Fluctuations in the number of organisms. environmental regulation. Fluctuations in the number of organisms Cyclic and non-cyclic fluctuations. Exogenous and endogenous factors of population dynamics. Connection with solar activity Moskaleva A.D., biology teacher

Question 1. What is population dynamics? What factors cause population fluctuations?

Population dynamics is the most important ecological process, characterized by a change in the number of organisms that make up them over time. Population changes are a complex process that ensures the stability of populations, the most efficient use of environmental resources by organisms, and finally, changes in the properties of the organisms themselves in accordance with the changing conditions of their life.

Population dynamics is closely dependent on indicators such as fertility and mortality, which are constantly changing depending on many factors. When the birth rate exceeds the death rate, the population increases, and vice versa: the number decreases when the death rate becomes higher than the birth rate. Constant changes in the living conditions of organisms lead to an increase in one or the other process. As a result, populations fluctuate.

Population fluctuations can be caused by seasonal changes in living conditions - factors: abiotic (temperature, humidity, light, etc.) or biotic (development of parasitic infections, predation, competition). In addition, the population dynamics is affected by the ability of individuals that make up the population to migrate - to fly, roam, etc.

Question 2. What is the significance of population dynamics in nature?

Dynamic population changes ensure the stability of populations, the most efficient use of ecological resources by the organisms that compose them, and finally, changes in the properties of the organisms themselves in accordance with the changing conditions of their life.

Question 3. What is the regulatory mechanisms-we? Give examples.

Populations have the ability to naturally regulate their numbers due to regulatory mechanisms that have the character of behavioral or physiological reactions of organisms to changes in population density. They are triggered automatically when the population density reaches either too high or too low values. material from the site

In some species, they manifest themselves in a severe form, leading to the death of an excess of individuals (self-thinning in plants, cannibalism in some animal species, throwing "extra" chicks out of the nest in birds), and in others - in a softened form. : expressed in a decrease in fertility at the level of conditioned reflexes (various manifestations of stress reactions) or by secreting substances that retard growth (daphnia, tadpoles - amphibian larvae) and development (often found in fish).

Interesting are the cases of limiting the size of the population by such changes in behavior with an increase in density, which ultimately lead to a mass migration of individuals.

For example, with an excessive increase in the population of Siberian silkworm butterflies, part of the butterflies (mainly females) scatter over distances of up to 100 km.

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Detailed solution paragraph § 80 in biology for students of grade 10, authors Kamensky A.A., Kriksunov E.A., Pasechnik V.V. 2014

1. What factors influence the population size?

Answer. In natural systems with a low level of species diversity, populations are strongly affected by abiotic and anthropogenic factors. It depends on the weather, the chemical composition of the environment and the degree of pollution. In systems with a high level of species diversity, population fluctuations are mainly controlled by biotic factors.

All environmental factors, depending on the nature of their influence on the population size, can be divided into two groups.

Factors independent of population density change the size of populations in one direction, regardless of the number of individuals in them. Abiotic and anthropogenic (with the exception of human environmental activities) factors affect the number of individuals, regardless of population density. Thus, severe winters reduce the number of populations of poikilothermic animals (snakes, frogs, lizards). The thick layer of ice and the lack of sufficient oxygen under the ice reduce the number of fish populations in winter. Dry summers and autumns followed by frosty winters reduce the populations of the Colorado potato beetle. Uncontrolled shooting of animals or trapping of fish reduces the regenerative capacity of their populations. High concentrations of pollutants in the environment have a negative impact on the abundance of all sensitive species.

The capacity of the environment (the maximum population size) is determined by the ability of the environment to provide the population with the necessary resources: food, shelter, individuals of the opposite sex, etc. When the population size approaches the capacity of the environment, there is a shortage of food due to its increased eating. And then the mechanism of population size regulation through intraspecific competition for a resource is put into action. If the population density is high, it is regulated by an increase in mortality as a result of increased competition. Some individuals die either due to lack of food (herbivores), or as a result of biological or chemical warfare. An increase in mortality leads to a decrease in density. If the population density is low, it is replenished by increasing the birth rate due to the renewal of food resources and the weakening of competition.

