Fluctuations in the number of organisms ecological regulation. Fluctuations in the abundance of organisms Cyclic and non-cyclical fluctuations
lesson type - combined
Methods: partially exploratory, problem presentation, reproductive, explanatory-illustrative.
Target:
Students' awareness of the significance of all the issues discussed, the ability to build their relationship with nature and society based on respect for life, for all living things as a unique and priceless part of the biosphere;
Tasks:
Educational: to show the multiplicity of factors acting on organisms in nature, the relativity of the concept of "harmful and beneficial factors", the diversity of life on planet Earth and the options for adapting living beings to the entire range of environmental conditions.
Developing: develop communication skills, the ability to independently acquire knowledge and stimulate their cognitive activity; the ability to analyze information, highlight the main thing in the studied material.
Educational:
To cultivate a culture of behavior in nature, the qualities of a tolerant person, to instill interest and love for wildlife, to form a stable positive attitude towards every living organism on Earth, to form the ability to see beauty.
Personal: cognitive interest to ecology.. Understanding the need to obtain knowledge about the diversity of biotic relationships in natural communities for conservation natural biocenoses. The ability to choose the target and semantic settings in their actions and deeds in relation to wildlife. The need for fair evaluation of one's own work and the work of classmates
cognitive: the ability to work with various sources of information, convert it from one form to another, compare and analyze information, draw conclusions, prepare messages and presentations.
Regulatory: the ability to organize independently the execution of tasks, evaluate the correctness of the work, reflection of their activities.
Communicative: participate in the dialogue in the classroom; answer questions from a teacher, classmates, speak to an audience using multimedia equipment or other means of demonstration
Planned results
Subject: know - the concepts of "habitat", "ecology", "environmental factors" their influence on living organisms, "connections of living and non-living";. Be able to - define the concept of "biotic factors"; characterize biotic factors, give examples.
Personal: make judgments, search and select information; analyze connections, compare, find an answer to a problematic question
Metasubject: links with such academic disciplines like biology, chemistry, physics, geography. Plan actions with a set goal; find the necessary information in the textbook and reference literature; to carry out the analysis of objects of nature; draw conclusions; formulate your own opinion.
Form of organization learning activities - individual, group
Teaching methods: visual and illustrative, explanatory and illustrative, partially exploratory, independent work with additional literature and textbook, with DER.
Receptions: analysis, synthesis, conclusion, transfer of information from one type to another, generalization.
Learning new material
Population dynamics
The population size is determined mainly by two phenomena - fertility and mortality
In the process of reproduction, the number of individuals in a population increases, theoretically it is capable of unlimited growth in numbers (curve 1 in the figure), however, environmental factors limit this growth, and the real curve (curve 2) of population growth approaches the value of the limiting number. The space enclosed between the theoretical curve and the real one characterizes the resistance of the medium.
The total population size is subject to seasonal, long-term periodic fluctuations in abundance, as well as non-periodic ones (for example, outbreaks of mass reproduction of pests). These changes in numbers are the dynamics of the number of populations.
There are conditional reasons for fluctuations in the number of populations.
In the presence of available food, the population size grows, but at its maximum value, food becomes a limiting factor, and its lack leads to a decrease in the number.
Population ups and downs can occur in the process of competition between several populations for one ecological niche.
Abiotic factors ( temperature regime, humidity, chemical composition of the environment, etc.) have a strong influence on the size of the population and often cause significant fluctuations.
Population density usually has a certain optimum. With any deviation of the number from this optimum, the mechanisms of its intrapopulation regulation come into effect.
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, which sharply reduces the active part of the population. An excessive increase in population density often stimulates cannibalism(from French cannibal - cannibal). A striking example is the phenomenon of eating their own eggs by flour beetles. Cannibalism is observed in some species of fish, amphibians and other animals. Cannibalism is known in more than 1300 animal species.
One of the important mechanisms of intrapopulation regulation of abundance is emigration- eviction, relocation of part of the population to less preferred habitats of the same range. In some species of aphids, an increase in population density is accompanied by the appearance of winged individuals capable of
settle down. During overcrowding, emigration occurs in a number of mammals (especially in murine rodents) and birds.
Drop in population density below the optimal level
(for example, with increased extermination of rats) causes an increase in fertility and stimulates their earlier puberty.
Some mechanisms of population size regulation can simultaneously prevent intraspecific competition. So, if a bird marks its nesting area by singing, then another pair of the same species nests outside it. Marks left by many mammals limit their hunting
site and prevent the introduction of other individuals. All this removes intraspecific competition and prevents excessive population compaction.
