When a population stops growing, its density tends to fluctuate around its upper asymptotic growth level. Such fluctuations can arise either as a result of changes in the physical environment, as a result of which the upper limit of abundance increases or decreases, or as a result of intra-population interactions, or, finally, as a result of interaction with neighboring populations. After the upper population limit (TO) will be achieved, the density may remain at this level for some time or immediately drop sharply (Fig. 7.7, curve 1). This drop will be even sharper if the resistance of the environment does not increase gradually, as the population grows, but appears suddenly (curve 2). In this case, the population will be realized
Rice. 7.7. Various types fluctuations in population density. Explanations in the text.
increase biotic potential. However, exponential growth cannot continue for long. When the exponential reaches the paradoxical point of tending to infinity, as a rule, a qualitative leap occurs - a rapid increase in numbers is replaced by massive cell death or death of individuals. An example of such fluctuations is an outbreak of insect reproduction, followed by their mass death; proliferation and death of algae (“blooming” of water bodies). A situation is also possible in which the population size “jumps” over the limit level (curves 3, 4), If nutrients and other factors necessary for life are accumulated even before the population begins to grow. This, in particular, can explain why new ponds and lakes are often richer in fish than old ones.
There are two main types of population fluctuations (Fig. 7.8). In the first type, periodic environmental disturbances such as fires, floods, hurricanes and droughts often lead to catastrophic, density-independent mortality. Thus, the population of annual plants and insects usually grows rapidly in spring and summer, and sharply declines with the onset of cold weather. Populations whose growth produces regular or random bursts are called
Rice. 7.8. Fluctuation of population density in opportunistic (1) and equilibrium (2) species.
Huddling opportunistic(Fig. 7.8, 1). Other populations, so-called equilibrium(typical of many vertebrates) are usually in a state close to equilibrium with resources, and their density values are much more stable (Fig. 7.8.2).
The two population types identified represent only the extremes of a continuum, but such a distinction is often useful when comparing different populations. The significance of contrasting opportunistic populations with equilibrium ones is that the independent and density-dependent factors acting on them, as well as the events that occur during this process, have different effects on natural selection and on the populations themselves. McArthur and Wilson (1967) called these opposing types of selection r-selection and K-selection, according to the two parameters of the logistic equation. Some characteristic features of r- and K-selection are given in the table.
In terms of time, population fluctuations occur non-periodic And periodic. The latter can be divided into fluctuations with a period of several years and seasonal fluctuations. Non-periodic fluctuations are of an unexpected nature.
IN Pacific Ocean, especially in the Great Barrier Reef region northeast of Australia, there has been an increase in numbers since 1966 starfish crown of thorns, Acanthaster planci. The crown of thorns, having previously been small in number (less than one individual per 1 m2), reached a density of 1 individual per 1 m2 by the early 1970s. The starfish causes great harm to coral reefs, as it feeds on the polyps that make up their living part. She “cleared” a 40-kilometer strip of reefs off the island of Guam in less than three years. None of the hypotheses proposed to explain the sudden increase in the number of starfish (the disappearance of one of its enemies - gastropod triton horn, Charonia tritonis, which is mined for its shells containing mother of pearl; increase in content in sea water DDT and, in connection with this, a violation of the natural balance; influence of radioactive fallout) cannot be considered satisfactory.
Under favorable conditions, populations experience an increase in numbers and can be so rapid that it leads to a population explosion. The totality of all factors contributing to population growth is called biotic potential. It is quite high for different species, but the probability of the population reaching the population limit under natural conditions is low, because this is counteracted by limiting factors. The set of factors limiting population growth is called medium resistance. The state of equilibrium between the biotic potential of a species and the resistance of the environment that maintains the constancy of population numbers is called homeostasis or dynamic equilibrium. When it is violated, fluctuations in population size occur, i.e. its changes.
Distinguish periodic and non-periodic fluctuations in population numbers. The first occur over the course of a season or several years (4 years - a periodic cycle of cedar fruiting, an increase in the number of lemmings, arctic foxes, polar owls; after a year, apple trees bear fruit in garden plots), the second are outbreaks of mass reproduction of certain pests useful plants, in case of disturbances in habitat conditions (droughts, unusually cold or warm winters, too rainy growing seasons), unexpected migrations to new habitats. Periodic and non-periodic fluctuations in population numbers under the influence of biotic and abiotic environmental factors, characteristic of all populations, are called population waves.
