difference between births and deaths in populations: r = b - d, where b is the number of births, and d- the number of dead individuals in the population for the same period of time.

General changes in population size are formed due to four phenomena: fertility, mortality, introduction and eviction of individuals (immigration and emigration).

8.5.2. fertility

fertility is the number of new individuals that appear in a population per unit of time per a certain number of its members.

Distinguish absolute and specific birth rate. The first is characterized by the total number of born individuals. For example, if in a population of reindeer, numbering 16 thousand heads, 2 thousand deer appeared in a year, then this number expresses the absolute birth rate. The specific is calculated as the average change in the number per individual over a certain time interval, and in this case it will be 0.125, i.e. one newborn per 8 members of the population per year.

The birth rate depends on many factors. Of great importance is the proportion of individuals capable of reproduction in a given period, which is determined by the ratio of sexes and age groups. The frequency of the sequence of generations is also important. So, among insects they distinguish monovoltine and multivoltine kinds. The first give one, the second - several generations per year. For example, aphids have up to 15 parthenogenetic generations per season. According to the number of periods of reproduction during life, monocyclic and polycyclic species are distinguished. monocyclicity, or single reproduction, usually characteristic of species with a short lifespan in a mature state (salmon fish, mayflies, May beetles and many other insects). Polycyclicity characterized by repeated reproduction of individuals and is inherent in most vertebrates and a number of invertebrates, such as crustaceans.

Plants secrete monocarpic and polycarpic species, i.e., with single and multiple reproduction during life. For the magnitude of the birth rate, the ratio of the reproduction period and the total life expectancy is also important. This period for female Drosophila is about 65%, for the gregarious locust Schistocerca - 15%, and for mayflies - only from 0.5 to 1% of the duration of their existence. An important role is played by the fertility of individuals. However, the reproduction of a population, as a rule, is not directly proportional to fertility. Fertility is highly dependent on the degree of development of care for offspring or the provision of eggs with nutrient materials. Among fish, the largest number of eggs are spawned by species with pelagic eggs - herring, cod, flounder, etc. For example, Sakhalin herring produces 38-46 thousand small, fractions of a millimeter, eggs. Salmonids that bury their eggs in the ground develop fewer eggs, but larger ones. The average fecundity of Amur pink salmon is 1300–1500 eggs with a diameter of 4–6 mm. The largest eggs are in sharks and chimeras, up to 6–8 cm. The eggs of these fish also have a dense protective shell. Their number is very small - a few pieces per female.

In species that protect and feed their young, fertility is sharply reduced. The clutch size in birds of different species varies not thousands of times, as in fish, but ranges from one egg (in some predators, penguins, auks, etc.) to 20–25 (the maximum number of eggs in chickens, for example, in gray partridges).

D. Lek established on the example of birds that their selection favors not maximum fertility, but the most efficient, i.e., the number of eggs at which the offspring is the most viable. Birds spend a huge amount of energy on feeding their chicks. Small birds bring food to the nest hundreds of times a day: the redstart - more than 200, the great tit - about 400, and the wren - up to 600 times. If the clutch size is greater than the usual limit, the chicks are underfed and their viability is reduced.

Greater fecundity is also developed in species under conditions of higher mortality, especially under strong pressure from predators. Selection for fertility compensates for the high mortality rate in populations. Therefore, with high fecundity, population growth can be very low. In different populations of the same species, fertility is usually the higher, the more unfavorable the habitats they occupy. So, in many mammals - hares, mice, voles - the number of cubs in the litter at the borders of the range is greater than in its center.

8.5.3. Mortality

Table 1

Partridge Survival Table (according to Schwerdpfeger, 1968)

Rice. 123. Different types of survival curves

The ideal case is if all individuals of the same generation live to the biological limit of age, and then die off within a short period of time. The curve describing such a dependence of the number of generations on time first runs parallel to the horizontal axis, and then bends steeply downwards (Fig. 123). In nature, such a course of mortality is not characteristic of any species, but there is an approximation to it, for example, in such insects with well-protected larvae as gold beetles, in large mammals with a high degree of survival of young animals. The curve describing mortality in modern human populations is close to the same type. The average life expectancy of an individual in such populations is high and approaches the maximum.

The most common variant in nature is the increased death of individuals in the early period of life. Adult forms are more protected or hardy. The mortality curve in such populations drops sharply towards the horizontal axis at the very beginning. Its slope reflects the rate of generation loss. Thus, in most fishes, a significant part of the population dies even at the stage of spawning, the screening of fry is also very large, and only among adults does the mortality become lower. In mice and voles fed by their mothers, death becomes more frequent after the young leave the nest, when they start an independent life. The average lifespan of an individual is much less than the maximum. The same picture is characteristic of many passerine birds (Fig. 124). Humans also had high infant mortality for almost all of history, which dropped dramatically with the advancement of medicine. This led to a change in the type of survival curve in generations and a rapid increase in the population of the Earth, called the "population explosion".

Rice. 124. The population pyramid of the finch Fringilla coelebs on the Curonian Spit: 1-11 – age of birds, years (from V. A. Paevsky, 1985)

The third variant of changes in the survival rate of generations is relatively rarely observed - a relatively uniform dropout due to random causes throughout the life cycle, without pronounced critical periods of increased mortality. On the graph, this is expressed by a straight line between the initial and zero generation numbers. Such a course of mortality has been noted, for example, in freshwater hydras kept in an aquarium.

8.5.4. Population survival strategies

Differences in the biotic potential of species depend on their size, systematic affiliation, and other reasons, but other things being equal, they are related to mortality in populations. This regularity, noticed by C. Darwin, was substantiated in the works of Academician I. I. Schmalhausen in the 40s of the last century. If a species is exposed in nature to mass indiscriminate elimination, i.e., death from numerous enemies, from which he is powerless to avoid, or is suppressed by other extreme circumstances, then the only direction of selection becomes an increase in reproduction. In this case, the probability of accidental preservation of offspring increases and the species avoids extinction. With indiscriminate elimination, differences between individuals do not matter for their survival, since the power of the impact of destructive factors is too high. At selective elimination, when mortality is largely determined by differences between individuals, selection improves various forms of morphophysiological adaptations that increase the resistance of the species to the influence of unfavorable conditions. Thus, a high biotic potential is an evolutionary response of a species to the pressure of unfavorable environmental influences that cause high mortality.

In the late 60s, this idea was revived in the concept TO- and G-selection put forward by American ecologists R. MacArthur and E. Wilson. They proposed to distinguish between two main reproduction strategies of organisms that ensure survival in different conditions, denoting them through the coefficients included in the population growth equation. At r-strategies selection is based on high fecundity, turnover of generations, and the ability to quickly spread, which allows species to quickly restore their numbers after a sharp decline. At K-strategy selection improves various forms of care for offspring, which reduces fertility. At the same time, the duration of life cycles increases and the mechanisms for sustainable maintenance of numbers in biocenoses are improved. Naturally, between the extreme forms there are all intermediate options. Elements TO- and G-survival strategies can be traced in all systematic groups of organisms. Even within a species, in populations living in different conditions, one or another direction of selection is intensified.

8.5.5. resettlement

The eviction of individuals from a population or its replenishment by newcomers is a natural phenomenon based on one of the most important biological features of a species, its dispersal ability. In each population, some individuals regularly leave it, replenishing neighboring ones or populating new territories not yet occupied by the species. This process is often called population dispersion. Settlement leads to the occupation of new biotopes, the expansion of the common range of the species, and its success in the struggle for existence.