As I. I. Shmalgauzen (1884-1963) notes, all biologists
logical systems are characterized by a greater or lesser ability to self-regulate, i.e. Homeostasis is the ability of a living system (including a population) to maintain a stable dynamic balance in changing environmental conditions. Dynamic equilibrium is the fluctuation of the population size within some average value.
The first attempt to identify the mechanisms of homeostasis in wildlife was made by K. Linnaeus
(1760). The generalized concept of homeostasis and the term itself were proposed by W. Kennon (1929).
The homeostatic system is, first of all, each individual, and beyond
the already population.
An important mechanism for regulating abundance is the stress response.
For a person, the phenomenon of stress was first described in 1936 by G. Selye. In response to the negative impact of any factors, two types of reactions occur in the body: specific, depending on the nature of the damaging agent
(for example, an increase in heat production under the influence of cold), and a non-specific stress reaction (stress) as a general effort of the body to adapt to changing conditions in nature, there are many forms of stress:
anthropogenic (occurs in animals under the influence of
human activities);
neuropsychiatric (manifested with incompatibility in-
individuals in a group or as a result of population overcrowding);
thermal, noise, etc.
Questions and tasks
1. What is called the resistance of the environment? What is the ecological meaning of this concept?
2. What are the main reasons for population fluctuations.
3. Give a description of the population as a self-regulating system. What is called population homeostasis?
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 low level species diversity, the number of populations is strongly affected by abiotic and anthropogenic factors. It depends on the weather chemical composition environment and the degree of its pollution. In systems with high level species diversity fluctuations in populations 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 environment adversely affect the abundance of all sensitive species.
The capacity of the environment (maximum population size) is determined by the ability of the environment to provide a population 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 allotment chemical substances 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. So, further crossing of the tiger on 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. It also threatens rare plants(orchid "lady's 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 were registered in animals living in the tundra - lemmings, snowy owls, arctic foxes. Seasonal fluctuations in abundance are also characteristic of many insects, mouse-like rodents, birds, and small aquatic organisms.
"There are certain upper and lower limits for 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 in 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 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 period these phenomena are practically eliminated.
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).
The fluctuations in the number of some species of birds (for example, the city sparrow) or fish (bleak, vendace, gobies, etc.) observed from year to year 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. various kinds animals and plants 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).
Dying and decomposition of wood returns to the forest soil nutrients. 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 leafworm periodically rejuvenates the ecosystem, as it were. coniferous forest, 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
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 populations 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 in 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 from stability living conditions,
џ duration species life,
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).
/ Chapter 9. Organism and environment Task: §9.6. Fluctuations in the number of organisms. Environmental regulation
Answer to Chapter 9. Organism and environment Task: §9.6. Fluctuations in the number of organisms. Environmental regulation
Ready-made homework assignments (GDZ) Biology Pasechnik, Kamensky Grade 9
Biology
Grade 9
Publisher: Bustard
Year: 2007 - 2014
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 change is difficult process, which ensures the stability of populations, the most efficient use of ecological 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, illumination, 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 are regulatory mechanisms? 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.
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 milder form: they are expressed in a decrease in fertility at the level of conditional reflexes (various manifestations of stress reactions) or by secreting substances that retard growth (daphnia, tadpoles - amphibian larvae) and development (often found in fish).
Of interest are the cases of limiting the size of the population by such changes in behavior with increasing density, which ultimately lead to mass migration of individuals.
For example, with an excessive increase in the population of Siberian silkworm butterflies, part of the butterflies (mainly females) fly apart over distances of up to 100 km.
Can lead to population fluctuations?
All life on earth is changing.
Changes underlie the evolution of organisms, the basis for the development of all ecological systems without exception.
Among the most important ecological processes is the dynamics of populations, i.e., changes in the number of their constituent organisms.
Populations could not exist in changing environmental conditions without changing along with them. Population changes are a complex process that ensures the stability of populations, the most efficient use of ecological resources by organisms, and finally, changes in the properties of the organisms according to the changing conditions of their lives.
Let us consider the mechanisms of changes in the number of populations.
Each population of plants or animals can be characterized by the rate of reproduction, or fertility. Fertility is expressed as the number or proportion of individuals (eggs, seeds) born (hatched, laid) in populations per unit of time. Fertility is determined by the properties of both organisms (for example, the fertility of females) and their populations (composition, abundance, etc.).
In any natural population, the number of individuals born always exceeds the number of their parents. This is easy to verify by remembering how many seeds one plant produces or how many cubs are born, for example, a cat, a she-wolf, a starling, a frog or a fish. Due to the birth rate, the population tends to unlimited growth.
However, not all individuals of new broods can survive to adulthood and leave offspring. Some of them are dying. The rate at which organisms die is called mortality. Mortality is expressed as the number or proportion of individuals dying per unit of time. Mortality limits population growth.
