The population is determined by the fertility ratio and mortality, then all factors that can have an effect on its growth affecting at least one of these processes, shifting the balance between them into one direction or the other. In case of exceeding fertility over mortality, the population is usually increasing (if you exclude migration).
Note 1.
As a rule, with an increase in population density, the growth rate is gradually decreasing to zero, or it fluctuates in a positive, then in the negative side under the action of the dynamics of environmental environmental factors. With the predominance of mortality over the birth rate, the size of populations is reduced.
Factors of stopping growth
Stop the growth of the population and its stabilization at some density level can occur under the influence of various factors. Weight they are interconnected by a negative feedback mechanism. For example, a shortage of any resource (for example, food) enhances intraspecific competition, which reduces the population and leads to the establishment of a new balance sheet of resource supply.
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The resistance of the medium in the amount is determined by the entire combination of limiting factors that impede the implementation of its maximum reproductive potential. This includes both external factors (resourcelessness, biotic relationships, abiotic factors) and intrapopulation regulatory mechanisms. Some of these factors acts regardless of the population density, others depend on it, and their impact may increase in proportion to increasing density, or faster pace.
Experimental studies of population growth
In natural conditions, it is extremely difficult to study the interaction of the growth in the growth of the population of factors, since we are usually the populations that have long existed on a certain territory, the density of which was formed in these conditions of the environment, and the medium itself has experienced the impact of these populations for many generations.
Note 2.
Laboratory populations are very promising in this regard, as well as the study of acclimatization facts and, especially, reactions of organisms. In the latter case, we have a unique opportunity to follow the growth of the population in its natural environment (but not transformed by the population due to the long absence of the studied form in this territory of the organisms) in fact, "from scratch".
All similar observations show a general pattern. In the initial stage, when a population is formed from several individuals in a new place, the reproduction is slow. Many individuals do not participate in reproduction, for some reasons to lose the rest, or not finding their place in the current social structure.
After the formation of normal for the population of the polo-age and socio-etological structure, its reproduction increases. At this stage, the number of individuals is still slightly, the density of their small and intraspecific competition is practically absent, and natural enemies often have not yet managed to master the new kind of food. Therefore, the individuals show a high reproductive potential close to theoretically possible, and the explosive growth of the population occurs.
Further events depend on the presence of the internal mechanisms for regulation of the number. If they are, then with increasing density, they begin to actively affect the population, reducing its reproduction due to the territoriality, the tension of hierarchical relations, stress reaction, etc. If this does not occur, then as the population approaches the limit of the biological capacity of the medium, its growth is not only slow, and may even be accelerated. This is due to the growth of fertility with increasing mortality.
Although this mechanism works short time, still the population density may have time to rise much higher than the permissible limits. This causes a catastrophic decline in numbers, usually two orders of magnitude or more.
After that, the medium is partially restored, a new increase in the number begins, and the situation is repeated. However, the new capacity of the medium is usually lower, and both growth and the subsequent decline becomes less disastrous. Gradually, for several similar cycles, the population adapts to the medium, its ecological niche begins to more significantly comply with local conditions, and the density specializes at a certain level.
Dynamics of populations - These are the processes of changing its basic biological indicators (numbers, biomass, structures) in time depending on environmental factors. DP is an adaptive answer to the conditions of the view, one of the most significant biological and environmental phenomena, because The life of the population is manifested in its dynamics.
An important process of population dynamics is growth numerous (or simply "population growth"), which occurs when the organisms are mastered by the organisms of habitats or after the transferred catastrophe. The nature of growth is different.
The growth of populations in any species, from bacteria to humans, has common patterns. Theoretically, the number of any species can grow infinitely due to the fact that the reproduction usually occurs in geometric progression. The number of descendants that can leave after himself one of the individual is called biotic potential of type . The descendants can only be two, as with a simple division of the cells in half (bacteria, amoebas, infusoria), and maybe hundreds of millions (the number of eggs, postponed for the life of a narrow-minded fish, the number of small seeds in many wood plants, etc.) .
Population growth dynamics
N.t. = N.t.-1 + B. – D. + C. – E.
Nt - the number of individuals at the moment
NT-1- number of individuals in the previous point,
B- the number of born,
D- the number of dead
C - the number of immigrants,
E- The number of emigrants
Grosty curves.
Two basic forms of growth curves are isolated - purely exponential (J-shaped) and sigmoid (S-shaped).
