Structure and properties of populations. Fundamentals of population ecology
Population structure is usually understood as its spatial, age, sex and genetic composition.
Sometimes they talk about the ecological structure of a population, including in this concept the distribution of individuals in the population by sex and age, often expressed in the form of so-called “life tables”. IN Lately the generalized concept of biochorological is often used population structure(from Greek choros - space).
The spatial structure of the population, i.e. the distribution of individuals within the population, is characterized by unevenness. This unevenness is determined by the heterogeneity of biogeocenotic conditions and determines the existence within the population of smaller spatial groupings of individuals - settlements occupying one or another part of its range; between such settlements there remain unoccupied or sparsely populated spaces.
An example of differences in the spatial structure of populations different types serves as the distribution of individuals of the main background species of plants and animals on a small (area of 100 ha) island. Skolme off the southwest coast of England. Certain plant species (agrostis bentgrass, Pteridium bracken, Armeria) and typical communities (swamp vegetation, hummock grass) form patches of unequal size on the island. Groupings of individual species of birds and mammals stand out even more clearly: 35,000 pairs of adult birds of the small petrel Puffinus puffinus are united in 8 colonies, 900 pairs of black-billed bird Larus fuscus - 10 nesting groups of different sizes. Nesting grounds of the herring gull L. argentatus and puffins Fratercula arctica - about 500 and 5000 pairs, respectively, border the island along the coasts with a broken ribbon of complex configuration.
In terms of the number of rabbits Oryctolagus cuniculus (in autumn up to 100 individuals per 1 ha) Scocolm is the most densely populated area of England, and yet the distribution of its settlements (dense and sparse) is uneven and forms a complex mosaic of patches. House mice living here in the wild are characterized by a combination of “lace” and “spotted” types of distribution in the same territory. Tagging made it possible to find out that, despite the apparent isolation of some groups of individuals, a number of individuals are characterized by movements that cover the entire studied territory.
Not all of the species considered form true populations on the island. There is no doubt that black whales and herring gulls inhabiting the island. Skokolm are only parts of some populations of these large birds. The issue of population independence is not entirely clear for colonies of puffins and petrels - their settlements on the island may be real populations, or only part of the population. The same can be said about the population of plant species if their pollen is effectively distributed over tens or hundreds of kilometers. But the population of rabbits and house mice on the island can rightfully be called populations.
The decision of whether to consider the group of individuals under study as a population or to consider it only a spatially isolated part of the population depends on how large the exchange of individuals is between neighboring groups. When studying a population snapping lizard Lacerta agilis in Western Altai, groups ranging from several individuals to hundreds of thousands of individuals were found.
A population here can rightfully be called a group of individuals of level IV with an exchange of about 0.01% of individuals of each generation and with a duration of existence of hundreds of generations. Before this - the population level - in the species population of the sand lizard, groups of individuals of two lower levels of integration (I and II) are clearly distinguished. They exist for a relatively short time (several generations) and widely exchange individuals, due to which their genetic composition changes sharply and quickly, it is unstable. In addition, the very fragility of such associations determines their ephemeral nature. The third level of integration in a number of cases can probably be recognized as a real population one. This depends in no small part on how strong the local selection pressure will be: with strong selection directed against newcomers, they will not be able to significantly change the genetic structure of the group; with a weak one, after just a few generations the genetic structure of groups exchanging individuals on such a wide scale (about 3-4% in each generation) is leveled out, and they form a single genetic totality.
From the foregoing it is clear that the essential point in determining the independence of a particular group of individuals is its sufficient number and isolation from neighboring similar populations. The measure of isolation can theoretically be the level of transfer of alleles from one gene pool to another, or practically the level of exchange of individuals. In turn, this level depends on the radius of individual activity, i.e., the distance that the average individual actually covers during its life through passive or active movement. In plants, for example, it can be determined by the distance of pollen distribution, the distance from the parents of their vegetative descendants (cuttings, buds, shoots, tumbleweed forms, etc.), seed dispersal (by wind, water, when carried by animals, etc.) . P.). It has been established that the population size of rodents, reptiles and amphibians is relatively small, while that of birds and widely migratory mammals is significant. The main reason for this is considered to be differences in the mobility of individuals of the mentioned groups of animals.
The size of the radius of individual activity is closely related to the nature of the use of the territory. Species that lead a sedentary lifestyle are characterized by an intensive type of space use (I. A. Shilov, 1977), in which individuals or their groups (in higher vertebrates, mainly family ones) use the resources of a relatively limited territory for a long time. The radii of individual activity and the territories of intrapopulation groups in such species are usually small, although they vary depending on the level of abundance. Species characterized by a nomadic lifestyle or other types of migration are characterized by an extensive type of space use, in which the food resources of the area are developed during the regular movements of groups of individuals. The radius of individual activity and the size of the territory of intrapopulation groups in such species are significant. There is no sharp boundary between intensive and extensive methods of using space, since during the nesting period populations often lead a sedentary lifestyle with intensive use of space, and then switch to a migratory lifestyle with an extensive type of use of the territory.
As a result, the spatial structure of populations will differ between species. Prides of lions, harems of eared seals, packs of wolves and jackals, pods of killer whales - these are just a few of a number of spatial-genetic groupings known in mammals.
Several terms have been proposed to designate such small, relatively independent, but short-lived groups of individuals, consisting of intensively interbreeding individuals and their descendants. In recent years, the term dem (from the Greek demos people) has been used in zoological literature to designate such groups. For plants and fungi, the question of the spatial structure of the population has not been developed in as much detail as for vertebrates, which is due to the specifics of their structure and way of life. We can assume that each population has its own spatial structure, which is maintained in a form adaptive for a given place and time. At the same time, the placement of individuals in space represents the “morphological” aspect of the spatial structure of the population, and the system of relationships between them is its functional aspect, which is discussed in more detail below.
One of the common mistakes in population research is obtaining not population characteristics, but characteristics of individual demes or other short-term populations of individuals, i.e. obtaining a non-representative sample. For example, when studying the sand lizard Lacerta agilis on the Kalbinsky ridge, we first encountered a group of individuals of 7 individuals, two of which (29%) were melanistic. However, it would be wrong to consider such a high frequency of melanists as a population characteristic: among the dozens of other individuals subsequently caught, there were no melanists.
The age structure of a population, i.e. the ratio of groups of individuals of different ages, is one of the most important characteristics of a population.
The age of any organism can be expressed in units of astronomical time - minutes, hours, months, years. Age can also be judged by the degree of development of the organism or its physiological maturity, i.e., determine physiological age. Unfortunately, the possibilities of determining the astronomical (calendar) age of individuals in natural populations are quite limited. They come down to the study of recording structures (M.V. Mina, G.A. Klevezal, 1976): layering of tree trunks, bones and teeth in vertebrates, rings or branches on the horns of some ungulates, age-related changes coloring, feather or coat structure, etc.
Determination of physiological age is carried out by changes in body size and shape, color, state of the generative system, maturation of the skeletal system, involution of glands (for example, the thymus), abrasion of the chewing surface of the teeth and other signs.
In the process of postnatal development of an individual, its morphophysiological and other characteristics and properties change; accordingly, age groups and populations differ in the specificity of the same characteristics.
