Fertility in mammals is called fertility, and the dynamics of populations depends on the result of reproduction of individuals and their immigration from neighboring populations. The main indicator of the rate of reproduction is the average number of offspring for a certain time in a female. Population dynamics may show negative values as a result of mortality and emigration (for example, in humans). This applies mainly to highly developed countries, but in third world countries this indicator is steadily going up from year to year.
Human society
Population indicators are most dependent on mortality - this is the average number of deaths per year. Calculation is carried out as a percentage or in the number per thousand individuals. Mortality statistics show the lowest rates in developed countries, where the level of medical care is high. Conversely, a high mortality rate is observed in countries where it is present in insufficient quantities.
Scientists who study the dynamics of populations should know not only the total number of deaths, but also the number of individuals that die before they reach puberty and leave offspring. If two offspring each pair manages to leave and they live to adulthood, such a population is considered stable. The mortality statistics in dynamics depends on the age of dying individuals and is displayed in charts on survival curves without fail.
Survival
Three main types of such curves are used. The first, reflecting the main factor in mortality, is natural aging. The second curve shows early mortality in the population. And the third curve shows constant mortality throughout the life of individuals in the population. Survival curves on the plots of populations of different species usually occupy intermediate positions between extreme forms. In parallel, the increase in the number of individuals is also analyzed. If fertility statistics show an excess of mortality in an isolated population, the population will grow. The beginning of growth in a curve is an exponent.
But it always happens over time that food and its reserves are exhausted in a given habitat. Population dynamics almost immediately react to a decrease in nutrient products. The curve takes the form of the letter S. It can be said to be an intelligent biological species. In other populations, population growth occurs without any control, eventually going too far until a catastrophe occurs - a population collapse due to depletion of resources. Then the curve takes the form of the letter J.
Strategies
If the breeding rate does not depend on the density of the species, such populations are called r-strategists, their sizes are far from stability and may even exceed the capacity of a given medium for some time. Usually it is a biological species with small size and short life span: small insects, microorganisms, annual plants. They quickly populate new spaces, but just as quickly and are crowded out by competitors.
Slowly breeding k-strategists control the density of their own population, their numbers always stabilize at the proper value, they have a certain structure and development dynamics. This is primarily a man, large animals and birds, trees. Fertility statistics show that the population is controlled by changes in external conditions: lack of food, the appearance of predators and the like. But there are also internal factors that can restrain the birth rate. For example, well-studied territorial behavior: taking care of the offspring of fish, for example, when they protect the nesting territory from intrusions of their relatives. There is also overpopulation, which drastically reduces fertility and even the degree of care for offspring.
The control
The reasons for the massive increase in the birth rate are most often climatic and weather factors, and in recent centuries human activity has been added to this. Most populations have feedback mechanisms to control abundance. The structure and dynamics of populations are interconnected so as not to exceed the limit beyond which disaster occurs. For example, the caterpillars of many species of butterflies die massively from hypothermia if they left the eggs ahead of time or when the cold had not yet ended.
Or they die of hunger if they are late to hatch when the leaves have already become large. If all these caterpillars appeared simultaneously and on time, then inevitably overpopulation. Then they are limited by another factor - parasites or predators. All these regulatory and controlling factors are entirely dependent on population density. In the same way, the number and resettlement beyond the boundaries of their own territories is regulated. For example, locusts emptying clouds all around, squirrels or lemmings migrate more calmly.
Change in numbers
The abundance, as well as the age and sex composition in a population changes if this is due to fluctuations in the environment, processes that occur within the population, in interactions with other species, and for many other reasons. In general, such changes are due to three main factors: fertility, mortality, and migration.
The latter are the movement of individuals of the population or replenishment by aliens. This phenomenon is naturally based on the most important biological trait of each species - the ability to resettle. Some parts of individuals regularly leave the population, adding to neighboring or settling in new, unoccupied territories. Thus, new biotopes are being developed and the range of this species is expanding.
Resettlement
Resettlement functions are usually performed during certain periods of the life cycle. For example, insects use their stage of adulthood (imago) for this, mammals and birds migrate at the age of growing young animals, plants disperse and give spores and seeds to the wind, aquatic inhabitants of the attached lifestyle multiply and settle using floating larvae, and so on.