Sigmoid, or S-shaped The curve describes the situation in which in the new habitat population its density at first increases slowly (the lag phase corresponding to the period of adaptation to the conditions), and then quickly. After some time, the growth rate slows down and becomes ultimately zero: the birth rate is fully balanced mortality. It is said that the curve goes to the plateau. The slowdown in the population growth is explained by an increase in intraspecific competition for resources, such as food or nesting sites. As a result, the mechanism of negative feedback increases the mortality rate of individuals and their reproduction slows down (a smaller percentage of pairing animals, an increase in the number caused by stress of miscarriages, etc.).
Exponential growth without going to the plateau ( J-shaped curve ) Corresponds to the situation in which after the initial adaptation period (Lag-Phase), the number of individuals increases sharply, but then growth is suddenly terminated when there was no resistance to the medium. Such an increase in the population is called independent of the density, since it is no longer restrained until the last moment followed by the mass death of individuals. This death in turn can be called up either by the adverse season, or the end of the breeding season of the organisms themselves or their main victims.
Fig. 5 types of population growth curves (population growth models):
a - j-shaped; b - s-shaped; K - Maintenance Capacity Environment(maximum population size that can exist volume and with regular feeding).
Factors of population dynamics.
The increase in population growth in nature occurs as a result of a number of factors. They are divided into two fundamentally different categories: modifiers and regulators:
Modifier factors they affect the number of population unilaterally, without feeling any influence on its part. This is, first of all, abiotic phenomena: droughts, pouring rains, floods, storms, low temperatures, etc. The growth and fertility of individuals obeys the rule of optimum, so all deviations from it change (modify) the number of populations in the direction of reduction, and the approximation to the optimum stimulates an increase in the number.
Regulator factors not only affect the number of populations, but also themselves change their strength depending on its density. With a drop in the number, the impact of regulators weakens, with an increase - enhances. There is so-called negative feedbackwhich is able to restrain the growth of the population. It should be noted that excessive, uncontrollable, the increase in the number is ultimately destroyed for any kind, because It can completely delete and undermine the resources of the medium. In evolution, there were many mechanisms for regulating the multiplay of species.
Types of numbers of numbers.
1. Stable type - It is characterized by a small amplitude and a long period of fluctuations in numbers. Externally, it is perceived as stable. This type of characteristic of large animals with a large lifespan, late onset of half-plant and low fertility. This corresponds to a low mortality rate. For example, hoofs (period fluctuations of 10-20 years), cetacean, hominids, large eagles, some reptiles.
2. Labile (fluctuating) type - It is distinguished by natural fluctuations in the number with a period of about 5-11 years and a significant amplitude (in tens, sometimes hundreds of times). Characterized seasonal changes in abundance associated with the frequency of reproduction. This type of one's own animal with a lifetime is 10-15 years old, earlier sexual ripening and high fertility. This includes large rodents, hazing, some predatory, birds, fish and insects with a long development cycle.
3. Ephemeral ( explosive type) The speakers are distinguished by an unstable numericality with deep depression, replacing outbreaks of mass reproduction, at which the number increases hundreds of times. Her differences are carried out very quickly. The total cycle length is usually up to 4-5 years, of which the peak of the number takes most often 1 year. This type of speakers is characteristic of short-lived (no more than 3 years) species with imperfect adaptation mechanisms and high death (small rodents and many types of insects).
Height - This is an increase in the total mass in the development process, leading to a constant increase in the size of the body. If the body did not grow, he would never have become more fertilized egg.
Growth is ensured by the following mechanisms: 1) an increase in cell size, 2) by increasing the number of cells, 3) by an increase in the non-tossing agent, the products of the vital cells of the cells. The concept of growth includes a special shear of metabolism, favorable synthesis processes, water intake and the deposition of the intercellular substance. Growth occurs on cellular, fabric, organ and organisomic levels. An increase in mass as a whole to the body reflects the growth of the components of its organs, tissues and cells.
Distinguish two types of growth: limited and unlimited. Unlimited growth continues throughout ontogenesis, right up to death. Such growth possess, in particular, fish. Many other vertebrates are characterized by limited growth, i.e. Quickly quickly go to the plateau of its biomass. The generalized curve of the dependence of the organism's growth on time with a limited growth has an S-shaped form (Fig. 8.18).
Fig. 8.18. Generalized curve of body growth dependence on time
Prior to development, the body has some source sizes, which for a short time practically do not change. Then the slow, and then the rapid increase in mass. Some time, growth rate can remain relatively constant and the slope of the curve does not change. But soon there is a slowdown in growth, and then the increase in the size of the body is terminated. After reaching this stage, the equilibrium is established between the material spending and the synthesis of new materials that provide an increase in the mass.