The age structure of populations can be expressed primarily by the ratio of the average durations of the pre-reproductive, reproductive and post-reproductive stages of individual development characteristic of a given population. In more detail, the age structure of the population can be presented in the form of age pyramids, which take into account the relative numbers of each age group.
The age structure of a population reflects such important processes as the intensity of reproduction, the mortality rate, and the rate of population renewal. It is closely related to the genetic characteristics of the generations that form a population and to the characteristics of the specific conditions of development of individual generations.
To accurately describe the age structure of the population, we will give definitions of some terms.
A generation (generation) includes all the offspring of individuals of the previous generation (children - parents). This concept is genetic. The transmission of hereditary material occurs through generations. Taking into account the number and ratio of generations is one of the important conditions for studying the structure of populations. The duration of a generation corresponds to the average reproductive age in a given population of individuals of a certain species and can serve as the basis for establishing the duration of existence of the population and its rank (recall that a population exists for many - hundreds and thousands - generations).
Offspring- simultaneously born individuals from a certain set of parents (which can belong to one or several generations). It ensures an increase in population size within a certain time frame. This is an ecological concept associated with the study of the demographic characteristics of a population: density, size, birth rate.
Age group- a group of individuals of the same age. Classification age groups often conditional (for example, “young”, “adults”, “old”), but it allows us to identify comparable groups of individuals in the studied samples from different populations. The age structure depends on the different combination of individuals of different generations, offspring and age groups in the population. In species that reproduce once in a lifetime (salmon and some other fish, many insects, annual plants), the age structure of populations is very simple, since a generation is both an offspring and an age group. The population then contains a maximum of two generations, two offspring and two age groups (if the parent generation does not die off before the generation of children is formed).
One generation may include more than one offspring and one age group. The simplest example of such a combination can be observed in the population of the shrew Sorex araneus. The parent generation of shrews gives one or two offspring during the spring-summer period and completely dies out by autumn. The daughter generation, which includes two offspring, in the fall consists of still immature individuals that reach maturity in the spring.
More often, a generation consists of individuals from several offspring and age groups (repeatedly reproducing animals and perennial plants). The age structure of the population in this case is very complex, especially since then the formation of one offspring occurs from individuals belonging to different generations. This phenomenon is observed, for example, in many voles that give two or three litters per season: if young individuals from the first litter quickly become involved in reproduction, then the pre-autumn litters can consist of individuals of two generations.
The age structure of long-lived animal species and especially plants is even more complex. It is difficult to imagine a combination of generations and offspring in forest plantations, where representatives of a number of successive generations bear fruit for tens (sometimes hundreds) of years, together producing one offspring each season. In addition, individual seeds can be stored without germinating for tens or hundreds of generations and, once in favorable conditions, can easily be integrated into the gene pool of descendant populations. The following real-life situation turns out to be similar to this: an acorn fallen from a 1000-year-old oak tree can germinate in 20-30 years; pollen from this young oak tree can pollinate the flowers of the parent oak tree, which is 50 generations older than its crossing partner. In these and other similar cases, one age group may include more than one offspring and generation.
Methods for studying the age structure of populations are determined by the purpose of the study. Ecologists study the morphophysiological characteristics of different age groups, age structure as indicators current state populations, identify trends in changes in numbers, etc. With an evolutionary approach, the main attention of the researcher is drawn to identifying the characteristics of generations, through which the transformation of the genotypic and phenotypic structures of the population is carried out.
In close unity with the age structure, the sexual structure of the population should be considered, which also determines the intensity of reproduction, population dynamics and features of the geno- and phenotypic composition of the population.
Sex structure of the population, i.e., the numerical sex ratio, varies extremely among different species of animals and plants, in different populations of the same species, at different times and in different age groups in the same population.
The genetic mechanism ensures a certain primary sex ratio during the formation of zygotes, usually in a 1:1 ratio. Other relationships are also possible. For example, the wood lemming Myopus schisticolor is consistently female dominated due to the presence of XXY females.
Due to the different viability of male and female organisms, the primary sex ratio differs markedly from the secondary sex ratio characteristic of newborns (regardless of the “mode of birth”), and even more from the tertiary sex ratio (at the onset of puberty). In humans, for example, the secondary sex ratio (on average for many populations) is 100 girls per 106 boys; by the age of 16-18 it levels out as a result of increased male mortality, and by the age of 50 it is 85 men per 100 women.
The secondary and tertiary sex ratio varies significantly both among different species of animals and plants, and within species, but there is little accurate data. In recent years, the attention of researchers has been drawn to the variability of the tertiary sex ratio within the species' range and the change in this indicator within one population over time.
Genetic structure of the population, i.e. frequencies of alleles and genotypes, - most important characteristic populations.
To understand the basic genetic processes occurring in a population, it is necessary to remember that it is not traits as such that are transmitted through generations, but certain hereditary structures. Moreover, each genotype determines a certain range of possibilities for the development of a trait depending on environmental conditions. In other words, each genotype is characterized by a certain reaction norm. The implementation of any trait in the ontogenesis of an individual is determined, as a rule, by many genes; on the other hand, a gene usually affects not one, but many traits. The limits of the reaction norm of each genotype are expressed by the set of phenotypes that can develop from this genotype under all environmental conditions that do not lead to death.
This results in an ambiguous correspondence between genotype and phenotype and, therefore, the impossibility of unambiguously judging the genetic structure of a population based on phenotypes.
S.S. Chetverikov and his students showed the enormous genetic heterogeneity of natural populations; It is rare to find genetically identical individuals in a population.
Genetic heterogeneity of populations is maintained both due to newly emerging mutations of various types, and due to the processes of recombination of the genetic diversity already existing in the population under the influence of evolutionary factors - natural selection, migration, random processes, a certain system of crossings.
Genetic heterogeneity allows the population and the species as a whole to use not only newly emerging hereditary changes, but also those that arose a long time ago and exist in the population in a latent form (“mobilization reserve” of variability) to adapt to constantly changing conditions of existence.
A manifestation of genetic heterogeneity and one of the important features of the genetic structure of natural populations is intrapopulation polymorphism, i.e. long-term coexistence in a population of two or more genetically various forms(in such ratios that the frequency of even the rarest form cannot be explained only by repeated mutations).
There are heterozygous and adaptation polymorphisms. The first is established as a result of the greater fitness of heterozygotes, the second - as a result of selection in different environmental conditions of genetically different forms within the population. A classic example of the latter is seasonal changes in the frequency ratios of the red and black forms of the ladybug Adalia bipunctata; the former tolerate cold better in winter, the latter reproduce more intensively in summer.
Despite heterogeneity, any population is united and represents a complex genetic system that is in dynamic equilibrium. From a genetic-evolutionary point of view, a population is a minimal system that can exist for a theoretically unlimited number of generations. It is sometimes said that a population is smallest system, possessing its own evolutionary destiny, which smaller groups of individuals do not have. Therefore, a population is an elementary evolutionary unit.
Populations: structure and dynamics Lecture 7.
Moskaluk T.A.
Bibliography
Stepanovskikh A.S. General ecology: Textbook for universities.
M.: UNITY, 2001. 510 p.
Radkevich V.A. Ecology. Minsk: Higher School, 1998. 159 p.
