In zoological parlance, gradation denotes the temporary mass reproduction of an animal species. It occurs in many types of insects, such as bark beetles and locusts , but also in mammals such as lemmings and field mice . If the weather is good and the food is good, they can multiply explosively. This exponential growth rate is known as progressation . Diseases, increased occurrence of predators , changed weather and food conditions, or direct anthropogenic interference ( e.g. large-scale use of pesticides ) stop the gradation and lead into the retrogradation phase , in which the mass increase collapses and the population density often falls below the regional average this type sinks.
In the case of predators who have also reacted with increased reproduction to the mass multiplication of their food animals, the collapse of a gradation often leads to an area leaving an area called evasion , which must not be confused with the cyclical animal migrations . An example of animals with such behavior are the invading birds .
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