Biological warfare is the killing of competitors within a population by direct attack (predators of the same species). A sharp decrease in food resources can lead to cannibalism (eating their own kind). Chemical warfare is the release of chemicals that retard growth and development or kill young individuals (plants, aquatic animals). The manifestation of chemical warfare can be observed in the development of tadpoles. At high density, larger tadpoles release substances into the water that inhibit the growth of small individuals. Therefore, only large tadpoles complete their development. After that, small tadpoles begin to grow.

The regulation of population size through the amount of food resources is clearly seen in the example of the interaction between predator and prey populations. They have a mutual influence on the abundance and density of each other, causing repeated ups and downs in the numbers of both populations. Moreover, in this system of fluctuations, the increase in the number of predators lag behind the increase in the number of prey in phase.

An important mechanism for the regulation of numbers in overcrowded populations is the stress response. An increase in population density leads to an increase in the frequency of meetings between individuals, which causes such physiological changes in them that lead either to a decrease in the birth rate or to an increase in mortality, which is the reason for the decrease in the population size. Stress does not cause irreversible changes in the body, but only leads to temporary blocking of some body functions. With the elimination of overpopulation, the ability to reproduce is quickly restored.

All population density-dependent mechanisms of population regulation are switched on before the environmental resources are completely exhausted. Due to this, self-regulation of numbers is carried out in populations.

2. What examples of cyclic fluctuations in population size do you know?

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

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

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

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

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

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

"There are certain upper and lower limits on average population sizes that are respected in nature or that theoretically could exist for an arbitrarily long period of time."

Example. In migratory locusts, at low numbers, the larvae of the solitary phase are bright green in color, and the adults are gray-green in color. During the years of mass reproduction, the locust passes into a staged phase. The larvae acquire a bright yellow color with black spots, while adults become lemon yellow. The morphology of individuals also changes.

Questions after § 80

1. What is population dynamics?

Answer. Population dynamics are the processes of changes in its main biological indicators over time. The main importance in the study of population dynamics is given to changes in abundance, biomass, and population structure. Population dynamics is one of the most significant biological and ecological phenomena. We can say that the life of a population is manifested in its dynamics.

A population cannot exist without constant changes, due to which it adapts to changing living conditions. Indicators such as fertility, mortality and age structure are very important, but none of them can be used to judge the dynamics of the population as a whole.

An important process of population dynamics is population growth (or simply “population growth”), which occurs when organisms settle in new habitats or after a catastrophe. The nature of growth is different. In populations with a simple age structure, growth is rapid and explosive. In populations with a complex age structure, it is smooth, gradually slowing down. In any case, the population density increases until factors limiting the growth of the population begin to act (the restriction may be associated with the full use of the resources consumed by the population or with other types of restrictions). In the end, a balance is reached, which is maintained.

2. What is the phenomenon of population regulation? What is its significance in the ecosystem?

Answer. When the growth of the population is completed, its numbers begin to fluctuate around some more or less constant value. Often these fluctuations are caused by seasonal or annual changes in living conditions (for example, changes in temperature, humidity, food supply). Sometimes they can be seen as random.

In some populations, population fluctuations are of a regular cyclic nature.

The best-known examples of cyclical fluctuations include fluctuations in the abundance of certain mammalian species. For example, cycles of three- and four-year periodicity are characteristic of many mouse-like rodents (mice, voles, lemmings) and their predators (polar owl, arctic foxes).

The best-known example of cyclic fluctuations in insect populations is the periodic outbreaks in acridoids. Information about the invasion of the wandering locust dates back to ancient times. Locusts live in deserts and dry areas. For many years, it does not migrate, does not harm crops and does not attract much attention to itself. However, from time to time the density of locust populations reaches monstrous proportions. Under the influence of crowding, insects undergo a series of changes in their appearance (for example, they develop longer wings) and begin to fly to agricultural areas, eating everything in their path. The reasons for such population explosions are apparently due to the instability of environmental conditions.

3. What role do abiotic and biotic factors play in changing population density?

Answer. The reasons for sharp fluctuations in the number of populations of some organisms can be various abiotic and biotic factors. Sometimes these fluctuations are in good agreement with changes in climatic conditions. However, in some cases, it is impossible to explain changes in the size of a particular population by the influence of external factors. The causes that cause population fluctuations may lie in themselves; then one speaks of internal factors of population dynamics.