Bigon M., Harper J., Townsend K. Ecology. Individuals, populations and communities / Transl. from English M.: Mir, 1989. Vol. 2.. Shilov I.A. Ecology. M.:
graduate School
, 2003. 512 p. (LIGHT, cycles)
1. The concept of population. Population types
2. Main characteristics of populations
3. Structure and dynamics of populations
4. The dual nature of population systems
a) evolutionary and functional essence of the population
b) biological inconsistency of population functions (Lotka-Volterra model; law of emergence)
5. Fluctuations in numbers
6. Ecological strategies of populations(populus - from Latin people. population) is one of the central concepts in biology and denotes a collection of individuals of the same species that has a common gene pool and a common territory. It is the first supraorganismal biological system. From an ecological perspective, a clear definition of a population has not yet been developed. The interpretation of S.S. has received the greatest recognition.
Schwartz, a population is a grouping of individuals, which is a form of existence of a species and is capable of independently developing indefinitely.
The main property of populations, like other biological systems, is that they are in continuous movement and constantly changing. This is reflected in all parameters: productivity, stability, structure, distribution in space. Populations are characterized by specific genetic and environmental characteristics that reflect the ability of systems to maintain existence in constantly changing conditions: growth, development, stability. The science that combines genetic, ecological, and evolutionary approaches to the study of populations is known as population biology.
EXAMPLES. One of several schools of fish of the same species in the lake; microgroups of Keiske lily of the valley in white birch forests, growing at the bases of trees and in open places; clumps of trees of the same species (Mongolian oak, larch, etc.), separated by meadows, clumps of other trees or shrubs, or swamps.
Ecological population –
a set of elementary populations, intraspecific groups, confined to specific biocenoses. Plants of the same species in a cenosis are called a cenopopulation. The exchange of genetic information between them occurs quite often.– a set of ecological populations inhabiting geographically similar areas. Geographic populations exist autonomously, their habitats are relatively isolated, gene exchange occurs rarely - in animals and birds - during migration, in plants - during the spread of pollen, seeds and fruits. At this level, the formation of geographical races and varieties occurs, and subspecies are distinguished.
EXAMPLES.
The geographical races of Dahurian larch (Larix dahurica) are known: western (west of the Lena (L. dahurica ssp. dahurica) and eastern (east of the Lena, distinguished in L. dahurica ssp. cajanderi), northern and southern races of the Kuril larch. Similarly M.A. Shemberg (1986) identified two subspecies of stone birch: Erman birch (Betula ermanii) and woolly birch (B. lanata). 1000 km, to the north - 500 km. Zoologists distinguish tundra and steppe populations of the narrow-skulled vole (Microtis gregalis). The species "common squirrel" has about 20 geographical populations, or subspecies.
2. Main characteristics of populations Number and density are the main parameters of a population. Number – the total number of individuals in a given territory or in a given volume. Density
– the number of individuals or their biomass per unit area or volume. In nature, there are constant fluctuations in numbers and density. Population dynamics
and density is determined mainly by fertility, mortality and migration processes. These are indicators that characterize population changes during a certain period: month, season, year, etc. The study of these processes and the causes that determine them is very important for forecasting the state of populations. Fertility is distinguished between absolute and specific. Absolute fertility is the number of new individuals appearing per unit of time, and specific
- the same quantity, but assigned to a certain number of individuals. For example, an indicator of a person's fertility is the number of children born per 1000 people during the year. Fertility is determined by many factors: environmental conditions, the availability of food, the biology of the species (the rate of sexual maturation, the number of generations during the season, the ratio of males and females in the population). According to the rule of maximum fertility (reproduction) in the maximum possible number of new individuals appears in populations;
Fertility is limited by the physiological characteristics of the species.
EXAMPLE. A dandelion can cover the entire globe in 10 years, provided that all its seeds germinate. Willows, poplars, birches, aspens, and most weeds produce exceptionally abundant seeds. Bacteria divide every 20 minutes and within 36 hours can cover the entire planet in a continuous layer. Fertility is very high in most insect species and low in predators and large mammals. Mortality, like birth rate, it can be absolute (the number of individuals that died during certain time), and specific.
It characterizes the rate of population decline from death due to disease, old age, predators, lack of food, and plays
main role
in population dynamics.
There are three types of mortality:
Same at all stages of development; occurs rarely, under optimal conditions; Increased mortality at an early age; characteristic of most species of plants and animals (in trees, less than 1% of seedlings survive to maturity, in fish - 1-2% of fry, in insects - less than 0.5% of larvae); High death in old age; usually observed in animals whose larval stages take place in favorable, little-changing conditions: soil, wood, living organisms.
Stable, growing and declining populations. The population adapts to changing environmental conditions by updating and replacing individuals, i.e. processes of birth (renewal) and decline (death), supplemented by migration processes. In a stable population, the birth and death rates are close and balanced. They may not be constant, but the population density differs slightly from some average size, domestic cats in separate families). When plants become over-densified (usually coincides with the beginning of the closeness of the cover, crown canopy), differentiation of individuals begins in size and state of life, self-thinning of populations, and in animals (usually coinciding with the achievement of sexual maturity of young animals) migration to adjacent free areas begins.
If the mortality rate exceeds the birth rate, then such a population is considered to be declining.
In the natural environment, it decreases to a certain limit, and then the birth rate (fertility) increases again and the population goes from declining to growing. Most often, populations of undesirable species are growing uncontrollably, while populations of rare, relict, and valuable species are declining, both economically and aesthetically.
3. Structure and dynamics of populations
The dynamics, condition and reproduction of populations are consistent with their age and sex structure. The age structure reflects the rate of population renewal and the interaction of age groups with the external environment. It depends on the characteristics of the life cycle, which differ significantly among different species (for example, birds and mammalian predators), and external conditions. In the life cycle of individuals there are usually three: age periods pre-reproductive, reproductive and post-reproductive
.
Plants are also characterized by a period of primary dormancy, which they go through in the stage of feeding seeds. Each period can be represented by one (simple structure) or several (complex structure) age stages. Annual plants and many insects have a simple age structure. A complex structure is typical for tree populations of different ages and for highly organized animals.
The more complex the structure, the higher the adaptive capabilities of the population.
One of the most famous classifications of animals by age is G.A. Novikova:
Newborns - until the moment of sight;
Young – growing individuals, “teenagers”;
Sub-adults – close to sexually mature individuals;
Adults are sexually mature animals;
Old are individuals that have stopped reproducing. In geobotany, N.M.’s classification of plants by age has gained recognition. Chernova, A.M. Bylovoy: Dormant seeds;
Seedlings (shoots) are plants of the first year of life, many of them live off
Immature - have transitional characteristics from juvenile to adult plants, they are still very small, they have a change in the type of growth, branching of shoots begins;
Virginile – " adult teenagers", can reach the size of adults, but there are no regenerative organs;
Young generative - characterized by the presence of generative organs, the formation of the appearance typical of an adult plant is completed;
Middle-aged generative - characterized by maximum annual growth and maximum reproduction;
Old generative - plants continue to bear fruit, but their shoot growth and root formation completely stop;
Subsenile - bear fruit very weakly, vegetative organs die off, new shoots are formed due to dormant buds;
Senile - very old, decrepit individuals, features of juvenile plants appear: large single leaves, shoots.