Settling functions are performed in a certain period of the life cycle: in insects, mainly at the adult stage, in thyroglyphoid mites, by special deutonymphs-hypopuses, in most birds and mammals, by growing young. In plants, seeds and spores are dispersed or dispersed; sessile animals are dispersed by swimming larvae or special generations during metagenesis.

Each type is characterized by its own rate of dispersion. It is estimated that about 1% of the young of hare regularly leave their birthplaces, while in populations of the great tit, on average, only a third of the young remain in the area where they hatched from eggs.

The dispersion is usually not directed; the dispersal of individuals occurs in various directions from the places of hatching.

Settling dispersion serves as a means of communication between populations. It increases as the population density increases.

In the period of population depression, on the contrary, the flow of invaders into the population increases. In sedentary animals with well-pronounced territorial instincts, aggressive behavior towards newcomers during a period of low population strength weakens, and the invaders occupy free areas.

A number of populations occupying unsuitable habitats are often unable to maintain their numbers through reproduction and may be maintained primarily through immigration. Such populations V.N. Beklemishev called dependent.

Settlement movements, according to N.P. Naumov, lead to the exchange of individuals between populations, increase the unity and overall stability of the species, since those adaptations that have arisen in local conditions, but are of general importance, can gradually spread within the entire species range. The penetration of dispersing individuals into territories that are not yet occupied by a species, their settlement and the formation of new populations are called invasion.

8.5.6. Population Growth Rate

Graphs of growth in the number of members of any natural population under new conditions for it are very different from the exponent. The curve, after a rise of varying degrees of steepness, turns parallel to the horizontal axis, which marks the establishment of a certain limiting population size, which is then maintained for a more or less long time (Fig. 122, 125). Such a course of the curve shows that in nature, some reasons restrain the excessive growth of the population, preventing it from realizing its biotic potential, and limit its number to certain limits.

Rice. 125. The number of beetles in a culture started with one pair of Rhizopertha dominica per 10 g of wheat (according to J. Varley, 1978)

Wheat was sieved every week and supplemented to 10 g

Changes in population growth rates can be different.

In one case, the growth rate is high from the outset and constant regardless of the increasing density, which corresponds to an avalanche-like, exponential increase in the population size. When a certain population density is reached, the growth rate drops immediately to zero. This means that the population stops reproducing abruptly. In nature, this type of environmental capture is found in species for which the speed of mastering resources is vital, for example, nematodes, ticks, some insects that live in rotting plant residues that quickly change quality, heaps of ungulate manure, etc. Small nematodes inhabiting horse and cow dung, can complete their life cycle in a few hours, while species close to them, but living in other, more constant conditions, develop from two to three weeks. The speed of mastering the environment is also important for preventing competition from other applicants. After mass reproduction and rapid consumption of available resources in populations of such species, reproduction stops and individuals settle by migrating to insects.

In another case, changes in population growth rates are inversely related to density. A sparse population under favorable conditions quickly increases its numbers, but the more individuals it becomes, the smaller the proportion of the next offspring, until the reproduction rate is equal to mortality (Fig. 126). Then the population growth rate drops to zero, and the total number stabilizes in accordance with the resources available to the population. This character of the formation of new populations is mainly characteristic of species in which success in reproduction at a low population level is not limited by the obligatory group way of life, the need for the sexes to meet, and other reasons (for example, in small parthenogenetic crustaceans, plants with apomixis, etc.). However, in most species, the highest population growth rate is observed only at a certain optimal density. If the population is very sparse, it makes it difficult for the sexes to meet, protect the young, show the group effect in animals, pollinate in plants, so the population grows very slowly at first.

Rice. 126. Dependence of fertility on density in the laboratory population of daphnia and in the wild population of the great tit (according to Yu. Odum, 1975)

As is known, the appearance of offspring primarily depends on the number of producers - individuals in the generative age state. At first, the growth in the number of producers is accompanied by a slow increase in the population, then, in a certain range of densities, the dependence is extremely pronounced, and even a small increase in the proportion of producers causes a rapid increase in the population until it reaches a certain level, which subsequently does not change, no matter how much the reproductive part of the population increased.

Such a relationship was first predicted by the French mathematician Verhulst in the middle of the 19th century for the human population, and later proved by the Englishman Pearl (1925) for animal populations in an environment where food resources have a certain replenishment limit.

The establishment of a certain level of population density after a certain period of growth does not mean at all that there are no more quantitative changes in populations. On the contrary, any population is always dynamic and constantly subject to population fluctuations, however, the range of all daily, seasonal and annual changes in populations is still much less than theoretically possible, corresponding to the realization of the entire biotic potential. Population fluctuations occur with different ranges around a certain average value, which corresponds to the horizontal part of the curve on the graph of population growth and stabilization.

High breeding potential plays a big role in the survival of the species. Populations reduced to a low level of abundance can quickly recover with a favorable change in conditions. Some species can resist being eaten away by various consumers or the threat of being squeezed out by competitors only by mass reproduction. The high breeding rate contributes to the rapid development of new spaces by the species.

However, unlimited reproduction is also fraught with great danger for any population, as it can lead to a rapid undermining of environmental resources, lack of food, shelters, space, etc., which will inevitably entail a general weakening of the population. Overcrowding is so unfavorable for any species that in the course of evolution, various forms have developed as a result of natural selection a wide variety of mechanisms that help prevent an excess of individuals and maintain a certain level of population density.

8.5.7. Dynamics of plant coenopopulations

All dimensional and quantitative characteristics of plants in cenopopulations vary over a wide range.

Changes include indicators such as total abundance and density, phytomass, area occupied by the population, projective cover, and age spectrum.

In most meadow plants, fluctuations in the number of seedlings, both during the season and over the years, range from 1–2 to 100–1000 per m2. In the steppe cenoses, in some years shoots may not appear at all. Their mass extinction occurs, for example, during the onset of drought, eating by phytophages, and oppression by adult plants. The age spectra of cenopopulations of different species have varying degrees of dynamism: 1) population waves (i.e., abundance waves) move gradually, while the type of the age spectrum does not change and it remains complete. This occurs with regular, but relatively small recruitment of young individuals; 2) population waves move quickly, the age spectrum can be broken and incomplete (Fig. 127). There are various transitions between these two types of dynamics.

Great lability of all population indicators is characteristic of reactive species, explerents, which are able to very quickly seize the vacant areas, but also free them when crowded out by competitive species.

Often there is a unidirectional irreversible change in the cenopopulation or its individual loci from inception to maturity and aging. Ultimately, the cenopopulation or locus in a given area disappears. This type of dynamics is called successive. For example, with intensive pasture load in the coenopopulations of annual bluegrass on fallows, the density of the coenopopulation decreases, young groups fall out of it, rapid general aging occurs, and the coenopopulation disappears. In some cases, individual loci within the cenopopulation are characterized by a successive type of dynamics, while the population itself generally remains stable.

Rice. 127. Annual changes in the population flow in the bent grass (according to E. I. Kurchenko, 1975)

In broad-leaved forests, the cenopopulations of the yellow goose onion plant exist in the form of separated loci. This species belongs to the group of explerents, i.e., it is able to capture the vacated territory very quickly due to the high growth rate and high energy of vegetative reproduction. Often the locus is occupied by one clone, the beginning of which is given by one juvenile individual. Having passed into the immature state, it begins to multiply, forming new juvenile individuals. A significant part of juvenile plants then passes into a dormant state, while normally developing plants successively pass through all age states up to generative. As a result, all or almost all individuals of the clone can go into a dormant state. This completes the development of the locus. This process takes 10–25 years. But in nature, this rarely happens, since even minor disturbances of the soil and forest floor by burrowing animals lead to the awakening of dormant bulbs. The development of the locus again begins with the juvenile age of the plants, and the process of clone development becomes cyclical. Since different loci develop asynchronously in time and space, the entire coenopopulation undergoes fluctuation changes. fluctuations - these are reversible, multidirectional changes, when periods of aging and rejuvenation of the cenopopulation alternate and generations continuously replace each other. Thus, the population retains its occupied area.