Any population has a strictly defined structure: genetic, age-sex, spatial, etc., but it cannot consist of fewer individuals than necessary for the stable development and resistance of the population to environmental factors. This is the principle of minimum population size. Any deviations of population parameters from optimal values are undesirable, but if their excessively high values do not pose a direct threat to the existence of the species, then a decrease to a minimum level, especially in population size, poses a threat to the species.
However, along with the principle of minimum population size, there is also the principle (rule) of population maximum. It lies in the fact that the population cannot increase indefinitely. Only theoretically is it capable of unlimited growth in numbers.
According to the theory of H.G. Andrevarty - L.K. Bircha (1954) - theory of population size limits - the number of natural populations is limited by the depletion of food resources and breeding conditions, the inaccessibility of these resources, and a too short period of accelerated population growth. The theory of “limits” is supplemented by the theory of biocenotic regulation of population size by K. Fredericks (1927): population growth is limited by the influence of a complex of abiotic and biotic environmental factors.
Factors or reasons for fluctuations in numbers:
sufficient food supplies and food shortages;
competition of several populations for one ecological niche;
external (abiotic) environmental conditions: hydrothermal regime, illumination, acidity, aeration, etc.
Fluctuations (deviations) in numbers are caused by a variety of reasons. And they are not always the same for different species. Periodic fluctuations in population numbers, which have a 10-11 year period, are explained by the periodicity of solar activity: the number of sunspots changes with a period of 11 years. The amount of food is the reason for fluctuations in the Siberian silkworm: it gives an outbreak after a dry, warm summer. It can cause an outbreak of numbers and a combination of many circumstances. For example, red tides are observed off the coast of Florida. They are non-periodic and for their manifestation the following events are necessary: heavy rainfalls, washing away microelements from the land (iron, zinc, cobalt - their concentrations must coincide up to ten thousandths of a percent), reduced salinity of the bottom, a certain temperature and calmness off the coast. Under such conditions, dinoflagellate algae begin to rapidly divide. Theoretically, from one single-celled dinoflagellate, as a result of 25 successive divisions, 33 million individuals can arise. The water turns red from them. Dinoflagellates release a deadly poison into the water, causing paralysis and then the death of fish and other sea inhabitants.
A person can, through his activities, cause an outbreak of certain populations. The result of anthropic influence is an increase in the number of sucking insects (aphids, bedbugs, etc.) after treating fields with insecticides that destroy their enemies. Thanks to humans, rabbits and the prickly pear cactus in Australia, house sparrows and gypsy moths in North America, the Colorado potato beetle and phylloxera in Europe, the Canadian elodea, the American mink and the muskrat in Eurasia showed incredible outbreaks of numbers after entering these new territories where there were no their enemies.
Sharp non-periodic fluctuations in numbers may arise due to natural disasters. For example, outbreaks of fireweed and the associated insect community are common in fires. Long-term drought turns a swamp into a meadow and causes an increase in the number of members of the meadow biocenosis.
The evolutionary significance of population waves is that they:
change the frequencies of alleles (small numbers at the peak of the wave can manifest themselves phenotypically, and at the decline they can disappear from the gene pool);
at the peak of the wave, isolated populations merge, migration and panmixia increase, and the heterogeneity of the gene pool increases;
population waves change the intensity of natural selection and its direction.
Upon reaching the final growth phase, population sizes continue to fluctuate from generation to generation around some more or less constant value. At the same time, the number of some species changes irregularly with a large amplitude of fluctuations (insect pests, weeds), fluctuations in the number of others (for example, small mammals) have a relatively constant period, and in populations of third species the number fluctuates slightly from year to year (long-lived large vertebrates and woody plants).
In nature, there are mainly three types of population change curves: relatively stable, spasmodic and cyclical (Fig. 6.9).