A cenopopulation in which all of the listed stages are represented is called normal, complete.
In forestry and taxation, the classification of tree stands and plantings by age classes is accepted. For conifers:
Seedlings and self-seeding – 1-10 years, height up to 25 cm;
Young growth stage – 10-40 years, height from 25 to 5 m; under the forest canopy corresponds to small (up to 0.7 m), medium (0.7-1.5 m) and large-sized (>1.5 m) undergrowth;
Perch stage – middle-aged plantings 50-60 years old; trunk diameters from 5 to 10 cm, height – up to 6-8 m; under the forest canopy there is a young generation of the tree stand, or a thin tree with similar dimensions;
Maturing plantings – 80-100 years;
they may be slightly smaller in size than the mother tree; they bear fruit abundantly in open areas and in open forests; in the forest they may still be in the second tier, but do not bear fruit; under no circumstances are they assigned to the wheelhouse;
Mature forest stands - 120 years and older, trees of the first tier and stunted trees of the second tier; bear fruit abundantly, at the beginning of this stage they reach technical ripeness, at the end - biological;
Overmature - over 180 years old, continue to bear fruit abundantly, but gradually become decrepit and dry out or fall out while still alive. For deciduous species, the gradations and supports are similar in size, but due to their more rapid growth
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and by aging their age class is not 20, but 10 years. characterize its ability to reproduce and survive, and is consistent with fertility and mortality rates. In growing populations with a high birth rate, young (Fig. 2), not yet reproductive individuals predominate; in stable ones, these are usually multi-age, full-fledged populations in which a certain number of individuals regularly move from younger to older age groups; the birth rate is equal to the population decline. |
, dependence on natural conditions.
The study of the sexual structure of populations is very important, since both ecological and behavioral differences are strongly expressed between individuals of different sexes. EXAMPLE.
There are species in which sex is initially determined not by genetic, but by environmental factors, as, for example, in Arizema japonica, when a mass of tubers is formed, female inflorescences are formed on plants with large fleshy tubers, and male inflorescences are formed on plants with small ones. The role of environmental factors in the formation of the sexual structure in species with alternating sexual and parthenogenetic generations is clearly visible. At the optimal temperature in daphnia (Daphnia magna), the population is formed by parthenogenetic females, and when deviating from it, males also appear.
The spatial distribution of individuals in populations is random, group and uniform.
Random (diffuse) distribution – uneven, observed in a homogeneous environment;
relationships between individuals are weakly expressed. Random distribution is characteristic of populations in the initial period of settlement; plant populations experiencing severe oppression by community edifiers; populations of animals in which social communication is weakly expressed.
EXAMPLES.
relationships between individuals are weakly expressed. Random distribution is characteristic of populations in the initial period of settlement; plant populations experiencing severe oppression by community edifiers; At the initial stages of settlement and establishment - insect pests on the field; seedlings of expansive (pioneer) species: willow, choicenia, larch, lespedeza, etc., in disturbed areas (mountain ranges, quarries);
Group distribution is the most common; reflects the heterogeneity of living conditions or different ontogenetic (age) patterns of the population.
It ensures the greatest stability of the population.
This can be done using simple mathematical processing of accounting data. A plot or trial area is divided into counting plots of the same size - at least 25, or plant counts are carried out on counting plots of the same size located at approximately the same distance. The set of sites represents a sample.
By denoting the average number of individuals of a species on sites in a sample by the letter m, the number of sites (counts) in a sample by n, the actual number of individuals of a species on each site by x, we can determine the dispersion, or measure of dispersion s2 (deviation of the value of x from m):
s2 = S(m-x)2 /(n-1)
With a random distribution s2=m (provided there is a sufficient sample size). With a uniform distribution, s2=0, and the number of individuals on each site should be equal to the average. With a group distribution, s2>m is always, and the greater the difference between the deviation and the average number, the more pronounced the group distribution of individuals.
4. The dual nature of population systems
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a) evolutionary and functional essence of the population Attention should be paid to the dual position of the population in the ranks of biological systems belonging to different levels of organization of living matter (Fig. 4). On the one hand, the population is one of the links in the genetic-evolutionary series, reflecting the phylogenetic relationships of taxa different levels , as a result of the evolution of life forms: organism - population - species - genus - ... - kingdom In this series, the population acts as a form of existence of a species, the main function of which is survival and reproduction. Playing an important role in the microevolutionary process, a population is the elementary genetic unit of a species. Individuals in a population have characteristic structural features |
, physiology and behavior, i.e.
heterogeneity. These features are developed under the influence of living conditions and are the result of microevolution occurring in a particular population. Changes in populations in the process of adaptation to changing environmental factors and the consolidation of these changes in the gene pool ultimately determine the evolution of the species.
b) biological inconsistency of population functions
The “duality” of populations is also manifested in the biological inconsistency of their functions. They are composed of individuals of the same species, and, therefore, have the same ecological requirements for environmental conditions, and have the same adaptation mechanisms.
But the populations themselves contain:
1) high probability of intense intraspecific competition
2) the possibility of a lack of stable contacts and relationships between individuals. Intense competition occurs during overpopulation, leading to depletion of life-sustaining resources: food for animals, moisture, fertility and (or) light for plants. If the number of individuals is too small, the population loses properties of the system
, its stability decreases. Resolving this contradiction is the main condition for maintaining the integrity of the system. It lies in the need to maintain optimal numbers and optimal relationships between intrapopulation processes of differentiation and integration.
Lotka–Volterra model. As an example of the natural regulation of the process of intraspecific competition, we can cite the Lotka–Volterra rule, which reflects the relationships in the food chain of consumers and producers, or predator and prey. It is represented by two equations. The first expresses the success of encounters between prey and predator:
Fertility naturally depends on the efficiency (f) with which food is passed on to offspring, and on the rate of food consumption (a × C" × N).
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In Fig. Figure 5 shows the graphical Lotka–Volterra model. It allows us to show the main trend in the predator-prey relationship, which is that fluctuations in the population size of the predator are consistent with fluctuations in the population size of the prey. At the same time, the cycles of increase and decrease in the numbers of predators and prey are shifted in relation to each other. When the number of prey (food resource) is large, the number of predators increases, but not indefinitely, but until there is a tension with food. A decrease in food supplies leads to increased intraspecific competition and a decrease in the number of predators, and this, in turn, again leads to an increase in the number of prey. |
Law of emergence. As an integral system, a population can be stable only with close contacts and interaction of individuals with each other. Only in a herd can artiodactyls resist predators. Only in a pack do wolves hunt successfully. In forest communities, as a rule, young trees grow better in biogroups ( group effect), forest restoration in disturbed areas is better with abundant seeding and the rapid emergence of tree seedlings. Animals live in herds, birds and fish live in flocks.
