Factors causing a change in numbers can be divided into two groups:
– factors independent of population density (modifying) and
– Dependent on population density (regulatory) factors.
Abiotic factors are predominantly independent of population density. They act on the population in any of its numbers. Biotic factors — natural enemies (predators, parasites, pathogens) and food resources — are dependent on population density. Their number varies with the change in population size. Biotic relationships in the form of interspecific interactions are the main factor regulating the number of populations. Changes in the number of natural populations are associated with the biological characteristics of the species and the nature of the factors that control the number of these populations. For example, the Siberian cocoonworm. Typically, about 90% of the eggs of these butterflies are affected by egg-eating parasites, which restrains their numbers. However, in the most severe frosts, a significant portion of the parasites die. Therefore, the massive emergence of caterpillars that feed on needles occurs after frost. Outbreaks of Siberian cocoonweed sometimes lead to the death of conifers over a vast area.
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