The population has “biological characteristics” that it shares with all its constituent organisms (it also grows, differentiates, that is, it is subdivided into smaller subgroups, changes along with changes in living conditions).
At the same time, it has “group characteristics” that serve as unique characteristics of the group and are inherent only in the group of individuals as a whole.
For example, indicators such as abundance (total number of organisms), fertility (rate of population growth), mortality (rate of decline in numbers as a result of the death of individuals), age composition (ratio of the number of individuals of different ages) cannot be applied to individual organisms, they can only characterize the population as a whole. These indicators are called demographic indicators.
They cannot be applied to individual organisms; they can only characterize the population as a whole. An individual is born and dies, has a particular age, but in relation to an individual, one cannot speak of fertility, mortality or age structure – characteristics that make sense only at the group level.
The study of demographic indicators is of great practical importance. It is important to know demographic indicators to understand the laws governing the life of populations, to predict the constant changes taking place in them.
Example: When deforestation is deforestation, it is very important to know the speed of its regeneration. When fishing for commercial fish, it is necessary to take into account the ability of their populations to renew their numbers.
In all these cases, we are primarily interested in changes in the population as a whole, the ability to predict these changes, to regulate them (for example, a decrease in the number of pests of agricultural land).
It is imperative for this to know the causes and rate of population changes, as well as to be able to measure various parameters of these natural objects.
Let’s consider these indicators in more detail.
Abundance is the total number of individuals in a population.
The measure of the abundance of individuals in a population is the total population size, or its total biomass. However, the measurement of these indicators in relation to many animals is associated with great difficulties, therefore, the indicator that is usually used to judge the decrease or increase in the population size is density.
Population density is the number of individuals or their biomass per unit area or volume.
Example: 1000 shrubs per 1 ha of forest; 20 thousand individuals of euglena per 1 m³ of water.
Population density is used in cases where it is more important to know not the specific size of the population at a particular moment in time, but its dynamics, that is, the course of changes in population size over time. Density in these cases serves as a measure of relative abundance.
Fertility is the number of individuals born in a population over a given period of time.
Fertility will determine the possibility of the development of the population due to the reproduction of the individuals inhabiting it.
Distinguish between maximum fertility (sometimes called physiological, or absolute) and ecological, or just fertility.
The maximum (potential) fertility is the theoretical maximum of the rate of formation of new individuals under ideal conditions (when there are no external factors that inhibit the reproduction process).
The maximum fertility is largely determined by the ability of females to simultaneously produce any amount of offspring, that is, physiological fertility, as well as the total number of offspring (for a certain period).
Ecological fertility – gives an idea of the rate of increase in the population size under the actual living conditions of the group of individuals under consideration.
Ecological fertility is not constant and changes depending on the physical conditions of the environment and the composition of the population.
Species that do not take care of their offspring are characterized by high potential and low ecological fertility.
Example: an adult female cod spawns millions of eggs, of which, on average, only 2 individuals survive to adulthood.
The number of born individuals is gradually decreasing, as living organisms die. In ecology, the concept of mortality is used to characterize this process. It can characterize individual population subgroups or the population as a whole.
Mortality is the rate at which a population declines due to the death of individuals.
Mortality determines the average life span that characterizes a group of individuals. Life expectancy is longer in the group where mortality is lower.