In some meadow plants, the temporary cessation of inspermation and the simultaneous maturation and aging of individuals can lead to the fact that the age spectrum will lose the young part, become broken, incomplete (Fig. 128). When the renewal is restored, young individuals form a new population wave during the further development of the population, which will eventually replace the damped wave of the old part of the cenopopulations. Such wavelike fluctuation fluctuations in abundance and age structure have been traced, for example, in the soddy meadow grass. High dynamism is also observed in meadow grass crops, especially under the influence of various anthropogenic factors, such as high doses of fertilizers, irrigation, and multiple alienation.

Rice. 128. Dynamics of coenopopulations of the meadow grass soddy in the Oka meadows (according to L. A. Zhukova, 1986)

8.5.8. Population homeostasis

Maintaining a certain density is called population homeostasis. The ability of populations to homeostasis is based on changes in the physiological characteristics, growth, and behavior of each individual in response to an increase or decrease in the number of members of the population to which it belongs.

Rice. 129. Self-thinning in tree plantations (according to G. F. Morozov, 1928):

on the left - dominant and oppressed trees in the spruce forest; on the right - the course of thinning of trunks with age in pine (1), birches (2) and ate (3)

Rigid forms of intraspecific competition include, for example, the phenomenon self-thinning in plants (Fig. 129). With a high density of seedlings, some plants inevitably die as a result of oppression by physiologically stronger neighbors. A decrease in the number of plants occurs even if the seeds sown are genetically homogeneous. In this case, apparently, the difference in the size of the seeds, in the time of germination, and the details of the microenvironment matter. In one of the experiments with clover Trifolium subterraneum, 84 days after the emergence of seedlings, 650 out of 1250 plants remained on a plot of 1 m 2, and the influence of pests was excluded.

In perennial ryegrass, the main ecological unit is not the individual, but the shoot. It was found that at different seeding rates, from 6 to 180 kg/ha, at first the density of shoots varies from 30 to 1070 per 100 cm 2, but then in all cases it becomes equal to about 500, i.e., new shoots, and in the denser part dies off.

Due to the peculiarities of their growth, the population density of plants is usually regulated not only by changing the number of individuals per unit area, but also by changing the vegetative power of each. In thickened crops, plants are less leafy, with fewer shoots. With an increase in the density of crops, their total mass first increases in proportion to the number of seeds sown, and then remains at a constant level, while the average mass of individual individuals decreases accordingly. In this case, it is not the number of individuals in the population that stabilizes, but the total leaf photosynthetic surface of plants.

In animals, strict forms of regulation of population density usually manifest themselves only in those cases when the supply of food, water, or other resources is sharply limited, and the animals are either not capable of searching for resources in another territory at a given period, or these searches are ineffective. For example, in small freshwater bodies where there are no other fish species, perch populations can maintain their existence and regulate density by feeding adults on their own juveniles. The fry grow at the expense of small plankton, to which large perches are not adapted. Cannibalism is not common in animal populations.

Of particular interest are some relatively rare species in which the ability to kill competitors within the population is fixed evolutionarily in their behavior and even morphology. Similar examples are found among insects.

1 – Opius fletcheri; 2 – Galesus sylvestrii ( a- first age b- second age

Among the mechanisms that retard the growth of populations, in many species, an important role is played by chemical interactions of individuals. So, the water of the aquarium, which contained daphnia, is able to retard the growth of representatives of the same species and retains this ability for several days. Tadpoles release protein particles into the water, which retard the growth of other tadpoles. The larger the individual, the stronger it affects the smaller ones, since resistance to the same inhibitor concentration is in direct proportion to size. One large Rana pipiens tadpole can stun all others in a 75 liter tank. The generation that hatched in a short period of time from the eggs laid in the same water body is soon divided into two size groups: larger tadpoles that continue to grow and small ones that have slowed down their growth due to an unfavorable concentration of the metabolite. The ecological benefit of such a division of the population is that individuals with a hereditarily faster growth rate, using the food resources of the reservoir to the full, are able to quickly complete metamorphosis and complete replenishment joins the population. The remaining small tadpoles, after the first batch leaves the reservoir and the concentration of the inhibitor in it decreases, also have a chance to increase in size and reach the stage of metamorphosis, but much later. This second part of the recruitment can be regarded as a kind of reserve that enters the population only under sufficiently favorable conditions (if the temporary reservoir does not dry up, if the reproduction of algae, the main food of tadpoles, etc., continues in it).

The release of growth retardant products into the environment has been found in many plants and aquatic animals, especially fish.

Another mechanism for limiting the number of populations is such changes in physiology and behavior with an increase in density, which ultimately lead to the manifestation of instincts mass migration. As a result, most of the population is evicted outside the territory occupied during the settled period. This is especially pronounced in insects, which are characterized by phaseness - a sharp change in the morphology and physiology of individuals depending on the density of the population (Fig. 131). In the migratory locust Schistocerca in its permanent habitats in India, Pakistan, East Africa and Arabia, with a low abundance, the larvae of the solitary phase are bright green, and the adults are grayish-green or brown in color. During the years of mass reproduction, which occurs with a favorable combination of weather conditions, the locust passes into the gregarious phase. The larvae acquire a bright yellow color with black spots, the adult immature Schistocerca is intensely pink-lilac, the sexually mature one is lemon-yellow. The morphology of individuals also changes: the elytra lengthens, the shape of the pronotum, keel, limb proportions, etc. change. The transition from one phase to another takes about three generations. The process is stimulated by visual perception of a special kind and contacts with the help of antennae. This causes a whole series of reactions in the body of insects, leading to hormonal changes, in which the endocrine glands are involved.

Rice. 131. Nymphs of the 5th age of the locust-Schistocerca (according to N.S. Shcherbinovsky, 1952): on the left - gregarious form; right - single form

The gregarious phase is characterized by increased excitability and extreme voracity. The fertility of females is reduced, but they lay eggs with a high content of nutrients. The gregarious locust is always in a state of migratory activity. The larvae move in clusters - swarms, and adults scatter in giant flocks for hundreds and thousands of kilometers from their permanent habitats (Fig. 132). So, at the end of the last century, the mass of one of the flocks of Schistocerci that flew across the Red Sea was determined to be no less than 44 million tons.

On the borders of their temporary range, migratory locusts cannot breed, and these foci soon die out. The flocks either die, or, gradually thinning out, begin to migrate to the zone of permanent foci. In sparse populations, a transition to a solitary phase occurs again, after 2–3 intermediate generations. Thus, the dispersal of locust swarms does not ensure the formation of new permanent populations, but practically serves only as a mechanism for removing overpopulation in places favorable for breeding. In this case, a huge number of insects die. The flocks, carrying innumerable disasters on their way, are themselves doomed.

Rice. 132. Migratory Locust Invasion

The phase phenomenon was found not only in gregarious grasshoppers, but also in other invertebrates. In aphids, an increase in population density causes the appearance of a winged phase and the dispersal of insects with the formation of new settlements. Usually aphids produce several generations of wingless females, but in conditions of constant overpopulation, winged females develop in each generation. In a number of amoebae, chemical changes in the composition of the aquatic environment, caused by overcrowding of the population, stimulate the transition to the motile flagellar stage. As a result, there is a rapid dispersal of individuals in space.