Cases are known when, under conditions of overpopulation, a number of mammals undergo sharp changes in their physiological state. Such changes primarily affect the organs of the neuroendocrine system, affecting the behavior of animals, changing their resistance to diseases and various types of stress.

Sometimes this leads to increased mortality of individuals and a decrease in population density. White hares, for example, during periods of peak numbers often die suddenly from the so-called "shock disease".

Such mechanisms can undoubtedly be classified as internal population regulators. They are triggered automatically as soon as the density exceeds a certain threshold value.

In general, all factors influencing the size of the population (it does not matter whether they limit or favor the reproduction of the population) are divided into two large groups:

– population density independent;

- dependent on population density.

The second group of factors is often called regulatory or density control.

One should not think that the presence of regulatory mechanisms should always stabilize the population. In some cases, their action can lead to cyclical fluctuations in numbers even under constant living conditions.

Tell us about the seasonal changes in the populations of animals and plants that you know (recall personal observations).

Answer. In many species of animals and plants, population fluctuations are caused by seasonal changes in living conditions (temperature, humidity, light, food supply, etc.). Examples of seasonal fluctuations in the number of populations are demonstrated - flocks of mosquitoes, migratory birds, annual grasses - in the warm season, in winter, these phenomena are practically reduced to nothing.

Of greatest interest are the fluctuations in the number of populations that occur from year to year. They are called interannual as opposed to intra-annual, or seasonal. The interannual population dynamics can be of a different nature and manifest itself in the form of smooth waves of changes (number, biomass, population structure) or in the form of frequent abrupt changes.

In both cases, these changes can be regular, that is, cyclic, or irregular - chaotic. The former, unlike the latter, contain elements that repeat at regular intervals (for example, every 10 years the population reaches a certain maximum value).

Observed from year to year fluctuations in the number of certain species of birds (for example, the city sparrow) or fish (bleak, vendace, gobies, etc.) give an example of irregular changes in the size of the population, usually associated with changes in climatic conditions or changes in environmental pollution living with substances that have a detrimental effect on organisms.

Observations of population fluctuations in the city of the great tit are interesting. Its number in the city in winter increases 10 times compared to summer.

Using additional literature, give examples of cyclic fluctuations in the number of animals or plants.

Answer. For natural populations, there are:

1) seasonal changes in numbers associated with seasonal changes in environmental factors,

2) fluctuations that are caused by weather changes. Seasonal changes in abundance are most pronounced in many insects, as well as in most annual plants.

Examples of significant population fluctuations are demonstrated by some species of northern mammals and birds, which have either 9-10- or 3-4-year cycles. A classic example of a 9- to 10-year fluctuation is the change in the abundance of hare and lynx in Canada, with peaks in hare abundance a year or more preceding peaks in lynx abundance.

To assess the dynamic state of plant populations, an analysis of age (ontogenetic) states is carried out. The most easily defined sign of a stable state of a population is a full-fledged ontogenetic spectrum. Such spectra are called basic (characteristic), they determine the definitive (dynamically stable) state of populations.

The most well-known examples of cyclic fluctuations include joint fluctuations in the abundance of some species of northern mammals. For example, cycles of three- and four-year periodicity are characteristic of many northern murine rodents (mice, voles, lemmings) and their predators (polar owl, arctic fox), as well as hares and lynxes.

In Europe, lemmings sometimes reach such high densities that they begin to migrate out of their overcrowded habitats. In both lemmings and locusts, not every case of an increase in numbers is accompanied by migration.

Sometimes cyclic fluctuations in population size can be explained by complex interactions between populations of different animal and plant species in communities.

Consider, as an example, fluctuations in the abundance of certain insect species in European forests, such as the pine moth and larch moth, whose larvae feed on tree leaves. The peaks of their numbers are repeated in about 4-10 years.

Fluctuations in the abundance of these species are determined both by the dynamics of tree biomass and fluctuations in the abundance of insect-eating birds. As the biomass of trees in the forest increases, the largest and oldest trees become susceptible to budworm caterpillars and often die from repeated defoliation (loss of leaves).

The dying off and decomposition of wood returns nutrients to the forest soil. They are used for their development by young trees that are less sensitive to attack by insects. The growth of young trees is also facilitated by an increase in illumination due to the death of old trees with large crowns. In the meantime, the birds are reducing the number of budworms. However, as a result of the growth of trees, it (number) again begins to increase and the process repeats.