Rice. 6.9. Basic curves of population changes various types:
1 - stable; 2 - cyclical; 3 - spasmodic
Species whose numbers from year to year are at the level of the supporting capacity of the environment have sufficient stable populations(curve 1 ). This consistency is typical for many species. wildlife and is found, for example, in unspoiled tropical wet forests, where the average annual precipitation and temperature vary very little from day to day and from year to year.
In other species, population fluctuations are regular. cyclical character (curve 2 ). Examples of seasonal fluctuations in numbers are well known. Clouds of mosquitoes; fields overgrown with flowers; forests full of birds - all this is typical for the warm season in the middle zone and disappears almost to nothing in winter.
A widely known example is cyclical fluctuations in the number of lemmings (northern herbivorous mouse-like rodents) in North America and Scandinavia. Every four years, their population density becomes so high that they begin to migrate from their overpopulated habitats. At the same time, they die en masse in fiords and drown in rivers, which to date has not had a sufficient explanation. Cyclic invasions of wandering African locusts into Eurasia have been known since ancient times.
A number of species, such as the raccoon, generally have fairly stable populations, but from time to time their numbers sharply increase (jump) to highest value, and then drops sharply to some low but relatively stable level. These species are classified as populations with abrupt growth in numbers(curve 3 ).
A sudden increase in numbers occurs with a temporary increase in the environmental capacity for a given population and may be associated with an improvement climatic conditions(factors) and nutrition or a sharp decrease in the number of predators (including hunters). After exceeding the new, higher capacity of the environment, mortality in the population increases and its size decreases sharply.
Rice. 6.10. Increasing the supporting capacity of the environment for the human population (according to T. Miller), the scale along the axes is relative
Throughout history in different countries Cases of collapse of human populations have been observed more than once, for example in Ireland in 1845, when the entire potato crop was destroyed as a result of fungal infection. Because the Irish diet relied heavily on potatoes, by 1900 half of Ireland's eight million people had starved to death or emigrated to other countries.
Nevertheless, the human population on Earth, in general, and in many regions in particular, continues to grow. Through technological, social and cultural changes, people have repeatedly increased the planet's supporting capacity (Fig. 6.10). Essentially, they were able to change their ecological niche by increasing food production, controlling disease, and using greater amounts of energy and material resources to make normally uninhabitable areas of the Earth habitable.
On the right side of Fig. Table 6.10 shows possible scenarios for further changes in the actual number of people on the planet if the supporting capacity of the biosphere is exceeded.
In ecosystems that are simple in structure (agrobiogeocenoses, desert, semi-desert and tundra ecosystems), the community of organisms is highly exposed to physical stress. In such biogeocenoses, the number of populations is largely influenced by weather patterns, water and air currents, the chemistry of the environment and the degree of its pollution. In natural biogeocenoses with a complex structure and rich species diversity, consisting of large quantity populations, fluctuations in numbers are mainly controlled biotic factors. Therefore, when studying the reasons that cause fluctuations in the size of a particular population, it is necessary to have a clear idea of how independent , so and about density-dependent factors.
The first include factors that constantly affect the population. These are abiotic and, above all, climatic factors of mortality. Unfavorable weather can cause the death of individuals in a population that have not yet reached a stable phase of development. The influence of temperature, light, and humidity on life expectancy, fertility, mortality, and other properties of organisms is well known. Moreover, climatic factors have a direct and stronger effect on poikilothermic animals than on homeothermic ones. The latter, having perfect physiological mechanisms, become relatively independent of the external environment. The reduction in population numbers during sharp drops in temperature is more noticeable in insects than in birds and especially in mammals.
The effect of climatic factors does not always manifest itself immediately, immediately. For example, in the taiga, favorable weather conditions lead to a high seed yield in a year, and an increase in the animal population with abundant food is observed only after two years. In this case, weather conditions operate regardless of population density.
Regardless of the density, other factors also come into play. Thus, the number of hollows in trees in a particular forest determines the number of cavity nesters. It goes without saying that the number of hollows does not depend in any way on the population density of hollow nesters. On the other hand, living space can limit population size. For example, the number of ptarmigan and a number of mammals (muskrat, etc.) is sharply reduced if they do not find suitable habitats, even with a favorable combination of other factors.