Territorial behavior animals, developed in the course of evolution as a system of instincts, is the most effective mechanism for restraining the growth of the population in a given area. Marking and protection of sites, which do not allow the reproduction of "foreign" individuals on them, lead to the rational use of the territory. In this case, the excess part of the population does not reproduce or is forced to move out of the occupied space. The same applies to bred offspring, among which only a certain part, due to the natural death of adults, occupies the vacant plots.

Evictions as a response to the growing population density are characteristic of many species of birds and mammals. In addition to the usual settling dispersion of young animals, a number of species with sharp fluctuations in numbers are characterized by mass movements - invasion. They occur irregularly, only in the years of breeding outbreaks, and do not have a constant direction. Such invasions are described, for example, in tundra lemmings, squirrels of Siberia and North America, etc. During invasions, some individuals remain in place, and young ones predominate among emigrants.

An increase in population density may be accompanied by such changes in the physiology of individuals that lead to a decrease in the birth rate and an increase in mortality. In mammals, the phenomenon is known stress, which was first described in 1936 by the physiologist G. Selye for humans. In response to the negative impact of any factors, two types of reactions occur in the body: 1) specific, depending on the nature of the damaging agent (for example, increased heat production under the action of cold), and 2) nonspecific reaction tension as a general effort of the body to adapt to changing conditions. This general reaction is made up of a series of physiological and morphological changes that gradually unfold as a single process. The stress response, or stress, occurs in response to any negative environmental influences, including the deviation of the population density from the optimum.

An important role in the development of stress is played by the signals of the cerebral cortex, which change the activity of the hypothalamus, the central link of the autonomic nervous system. In turn, the activity of the hypothalamus causes changes in the functioning of the pituitary-adrenal hormonal system. In a state of stress in animals, the adrenal cortex greatly increases and the concentration of corticosteroid hormones secreted by this organ increases, as well as a number of other changes in the hormonal balance of the body. In females in the population, ovulation disturbances, resorption of embryos become more frequent, lactation stops early, instincts for caring for offspring fade away, etc., the number of broods and the number of young in them decreases. Ultimately, all this leads to inhibition of population growth. In mouse-like rodents kept in cages of the same size, an inversely proportional relationship between the number of animals in the cage and the mass of their reproductive organs is clearly manifested. In a state of stress in animals, even with a sufficient supply of food, resistance to the harmful effects of the environment decreases, which increases mortality.

The behavior of animals depends primarily on population density. In many species, under conditions of crowding, the level of aggressiveness increases, the reaction to individuals of the opposite sex, young animals, etc., changes.

The stress response as a mechanism regulating fertility is especially pronounced in animals with a well-defined system of hierarchical subordination in groups.

The stress response is characteristic of submissive animals; their reproductive function is also inhibited. Dominant individuals do not show stress reactions. In overcrowded populations, stress spreads to most individuals and, apparently, inhibits reproduction.

A stressful state does not cause irreversible changes in the reproductive system, but only leads to a temporary hormonal blocking of its function. After overcrowding is eliminated, the ability to reproduce can be restored in a short time.

The patterns of stress caused by overpopulation are studied mainly in laboratory animals. However, numerous facts recorded in natural populations suggest that, under natural conditions, stress plays a significant role in regulating the abundance and structure of populations and the behavior of mammals (Fig. 133).

Rice. 133. The dependence of the intensity of reproduction on the population density in the population of the little ground squirrel (according to M. R. Magomedov, 1995)

Rice. 134. Population dynamics of the Siberian lemming in Alaska (after Bunnel et all., 1975)

For example, in the dynamics of populations of a number of tundra lemmings, regular cycles of three-four-year periodicity with an amplitude of fluctuations up to 600 times were recorded (Fig. 134). The peak phase in such fluctuations is usually limited to one season, followed by a sharp decline, a phase of population depression and a subsequent increase. With an increase in the number, an increase in fertility, an increase in the rate of maturation of young animals, a complication of the age structure of the population, and a decrease in newborn mortality are recorded. During the peak period, there is a sharp decline in reproduction and, at the same time, mortality increases in all age groups. Serious destructive changes are found in the ovaries of females of all ages, mass death of follicles is observed in the early stages of development. A year or two after the decline in numbers, the overall intensity of reproduction remains medium, and mortality is high, and then again all reproduction indicators begin to grow. In animals born at a low population, the normal functioning of the ovaries is restored.

At different stages of this cycle, the hormonal state of animals belonging to different generations changes greatly. At the peak of abundance, excessive activity of the adrenal and thyroid glands is noted, which sharply inhibits the reproductive functions of the body. During the cycle, in successive generations, not only the functioning of individual glands changes, but also the entire endocrine system of animals. After several generations, during a period of minimum abundance, the state of the endocrine system normalizes and ensures the restoration of the efficiency of the reproduction process. The specific reasons for such hormonal differences in different generations are related to the fact that the viability and endocrine characteristics of the organism are formed in the embryonic period and are largely determined by the physiological state of the parents.

Thus, the dynamics of the number of lemmings can be represented as an autoregulated process in which endocrine mechanisms play an important role.

All the above examples of interaction between members of a population, from “hard” forms—direct destruction of one individual by another—to a decrease in reproductive abilities as a conditioned reflex to an increase in the frequency of contacts, are different forms of limiting the growth of populations. These inhibitory mechanisms are activated until the complete depletion of environmental resources in response to signals indicating the threat of overpopulation.

Population (populus - from lat. people. population) is one of the central concepts in biology and denotes a set of individuals of the same species that has a common gene pool and has a common territory.

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

An elementary (local) population is a collection of individuals of the same species occupying a small area of ​​a homogeneous area. Between them there is a constant exchange of genetic information. (One of several schools of fish of the same species in the lake)

Ecological population - a set of elementary populations, intraspecific groups confined to specific biocenoses. Plants of the same species in a cenosis are called a coenopopulation. The exchange of genetic information between them occurs quite often. (Fish of the same species in all flocks of a common reservoir).

Geographical population - a set of ecological populations that inhabited geographically similar areas. Geographical populations exist autonomously, their ranges are relatively isolated, gene exchange occurs rarely - in animals and birds - during migrations, in plants - when carrying pollen, seeds and fruits. At this level, the formation of geographical races, varieties, subspecies are distinguished. (Geographic races of Dahurian larch (Larix dahurica) are known: western (west of the Lena (L. dahurica ssp. dahurica).

Population dynamics - periodic or non-periodic change in the size, sex or age composition of a population as a result of the action of abiotic (not dependent on the size and density of the population itself) and biotic (depending on the size and density of the population) factors.

There are three types of population dynamics: stable (change in population size by several times); changeable (fluctuations in numbers dozens of times); explosive (periodic excess of the average number of hundreds and thousands of times).

The concept of biotic potential.

Biotic potential in ecology, the ability of a species to withstand the adverse effects of the external environment. The term was introduced by the American ecologist R. Chapman (1925) in connection with the problem of animal population dynamics. According to Chapman, Biotic potential is a quantitative expression of the ability of organisms to withstand the resistance of the external environment. According to his theory, the potential fertility of animals is not realized, since it is suppressed by the unilateral influence of the external environment, with which the organisms are in antagonistic relations. According to modern views, this point of view looks simplified. Changes in the fertility and survival of animals occur both under the influence of abiotic factors and as a result of interspecific and intraspecific relationships. An important role in these processes is played by intrapopulation mechanisms that provide an active response of the population to external influences.