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At the same time, the population, as a system, acquires new properties that are not equivalent to a simple sum of similar properties of individuals in the population. For example, when daphnia, the food of perch, gather in a group, the group forms a protective biofield (Fig. 5), thanks to which the fish do not “notice” the food. One daphnia does not have such a biofield, and it quickly becomes prey for fish. The same pattern manifests itself when populations are combined into a biocenosis system - the biocenosis receives properties that none of its blocks have separately. This law, the law of emergence, was formulated by N.F. Reimers. |
5. Fluctuations in numbers
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At favorable conditions in populations there is 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 opposed by limiting (limiting) factors. The set of factors limiting population growth is called environmental resistance. The state of equilibrium between the biotic potential of a species and the resistance of the environment (Fig. 6), which maintains the constancy of the population size, is called homeostasis or dynamic equilibrium. When it is violated, fluctuations in the population size occur, that is, changes in it. |
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 on garden plots), the second are outbreaks of mass reproduction of some pests of beneficial plants, due to 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 a biotic factors environments common to 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 ones are undesirable, but if excessive high values
relationships between individuals are weakly expressed. Random distribution is characteristic of populations in the initial period of settlement; plant populations experiencing severe oppression by community edifiers; they 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. Very many species in the Far East are characterized by minimal population sizes: Amur tiger,, polar bear, mandarin duck, many butterflies: Maka's tail-bearer and Ksuta's tail-bearer, admiral, zephyrs, beauty Artemis, Apollo, relict longhorned beetle, stag beetle; from plants: all araliaceae, orchids, whole-leaved fir, dense-flowered pine, Manchurian apricot, hard juniper, pointed yew, two-row lilies, calloused lilies, Daurian lilies, etc., Ussuri fritillary, Kamchatka trillium and many other species.
However, along with the principle of minimum population size, there is also the principle, or 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 limits, the number of natural populations is limited by the depletion of food resources and breeding conditions, the unavailability of these resources, too
short period accelerating 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.?
What are these factors or
reasons for population fluctuations
Sufficient food supplies and food shortages;
Competition between several populations for one ecological niche;
External (abiotic) environmental conditions: hydrothermal regime, illumination, acidity, aeration, etc.
6. Ecological strategies of populations
Whatever the adaptations of individuals to living together in a population, whatever the adaptations of the population to certain factors, all of them are ultimately aimed at long-term survival and continuation of oneself in any conditions of existence.– it is possessed by rapidly reproducing species (r-species); it is characterized by selection for increased population growth rates during periods of low density. It is typical for populations in environments with sudden and unpredictable changes in conditions or in ephemeral, i.e. existing a short time(drying puddles, water meadows, temporary watercourses)
The main characteristics of r-species: high fertility, short regeneration time, high numbers, usually small sizes of individuals (plants have small seeds), small life expectancy, large expenditures of energy on reproduction, short-lived habitats, low competitiveness. R-types quickly and in large quantities inhabit unoccupied territories, but, as a rule, quickly - within the life of one or two generations - they are replaced by K-species.
r-species include bacteria, all annual plants (weeds) and insect pests (aphids, leaf beetles, stem pests, gregarious locusts). Among perennials - pioneer species: Ivan-tea, many cereals, wormwood, ephemeral plants, from tree species– willows, white and stone birches, aspen, choiceniya, and conifers – larch;
They appear first on disturbed lands: burnt areas, mountain ranges, construction quarries, and along roadsides. K-strategy –
this strategy is possessed by species with a low reproduction rate and high survival rate (K-species); it determines selection for increased survival at high population densities approaching the limit.
The main characteristics of K-species: low fertility, significant life expectancy, large sizes of individuals and seeds, powerful root systems, high competitiveness, stability in the occupied territory, high specialization of lifestyle. The reproduction rate of K-species decreases as the maximum population density approaches and increases rapidly at low densities; parents take care of their offspring. K-species often become dominant in biogeocenoses.
K-species include all predators, humans, relict insects (large tropical butterflies, including Far Eastern butterflies, relict longhorned beetle, stag beetle, ground beetles, etc.), a single phase of locusts, almost all trees and shrubs. The most striking representatives of plants are all conifers, Mongolian oak, Manchurian walnut, hazels, maples, forbs, and sedges.
relationships between individuals are weakly expressed. Random distribution is characteristic of populations in the initial period of settlement; plant populations experiencing severe oppression by community edifiers; In the forests on the ecological profile "Mountain Taiga" in the spring, before the leaves bloom on the trees, ephemeroids rush to bloom, bear fruit and finish the growing season: corydalis, Adonis Amur, anemone, oriental violet (yellow).
Under the forest canopy, peonies, lilies, and crowberry begin to bloom.
In open areas in the dry oak forests of the southern slope, sheep fescue and roseate grass grow. Oak, fescue and other species are K-strategists, marianberry is r-strategist. 40 years ago, after a fire, parcels of aspen (r-species) formed in the fir-broadleaved forest type. Currently, aspen is leaving the forest stand, being replaced by K-species: linden, oak, hornbeam, walnut, etc.
Any population of plants, animals and microorganisms is a perfect living system, capable of self-regulation and restoration of its dynamic balance. But it does not exist in isolation, but together with populations of other species, forming biocenoses. Therefore, interpopulation mechanisms that regulate relationships between populations of different species are also widespread in nature. The regulator of these relationships is a biogeocenosis consisting of many populations of different species. In each of these populations, interactions occur between individuals, and each population has an impact on other populations and on the biogeocenosis as a whole, just as the biogeocenosis with its constituent populations has a direct impact on each specific population.. ... For a biogeocenosis, this means establishing and maintaining its heterogeneous composition and the established relationships between components. When the conditions of existence change, the stationary state is, of course, disrupted.
There is a re-evaluation of the norm and options, and, consequently, a new transformation, i.e. further self-development of these systems...” At the same time, the relationships between links in the biogeocenosis change, and in populations there is a restructuring of the genetic structure. Return to ARTICLES BY STAFF OF BSI FEB RAS Return TO HOME PAGE site Botanical Garden
FEB RAS Population
in ecology they call a group of individuals of the same species interacting with each other and jointly inhabiting a common territory. Members of the same population have no less impact on each other than physical environmental factors or other species of organisms living together. In populations, all forms of connections characteristic of interspecific relationships are manifested to one degree or another, but the most pronounced are mutualistic (mutually beneficial) and competitive. Specific intraspecific relationships
- these are relationships associated with reproduction: between individuals of different sexes and between parent and daughter generations.
During sexual reproduction, the exchange of genes transforms the population into a relatively integral genetic system. If cross-fertilization is absent and vegetative, parthenogenetic, or other modes of reproduction predominate, genetic connections are weaker and the population is a system of clones, or pure lines, sharing the environment. Such populations are united mainly by ecological connections. In all cases, populations have laws that allow limited environmental resources to be used in this way to ensure the preservation of offspring. This is achieved mainly through quantitative changes in the population. Populations of many species have properties that allow them to regulate their numbers. Maintaining optimal numbers under given conditions is called population homeostasis. The homeostatic capabilities of populations are expressed differently in various types
. They are also carried out through the relationships of individuals.
Thus, populations, as group associations, have a number of specific properties that are not inherent in each individual individual.
1) Main characteristics of populations: number
2) – the total number of individuals in the allocated territory; population - the average number of individuals per unit area or volume of space occupied by a population; population density can also be expressed in terms of the mass of population members per unit of space;
3) birth rate– the number of new individuals that appeared per unit of time as a result of reproduction;
4) mortality - an indicator reflecting the number of individuals who died in a population over a certain period of time;
5) population growth– the difference between fertility and mortality; the increase can be both positive and negative;
6) growth rate - average increase per unit of time.