Fig. 8.19. Changes in growth rate depending on the development stage of the human body.
BUT - fret and in the first two years after birth, B. - at the beginning of the postnatal period
The most important growth characteristic is its differentiality. This means that the growth rate of non-etinakov, firstly, in various parts of the body and, in - second, at different stages of development. Obviously, differential growth has a huge impact on morphogenesis.
An equally important feature is the property of growth as equipality. This means that, despite the emerging factors, the individual seeks to achieve a typical species size. Both differential and growth equifinality indicate the manifestation integrity developing the body.
The speed of overall growth of the human body depends on the stage of development (Fig. 8.19). The maximum growth rate is characteristic of the first four months of intrauterine development. This is explained by the fact that the cells at this time continue to share. As the fetus grows, the number of mitoses in all tissues decreases, and it is assumed that after six months of intrauterine development, there are almost no formation of new muscle and nerve cells, except for neuroglia cells.
Fig. 8.20. Growth curves of individual organs and tissues
compared to a generalized growth curve (clarification, see the text)
Further development of muscle cells is that the cells become greater, their composition changes, the intercellular substance disappears. The same mechanism acts in some tissues and in postnatal growth. The growth rate of the body in postnatal ontogenesis is gradually decreasing for a four-year-old age, then some time remains constant, and at a certain age it makes a jump again, called pubertal growth jump. This is due to the period of puberty.
The difference in the growth rate of organs and tissues is shown in Fig. 8.20. The growth curves of most skeletal and muscle organs repeat the course of the overall growth curve. The same applies to changes in the size and individual organs: liver, spleen, kidneys. However, the growth curves of a number of other tissues and organs differ significantly. In fig. 8.20 shows the overall growth curve of the body and most of the other organs ( III), the growth of external and internal reproduction bodies ( IV), brain growth, as well as skull, eyes and ears ( II.), the growth of the lymphatic tissue of the almonds, a heart-shaped process, intestines and spleen ( I.).
The value of various growth rates of organs and tissues for morphogenesis is clearly seen from fig. 8.21. Obviously, in the fruit and postnatal periods, the growth rate of the head decreases compared with the rate of leg growth.
Fig. 8.21. Human body proportions in embryogenesis and after birth
Fig. 8.22. Forms of proliferation growth.
BUT - multiplicative; B - accretion (clarification See in the text)
Pubertal growth jump characterizes only man and monkeys. This makes it possible to evaluate it as a stage in the evolution of primates. It correlates with such a feature of ontogenesis, as an increase in the period of time between the completion of feeding and sex ripening. In most mammals, this interval is small and there is no puberty growth jump.
As mentioned above, the growth is carried out at the expense of such cell processes as an increase in cell size and an increase in their quantity. Several types of cell growth are isolated.
Auxtent - Growth going by increasing cell size. This is a rare type of growth, observed in animals with a constant number of cells, such as provicrats, round worms, insect larvae. The growth of individual cells is often associated with polyploidization of the nuclei.
Proliferation - Growth occurring by reproduction of cells. It is known in two forms: multiplicative and accretion.
Multiplicative The growth is characterized by the fact that both cells arising from the division of the general cell are re-entering again (Fig. 8.22, BUT). The number of cells grows in geometric progression: if n - division number, then N. N \u003d 2. N.. Multiplicative growth is very effective and therefore in pure form almost never occurs or it ends very quickly (for example, in the embryonic period).
Accretion The growth is that after each subsequent division, only one of the cells is again divided, while the other ceases division (shaded, fig. 8.22, B.). In this case, the number of cells grows linearly. If a p - division number, then N. N \u003d 2. n. This type of growth is associated with the separation of the organ on the cambial and differentiated zone. Cells are moving from the first zone to the second, while maintaining a constant relationship between zones. Such growth is characteristic of the organs where the cellular composition is renewed.
The spatial organization of growth is complex and natural. It is with it that the species specificity of the form is largely connected. This is manifested in the form almeometric Growth. His biological meaning is that the body during growth should be preserved not geometric, but a physical similarity, i.e. Do not exceed certain relationships between body weight and sizes of support and motor organs. Since with the growth of the body, the mass increases in the third degree, and the sections of bones in the second degree, so that the body is not crushed by its own weight, the bones should grow in thickness disproportionately quickly.