If we consider the existence of coniferous forests over long periods of time, it becomes clear that the leaf roller periodically rejuvenates the ecosystem of the coniferous forest, and is an integral part of it. Therefore, the increase in the number of this butterfly does not represent a catastrophe, as it may seem to anyone who sees dead and dying trees at a certain stage of the cycle.

The reasons for sharp fluctuations in the number of some populations can be various abiotic and biotic factors. Sometimes these fluctuations are in good agreement with changes in climatic conditions. However, in some cases, it is impossible to explain changes in the size of a particular population by the influence of external factors. The causes that cause population fluctuations may lie in themselves; then one speaks of internal factors of population dynamics

fluctuations in the number of organisms.
ENVIRONMENTAL REGULATION

Tasks : to acquaint with the ecological characteristics of the population, to identify regulatory mechanisms.

Content elements: population dynamics, birth rate, mortality, regulatory mechanisms, cyclic fluctuations in numbers.

Lesson type: combined.

Equipment: tables showing the population structure of the species, cyclic fluctuations in the number of species.

During the classes

I. Organizational moment.

II. Checking students' knowledge.

Biological dictation.

1. Competition is a relationship between ...

2. A symbiotic relationship is established between...

3. The stomach and intestines of ruminant mammals are constantly inhabited by bacteria that cause fermentation. This is an example…

4. An example of competition is the relationship between ...

5. Layering is an example of such interspecies relationships as ...

6. If both species benefit from the interaction, then this is an example ...

7. If individuals of one species eat individuals of another species, this form of relationship illustrates ...

8. What is the name of the form of relationship between nodule bacteria and legumes?

9. Seeds of the series are spread with the help of man. This is
example...

10. What is the name of the form of the relationship between a shark and a fish-stuck-paly?

III. Learning new material.

As you know,population - a group of individuals of the same species interacting with each other and living together in a common area.

Populations are dynamic. They are constantly changing. The mobility and strength of these changes reflectdynamic characteristics . The state of the population is characterized by such indicators as the birth rate, mortality, introduction and eviction of individuals, number, growth rate. This takes into account time.

Population size is the total number of individuals in it. This value is characterized by a wide range of variability, but it cannot be below certain limits. Reducing the population beyond these limits can lead to the extinction of the population.

Density populations is the number of individuals per unit area or volume. With an increase in the population, its density, as a rule, increases; it remains the same only in the case of dispersal of individuals and expansion of the range.

Spatial structure populations are characterized by the peculiarities of the distribution of individuals in the occupied territory and may change over time; it depends on the season of the year, population size, age and sex structure, etc.

Sex structure reflects a certain ratio of males and females in a population. Changes in the sexual structure of a population affect its role in the ecosystem, since males and females of many species differ from each other in their diet, rhythm of life, behavior, etc. The predominance of the proportion of females over males ensures more intensive population growth.

Age structure of the population reflects the ratio of different age groups in populations, depending on life expectancy, the time of onset of puberty, the number of offspring in a litter, the number of offspring per season, etc.

ecological structure populations indicates the attitude of various groups of organisms to environmental conditions.

fertility is the number of young individuals born per day, month or year, andmortality is the number of deaths during the same period.

Regulatory mechanisms – processes that automatically regulate population stability. Necessary when increasing or decreasing the number. Regulatory mechanisms causecyclical population fluctuations , which depend:

џ on the stability of living conditions,

џ life span of the species,

џ amount of food,

џ ability to reproduce,

џ human influence.

Conditions for population stability

IV. Consolidation of the studied material.

Solving environmental problems.

Task1.

The genetic mechanism of sex determination ensures that offspring are split by sex in a ratio of 1: 1. In populations of many animals, the ratio of females to males can deviate markedly from 1: 1. What do you think can cause such deviations? Can they be adaptive?

Task2.

Many animals spend part of the year alone or in pairs, and in some seasons form flocks. Give examples of such animals and analyze what features of their lifestyle this phenomenon is associated with.

Task3.

The two extreme types of distribution of individuals in space are uniform (in which the probability of being near another individual is less than the probability of being at some distance from it) and group (individuals form groups). Under what conditions do these two types of distribution arise? Consider the issue separately for animals and plants, and then draw general conclusions.