A population is characterized by a certain organization. The distribution of individuals across the territory, the ratio of groups by sex, age, morphological, physiological, behavioral and genetic characteristics reflect population structure. It is formed, on the one hand, on the basis of the general biological properties of the species, and on the other, under the influence of abiotic environmental factors and populations of other species. The structure of populations therefore has an adaptive character. Different populations of the same species have both similar structural features and distinctive ones that characterize the specific environmental conditions in their habitats.
8.2. Population structure of the species
Each species, occupying a specific territory (area), represented on it by a system of populations. The more complex the territory occupied by a species is, the greater the opportunities for the isolation of individual populations. However, to no lesser extent the population structure of a species is determined by its biological characteristics, such as the mobility of its constituent individuals, the degree of their attachment to the territory, and the ability to overcome natural barriers.
8.2.1. Degree of isolation of populations
If members of a species are constantly moving and mixing over large areas, the species is characterized by a small number of large populations. For example, reindeer and arctic foxes have great migratory abilities. Tagging results show that Arctic foxes move hundreds and sometimes more than a thousand kilometers from breeding sites during the season. Reindeer make regular seasonal migrations also on a scale of hundreds of kilometers. The boundaries between populations of such species usually pass along large geographical barriers: wide rivers, straits, mountain ranges, etc. In some cases, a mobile species with a relatively small range can be represented by a single population, for example, the Caucasian tur, whose herds constantly roam throughout two main ridges of this mountain range.
When weak developed abilities Before migration, many small populations are formed within the species, reflecting the mosaic nature of the landscape. In plants and sedentary animals, the number of populations is directly dependent on the degree of heterogeneity of the environment. For example, in mountainous areas, territorial differentiation of such species is always more complex than in flat open spaces. An example of a species in which the multiplicity of populations is determined not so much by environmental differentiation as by behavioral features is Brown bear. Bears are distinguished by their great attachment to their habitats, therefore, within their vast range, they are represented by many relatively small groups that differ from each other in a number of properties.
The degree of isolation of neighboring populations of the species varies greatly. In some cases, they are sharply separated by territory unsuitable for habitation and are clearly localized in space, for example, populations of perch and tench in lakes isolated from each other or populations of the plate-toothed rat, white-whiskered warbler, Indian warbler and other species in oases and river valleys among deserts.
The opposite option is the complete settlement of vast territories by the species. This distribution pattern is typical, for example, of small ground squirrels in dry steppes and semi-deserts. In these landscapes, their population densities are universally high. Some unsuitable areas for life are easily overcome when young animals are resettled, and in favorable years temporary settlements appear on them. Here, boundaries between populations can be distinguished only conditionally, between areas with different population densities.
An example of a continuous distribution of a species is the seven-spotted ladybug.
Within the same species there can be populations with both clearly distinguishable and blurred boundaries (Fig. 95).
There is an exchange of individuals between populations, which can be either fairly regular or episodic. During the seasonal migrations of crows, for example, some young birds annually remain in wintering areas, forming pairs with representatives of the sedentary population. Communication between the population of individual fish species in lakes occurs much less frequently, for example, in years with particularly heavy floods, when separate reservoirs are connected into a single water system.
Connections between populations support the species as a whole. Too long and complete isolation of populations usually leads to the formation of new species.
Differences between individual populations are expressed in varying degrees. They can affect not only their group characteristics, but also the qualitative features of the physiology, morphology and behavior of individual individuals (Fig. 96). These differences are created mainly under the influence of natural selection, which adapts each population to the specific conditions of its existence.
Snowshoe hares from different parts habitats differ in color, size, and structure of the digestive system. For example, the length of the cecum in hares of the Yamal Peninsula is 2 times greater than in representatives from the forest-steppe Urals. This is due to the nature of the diet, the different proportion of roughage in the diet.
8.3.3. Spatial structure of plant and animal populations
Types of distribution of individuals in space. The space occupied by a population provides it with the means to live. Each territory can support only a certain number of individuals. Naturally, the complete use of available resources depends not only on the total population size, but also on the distribution of individuals in space. This is clearly manifested in plants, the feeding area of which cannot be less than a certain limiting value. By intercepting nutrients and water with its roots, shading the space, and releasing a number of active substances, each plant extends its influence to certain area, therefore, the optimal interval for a population is such an interval between neighboring specimens at which they do not negatively influence each other, but at the same time there is no underutilized space.
Rice. 104. The main options for placing colonies of large gerbils (according to E.V. Rothschild, 1966):
1 – continuous uniform settlements; 2 – continuous lace settlements; 3 – narrow-belt; 4 – wide-belt; 5 – small-island; 6 – large-island; 7 – separate clusters of colonies
In nature, an almost uniform, ordered distribution of individuals in an occupied territory is rarely found, for example, in dense populations of sessile marine polychaetes or in pure thickets of some plants. However, most often the members of the population are distributed unevenly in space (Fig. 104), which is due to two reasons: firstly, the heterogeneity of the occupied space, and secondly, some features of the biology of the species that contribute to the emergence of clusters of individuals. In plants, such aggregation occurs, for example, during vegetative propagation, with weak distribution of seeds and their germination near the mother; in animals - with a group lifestyle in families, herds, colonies, at concentrations for reproduction, overwintering, etc.
The uneven distribution of population members can manifest itself in two extreme variants with all sorts of transitions between them: 1) in a pronounced mosaic with unoccupied space between individual clusters of individuals and 2) in a distribution of a random, diffuse type. An example of the first is the nesting of rooks, settling in colonies in groves or parks, adjacent to favorable feeding grounds. Diffuse distribution occurs in nature if members of a population are relatively independent from each other and live in a homogeneous environment for them. This is, for example, the placement of mealworms Tribolium confusum in flour, mayfly larvae in stream water, karakurt spider burrows in meadows, etc.
In each specific case, the type of distribution in the occupied space turns out to be adaptive, that is, it allows optimal use of available resources. The ways in which rational placement is achieved are determined by the system of relationships between members of the population.
Plants in a coenopopulation are most often distributed extremely unevenly, forming more or less isolated groups, clusters, the so-called microcoenopopulations, subpopulations or cenopopulation loci. These clusters differ from each other in the number of individuals, density, age structure, and extent. Often the denser center of the aggregation is surrounded by individuals located less densely.
The spatial heterogeneity of the cenopopulation is associated with the nature of the development of clusters over time. In crescent alfalfa, for example, seeds usually fall in the immediate vicinity of the mother plant, so clusters of young (seedlings, juvenile and immature) are formed near abundantly fruiting middle-aged generative plants. These clusters are characterized by high density. As individuals move into the next age states, the clusters change their age structure and become thinner. At the same time, new germs are engrafted within the cluster, it becomes denser, its structure becomes more complex, and the territory it occupies expands. If the engraftment of the primordia occurs outside the cluster, then a new one arises. Clusters can partially merge with each other, i.e., reach a higher level of aggregation.
Thus, the life of a cenopopulation proceeds in the form of asynchronous age-related changes in various loci, while its spatial structure also changes, because the configuration, extent of loci and their location in the phytocenosis change.