Regulation of growth is complex and diverse. The genetic constitution and the factors of the external environment are of great importance. Almost every kind has genetic lines characterized by the limit sizes of individuals, such as dwarf or, on the contrary, giant forms. Genetic information is concluded in certain genes determinizing body length, as well as in other genes interacting with each other. The implementation of all the information is largely due to the action of hormones. The most important of the hormones is somatotropin, allocated by the pituitary gland from the moment of birth to the adolescence. Thyroid hormone - Tyroxin - plays a very large role throughout the entire growth period. From adolescent age, the growth is controlled by steroid hormones of adrenal glands and gonad. Of the factors of the environment, food, time of year, psychological impacts have the greatest importance.
Interesting is the dependence of the ability to increase from the age stage of the body. Fabrics taken at different stages of development and cultivated in a nutrient medium are characterized by different growth rate. The older the embryo, the slower its fabric grow in culture. Fabrics taken from an adult organism grow very slowly.
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Dynamic population indicators
Any population is theoretically capable of an unlimited increase in the number, if it does not limit the factors of the external environment. Even the most slowly multiplying view is able to produce as many individuals in a short time that there is no place for them on the globe. In just five generations, i.e. for 1-1.5 year old months, one single TLL may leave more than 300 million descendants. If all the embryos were preserved, and all the offspring survived, the number of any population through certain intervals would increase in geometric progression And this is despite the fact that only a few eggs or a cubs are produced in a lifetime, and others are thousands and even millions of embryos. Growth in geometric progression is expressed on the schedule exponential curve . IN AND. Vernadsky called this process "Pressure of life" .
In fact, in all living organisms, the ability to multiply is improved. This ability makes life a very powerful force on Earth. A huge live weight of organisms maintains a circulation of substances on the planet, creates rocks, soil, regulates the composition of the waters and the atmosphere. But in nature, the biotic potential of the population is never fully implemented. The main limiter on the way to the boundless reproduction of organisms - a lack of resources The most necessary: \u200b\u200bfor plants - mineral salts, carbon dioxide, water, light; for animals - food, water; For microorganisms - a variety of compounds consumed. The reserves of these resources are not infinite, in different parts of the planet, they have their limits, and the reproduction of species is contained. Second limiter - influence unfavorable conditions, slowing the growth and reproduction of organisms, even if there are resources necessary for this. Finally, the death of embryos or the younger of young individuals from enemies and diseases. So, thousands of acorns that annually produces one large oak, turn out to be eaten proteins, boars, jokes, mice, insects, or affected by mold fungi and bacteria, or dying in the prostivities stage for various reasons. As a result, only adult trees grow from the studios of the acorns.
General changes in the population number are made by four phenomena: fertility, mortality, instill and evict individuals (immigration and emigration).
Birth rate (rate of birth) – The number of new features that appeared in the population per unit of time as a result of reproduction.
Distinguish the maximum and actual fertility. Maximum Fertility - Maximum realization of the possibility of birth in the absence of limiting environmental factors. Actual fertility - Real realization of birth opportunity.
There are absolute and specific fertility. Absolute (general) fertility, or rate of birth – The number of individuals (eggs, seeds, etc.), born (delayed, produced, etc.) for some period of time.
Specific birth rate is the ratio of the birth rate to the initial number. This value depends on the intensity of the reproduction of individuals: for bacteria - an hour, for phytoplankton - a day, for insects - week or month, for large mammals - year.
The birth rate depends on many reasons. Of great importance is the proportion of individuals capable of reproduction, which is determined by the ratio of floors and age groups. The frequency of the generation sequence is also important. So, among insects distinguish mallinyal and polivolithic views. The first give one, the second - several generations for the year. For example, the tool over the season has up to 15 parthenogenetic generations. In terms of the number of reproduction periods, there are distinguished monocyclic and polycyclic views. Monocyciality, or one-time reproduction, is typically characteristic of a short life expectancy in a hawk state (salmon fish, bastards, may beetles and many other insects). The polycycle is characterized by re-reproduction of individuals and inherent in most vertebrate animals and a row of invertebrates, such as crustaceans.
Plants are distinguished monocarpical and polycarpic Views, i.e. with a single and multiple reproduction during life.
The multiplay of the population, as a rule, is not directly proportional to fertility. Fruitness strongly depends on the degree of development of care for the offspring or the provision of eggs with nutritional materials. Among the fish, the largest amount of eggs leave species with pelagic caviar - herring, cod, cambal, etc. For example, Sakhalin herring produces 38-46 thousand small, in the share of millimeter, and eggs. Salmon, burning caviar in the ground, develops a smaller number of eggs, but larger than large sizes. The average fecundity of the Amur Gorbushi 1300-1500 Irkin with a diameter of 4-6 mm. The largest caviar of the shark and chimeron, up to 6-8 cm. Eggs of these fish have besides a dense protective shell. Their number is very small - several pieces per female.