Both birth and death rates 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.
The ability to change allows populations to constantly adapt to changing living conditions. For example, the emergence of free resources leads to an increase in the birth rate, an increase in the number and expansion of the territorial boundaries of populations (as is observed when competitive pressure is weakened), and vice versa. Population fluctuations can be caused by seasonal changes in living conditions - temperature, humidity, light.
Sometimes the causes that cause population fluctuations may lie in themselves. This happens when the mortality or birth rate of organisms changes in response to changes in their numbers, more precisely, population density, i.e., the number of individuals per unit area.
Mechanisms of this kind are called regulatory, they work automatically when the population density reaches either too high or too low values.
Regulatory mechanisms may have the character of behavioral or physiological responses of organisms to changes in population density.
There are cases when, in conditions of overpopulation, a number of mammals there are sharp changes in the physiological state, which affects the behavior of animals, reduces their resistance to diseases and other adverse effects.
White hares, for example, during periods of peak numbers often die suddenly from “shock sickness”. In some fish species, at high population densities, adults switch to feeding on their juveniles, as a result of which the population begins to decline. An increase in mortality and a decrease in the birth rate under the influence of high density are observed in populations of many species of animals and plants. In all these cases, the population itself, or rather, its density, gives the signal for the activation of regulatory mechanisms.
The activation of regulatory mechanisms can cause cyclical fluctuations in population size.
An example of cyclic changes is given by 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, snowy owls, arctic foxes, etc.
In the course of evolution different types living organisms acquire different properties. This is reflected in the properties of their populations, in the peculiarities of population fluctuations. Populations of species adapted to existence in stable, albeit harsh conditions (penguins, whales, polar bears), as a rule, are not capable of rapid changes in numbers. Without human intervention, their numbers change smoothly, without sharp peaks or dips. Such a picture of dynamics is typical for organisms with a long development cycle, whose populations include many age groups. In the same water body, for example, the abundance of pike, whose population consists of 25 age groups, changes much more slowly than the abundance of eels, whose population includes only 6 age groups.
Other species living in temperate zones, especially annual animals (most insects) and plants (some types of grasses), are capable of rapid and drastic changes in numbers. These changes are wide-ranging. During the years of minimum and maximum abundance, the number of such species can differ by tens, hundreds, and sometimes thousands of times. These species are characterized by “population explosions. - sharp explosive increases in numbers that occur almost suddenly. This happens when there are especially favorable conditions for the reproduction of organisms. Populations of this type, as a rule, are the first to populate new habitats in communities that are at the early stages of their development.
In mature ecosystems, including many different species of plants, animals and microorganisms, where biotic relationships are developed and there is a strict distribution of used resources, relationships such as competition or predation become the main reason for fluctuations in the number of individual species.
Biotic relationships act as a kind of regulators, they suppress "population explosions", transform chaotic changes into the form of regular periodic fluctuations, and in a number of cases stabilize the number of organisms.
Here we are faced with important properties that ecological systems of different levels of organization (communities, populations, ecosystems) are endowed with:
the functioning of an individual element of the system is determined by its connections with other elements;
individual elements are interchangeable: the loss of one leads to the fact that its functions begin to perform
another element occupying a similar position in the system.
This is another type of regulation.
Communities, as it were, regulate the changes that occur in individual populations. Populations, on the other hand, help the ecosystem to preserve its properties even with the loss of one or another of its elements. With the disappearance of one species, its place is taken by another, similar to the first one in position in the trophic structure of the community.
An example is the usual changes in the species composition of fish in water bodies where fishing is developing. The decrease in the number of the most valuable species due to fishing often leads to an increase in the number of so-called "weedy" fish, which are not of interest to fishermen. Species richness is declining, although total strength fish population remains unchanged.
Microevolutionary processes are closely related to population dynamics. The likelihood of changes in the gene pool (violation of its balance) especially increases when the population size is low. Consequently, in years of low abundance, microevolutionary processes should proceed more actively. If we take into account that the decrease in the number of organisms occurs with sharp changes in external factors, we can understand that at the same moments motive selection begins to intensify. In other words, when faced with changes in living conditions, the population responds to them not only by changes in numbers, but also by changes in the organisms themselves: individuals with only those properties that are useful in given specific conditions remain in the population.
During periods of population growth, the acquired changes are fixed in the population. Stabilizing selection begins to act. This is how organisms adapt to new conditions of life.
Population dynamics. Fertility. Mortality. regulatory mechanisms. Cyclic population fluctuations.
1. What is population dynamics? What factors cause population fluctuations?
2. What is the significance of population dynamics in nature?
3. What are regulatory mechanisms? Give examples.
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology Grade 9
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