Fertility.

fertility- a demographic process characterized by the frequency of births in a certain population group: the number of live births per 1,000 population in 1 year. The worldwide birth rate in 1985-90 was 27.1%; the highest birth rate, according to UN estimates, was observed in Kenya - 53.9%, the lowest in San Marino - 9.3% (1985). In Russia in 1990 - 13.4%. Along with mortality, infant mortality and life expectancy, it is an important indicator of the natural movement of the population.

Mortality.

Mortality - the intensity of the process of death of individuals in the population. Mortality is expressed as the number of individuals that died or died per defined. a period in a certain territory or water area in relation to their conditional number (to 100 or 1000); sometimes they use a specific estimate of mortality - per individual per unit of time. The time period for which mortality is estimated can vary from hours and days for small organisms (bacteria, protozoa) to a year for large ones (mammals, birds).

Often used in ecological literature, the expression "natural balance" means a state of balance (dynamic equilibrium) that is characteristic of most populations in a community; it would be completely wrong to understand in this case equilibrium as a static state. The study of fluctuations in the number of animals is the most important area of ​​ecology, influencing such seemingly distant areas of science and activity as genetics, agriculture and medicine.

Seasonal and cyclical (generally covering several years) population fluctuations have long been of interest to naturalists who have tried to establish correlations between observed population processes and various climatic factors. In practical terms, this problem is very important: forecasts of mass reproduction of harmful insects or outbreaks of epidemics depend on its solution. Quite independently, specialists studying the mechanisms of natural selection became interested in the mathematical description of the distribution of new genetic variants of organisms in a population. In order to make the appropriate calculations, it was necessary to have data on the actual population density and how quickly it changes. The rate at which a new genetic variant spreads will obviously differ depending on whether the population is increasing, decreasing or remaining stable in a given period. Geneticists have found that the distribution of genes in a population can be in the nature of regular cyclic fluctuations. In general, the study of animal population dynamics is extremely important for solving a variety of biological problems. The dynamics of plant populations has been studied to a lesser extent, perhaps due to the relative stability of their distribution.

When studying population dynamics, such an important concept as “biotic potential” is widely used, i.e. the rate of reproduction characteristic of a given species (the value of which is affected by the sex ratio, the number of offspring per female, and the number of generations per unit of time). The biotic potential of many organisms, especially the smallest ones, is enormous, and if nothing restrained the growth of their populations, they would very quickly populate the entire Earth. The size of any existing population can be represented as the ratio of biotic potential to environmental resistance, i.e. to the sum of all factors hindering the growth of the population of this species. Since real populations of plants and animals are more or less stable over time, environmental resistance to species with high biotic potential should be quite strong.

Under favorable conditions, population growth is observed and can be so rapid that it leads to a population explosion. The totality of all factors contributing to the growth of the population is called the biotic potential. It is quite high for different species, but the probability of reaching the population limit in natural conditions is low, because. this is opposed by limiting (restricting) factors. The set of factors that limit the growth of the population is called environmental resistance. The state of equilibrium between the biotic potential of a species and the resistance of the environment, which maintains the constancy of the population, is called homeostasis or dynamic equilibrium. When it is violated, fluctuations in the size of the population occur, i.e., changes in it.

The maintenance or growth of numbers depends not only on the rate of reproduction (number of newborns, eggs laid, seeds or spores produced per unit of time). No less important is the replenishment of the adult composition of the population at the expense of offspring. A high reproduction rate at low recruitment rates cannot significantly increase its abundance.

For example - fish spawn thousands or millions of eggs, but only a negligible part survives and turns into an adult animal. Plants disperse a huge amount of seeds.

Conversely, the size of the population can grow due to an increase in the recruitment rate at a low reproduction rate. This applies to humans (the birth rate is low, but infant mortality is low, so almost all children survive to adulthood).

Another important factor leading to population growth is the ability of animals to migrate, and seeds to disperse in new territories, adapt to new habitats and populate them, the presence of protected mechanisms and resistance to adverse environmental conditions and diseases.

Biotic potential is a set of factors that contribute to an increase in the number of a species.

Therefore: the growth, decline and constancy of the population depends on the relationship between the biotic potential and the resistance of the environment.

The principle of population change: it is the result of an imbalance between the biotic potential and the resistance of its environment.

Such an equilibrium is dynamic, i.e. continuously adjustable, because environmental resistance factors rarely remain unchanged for a long time. For example: in one year the population decreased due to drought, and the next year it fully recovered with heavy rains. Such fluctuations continue indefinitely. Equilibrium is a relative concept. Sometimes the amplitude of deviations is small, sometimes significant, but as long as the reduced population is able to restore its former size, it exists.

Equilibrium in natural systems depends on population density, i.e. number of individuals per unit area. If the population density grows, the resistance of the environment increases, in connection with which the mortality rate increases and the population growth stops. Conversely, with a decrease in population density, the resistance of the environment weakens and the former number is restored.

Human impact on nature often leads to the extinction of the population, because. does not depend on population density. Destruction of ecosystems, environmental pollution equally affect populations with both low and high density.

In addition, the biotic potential depends on the critical population size. If the population size (of deer, birds or fish) falls below this value, which guarantees reproduction, the biotic potential tends to zero and extinction is inevitable.

Existence may be endangered even when many members of a species are alive but living at home, i.e. isolated from each other (parrots).



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ECOLOGY,(from the Greek óikos - dwelling, residence) - a science that studies the organization and functioning of populations, species, biocenoses (communities), ecosystems, biogeocenoses and the biosphere. In other words, it is the science of the relationship of organisms with each other and with the environment. The term "ecology" was proposed by the German zoologist E. Haeckel in 1866, but became widespread only at the beginning of the 20th century. The subject matter of this science is not new. The study of animals and plants in their natural habitats was previously occupied, by the definition of old authors, "natural history" and "bionomy".

For many years, ecology remained a purely specialized scientific discipline, little known to the general public. However, since the late 1960s, environmentalists have increasingly begun to warn of adverse changes in the environment caused by rapid population growth and the development of industrial technology. The state of the habitat began to concern public opinion, and environmental and government organizations began to turn to environmentalists for help in solving problems caused by water and air pollution or the thoughtless use of herbicides and pesticides.

The development of biological sciences has gone in two main directions: one is based on the taxonomy of the studied animals and plants, the second - on the methods and approaches used in this area of ​​biological knowledge. The first direction includes such well-defined branches of biology as, for example, mycology (the science of fungi), entomology (the science of insects) or ornithology (the science of birds). It is more difficult to separate the separate biological disciplines related to the second direction. For example, the study of the structure of animals and plants is carried out within the framework of several sciences: cytology, histology, anatomy. The functioning of various living structures - from cells and tissues to organs and the whole organism - is the subject of physiology. However, the traditional approach of the physiologist can be gradually transformed and become an ecological approach if the main emphasis is placed on the study of the reactions and behavior of the whole organism, as well as the relationships of organisms of the same or different species. It is quite characteristic that some information about the behavior of animals and their reactions to external factors (for example, to light or heat) is given both in textbooks of ecology and in textbooks of physiology.

The difference between ecology and physiology, in general terms, boils down to the fact that the former seeks to study animals and plants in natural conditions, while the latter studies organisms within the walls of the laboratory. Of course, the value of field studies will be of little value if their results are not compared with laboratory data obtained in the study of the reactions of isolated organisms to certain influences produced under strictly controlled conditions. As for laboratory physiological studies, they also make sense only if their data are compared with materials from observations of organisms in the natural environment. Although closely related disciplines, physiology and ecology nevertheless differ significantly from each other in methods, terminology and general approaches.

