Population dynamics: the difference between fertility and mortality, caring for the offspring, survival conditions

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 individual. The dynamics of populations may show negative values as a result of mortality and emigration (for example, in humans). This applies mainly to the highly developed countries, but in the countries of the third world this figure is steadily going upwards year by year.

Human society

Population indicators are most dependent on mortality, the average number of deaths per year. The calculation is carried out in percent or by the number per thousand individuals. Mortality statistics show the lowest marks in developed countries, where there is a high level of medical care. Conversely, a high mortality rate is observed in countries where it is present in insufficient quantities.

Scientists investigating 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 descendants leave each pair and they live to adulthood, such a population is considered stable. The mortality statistics in dynamics depend on the age of the perishing individuals and are displayed in the graphs on the survival curves without fail.

Survival

Three main types of such curves are used. The first, which reflects the main factor of mortality, is natural aging. The second curve shows the early mortality in the population. And the third curve shows the constant mortality throughout the life of individuals in the population. Survival curves on the graphs of populations of different species usually occupy intermediate positions between extreme forms. At the same time, the addition of the number of individuals is analyzed. If in an isolated population, fertility statistics show an excess of mortality, the population will grow. The beginning of growth in the curve is an exponential.

But it always happens so over time that food and its supplies are exhausted in this habitat. Dynamics of populations almost immediately on the reduction of nutrient products reacts. The curve takes the form of a letter S. This, you can say, is a clever biological species. In other populations, the growth in numbers occurs without any control, ultimately going too far until a catastrophe occurs - the collapse of the population due to depletion of resources. Then the curve takes the form of the letter J.

Strategies

If the reproduction rate does not depend on the species density, such populations are called r-strategists, their dimensions are far from stable and for some time may even exceed the capacity of the given medium. Usually it is a biological species with small sizes and small lifespan: small insects, microorganisms, annual plants. They quickly populate new spaces, but are also quickly replaced by competitors.

Slowly multiplying k-strategists control the density of their own population, their numbers always stabilize on the proper value, they have a certain structure and dynamics of development. This is primarily a person, large animals and birds, trees. The fertility statistics show that the population is controlled by changing external conditions: lack of food, the appearance of predators and the like. But there are internal factors that can curb the birth rate. For example, well-studied territorial behavior: caring for offspring in fish, for example, when they protect nesting territory from the intrusions of their relatives. There is also overpopulation, which drastically reduces fertility and even the degree of care for the offspring.

Control

The reasons for the massive increase in the birth rate are most often climatic and weather factors, and in recent centuries, this has been supplemented by human activities. Most populations have a feedback mechanism for regulating abundance. The structure and dynamics of populations are interrelated in order not to exceed the limit beyond which a catastrophe occurs. For example, caterpillars of many species of butterflies die massively from hypothermia, if they left the eggs ahead of time or when the colds are not over.

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 overpopulation is inevitable. Then they are limited by another factor - parasites or predators. All these regulatory and controlling factors entirely depend on the density of the population. In the same way, the number and distribution outside its own territories are regulated. For example, locusts that are devastating all around the clouds migrate, proteins or lemmings migrate more peacefully.

Change in the number

The number, as well as the age and sex composition in the population, change if it is due to fluctuations in the environment, processes that occur within the population, interactions with other species, and for many other reasons. In general, such changes are formed due to three main factors: fertility, mortality and migration.

The latter are the displacement of individuals of the population or the replenishment by aliens. The phenomenon is naturally based on the most important biological feature of each species - the ability to resettle. Some parts of individuals regularly leave the population, adding to neighboring or living new, unoccupied territories. So new biotopes are developed and the range of this species is expanding.

Resettlement

Resettlement functions are usually performed at certain periods of the life cycle. For example, insects use for this their stage of adulthood (imago), mammals and birds migrate in the age of growing young animals, plants scatter and give up to the wind spores and seeds, water inhabitants of the attached way of life reproduce and settle with floating larvae and so on.

A number of populations that occupy places that are of little use for others, most often can not support the number of reproduction and preserve the population only through migration. In principle, absolutely any biological species is capable of growing unlimitedly in numbers - purely theoretically. However, this growth is always limited by environmental factors.

Theories and Reality

If such a hypothetical variant were possible, then the population growth and its velocity would depend only on the amount of the biotic potential inherent in the species. That is, this is the indicator that reflects the maximum number of offspring from one individual or one pair for a certain unit of time. If all the offspring survived, the population size - absolutely any - would increase according to an exponential curve, that is, in geometric progression.

Real life shows us completely different patterns and, of course, the growth curve of the population in the form of the letter S. Usually the number initially increases slowly, then much faster, and then the environment begins to resist, slowing this growth. Thus, 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 ceased. The dynamics of populations does not stand 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 habitat conditions, as well as fluctuations associated with the periodic fluctuations. The first are particularly pronounced in the life of numerous insects and in annual plants.

Also, significant fluctuations in fertility and mortality, changing the population size, show 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, the Canadian lynx and the white hare have a ten-year cycle of fluctuations in numbers, with hares first at the peak of population growth, and next year - lynxes.

Plants

The dynamic state of populations among plants is assessed using analysis of ontogenetic, that is, age states. The most easily determined sign of stability is the full-length age spectrum, that is, the basic, characteristic, determining the dynamically stable (definitive) state of populations.

The main properties of each population are the mechanisms regulating the number of individuals. All significant deviations are associated with bad consequences for the existence of the population. Therefore, adaptive factors that help maintain normal values of the numbers of individuals work to reduce or restore.

Biotic potential

Every population can be characterized by a biotic potential, that is, a possible offspring of one or a pair of individuals that are capable of biological reproduction. The higher the biotic potential, the lower the level of organization for this type of organism, which allows using it only at short intervals or in some cases. Conditions for reproduction may be environments rich in nutrients. This is an exponential type of growth. For the human population in modern conditions, it is also characteristic - due to the reduction in child mortality in general.

Changes in numbers have their own periods, that is, population waves or population waves. Large changes in either direction, in comparison with the mean values, basically have negative consequences for the subsequent life of the population. High numbers - lack of food, small - threat of extinction, as, for example, in the Amur tiger. The dynamics of populations can be divided into two components: dependent on the number and not. The latter is typical for an exponential growth curve, and the first for a logistic one.

The genotype of the population

Each population has a huge number of different genes that make up the gene pool. Genes can be in several forms (so-called alleles). The number of individuals in the population that carry a particular allele is determined in this case by the frequency of this allele. The genetic structure of the population is characterized by the frequency of genotypes and the frequency of alleles. The population is a hereditary heterogeneous set of individuals of the same species, and therefore it must be contrasted with a clean line, that is, what is called a hereditary homogeneous collection of individuals of a given species.

The signs of all the individuals that enter the population are a phenotype, and the totality of hereditary deposits is the genotype. Uniformity 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 ones - temporary ones, which prevent the merging of populations.

Interesting Facts

Care for the progeny of fish in different populations is different. Many species do not care about it 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, laying fish-moon.

If the eggs are laid aside a little, without the care of the offspring the population will simply die out. Therefore, in some species, males protect the nest with clutch for up to two weeks, while others invent ingeniously hide caviar, for example, in the mantle cavity of mollusks or even on their own body grow a special pouch, like male seahorses. Tilapia generally grows offspring in her mouth. The entire diversity of populations existing on Earth will never tire of surprising a person.