The factors causing changes in numbers can be divided into two groups:
- population density independent (modifying) factors and
- population density-dependent (regulatory) factors.
Predominantly abiotic factors are considered independent of population density. They act on the population at any size.
Example: especially harsh winters cause the death of wintering cabbage whites, regardless of whether a large or small number of individuals make up this population in a given winter period. Or, conversely, favorable wintering conditions can contribute to an increase in the number of individuals both in small populations and in large ones.
Abiotic environmental factors can cause significant fluctuations in population size.
The population density-dependent factors include biotic factors – natural enemies (predators, parasites, pathogens) and food resources. Their number changes along with the change in population size.
Example: it was found that as soon as the population density of a particular prey species increases, the population of its predators also increases. Parasites and diseases can have the same effect on the host population.
Sometimes the reasons for fluctuations in population size may lie in themselves. This happens when the mortality or fertility of organisms changes in response to changes in their population density, that is, the number of individuals per unit area.
Mechanisms of this kind are called regulatory, they are triggered automatically when the population density reaches either too high or too low values.
Regulatory mechanisms can be behavioral or physiological reactions of organisms to changes in population density. Cases are known when under conditions of overpopulation in a number of mammals there are sharp changes in the physiological state, which affects the behavior of animals, reduces their resistance to diseases and other adverse effects.
Example: White hares often die suddenly from “shock sickness” during peak population periods. In some fish species, at a high population density, adults switch to feeding on their juveniles (cannibalism), as a result of which the population begins to decline.
An increase in mortality and a decrease in fertility under the influence of high density are observed in populations of many species of animals and plants. In all these cases, the signal for triggering regulatory mechanisms is given by the population itself, or rather its density.
The triggering of regulatory mechanisms can cause cyclical fluctuations in the population size.
An example of cyclical changes is given by fluctuations in the abundance of some species of northern mammals. For example, cycles of three- and four-year periodicity are characteristic of many northern mouse-like rodents – mice, voles, lemmings, polar owls, Arctic foxes, etc.
As a rule, the higher the population density, the stronger the influence of regulatory factors. Without them, the population size could grow indefinitely, which would lead to the complete destruction of food sources. At the same time, the regulating factors, in contrast to the modifying ones, never bring the population size to zero due to the fact that the strength of their action decreases as the population size decreases.
Thus, the peculiarity of the action of factors depending on density is to smooth out sharp fluctuations in the number, due to which the population size is maintained at a certain optimal level.