The triggering of regulatory mechanisms can cause fluctuations in the number of populations.
Three main types of population dynamics can be distinguished: stable, cyclical, and abrupt (explosive).
The stable type is distinguished by a small range of fluctuations (sometimes the number increases several times). It is characteristic of species with well-pronounced mechanisms of population homeostasis, high survival rate, low fertility, long life span, complex age structure, and developed care for offspring. A whole complex of efficiently working regulatory mechanisms keeps such populations within certain density limits.
Example: the dynamics of the number of large mammals and birds, as well as a number of invertebrates.
Cyclic type. Oscillations occur in a significant range of densities, differing by one or two orders of magnitude. The return to a stable state is quick. Regulatory mechanisms do not lose control over the population size, increasing their efficiency following an increase in density. Such a course of numbers is widespread in different groups of animals.
Example: seasonal fluctuations in the abundance of many species: mosquito clouds; fields overgrown with flowers; forests full of birds – all this is typical for the warm season in the middle lane and practically disappears in winter. Fluctuations in the number of some species of northern mammals: cycles of three- and four-year periodicity are characteristic of many northern mouse-like rodents – mice, voles, lemmings, snowy owls, Arctic foxes, etc.
With a spasmodic (explosive) type of population dynamics, the termination of the action of modifying factors does not cause a rapid return of the population to a stable state.
Populations of species with this type of population dynamics are periodically characterized by extremely high and unusually low levels of abundance. Such a course of numbers is found most often in species with a short lifespan, high fertility, and a rapid turnover of generations. It is characteristic, for example, of some insects (locusts, forest pests – barbel, bark beetles, a number of lepidoptera and sawflies, etc.), among mammals it is noted in many species of murine rodents.
Example: information about the invasion of the wandering locust goes back to ancient times. Locusts live in deserts and dry areas. For many years, it does not migrate, does not devour crops and does not attract attention. However, from time to time, the density of locust populations reaches monstrous proportions. Under the influence of crowding, insects undergo a number of changes in their appearance (for example, they develop longer wings) and begin to fly to agricultural areas, eating everything in their path. The reasons for such population explosions are apparently due to the instability of environmental conditions.