Ecology in a broad sense, as the study of organisms and biological processes in natural conditions, covers the fields of several independent sciences. Thus, the ecological sciences undoubtedly include limnology, which studies life in fresh waters, and oceanology, which studies organisms living in the seas and oceans. In fact, an ecological approach to purely medical problems is demonstrated by epidemiology, which studies the processes of the spread of diseases. Many issues of human biology and sociology are sometimes interpreted from the standpoint of ecology.

HABITAT

The habitat can be defined as the totality of all external factors and conditions affecting an individual organism or a certain community of organisms. Thus, this complex concept implies that it is very difficult, and sometimes impossible, to isolate individual factors in the environment of an organism. In ecological terms, each animal or plant is associated with its own specific habitat, the description of which is, first of all, a statement of the conditions in which this animal or plant exists. For the sake of convenience, all conditions can be subdivided into physical (climatic), chemical and biological.

Climate.

The ecologist pays special attention to the climate, but, as a rule, he is not satisfied with the standard data provided by meteorological stations. After all, for an ecologist, first of all, those conditions are important in which the real life of specific animals or plants takes place, for example, the microclimate characteristic of the forest floor, the coastal strip of a lake, or the core of a decaying log. The ecologist must also take into account climate change in space and time. He needs to explore the many climatic gradients in the area. Some of them - for example, depending on the geographical latitude or height above sea level - are quite obvious. Others, such as those related to the depth of the pond, the height of the tiers in the forest, or the transition from woodland to grassland, need to be specially studied. Changes in climate over time may include such phenomena as the cyclical dynamics of various indicators during the day, irregular fluctuations from one day to another, as well as long-term climatic cycles and changes associated with geological processes.

Assessment of climatic conditions by an ecologist has three levels, each of which has its own study methodology; this is the geographic climate, the climate of a particular habitat (“ecoclimate”) and the climate of the immediate environment of the organism (“microclimate”). The geographic climate, about which meteorological stations collect data, serves not only as a standard against which data from more specialized studies are compared, but also as a basis for analyzing the large-scale distribution of certain organisms. However, information on geographic climate by itself is meaningless without additional information about climatic conditions in specific habitats. For example, from the report of the weather station about the observed frosts, it is not clear where they, in fact, were - in the open area where the instruments were located, or in the forest, where animals or plants of interest to the ecologist live. Sometimes temperature and humidity differ sharply even in neighboring biotopes. Similarly, the stratification of physical conditions observed in the soil, water body or forest is of great importance. Sometimes, in order to understand the behavior of an animal, the ecologist needs to know the conditions of temperature and humidity under the cover of foliage, on the surface film of water or in the pulp of the fruit, in the course made by the insect larva.

Chemical environment.

The chemical composition of the environment is usually given special attention by researchers dealing with aquatic organisms. The properties of dissolved substances and their concentration, of course, are important in themselves as conditions that provide nutrition (primarily for plants), but they also have other effects. For example, salinity can affect the specific gravity of organisms and the osmotic pressure inside cells. The reaction of the environment (acidic or alkaline) and the composition and content of dissolved gases are also important for organisms. In the terrestrial environment, the chemical characteristics of the soil and soil moisture have a significant impact on vegetation, and through it, on animals.

Biotic environment.

Biotic factors of the environment are manifested through the relationship of organisms that are part of the same community. It is possible to study plants or animals in "pure cultures", without connections with other living beings, only in the laboratory. In nature, many species are closely interrelated, and their relationship to each other as components of the environment can be extremely complex. As for the connections between the community and the surrounding inorganic environment, they are always bilateral, mutual. Thus, the nature of the forest depends on the corresponding type of soil, but the soil itself of one type or another is formed to a large extent under the influence of the forest. Likewise, the temperature, humidity, and light in a forest are determined by vegetation, but the resulting climatic conditions in turn affect the community of organisms living there.

limiting factors.

When analyzing the distribution of individual organisms or entire communities, ecologists often turn to the so-called. limiting factors. An exhaustive description of a particular environment is not only impossible, but also unnecessary, since the distribution of animals and plants (both in geographical areas and in individual habitats) can be determined by just one factor, for example, extreme (for these organisms) temperatures, too low (or too high) salinity or lack of food. However, it is not easy to isolate such limiting factors, and attempts to establish a direct relationship between the distribution of organisms and some external factor are far from always successful. For example, laboratory experiments show that some animals living in brackish and marine waters are able to tolerate changes in salinity over a wide range, and their apparent confinement to a narrow range of values ​​\u200b\u200bof this factor is simply due to the presence of suitable food in the appropriate places.

BIOLOGICAL COMMUNITIES

One of the main areas of ecological research is the study of plant and animal communities, their description, classification and analysis of the relationships of the organisms that form them. The term "ecosystem", also often used by ecologists, refers to a community in conjunction with the conditions of its existence, i.e. with non-living (physical) components of the environment.

Plant communities have been studied better than animal communities. This is partly due to the fact that it is the nature of the vegetation that largely determines the composition of the animals living in certain places. In addition, plant communities are more accessible to the researcher, while direct observations of animals are not always possible, and even in order to simply estimate their numbers, ecologists are forced to turn to indirect methods, such as trapping with various devices. When classifying and describing communities, terminology developed by botanists is usually used.

Community classification.

Although there are numerous schemes for classifying communities, none has become universally accepted. The term "biocenosis" is often used to refer to a separate community. Sometimes a hierarchical system of communities of increasing complexity is distinguished: “consortia”, “associations”, “formations”, etc. The widely used concept of "habitat" refers to a set of environmental conditions necessary for one or another specific plant or animal species or for a particular community. Obviously, there is a certain hierarchy of communities and habitats. For example, a lake is a large ecological unit within which it is possible to distinguish communities of organisms associated with the shore, shallow waters, deep sections of the bottom, or the open part of the reservoir. In the coastal zone community, in turn, one can distinguish smaller and more specialized groups of species that live near the surface of the water, on plants of a certain type, or in mud deposits at the bottom. There are, however, great doubts as to whether these communities should be classified in detail and one or another name should be rigidly assigned to them.

The names of some ecological communities are used very widely by biologists. Such, for example, are the terms "plankton", "nekton" and "benthos". Plankton is a collection of small, mostly microscopic, organisms that live in the water column and are passively carried by currents. Nekton is made up of larger and actively moving aquatic animals (for example, fish). Benthos include organisms that live on the bottom surface or in the thickness of bottom sediments. Both in the seas and in lakes, planktonic organisms are numerous and diverse. It is they that serve as a food base for larger animals, and in the ocean they practically determine the existence of all other inhabitants of the water column.

Biological communities are often distinguished by "dominant" or "subdominant" species. This approach is convenient from a practical point of view, especially when it comes to terrestrial ecosystems of the temperate zone, where one type of grass can determine the appearance of the steppe, and one type of tree can determine the type of forest. The concept of dominant species, however, is poorly applicable to the tropics, as well as to communities of organisms that inhabit the aquatic environment.

Community succession.

Ecologists have traditionally paid great attention to the study of "succession", i.e. a regular sequence of changes associated with the development and aging of communities or the change of communities in a particular area. Succession is most easily observed in Western Europe and North America, where human activity, as relentless as a geological process, has radically altered natural landscapes. In place of the destroyed virgin forests, a slow natural change of species occurs, ultimately leading to the restoration of a relatively stable and little changing "climax" (mature) forest community. Most of the territories located around the ancient centers of Western civilization and available for ecological research are occupied by unstable transitional communities that have developed on the site of climax communities destroyed by man.