Density-dependent factors typically influence the rate of population growth. In this case, it can change in three directions.
In species with strong fluctuations in numbers (mouse-like rodents, insects), population growth rates usually stabilize at high population densities, i.e. remain almost unchanged until the population reaches its maximum size. At maximum density, the growth rate drops sharply.
The third direction, due to the influence of density-dependent factors, is that population growth rates can be maximum even at average densities. But even in this case, the population density, having reached a maximum, begins to decrease. This is especially true for some birds and insects.
7 Intrapopulation regulation of population numbers
Population density usually has a certain optimum. With any deviation of the number from this optimum, the mechanisms of its intrapopulation regulation begin to operate. One of the main mechanisms contributing to the establishment of stable stability in a population is the action of density-dependent factors. Abiotic factors also influence population mortality, but do not independently create its sustainable stability.
Regulation of population numbers in different species of animals and plants is carried out differently. However, in each of them the optimum density is established in a certain way.
An increase in the population density of many insects is accompanied by a decrease in the size of individuals, a decrease in their fertility, an increase in the mortality of larvae and pupae, a change in the rate of development and sex ratio, as well as an increase in the number of diapausing individuals, which sharply reduces the active part of the population.
Often, when population density increases excessively, cannibalism is stimulated. A striking example is the phenomenon of mealworms eating their own eggs. Cannibalism is observed in some species of fish, amphibians and other animals.
One of the important mechanisms of intrapopulation regulation of numbers is emigration, the intensity of which is stimulated by an increase in population density. This is quite typical for many insects, in which, at a certain population density, there is an eviction of some individuals, sometimes significant ones, into less preferred habitats of the same range. In some aphid species, an increase in population density is accompanied by the appearance of winged individuals capable of dispersal. When populations become overcrowded, emigration occurs in a number of mammals (especially mouse-like rodents) and birds.
The regulatory role of intraspecific competition for limited resources has been sufficiently studied. In carrion flies, from the huge number of eggs laid on a corpse, so many larvae emerge that there is not enough food for everyone. As a result, their mortality rate at early ages increases catastrophically. A similar phenomenon was found in bark beetles), lasius ants, some dragonflies and other insects.
In the simplest cases, intrapopulation regulatory mechanisms of numbers manifest themselves in the form of direct competition for resources necessary for life, the quantity of which is insufficient to meet the needs of all individuals. It is known that the population density of the codling moth and cabbage moth is regulated by competition for food and pupation sites. Intraspecific competition in some flies, when the population density increases to a certain level, leads to a drop in the mass of pupae, which is accompanied by increased mortality.
The problem is important "minimum viable population" , the essence of which is to determine the minimum population size that would guarantee its existence for a sufficiently long period. At the same time, a drop in population density below the optimal level, for example with increased extermination of rats, causes an increase in fertility and stimulates their earlier puberty.
Some mechanisms for regulating population numbers can simultaneously act as mechanisms that prevent intraspecific competition. So, if a bird marks its nesting site by singing, then another pair of the same species nests outside it. The marks left by many mammals limit their hunting area and prevent the entry of other individuals. All this reduces intraspecific competition and prevents excessive population densification.
In plants, the regulatory mechanisms of population numbers are primarily intraspecific competition. It is usually associated with increased plant density. In over-compacted crops, for example, there is a decrease in the amount of seed production, which has great importance for agriculture and forestry. Most often, plants of the same species compete for light and moisture. In dense crops they shade each other, and with a limited amount of water they experience a lack of it. As a result, some of them die. This phenomenon is most typical for many garden crops and forest plants. There are always significantly more young plants in the forest than old ones. Intraspecific competition for moisture explains the frequent correct distribution of desert plants. It seems as if someone had seated them at a strictly defined distance from each other. In depressions in the terrain, in oases, this uniform sparseness of plant populations immediately disappears. Light-loving and relatively moisture-loving baobabs are distributed in the same way in African savannas.
However, it should be taken into account that a population is usually part of a community and that the sustainable existence of biocenoses is possible only with certain quantitative ratios of all components. This is the reason for the need to regulate numbers to ensure a stable state of both individual populations and biocenoses as a whole.
