Natural communities consist of populations of different organisms capable of self-reproduction. Each population is a grouping of individuals of one species, located in a particular area. View, as a rule, includes a variety of populations, the isolation between them is never absolute. Individuals of a population capable of migration and resettlement, their distribution depends on the geographical barriers in the range of species, as well as the nature of the habitat, the species numbers. Within the species there are separate groups: for example, populations of many mammals are composed of family groups or herds. On the relationships between individuals in the population itself and with the environment depends on the population structure and population dynamics. The population consists of many parts that are in certain ratios. So, age is determined by the number of individuals of different age sex structure - the ratio of males and females, the genetic and phenetic structure - relation in the population of individuals with different genotypes and phenotypes. The spatial structure is determined by the characteristics of placement of individuals in space. Often, individuals of one species form clusters or groups that are characteristic of territorial behavior. Animals protect their territory, signal to other individuals that the site is occupied. The spatial structure of populations can change over time: it depends on the season, the number and age of the population, etc. The dynamics of population size - a change in the number of individuals in the population, and population (or population) - the total number of individuals in the population. Due to the difficulty in determining the total population, in practice, determine the population density - the number of individuals per unit area (or volume - when inhabiting the water column or soil). The number of changes in population through migration, generational change, changing the ratio of earnings due to the birth of new individuals and the world from other populations, as well as a result of loss - the death or expulsion of individuals. The study of population dynamics is essential in order to predict outbreaks of pests or game animals. The ability of organisms to increase the number in a given period of time called the biotic potential of the form. The growth of large mammals is small - under favorable conditions, their numbers increase only 1.05-1.1 times, whereas the crustaceans (eg, aphids, daphnia) numbers for the year 1016-1030 can grow in time. The number of bacteria and unicellular algae under ideal conditions will grow exponentially (exponential growth). Absolute rate of growth of population is called the amount of change per unit time, and specific growth rate - the rate of growth per capita, and this value reflects the biotic potential. As a rule, the exponential growth is a short period of time and with an excess of resources (food, places for breeding, lack of adverse environmental factors). As the deterioration of the specific growth rate decreases and, after reaching a certain density, size of the population stops growing. This limiting density, which reaches a population in a specific context, called the capacity of the environment. When population density is equal to carrying capacity, the rate of resource consumption equals the rate of recurrence, ie,population size will fluctuate near some average level. When the density becomes greater carrying capacity and resources is small, increases in population mortality.In populations of multicellular organisms, the specific growth rate depends on the fertility and mortality, whereas in populations of microorganisms growth rate depends on the rate of cell division (the original cells are divided into subsidiaries, which determines the population growth). In multicellular organisms differentiate specific fertility (it is expressed in individuals per individual per unit of time - for example, a person express their birth rate per year per 1,000 persons) and the absolute fertility (number of individuals appearing in the population per unit time), and mortality ( absolute and specific) - the rate decrease population size due to loss of animals from predators, disease, old age or other reasons. If the birth rate is higher mortality, specific growth rate is positive, if the mortality rate is higher birth rates, the rate of growth will be negative - population size decreases. Fertility, mortality and population dynamics of the population is directly related to the age structure of populations. Therefore, in order to predict changes in the number of real natural populations should be aware of its age structure, sex composition, and fecundity of individuals of all ages. For these calculations using different mathematical models. They are quite complex and require payment for use of computers. Under natural conditions, changes in population size is oscillatory. The amplitude of oscillation depends on the characteristics of the species and the conditions of existence. Many large vertebrates (with relatively constant environmental conditions), the number fluctuates only a few times, whereas, for example in insects, the population size fluctuations is wider - at 40-50 times, and in some years, under favorable conditions, there are sharp outbursts of when it rises to tens or even millions of times. For example, outbreaks of locust population, occurring at intervals of approximately forty years. In addition to occasional outbreaks of, there may be regular and flash, which will be caused by various factors (temperature, food resources, etc.). Factors affecting population size, is divided into independent (abiotic factors) and dependent (biotic factors - competition, predation, food supply, spread of infections, etc.) on its density. The multiplicity of mechanisms of regulation of population size in the natural environment contributes to the fact that in natural populations is rare catastrophic population growth, resource erosion and loss of population. All of the adaptive response of organisms in nature are intended to increase the likelihood of survival and leaving offspring. Environmental Strategy - general description of the growth and reproduction of the species. It includes the growth rate of individuals during puberty, fertility and other characteristics.Environmental strategy depends on many factors, but especially important are those that have an impact on the strategy of growth and reproduction, that is, factors affecting mortality. Environmental strategies are very diverse, but mainly produce two extreme types of strategies, known as r-and K-strategy (the characters are from the logistic equation of growth of population). r-selection strategy based on increasing the rate of population growth during periods of low density, and K-strategy is caused by the selection for increased survival at high population densities close to carrying capacity. K-strategists are slowly evolving, are larger and longer life spans, form a small number of larger, well-protected offspring; r-strategy is more common in organisms living in an environment with sudden changes in environmental conditions - in the ephemeral (existing only briefly) biotopes in ephemeral pools. The number of such populations varies widely, but there is a high rate of reproduction, as a favorable period of time is very short-lived and individuals benefit quickly reach maturity. K-strategy is characterized by more than the populations living in stable environmental conditions, or in a naturally evolving (eg seasonal changes). Therefore, the K-strategists are less numerous progeny, but competitive. Between r-and K-strategies, many transitions, but the selection of these two basic types of strategies helps to explain the relationship between the different quality characteristics of the population and environmental conditions.