In areas less exposed to human influence, succession also occurs, although its manifestations are not so noticeable. For example, it is observed where a river changing its course forms a new bank of sediments, or where a sudden landslide frees the bare surface of a rock from the soil, or in a place in a forest where an old tree falls. Succession is clearly manifested in fresh water bodies. In particular, a lot of effort was spent on studying the processes of aging, or eutrophication, in lakes, leading to the fact that the area of ​​open water, gradually shrinking, gives way to a quagmire, and then to a swamp, which itself eventually turns into a terrestrial ecosystem with its characteristic vegetation succession. Pollution of water bodies and an increase in the influx of nutrients into them (for example, when plowing land and applying fertilizers) significantly accelerates the processes of eutrophication.

The study of the relationships between different groups of organisms in a community is, although not an easy, but very interesting task. The researcher who has undertaken its permission must use the entire body of biological knowledge, since any life processes are ultimately aimed at ensuring the survival, reproduction and settlement of organisms in accessible and suitable habitats for their life. Studying certain communities, the ecologist is faced with the problem of establishing the species belonging to the plants and animals that make up them. It is very difficult to describe the species composition of even a simple community, and this circumstance extremely hinders the development of research. It has long been noted that the observation of any animal is meaningless if it is not known what species it belongs to. However, it is clear that identifying all the organisms that live in a certain area is such a laborious task that it can become a life's work in itself. That is why it is considered expedient to conduct ecological research in regions whose flora and fauna are well studied. Usually these are temperate latitudes, not the tropics, where many plants and animals (primarily various invertebrates) have not yet been identified or insufficiently studied.

food chains.

Among the various types of relationships within the community, an important place is occupied by the so-called. food, or trophic, chains, i.e. those sequences of different types of organisms along which matter and energy are transferred from level to level, since some organisms eat others. An example of the simplest food chain is the series “birds of prey - mice - plants”. In almost every community, there is a set of interconnected food chains that form a single food web.

The basis of all food chains and, accordingly, the food web as a whole are green plants. Using the energy of the Sun, they form complex organic substances from carbon dioxide and water. That is why environmentalists call green plants producers, or autotrophs (i.e., self-feeding). In contrast, consumers (or heterotrophs), which include all animals and some plants, are not able to produce nutrients for themselves and, in order to replenish energy costs, must use other organisms as food.

In turn, among the consumers, a group of herbivores (or “primary consumers”) are distinguished, feeding directly on plants. Herbivores can be very large animals, like an elephant or a deer, and very small, like many insects. Predators, or "secondary consumers", are animals that eat herbivores and in this indirect way receive the energy stored in plants. Many animals act as primary consumers in some food chains, and as secondary consumers in others; since they can consume both plant and animal food, they are called omnivores. In some communities, there are also so-called. tertiary consumers (for example, a fox), i.e. predators that eat other predators.

Another important link in the food chain is decomposers (or decomposers). These include mainly bacteria and fungi, as well as some animals, such as earthworms, which consume the organic matter of dead plants and animals. As a result of the activity of decomposers, simple inorganic substances are formed, which, getting into the air, soil or water, again become available to plants. Thus, chemical elements and their various compounds are in constant circulation, passing from organisms to abiotic components of the environment and then back to organisms.

Unlike matter, energy is not subject to recycling, i.e. cannot be used twice: it moves only in one direction - from producers, for which the source of energy is sunlight, to consumers and further to decomposers. Since all organisms expend energy to maintain their life processes, each trophic level (in the corresponding link in the food chain) consumes a significant amount of energy. As a result, each subsequent level gets less energy than the previous one. So, primary consumers have less energy than producers, and secondary consumers get even less.

A decrease in the available amount of energy upon moving to a higher trophic level leads to a corresponding decrease in the biomass (i.e., total mass) of all organisms at this level. For example, the biomass of herbivorous animals in a community is much less than the biomass of green plants, and the biomass of predators, in turn, is many times less than the biomass of herbivores. Describing such relationships, ecologists often use the image of a pyramid, at the base of which are producers, and at the top are predators of the last (highest) link.

Niche concept.

A separate link in a particular food chain is usually called an ecological niche. The same niche in different parts of the world or different habitats is often occupied by somewhat similar but not related animals. For example, there are niches of primary consumers and large predators. The latter can be represented in one community by a killer whale dolphin, in another by a lion, and in a third by a crocodile. If we turn to the geological past, we can give a rather long list of animals that once occupied the ecological niche of large predators.

Commensalism and symbiosis.

Ecologists' attention to food chains may give the impression that the struggle of species for existence is primarily a struggle for the survival of predators and prey. However, it is not. Food relationships are not limited to predator-prey relationships: two animal species in the same community can compete for food, or they can cooperate in their efforts. A food source for one species is often a by-product of the activities of another. The dependence of scavengers on predators is just one example. A less obvious case is the dependence of organisms inhabiting small accumulations of water in hollows on the animals that make these hollows. This kind of benefiting by some organisms from the activities of others is called commensalism. If the benefits are mutual, they talk about mutualism or symbiosis. In fact, individual species in a community are almost always in a bilateral relationship. Thus, the population density of prey depends on the activity of predators; the reduction of the latter can lead to such a high population density of the victims that they begin to suffer from famine and epidemics. See also e COMMENSALISM; SYMBIOSIS.

Shelter.

Interspecies relations in the community are not limited to problems of food. Sometimes it is very important to have a shelter that protects from adverse climatic influences, as well as from all kinds of enemies. Thus, the trees in the forest are important not only as the basis of most food chains, but also as a purely mechanical framework that makes it possible for a complex community of various organisms to develop. It is on the trees that plants such as creepers and epiphytes hold, and many animals live. In addition, trees provide a certain protection of organisms from adverse environmental factors and create a special climate necessary for those who live under the forest canopy.

ECOLOGY OF SPECIES

An important part of ecology is the study of the life cycles of various animal and plant species (“bionomy”). It is impossible to understand the features of the structure and functioning of entire communities without a preliminary study of the needs and behavior of dominant species. Such research is usually referred to as "species ecology" (as opposed to "community ecology").

To get an idea about the peculiarities of the ecology of any kind of animals or plants, it is necessary to pay attention to how and at what speed these organisms grow, how and what they eat, how they reproduce, settle and survive climatically unfavorable periods. This requires observations in natural conditions, as well as laboratory experiments. Perhaps the weakest point in the study of communities is the practical impossibility of applying experimental methods to such complex objects. That is why our understanding of the structure of communities is largely based on the data that are obtained from the study of individual populations of the species that make up the community.

Change of habitat.

Territory,

those. a piece of space actively used by an animal and protected from intrusions by other individuals plays an important role in the regulation of relations between individuals of most of the studied birds and mammals. In some animals (for example, warblers or great tits), each male dominates a territory with clearly defined boundaries and does not allow competitors to enter it. In other cases (for example, in the howler monkeys studied by K. Carpenter in Panama), the site belongs to a group of individuals, sometimes quite large, which protects it from the invasion of other similar groups or individual individuals of the same species. As many ecologists believe, the factor limiting the size of populations is most often the availability of suitable territory, and not directly the lack of food. From the point of view of species distribution, the instinct to protect the territory is very important, as it ultimately allows animals to more evenly populate a certain space and use it more efficiently, maintaining an optimal population density.

Hibernation.