8 Population as a self-regulating system
Populations of animals, plants and microorganisms have the ability to naturally regulate density, i.e. the density, with more or less significant fluctuations, remains in a stable state between its upper and lower limits. This is ensured by the action of certain adaptive mechanisms. It is based on the fact that the supply of energy necessary for the survival of a particular population does not exceed a certain level and, thus, maintains the size of this population.
The tendency of living systems, including populations, to maintain internal stability through their own regulatory mechanisms is called I am homeostasis, and fluctuations in population numbers within some average value - their dynamic balance.
Biological regulation (dynamic equilibrium, homeostasis) of a population, or its automatic self-regulation, cannot be caused by abiotic factors independent of population density if they act in isolation from biotic ones. Only factors dependent on population density are able to regulate numbers and ensure their balance.
All biological systems are characterized by a greater or lesser capacity for self-regulation, i.e. To homeostasis. With the help of self-regulation, the overall existence of each system is maintained - its composition and structure, characteristic internal connections and transformations in space and time. Such homeostatic systems are, first of all, each individual, and then the population. Since self-regulating systems are not closed, they actively interact with the external environment and are therefore subject to change. Changes are not only cyclical with a return to the original state, but also historically irreversible. However, both are regulated in the direction of preserving the system, in the case under consideration - the population.
Self-regulation of the population is carried out by two mutually balancing buffer forces operating in nature. This is, on the one hand, the ability of organisms to reproduce, and on the other hand, reactions that depend on population density and limit reproduction.
Self-regulation is a necessary adaptation of organisms to maintain life in constantly changing conditions.
In the evolutionary development of organisms, changes concern not the individual, but their totality - the population. These changes are also regulatory in nature. That is why the population, as an elementary evolving unit, has not only a specific structure, but also the ability to self-regulate. At the same time, its number is regulated by the rate of reproduction, phenotypic diversity is natural selection, and genetic - by mutation, crossing, natural selection.
Populations - open systems. There are many channels through which information reaches the population. These input channels connecting the population with the external environment are specialized and controlled by the population itself. Therefore, all regulatory processes are always carried out due to the forces acting within the population. Therefore, biological regulation is self-regulation. However, despite the fact that populations have an internal mechanism of self-regulation, the action of which is aimed at maintaining the constancy of the structure, the latter does not remain unchanged in new environment, i.e., with changes in living conditions, the population also changes.
Since, when considering issues related to fertility, mortality, migration of individuals, with the influence of density-dependent and independent factors on the number of intraspecific groups, with intraspecific competition, the group effect, phase variability and other phenomena, the processes of self-regulation of population numbers have already been illustrated, we will limit ourselves to the following examples. It is well known that changes in environmental conditions can lead to a sharp increase in mortality. As a result, a signal appears in the population informing about a catastrophic reduction in numbers. This affects the physiology of all members of the population, which is manifested in the mobilization of its resources to minimize energy expenditure, to maintain normal life activity, and to increase the resistance of individuals to adverse factors. As a result, the rate of aging of individuals decreases, the relative number of females increases, and their fertility increases. This phenomenon has been studied in populations of many animals, especially insects, amphibians and mouse-like rodents.
Self-regulation with a sharp increase in population density has a diametrically opposite character. An overdensified population receives a corresponding signal, and its individual individuals, becoming cannibals, intensively exterminate their fellows. In addition, the fertility of females sharply decreases, and the mortality rate of the weakest individuals increases. As a result, after a relatively short period of time, the population size returns to normal.
An important mechanism of population regulation, manifested in an overcrowded population, is stress reaction (from the English stress - tension). If a population is exposed to some strong stimulus, it responds to it with a nonspecific reaction, which is called stress. There are many forms of stress in living nature: anthropic (occurs in animals under the influence of human activity); neuropsychic (manifests itself when individuals in a group are incompatible or as a result of population overdensification); thermal; noise, etc. For example, as a result of population overdensification, physiological changes occur in individual individuals that lead to a sharp reduction in the birth rate and an increase in mortality. In mammals this phenomenon is called stress syndrome . At the same time, animals become so aggressive ( brutal fights, intolerance of the presence of a neighbor, etc.) that their reproduction almost completely stops. In a stressful state, the adrenal cortex enlarges and the concentration of corticosteroid hormones increases. In females, ovulation is disrupted, embryos are reabsorbed, instincts to care for offspring do not appear, etc.
