Population Dynamics, Animal

the regular patterns of change in the number of individuals in the population of a given species during the course of a year (seasonal) or a number of years (perennial), determined by changes in the individual rates of birth (fertility), death, and migration (emigration or immigration).

Animal population dynamics is a species adaptation to the rhythms of change of local conditions. The number of individuals of slowly reproducing species (large predators, ungulates) with great longevity increases insignificantly during the course of a season. In contrast, the size of population of animals that bear several litters each year and mature rapidly may increase many times in the course of a single year or season. Thus, the size of population of many species of rodents increases under favorable conditions dozens, even hundreds, of times from spring to autumn; the population of many species of insects, such as Diptera, increases as much as 1,000 times. The higher the fertility of the species and the greater its capacity to increase the rate of reproduction under favorable conditions, the wider is the possible range of fluctuation of its population during the year. The connection between seasonal and perennial animal population dynamics is a function of the correlation between the average life span and the fertility, which depend on the morphophysiological adaptations of the species to its habitat and its innate degree of parental care.

The Soviet biologist S. A. Severtsov (1941) distinguished a number of types of animal population dynamics: from long-lived animals with few offspring and stable populations (ungulates) to “ephemera” with extremely unstable numbers, short lives, and high fertility rates (small rodents, many insects and other invertebrates). When mortality is high under natural conditions a cessation of reproduction or a decrease in its rate leads to a substantial decrease in population; the population is restored when there is a rise again in the rates of reproduction and survival. Under favorable environmental conditions a new period of reproduction begins and the population level increases, creating the preconditions for a population explosion of the species. Both presently active and preceding conditions have an influence on reproduction and survival. A particular perennial periodicity of population dynamics has been observed for certain vertebrate species, such as lemmings and certain predaceous mammals, for Which population maximums are reached every three or four years. There are indications in certain species of mammals and insects of an 11-year population cycle, reflecting the cyclic character of solar activity. The periodicity (rhythmicity) of animal population dynamics is distinct in relatively simple biocenoses (tundra, steppe, desert) and less pronounced or practically nonexistent in more complex biocenoses (especially in tropical rain forests).

Regulatory mechanisms (factors) of three types lie at the basis of animal population dynamics as a process of adaptation to local conditions: individual adjustment (adaptation), biocenotic regulation, and population regulation. Individual adaptations are expressed in metabolic adaptations to the physical and chemical conditions of the environment (temperature, humidity, atmospheric composition, salinity). Deviations of these conditions from the norms lead to a state of stress, by means of which the body overcomes the unfavorable influence of a factor (but only to a certain limit, beyond which the animal will die). Biocenotic regulation is expressed principally in the relations between organisms that serve as food and their animal-consumers (plants and herbivorous animals, predators and their prey, parasites and hosts). In population-biocenotic regulation the size of the population depends on the food base and on the population density or the dimensions of the inhabited space. After a drop in the number of animals the possibilities of the development of mass diseases and the influence of predators decrease, the availability of food increases, and the number of animals rises. The falling off of food availability and the worsening of other conditions, the results of high population numbers, exert a negative effect on subsequent reproduction and the viability of individual animals. A situation arises that favors the development of diseases and the influence of predators. A drop in numbers occurs, usually to the level at which energy expended in food gathering is compensated by the food consumed. A decrease in the number of animal-consumers creates the preconditions for a restoration in the number of those animals that serve as their food.

At the basis of population regulation lie certain neurohumoral mechanisms that inhibit or accelerate (depending on population density) the rate of reproduction (speed of sexual maturation, female fertility, male activity), the mobility of the animals, and mortality. Among the factors that depend on density, the speed of sexual maturation has special significance. Populational mechanisms play an important role in the life of both vertebrates and invertebrates. The basis of these mechanisms is intrapopulational organization (structure), or groupings of individuals (families, herds, flocks, colonies, parcels, or demes), that ensure relative orderliness in the use of a territory and the achievement of a certain optimal density of distribution. Such organization is maintained by means of chemical, optical, acoustic, electric, and mechanical means of signaling and communication, which are used by higher and lower animals alike.

Various factors may play a leading role in the population dynamics of various species and ecological groups. Thus, the population dynamics of predators is determined to a considerable degree by the state of the food base: the number of squirrels and many mice depends on the seed harvest, and the population size of grazing and browsing species (ungulates) is determined in large measure by predators and parasites, whose numbers are an immediate and concrete function of the number of prey.

Animal population dynamics in different years and in various parts of a species’ area of distribution may differ in both the character (amplitude) and the mechanisms of fluctuation. As a rule, the significance of biocenotic, and especially of populational, factors decreases the periphery of a territory, while environmental (especially climatic) factors, which act both directly and through the food base, acquire primary significance. Conversely, as the optimum population for a territory is approached, the factors that depend on population density (biocenotic and populational regulation) acquire increasing significance. There is a complex relationship between the density-dependent and the independent factors, the division of which is arbitrary. At different stages of the population dynamics curve different factors acquire primary significance. For example, the role of predators, as well as of many diseases, increases during a period of population decline. A sharp change in climatic factors, as a rule, produces nonperiodic changes in the size of the population that are superimposed on the basic population dynamics curve. Mass deaths may be caused by spring floods, unseasonal return of cold weather, deep snow cover, severe drought, and so on.

A study of the principles of animal population dynamics is necessary in order to build a scientific foundation for the rational use of beneficial animals and the control of harmful ones. Mathematical methods (in particular, simulation) are used in this kind of study. By influencing animals or the environment they inhabit, man changes animal population dynamics. In catching fish, birds, and fur-bearing animals or in exterminating pests, man artifically thins out their populations. This decreases the competition for food, shelter, and dwelling places and increases the chances for survival of the remaining individuals; deaths from the so-called natural mortality factors are sharply reduced and fertility increases. However, excessive destruction of animals, as well as destruction of their food reserves, nesting places, and places of shelter, leads to their disappearance, starting with the less favorable places of habitation. This dissociates populations and leads to their gradual extinction.

By improving the food supply and protective properties of a territory, man may increase the number of animals and make them more stable, even while continuing intensive exploitation.

While the term “population dynamics” is rarely used to refer to plants, it has much in common with the concept of harvest.