A number of populations that occupy places unsuitable for others, most often cannot maintain their numbers by reproduction and preserve the population only due to migration. In principle, absolutely any biological species can grow unlimitedly in numbers - purely theoretically. However, this growth is always limited by environmental factors.
Theories and Reality
If such a hypothetical option were possible, then the population growth and its speed would depend only on the magnitude of the biotic potential inherent in the species. That is, this is an indicator that reflects the maximum number of descendants from one individual or one pair for a certain unit of time. If all offspring survived, the population — absolutely any — would increase along an exponential curve, that is, in geometric progression.
Real life shows us completely different patterns and, of course, the population growth curve in the form of the letter S. Usually, the number at first increases slowly, then much faster, and then the environment begins to resist, slowing down this growth. Thus, an equilibrium is achieved in mortality, fertility and migration. A certain level of density is established after a period of growth, but this does not mean that quantitative changes in the population have stopped. Population dynamics never stands still, the number fluctuates, and the stationary level is always subject to fluctuations.
Natural populations
Natural populations are characterized by seasonal changes in numbers, which are associated with environmental conditions, as well as fluctuations associated with weather fluctuations. The former are especially pronounced in the life of numerous insects in annual plants.
Significant fluctuations in fertility and mortality, which change the population size, are also demonstrated by many species of birds and mammals in northern latitudes. Here the cycles are longer - from three to four to nine to ten years. For example, Canadian lynx and white hare have a ten-year cycle of fluctuations in numbers, with the first hare at the peak of population growth, and the next year - lynx.
Plants
The dynamic state of populations among plants is assessed using an analysis of ontogenetic, that is, age-related conditions. The most easily identifiable sign of resistance is the full-term age spectrum, that is, the basic, characteristic, defining dynamically stable (definitive) state of populations.
The main properties of each population are mechanisms for regulating the number of individuals. All significant deviations are associated with bad consequences for the existence of the population. And therefore, adaptation factors work to reduce or restore, contributing to the maintenance of normal values of the number of individuals.
Biotic potential
Each population can be characterized by biotic potential, that is, possible offspring from one or a pair of individuals that are capable of biological reproduction. The higher the biotic potential, the usually lower the level of organization for a given species of organisms, allowing it to be used only for short periods or in individual cases. Propagation conditions may include nutrient rich media. This is an exponential type of growth. For a human population in modern conditions, it is also characteristic - due to the decrease in child mortality mainly.
Changes in numbers have their periods, that is, population waves or waves of numbers. Major changes in any direction compared with the average values basically have negative consequences for the subsequent life of the population. High numbers - lack of food, small - the threat of extinction, as, for example, the Amur tiger. The dynamics of populations can be divided into two components: dependent on the number and not. The latter is characteristic of an exponential growth curve, and the first is a logistic one.
Population genotype
Each population has a huge number of very different genes that make up the gene pool. Genes can be in several forms (the so-called alleles). The number of individuals in the population that carry a specific allele, and in this case determine the frequency of this allele. The genetic structure of the population is characterized by the frequency of genotypes and the frequency of alleles. A population is a hereditary heterogeneous collection of individuals of the same species, and therefore it should be opposed by a clean line, that is, what is called a hereditary homogeneous collection of individuals of this species.
The characters of all individuals that make up the population are the phenotype, and the totality of the hereditary inclinations is the genotype. Homogeneity is not observed in any of the types, because the genotype is formed by the conditions of existence on the platform of interacting factors of heredity and selection (with variability and isolation). There are geographical genetic barriers (spatial) and there are ecological - temporary barriers that prevent the merging of populations.
Interesting Facts
The care for the offspring of fish varies in different populations. Many species do not care about him at all, leaving caviar unattended. As a result, most of the eggs die. This is because such fish have a very large number of caviar - up to three hundred million, for example, the moon fish lays.
If little eggs are laid, without concern for the offspring, the population will simply die out. Therefore, in some species, males protect the nest with masonry for up to two weeks, while others inventively hide eggs, for example, in a mantle cavity of mollusks or even grow a special pouch on their own body, like males of a seahorse. Tilapia generally produces offspring in its mouth. All the diversity of populations that exist on Earth will never tire of surprising a person.