In animals, due to their mobility, the ways of regulating territorial relations are more diverse compared to plants. Even sessile forms have a number of adaptations for rational placement in space. In ascidians and bryozoans, the growing edge of the colony, encountering a colony of another species, grows on top of it and ultimately suppresses it. But if colonies of the same species meet, each of them inhibits the growth of the neighboring one and they begin to spread in a different direction. When completely surrounded by colonies of its own species, vegetative reproduction stops, but the formation of reproductive products and mobile larvae increases.
The larvae of oysters, sea acorns, and sessile polychaetes, guided by chemical stimuli, usually settle in places where there are already individuals of the same species. Before final attachment, the settled larva is characterized by special searching behavior, which allows it to occupy a certain place in the aggregation. According to observations, cyprisoid larvae of the sea acorn Balanus balanoides usually attach at a distance of at least 2.5 mm from young and at least 2 mm from old individuals of their species. At the same time, they freely settle in close proximity and even on the surface of representatives of other species.
In higher animals, intrapopulation distribution is regulated by a system of instincts. They are characterized by special territorial behavior - a reaction to the location of other members of the population. Instincts that support the placement of individuals or groups in populations across a territory exist in birds, mammals, reptiles, a number of fish, and to a lesser extent in amphibians. They are also expressed in many species of invertebrates with a complex nervous system - a number of insects, spiders, crabs, octopuses, etc.
According to the type of use of space, all mobile animals are divided into two main groups: sedentary And nomadic.
During a sedentary existence, an animal uses a rather limited area of the environment throughout its entire or most of its life. Such animals are distinguished by their instincts of attachment to their area, and in the case of forced relocation, by the desire to return to well-known territory. This “feeling of home” is called “homing” in ecology. home- house). Many species return to their breeding grounds even after long and distant migrations. It is known, for example, that the same pair of starlings can occupy “their” birdhouse from year to year. Homing pigeons were even used for practical purposes - to carry mail.
A sedentary lifestyle has significant biological advantages. In well-known territory, the animal can navigate freely, spends less time searching for food, and takes the shortest route to shelters known to it. In addition, many sedentary species create a stocking system, lay paths, build additional nests and burrows, which helps them survive. For example, squirrels have a main nest where the young are hatched, and several additional ones in which the animals hide from bad weather or enemies. Squirrels also create a series of pantries, storing nuts, seeds, and mushrooms for the winter. Long-term use of a certain territory helps its more complete development.
On someone else's property, the animal's behavior changes. Observations of gophers have shown, for example, that such animals are characterized by fussiness, uncertainty of movement, often look around, find shelter only by chance and therefore die more often than the owners of the territory.
A decrease in the likelihood of death from predators in familiar territory has also been proven in experiments. For example, when releasing a long-eared owl into a room where there were hamsters, it turned out that the owl caught animals that had previously become familiar with the room’s furnishings five times less often than those that entered it for the first time.
However, a sedentary lifestyle poses the risk of rapid depletion of resources if population densities become too high. Sedentary species have developed adaptive behavioral features that ensure the delimitation of habitats between individuals, families or other intrapopulation groups. The total area occupied by the population is divided into separate individual or group areas, thereby achieving the orderly use of food supplies, natural shelters, breeding sites, etc.
Think about population. 6. Ecological strategies of populations- this is a collection of individuals of the same species with a common gene pool, interbreeding with each other and inhabiting a certain space for a long time.
Depending on time and space, the habitat of species varies. This depends on many environmental factors known to you. Not adapted to climatic conditions species are disappearing. Their place is taken by those who are more resilient and adapted. Populations of the same species exist separately from each other, as competition for the ecological niche appears. Therefore, populations of the same species occupy different territories.
Modern definitions of population are formulated in the works of Russian researchers S. S. Shvarts, A. M. Gilyarov, A. V. Yablokov. For example, according to the definition of S. S. Schwartz (1969), a population is “elementary groups of organisms of a certain species, long time maintaining their numbers in constantly changing environmental conditions." According to A.V. Yablokov, these are "groups of organisms of the same species inhabiting a certain territory, with a common evolutionary path of development."
Developing the ideas of his compatriots, A. M. Gilyarov gave a slightly different definition of population.
6. Ecological strategies of populations- is a collection of organisms of the same species with a common gene pool, inhabiting a certain space for a long time and maintaining sustainable reproduction of numbers. Within a population there is a constant struggle for existence, and groups of individuals of the same species are isolated from each other. They form local, ecological, geographical populations. This classification of the population was introduced by the famous Russian scientist N.P. Naumov.
A population as a biological unit has its own specific structure, properties and functions. The population structure is characterized by the number of individuals and their distribution in space. And the functions of the population are identical to the functions of other biological systems. Properties characteristic of a population are growth, development, reproduction, adaptability to constantly changing environmental conditions, and genetic characteristics.
Elementary (local) population- a collection of individuals of the same species occupying small areas of homogeneous territory.
The number of elementary populations in nature, the evolution of development and duration depend on the complexity and simplicity of the conditions in the biocenosis and its homogeneity.
In nature, the mixing of individuals of local populations blurs the boundaries between them.
Ecological population - is formed as a set of local populations. Basically, these are intraspecific groups adapted to exist in a certain biocenosis. For example, the common squirrel is widespread in various types forests. Therefore, such ecological populations as “pine” and “spruce” can be distinguished. They are weakly isolated from each other, so there are few differences between them.
Geographic population- these are ecological populations that cover a group of individuals inhabiting large territories with geographically homogeneous living conditions. Geographic populations are relatively isolated from each other and differ in fertility, size of individuals, and a number of ecological, physiological, behavioral and other features. Such long-term isolation of a population can gradually lead to the formation of a geographical race or new forms of the species. Such species are usually considered as a geographical speciation, a race, or as a synonym for that species. For example, more than 20 geographic populations of the common squirrel are known. The boundaries and sizes of populations in nature are determined by the characteristics of not only the territory being inhabited, but also by the properties of the population itself. The results of research by N.P. Naumov show that dividing a species into small territorial groups increases the diversity of the species and enriches its gene pool. Consequently, there is no absolute population in nature. Therefore, in the process evolutionary development of each species during settlement (migration) they constantly mix with each other. In plants, pollen is widely distributed over long distances by wind. As a result, different population forms are maintained within a species. Therefore, from an ecological point of view, the population does not yet have a single definition. The definition of S.S. Schwartz deserves the greatest recognition: “A population is an intraspecific grouping, a form of existence of a species with certain quantitative and qualitative parameters.”
The main indicators characterizing a population are numbers and density. Population size is the total number of individuals in a given area or volume. The number of organisms is never constant. It depends on the birth and death rates of individuals.
Population density determined by the number of individuals or biomass per unit area or volume, for example: 150 spruce plants per 1 hectare, or 0.5 g of daphnia per 1 m3 of water.
Population density varies depending on its size. Population density does not increase indefinitely; this requires the possibility of settlement or free space. Dispersal continues until the organisms encounter any obstacle. There are random, uniform and group distribution of populations.
WITHrandom settlement characteristic only for a homogeneous medium. For example, pests spread randomly in fields, but then, as they multiply, the spread becomes group or spotty.
Most common group settlement, and it can be random. For example, in a forest, trees are distributed first in groups and then evenly. In plants, dispersal occurs through the spread of spores, seeds, and fruits, while in animals, dispersal is rapid and passive. For example, foxes, moose and other ungulates are very active. Slow spread occurs in sedentary animals.
