Give examples of cyclical fluctuations in the numbers of animals or plants.

For natural populations distinguish:
1) seasonal changes in numbers associated with seasonal changes in environmental factors,
2) fluctuations that are caused by weather changes. Seasonal changes in numbers are most pronounced in many insects, as well as in most annual plants.
Some species of northern mammals and birds demonstrate examples of significant fluctuations in numbers, in which either 9–10 or 3–4 year cycles are observed. A classic example of 9–10-year-old fluctuations is a change in the number of white hare and lynx in Canada, with peak hare numbers for a year or more preceding peak lynx numbers.
To assess the dynamic state of plant populations, an analysis is made of age-related (ontogenetic) conditions. The most easily defined sign of a stable state of a population is a full-term ontogenetic spectrum. Such spectra are called basic (characteristic); they determine the definitive (dynamically stable) state of populations.
The most famous examples of cyclic fluctuations include joint 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) and their predators (polar owl, arctic foxes), as well as hares and lynxes.

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