Hibernation and hibernation are also directly related to the ecology of the species, since members of the same community can show completely different ways of experiencing unfavorable periods of the year. Hibernation is a special physiological state of the body, in which many of its normal functions are turned off or extremely slowed down, which allows the animal to be in a state of complete rest for a long time. Trying to precisely define the concept of hibernation usually leads to extremely cumbersome and inconvenient formulation, because there are actually many ways in which animals can survive a difficult winter period. For example, it is hardly possible to talk about real hibernation of bears, since their body temperature practically does not decrease during this period. The state of complete torpor in the American woodchuck, the winter sleep of the bear, the seasonal change of fur, and the change in behavior of the hares are all examples of different ways of solving the same problem, namely adaptation to seasonal cycles. As another such method, seasonal migration of animals to areas with a more favorable climate can be considered.

The study of the mechanisms of hibernation is mainly carried out by physiologists, since this requires laboratory studies of the hibernating animal, as well as direct experiments to identify the factors that determine the beginning and end of winter dormancy. Our understanding of these mechanisms is far from complete, perhaps because the problem itself is on the periphery of physiology and ecology and is not studied enough. There are various theories explaining the mechanisms of the onset of hibernation, its course and exit from hibernation, and it is possible that the factors controlling these processes are different in different species. The most important role is played by changes in temperature, nutritional conditions, the provision of the animal with fat reserves, as well as the length of daylight hours. If warm-blooded animals may or may not hibernate, then cold-blooded animals, such as insects in temperate latitudes, must inevitably be dormant in winter, since normal metabolic processes simply cannot proceed at such low temperatures.

Most insect species survive the winter as eggs. However, in many other animals, the egg is precisely that stage of the life cycle that is best adapted to developmental delay. The same can be said about the seeds and spores of plants. In a certain sense, plants resemble cold-blooded animals: due to low temperatures, the normal metabolism of these organisms in winter is impossible. In addition, plants are very sensitive to moisture loss during transpiration, and winter is a period of drought, since liquid water is usually not available at this time of the year in temperate latitudes. Over the course of evolution, perennials have adapted to the changing seasons, shedding their leaves for the winter and forming dormant, well-protected buds. It is curious that the preservation of plants in a temperate climate in winter, and in the tropics during the dry and hot season, is ensured by essentially the same mechanisms.

The so-called diapause (temporary cessation of development), observed in insects and other invertebrates, sometimes without apparent connection with changes in environmental factors, has long been the subject of research by ecologists and physiologists. Aestivation (summer hibernation), which serves to survive heat and drought, can also be considered as a special case of diapause. Aestivation is very common among insects, especially those living in the tropics. Like winter diapause, summer diapause is most often observed at the egg stage, although in some cases larvae and even adults are adapted to this state.

Spreading.

The study of the geographical distribution of animals and plants is also within the scope of ecology. Traditional zoogeography differs from ecology in that it relies primarily on data from the geological history of the Earth and pays special attention to the distribution of large taxonomic groups over the main biogeographic regions. In some cases, such an approach is absolutely necessary. So, without knowing the history of the continents, it is impossible to understand why marsupials are currently found only in Australia and America. However, the current boundaries of species distribution depend almost exclusively on environmental factors. In order to establish the reasons for this or that distribution of individual species or entire communities, it is necessary to identify the main limiting factors. For example, the northern limit of occurrence of any insect species in the Northern Hemisphere is often determined by whether the species has a mechanism for experiencing a long cold winter. Insects unable to enter diapause for the winter period are forced to live only in those areas where the climate allows them to remain active throughout the year. The geographical distribution of plants is determined mainly by the main climatic zones and the nature of the soils.

POPULATION DYNAMICS

Often used in ecological literature, the expression "natural balance" means a state of balance (dynamic equilibrium) that is characteristic of most populations in a community; it would be completely wrong to understand in this case equilibrium as a static state. The study of fluctuations in the number of animals is the most important area of ​​ecology, influencing such seemingly distant areas of science and activity as genetics, agriculture and medicine.

Seasonal and cyclical (generally covering several years) population fluctuations have long been of interest to naturalists who have tried to establish correlations between observed population processes and various climatic factors. In practical terms, this problem is very important: forecasts of mass reproduction of harmful insects or outbreaks of epidemics depend on its solution. Quite independently, specialists studying the mechanisms of natural selection became interested in the mathematical description of the distribution of new genetic variants of organisms in a population. In order to make the appropriate calculations, it was necessary to have data on the actual population density and how quickly it changes. The rate at which a new genetic variant spreads will obviously differ depending on whether the population is increasing, decreasing or remaining stable in a given period. Geneticists have found that the distribution of genes in a population can be in the nature of regular cyclic fluctuations. In general, the study of animal population dynamics is extremely important for solving a variety of biological problems. The dynamics of plant populations has been studied to a lesser extent, perhaps due to the relative stability of their distribution.

biotic potential.

When studying population dynamics, such an important concept as “biotic potential” is widely used, i.e. the rate of reproduction characteristic of a given species (the value of which is affected by the sex ratio, the number of offspring per female, and the number of generations per unit of time). The biotic potential of many organisms, especially the smallest ones, is enormous, and if nothing restrained the growth of their populations, they would very quickly populate the entire Earth. The size of any existing population can be represented as the ratio of biotic potential to environmental resistance, i.e. to the sum of all factors hindering the growth of the population of this species. Since real populations of plants and animals are more or less stable over time, environmental resistance to species with high biotic potential should be quite strong.

population pressure.

The biotic potential can also be characterized as a kind of "population pressure" that opposes the constant impact of various adverse environmental factors. If weather conditions improve for a while, the pressure of the main predator weakens, or other unpredictable changes occur that contribute to the development of this population, it will show rapid growth (manifestations of which are invasions of locusts or mice, and sometimes a decrease in the price of the fur of some become common furry animal).

population cycles.

The number of small animals with a short lifespan is subject to regular seasonal changes. One species can be massive in spring, another at the beginning of summer, and the third even later, and thus in one habitat there is a seasonal succession of dominant forms. Such changes in species are especially characteristic of planktonic communities, not only in the seas, but also in lakes. In addition, the number of species can vary greatly from year to year. In large mammals, cyclical changes in abundance cover a longer period, and researchers often use various indirect data, including fur harvesting statistics, to evaluate them. For example, lemmings and arctic foxes have four-year cycles, and they coincide on both sides of the Atlantic. Such fluctuations in abundance may be related to climatic cycles. A certain role is also played by the circumstance that with a high population density, epidemic diseases arise more easily, as a result of which the number is reduced to a minimum; in the future, it begins to gradually increase again, and the cycle repeats.

Changes in population size also occur over geological time periods as some species gradually give way to others. It is impossible to directly observe such processes because of their enormous temporal extent, but something similar can be seen in those cases when, due to human activity, comparable in effect to geological phenomena, some species rapidly disappear or new species are introduced into those areas where they were not before. This has been the case with rabbits introduced to Australia, European rats and mice introduced to the Americas, and many plant pests that have spread throughout the world.

Paleoecology.

Some fossil forms are so common that they can be used to reconstruct environmental conditions and community structure in past geological epochs. Of particular value for such a reconstruction are those cases where the deposits are entirely formed by the remains of organisms or contain clearly marked (for example, plant pollen or imprints of their leaves) layers. Studies of this kind, carried out primarily by botanists, are part of the task of paleoecology.

APPLIED ASPECTS

The study of human, animal or plant diseases from an ecological point of view is the main subject of epidemiology. This science has developed systems of measures to limit the spread of diseases such as malaria, typhoid, plague, yellow fever and sleeping sickness. Such measures usually include the control of disease-carrying insects. As with agricultural pests, this control must be based on a good knowledge of the ecology of the organisms concerned.

Literature:

Nebel B. environmental science. How the world works, tt. 1–2. M., 1993