In species that protect and feed their young, fertility is sharply reduced. The size of masonry in birds of different species varies not thousands of times, as in fish, and ranging from one egg (in some predators, penguins, cumbers, etc.) to 20-25 (the maximum number of eggs in chicken, for example, in gray partridges).
Large fertility is produced in species also in conditions of higher mortality, especially with strong pressure from predators. Fertility selection compensates for a high rate of death in populations. Therefore, with high fertility, the population growth increases can be quite low. In various populations of one species, fertility is usually the higher, the more unfavorable their habitats occupied by them. So, in many mammals - hares, mice, voles - the number of cubs in litter on the borders of the range is more than in his center.
Hypothesis of differentiated sex specialization (V. A. Geodokyan) - Floor differentiation occurs in two main directions of evolution: change (male floor) and preservation (female gender). The more in the population of women's individuals, the better its genotypic structure is preserved; The more male individuals, the greater the speed or the magnitude of the change in this structure.
Mortality (mortality rate) – The number of individuals who died in a population per unit of time (from predators, diseases, old age and other reasons). Mortality - the amount of bidding.
Distinguish the minimum and actual mortality. Minimum mortality - The minimum possible mortality rate. Actual mortality - The real mortality rate.
Distinguish absolute and specific mortality. Absolute (general) mortality or mortality rate – Number of individuals who died over time .
Specific mortality (a) - The ratio of mortality rate to the initial number.
Population growth rate – Changing the number of population per unit of time. The growth rate of the population can be positive, zero and negative. It depends on the rates of fertility, mortality and migration (instiges - immigration and evictions - emigration). The increase (profit) of the number occurs as a result of fertility and immigration of individuals, and a decrease in (loss) of the number - as a result of mortality and emigration of individuals.
The absolute and specific growth rate of the population is distinguished. Absolute (general) growth rate – Changing the population number over time .
Specific growth rate - The ratio of growth rate to the initial number.
In the absence of limiting environmental factors, the specific growth rate is equal to the magnitude , which characterizes the properties of the population itself and is called specific (congenital) population growth rate or biotic potential of the species.
The magnitude of biotic potential is very different from different species. For example, the female roeble is able to produce 10-15 kids, Trikhin postpone 1.8 thousand larvae, the female of the honey bee - 50 thousand eggs, the fish-moon - up to 3 billion eggs.
However, in nature, due to the action of limiting factors, the biotic potential of the population is never fully implemented. Its value usually develops as a difference between fertility and mortality in the population.
Factors restricting population growth
Despite the fact that the potential growth potential in all types of organisms is very high, in vivo usually growth as such does not occur at least for a long time. The number of populations is either quite stable, or differs in periodic oscillations with a rather large amplitude around a certain average. This is due to a number of factors that limit the growth of the population.
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Potential reproduction
The components of the population reproduction potential are: the duration of the reproductive period of individuals, multiplication multiplicity during life, lifespan, the number of offspring produced at a time, the duration of one reproductive cycle. All these parameters in different populations are changing widely.
The preproducement period (including the embryonic and post-emptilic development here) can be from dozens of minutes (some prokaryotes), up to one and a half dozen years old (man, elephants, some fish, invertebrates with long-term larval development, many trees, etc.).
During the course of life, the number of reproductive cycles can be defined and uncertain. The minimum number of reproductive cycles is equal to one. This is typical for annual plants, many invertebrates, some fish. For many plants and most vertebrates, an indefinite number of reproductive cycles is characterized, depending on the habitat conditions of the body. So, long-lived trees, many fish, some reptiles, birds can multiply at least a few dozen times for their lives.
The life expectancy of organisms is also changing widely - from tens of minutes (the interval between the divisions of some prokaryotes) to hundreds or even thousands of years (some plants). For some of the long-lived organisms, a post-repudent period is characterized, when in old age they lose the ability to reproduce. Others are missing or very poorly expressed.
In one reception, the multiplying individual can produce one descendant (in the division of most single-cell), or even less than one (with a multiplication of a pair of some higher animals, which one young one appears), or a significant amount of them. The record among animals here belongs to some fishes, while spawning tens of millions of eggs, and among plants - multiplying disputes.
Note 1.
The duration of one reproductive cycle in different organisms is from tens of minutes (some prokaryotes) to 3-5 years.