Homework: § 9.6 (repeat § 9.1–9.5).

Population size is the total number of individuals of a species present in a given area.

Under favorable conditions in populations, an increase in numbers 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 limiting (limiting) factors oppose this. The set of factors that limit the growth of the population is called environmental resistance. The state of balance between the biotic potential of the species and the resistance of the environment, maintaining the constancy of the population, is called homeostasis or dynamic equilibrium. If it is violated, fluctuations in the population size occur, i.e. her changes.

Distinguish periodic and non-periodic oscillations the number of populations. The former occur over the course of a season or several years (4 years - a periodic cycle of fruiting of cedar, an increase in the number of lemmings, arctic foxes, polar owls; a year later, apple trees bear fruit in garden plots), the latter are outbreaks of mass reproduction of some pests of useful plants, when environmental conditions are violated habitats (droughts, unusually cold or warm winters, too rainy growing seasons), unforeseen migrations to new habitats. Periodic and non-periodic fluctuations in the number of populations 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, sex and age, spatial, etc., but it cannot consist of a smaller number of individuals than is necessary for the stable development and resistance of the population to environmental factors. This is the principle of minimum population size.

However, along with the principle of the minimum size of populations, there is also the principle (rule) of the population maximum. It lies in the fact that the population cannot increase indefinitely. It is only theoretically capable of unlimited growth in numbers.

According to the theory of H.G. Andrevarty - L.K. Birch (1954) - the 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 population growth acceleration. The theory of "limits" is supplemented by the theory of biocenotic regulation of population size by K. Frederiks (1927): population growth is limited by the influence of a complex of abiotic and biotic environmental factors.

fluctuations(deviations) in numbers are caused by a variety of reasons. And they are not always the same for different species. For example, a change in the physical environment that raises or lowers the upper limit on population density or size; intrapopulation interactions; interaction with neighboring populations.

For natural populations, there are: a) seasonal changes, the magnitude of which is regulated by ontogenetic adaptations associated with seasonal changes in environmental factors; b) annual (interannual) changes. They are divided into 2 groups: - fluctuations due to the difference in the physical factors of the environment during the year, i. external (exogenous) factors in relation to populations. They are not regular and show a clear relationship with one or more major limiting physical factors (temperature, salinity of sea water, precipitation, etc.); - fluctuations associated with dynamic changes, i.e. with internal (endogenous) factors. They are often regular, so they can be called cycles.

Sharp oscillations (fluctuations) are characteristic of populations with an exponential increase in abundance, while damped oscillations in population abundance are characterized by a logistic type of growth, in which, due to the characteristics of the life cycle of their constituent species, there is some time delay in the response to an increase in density.

Periodic fluctuations in the number of populations with a 10-11-year period are explained by the periodicity solar activity: The number of sunspots varies with a period of 11 years. The amount of food is the reason for fluctuations in the Siberian silkworm: it flares up 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 not periodic and for their manifestation the following events are necessary: ​​heavy showers, washing away microelements from the land (iron, zinc, cobalt - their concentration should match up to ten thousandth of a percent), low salinity of the bottom, a certain temperature and calm near the coast. Under such conditions, dinoflagellates algae begin to divide intensively. Theoretically, from one single-celled dinoflagellate, as a result of 25 consecutive divisions, 33 million individuals can occur. The water turns red. Dinoflagellates release a deadly poison into the water, causing paralysis and then death of fish and other sea creatures.

Sharp non-periodic population fluctuations can occur as a result of natural disasters. For example, outbreaks of fireweed and the associated insect community are common in conflagrations. Long-term drought turns the swamp into a meadow and causes an increase in the number of members of the meadow biocenosis.

Factors, causing changes in population size is divided into:

Exogenous causes(mainly abiotic factors) changes in population size are rooted in external factors. These include changes in the number of predators, diseases, favorable or unfavorable weather conditions, solar activity.

Endogenous causes (biotic factors) population dynamics are caused by intrapopulation factors such as competition, intrapopulation aggression, and stress. Currently, most scientists believe that the endogenous causes that determine the change in population size are associated with habitat density. The higher the density, the higher the stress level in the population. An increase in stress leads to a suppression of the ability to reproduce, to a decrease in resistance to disease, and to an increase in mortality.