The nature of the signals perceived by the population as an “order” to action is very diverse, and the signaling system works flawlessly. Therefore, even extremely high density or mortality does not cause sharp disturbances in the population structure. This guarantees the restoration of the population within the optimum range in a relatively short period of time. This is how, for example, numerous outbreaks of mass reproduction of insect pests ended.
Consequently, any population of plants, animals and microorganisms is a perfect living system capable of self-regulation. At the same time, we must not forget that a population is the smallest evolving unit. It does not exist in isolation, but in connection with populations of other species. Therefore, extra-population mechanisms of automatic regulation, or more precisely, inter-population mechanisms, are also widespread in nature. In this case, the population is a regulated object, and the regulator is a biogeocenosis consisting of many populations of different species. The biogeocenosis as a whole and the populations of other species included in it significantly influence this specific population, and each population, for its part, affects the biogeocenosis of which it is a part.
Upon reaching the final growth phase, population sizes continue to fluctuate from generation to generation around some more or less constant value. At the same time, the number of some species changes irregularly with a large amplitude of fluctuations (insect pests, weeds), fluctuations in the number of others (for example, small mammals) have a relatively constant period, and in populations of third species the number fluctuates slightly from year to year (long-lived large vertebrates and woody plants).
In nature, there are mainly three types of population change curves: relatively stable, cyclical and spasmodic (Fig. 2.23).
Rice. 2.23.
7 - stable; 2 - cyclical; 3 - spasmodic
Species whose numbers from year to year are at the level of the supporting capacity of the environment have sufficient stable populations(curve /). This consistency is characteristic of many species of wildlife and is found, for example, in intact tropical rainforests, where the average annual rainfall and temperature vary very little from day to day and from year to year.
In other species, population fluctuations are regular. cyclical nature(curve 2). Examples of seasonal fluctuations in numbers are well known. Clouds of mosquitoes; fields overgrown with flowers; forests full of birds - all this is typical for the warm season in the middle zone and disappears almost to nothing in winter.
A widely known example is cyclical fluctuations in the number of lemmings (northern herbivorous mouse-like rodents) in North America and Scandinavia. Once every four years, their population density becomes so high that they begin to migrate from their overpopulated habitats; At the same time, they die en masse in fiords and drown in rivers, which to date has not been sufficiently explained. Cyclic invasions of wandering African locusts into Eurasia have been known since ancient times.
A number of species, such as the raccoon, generally have fairly stable populations, but from time to time their numbers sharply increase (jump) to a high value and then drop sharply to some low but relatively stable level. These species are classified as populations with abrupt growth in numbers(curve 3).
A sudden increase in numbers occurs with a temporary increase in the environmental capacity for a given population and may be associated with improved climatic conditions (factors) and nutrition or a sharp decrease in the number of predators (including hunters). After exceeding the new, higher capacity of the environment, mortality in the population increases and its size decreases sharply.
Throughout history, different countries have repeatedly witnessed cases of collapse of human populations, for example in Ireland in 1845, when the entire potato crop was destroyed as a result of infection by a fungus. Because the Irish diet relied heavily on potatoes, by 1900 half of Ireland's eight million people had starved to death or emigrated to other countries.
Nevertheless, the human population on Earth in general and in many regions in particular continues to grow. Through technological, social and cultural changes, people have repeatedly increased the planet's supporting capacity (Fig. 2.24). Essentially, they were able to change their ecological niche by increasing food production, fighting disease, and using large amounts of energy and material resources to make normally uninhabitable areas of the Earth habitable.
On the right side of Fig. Table 2.24 shows possible scenarios for further changes in the actual number of people on the planet if the supporting capacity of the biosphere is exceeded.
Rice. 2.24. Increasing the supporting capacity of the environment for the human population (according to T. Miller) 1
