Ecology (from Greek oikos - the house, the dwelling, a dwelling and logos - the doctrine, a thought) - science about functioning of ecological systems. Ecosystems consist of objects of live and inanimate nature. Populations (Latin populatio - the population) - basic elements of an ecosystem. All populations in the nature form a certain unity which develops and works under the laws.
To understand how the ecological system functions, it is necessary to know characteristics of the populations making this system. Population is characterized in general by demographic characteristics: birth rate; mortality; structure of individuals on age structure; number of individuals (their abundance).
Demographic characteristics reflect the speed of the processes happening in population. They make sense only for group of individuals: it is impossible to speak about birth rate and mortality in relation to a certain individual. Knowledge of demographic characteristics of population is important for forecasting of possible changes, both in the population, and in all community in general.
Population as set of organisms is in the best way characterized by abundance. An abundance measure - the population number (general biomass). However measurement of this indicator for many populations of animals is connected with great difficulties. Therefore, as a rule, instead of abundance for characteristic of population use a concept of density. Population density - number of individuals per unit area (biomass density). Examples of density of populations: - 300 trees on 1 hectare of the wood; - 4 million individuals of a chlorella on 1 cubic meter of water; - 100 kg of fish on 1 hectare of a surface of a reservoir. The ability of population to increase in number characterizes birth rate. The birth rate is a number of the individuals who were born for a certain period. Distinguish two types of birth rate: 1. The maximum rozhdayemostmaksimalny birth rate - a concept purely theoretical. Shows what maximum speed of the birth of new individuals in the absence of the constraining external factors. The maximum birth rate is defined only by physiological fertility of females.2. The ecological rozhdayemostyekologichesky birth rate considers already actual situation of life in population. Gives an idea of how the considered group of individuals will propagate in reality. Ecological birth rate - size changeable: it depends on structure of population and physical environmental conditions. The high potential birth rate and low ecological birth rate is characteristic of types which do not care for the posterity. For example, the female of a cod throws millions of berries, but from them live up to an adult state on average 2 individuals.