Biological Cycles

rhythmic repetition of biological phenomena in associations of organisms (populations, biocenoses), which serves as an adaptation to cyclic changes in their conditions of existence. Biological cycles are included in the more general concept of biological rhythms, which includes all rhythmically repeated biological phenomena. Biological cycles may be daily, seasonal (annual), or at intervals of many years.

Twenty-four-hour biological cycles are expressed in regular fluctuations in the physiological phenomena and behavior of animals during the course of a 24-hour day. At their base lie automatic mechanisms which are corrected by the effects of external factors: 24-hour variations in illumination, temperature, moisture, and so on.

At the base of seasonal biological cycles lie those metabolic changes which in animals are regulated by hormones. In various seasons the condition and behavior of organisms within a population or a biocenosis change: accumulation or consumption of reserve substances may occur; shedding of outer layers (molting), reproduction, animal migration, hibernation, and other seasonal phenomena begin or end. Being to a considerable degree automatized, these phenomena are corrected by external influences (weather conditions, food reserves, and so on).

Biological cycles lasting many years are conditioned by cyclic variations of climate and other conditions of existence (in connection with changes in solar activity and other cosmic and planetary factors). Such biological cycles occur in populations and biocenoses and are expressed in fluctuations in reproduction and the numbers of certain species, in resettlement of populations in new places, or in the extinction of parts of populations. These phenomena are the end result of cyclic changes in populations and biocenoses and of fluctuations in their conditions of existence, mainly climate.