Actively moving organisms have huge ranges, without sharp boundaries between populations, while sedentary organisms, on the contrary, have clearly demarcated populations. These include amphibians, reptiles, and mollusks. The size of the population's range depends on the size of the organisms, behavioral activity, food supply and other abiotic factors. For example, in insects and herbaceous plants the number of individuals can reach hundreds of thousands or more. In contrast, the numbers and densities of large animals and large woody plants are variable and related to human activity. In addition, feed factors play a special role.
Reduced feed yield in different years contributed to a sharp decrease in the population dynamics of squirrels, hares, chukars, and pheasants. Therefore, in nature, population instability is natural. However, in some cases, the population size is replaced by a sharp drop or increase. These processes occur frequently in nature. There are many reasons for their occurrence. These may be the gene pool of the species, environmental factors, growth rate, competition, excess food, etc.
A population in nature is capable of self-regulation of numbers. Each species has upper and lower limits for increasing numbers, beyond which it cannot go. Therefore, the population size is kept at optimal level. There are daily and seasonal fluctuations in the number of organisms. For example, in small animals, rodents, and some birds, fluctuations in numbers can be very significant. Thus, it is known that the number of rodents increases during the season by 300-500 times, and some insects - by 1300-1500 times. Such population outbreaks are common among locusts, infectious disease pathogens, viruses and bacteria, and cause enormous damage to agriculture and human life.
Sharp population declines are not permanent. In some cases, they lead to population extinction. The total lifespan of organisms is divided into three types, that is, there are three types of survival of organisms (Scheme 6).
Scheme 6
I- low mortality in the early stages of development and increased in the later stages (insects, large mammals); II - life expectancy is stable (some fish, birds, plants, etc.); III - maximum mortality in the early stages of development and low in adulthood (some fish, invertebrate animals)
Three types of survival.
The first type of survival is observed mainly in insects, large mammals, trees, people. Maximum mortality occurs in Last year(old age), where a large number of individuals have the same life expectancy, and, of course, the first type of curve varies depending on genes, life expectancy, and sexual characteristics.
The second type is characteristic of organisms where the mortality rate remains constant throughout life. These include coelenterate organisms of fresh water bodies.
The third type is characteristic of most organisms. It is characterized by increased mortality of organisms in the early stages of development, for example: fish, birds, and many invertebrates that are distinguished by their fertility. Plant mortality is 90-95%.
The obtained data on the patterns of survival of organisms play a large role in conducting theoretical studies and experiments with beneficial and harmful species of populations.
In addition to birth and death rates, migration has a major influence on population size or density. The population always strives to expand its range. This mainly depends on the size and density of the younger generation. However, the population cannot expand its range indefinitely; limiting factors or unfavourable conditions new habitats.
There are stable, growing and declining populations. A balanced intensity of birth and death rates forms a stable population. In addition, the stability of the population depends on genetic, historical, and biological conditions. In nature, population stability also depends on birth and immigration, mortality and emigration. Individuals appear in the population during immigration and decrease as a result of emigration.
Only with a balanced combination of these factors does a stable population form. Knowledge of the structure and patterns of population development is of great practical importance.
Population. Elementary population. Ecological population. Geographic population. Population size. Population density. Random settlement. Group settlement. Three types of survival.
1. There are different views of environmental scientists on population problems.
2. The main properties of the population are spatial distribution, numbers, density.
3. Fluctuations in population numbers depend on environmental factors.
4.;There are three types of survival of organisms. In nature, there are organisms with a high reproduction potential (locusts, etc.).
1.What is a population?
2.How are populations classified?
3.How widespread are populations?
1. What is the essence of the population definitions given by S. Schwartz, A. Yablokov, A. Gilyarov and N. Naumov?
2.Name the properties of the population and tell us about their content.
1.What factors prevent the widespread spread of populations?
2.Which of the three types of survival does a person belong to?
3. Explain the types of survival of saiga and carp using the diagram.
1.How do saiga populations move in Kazakhstan in winter and spring? Why?
2. How did the kulan populations appear in Kazakhstan and what do you know about their numbers?
The ratio of individuals by sex and especially the proportion of breeding females in the population have great importance for further growth of its numbers. In most species, the sex of the future individual is determined at the time of fertilization as a result of recombination of sex chromosomes. This mechanism ensures an equal ratio of zygotes by sex, but it does not follow from this that the same ratio is characteristic of the population as a whole. Sex-linked traits often determine significant differences in the physiology, ecology and behavior of males and females. The consequence of this is a higher probability of death of representatives of either sex and a change in the sex ratio in the population.
Ecological and behavioral differences between males and females can be pronounced. For example, male mosquitoes of the Culicidae family, unlike blood-sucking females, during the imaginal period either do not feed at all, or are limited to licking dew, or consume plant nectar. But even if the lifestyle of males and females is similar, they differ in many physiological characteristics: growth rates, timing of puberty, resistance to temperature changes, starvation, etc.
Differences in mortality appear even in the embryonic period. For example, among muskrats in many areas, among newborns there are one and a half times more females than males. In populations of Megadyptes antipodes penguins, no such difference is observed when chicks hatch from eggs, but by the age of ten, only one female remains for every two males. In some bats, the proportion of females in the population after hibernation sometimes decreases by up to 20%. Many other species, on the contrary, are characterized by a higher mortality rate of males (pheasants, mallard ducks, great tits, many rodents).
Thus, the sex ratio in a population is established not only according to genetic laws, but also to a certain extent under the influence of the environment.
In red forest ants (Formica rufa), males develop from eggs laid at temperatures below +20 °C; at higher temperatures, almost exclusively females develop. The mechanism of this phenomenon is that the muscles of the spermatic receptacle, where sperm is stored after copulation, are activated only at high temperatures, ensuring fertilization of laid eggs. From unfertilized eggs in Hymenoptera only males develop.
The influence of environmental conditions on the sexual structure of populations in species with alternating sexual and parthenogenetic generations is especially clear. At optimal temperatures, Daphnia Daphnia magna reproduces parthenogenetically, but at elevated or decreased temperatures, males appear in populations. The appearance of a bisexual generation in aphids may be influenced by changes in length daylight hours, temperatures, increasing population density and other factors.
Among the flowering plants there are many dioecious species in which there are male and female individuals: species of willows, poplars, white naplar, small sorrel, perennial woodweed, field thistle, etc. There are also species with female dioecy, when some individuals have bisexual flowers, and others are female, that is, with an undeveloped androecium. Typically, androsterile flowers are smaller than bisexual flowers. This phenomenon occurs in the families Lamiaceae, Cloveaceae, Teasulaceae, Campanaceae, etc. Examples of species with female dioecy are Marshall's thyme, oregano, field mint, ivy bud, drooping gum, forest geranium, etc. Populations of such species are genetically heterogeneous. Cross-pollination is facilitated in them, and proteroandry is more often observed - earlier maturation of anthers compared to pistils. Within the range of species, the sexual structure of plant populations is more or less constant, but changes in external conditions change the sex ratio. Thus, in the dry year of 1975 in the Trans-Urals, the number of female forms, for example, in steppe sage 10 times, in asparagus 3 times.
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