Dispersal of Animals

the gradual expansion of the region inhabited by certain animal species. The dispersal of animals is usually linked with a change in abiotic and biotic environmental conditions and in the population size of the animals. The state of a population of a given species, for example, an increase in number that causes greater population density, stimulates the dispersal of the animals. Hence, the general warming trend in Eurasia in the 20th century has influenced the northerly shift in the northern boundary of distribution for a number of animal species.

Dispersal can be either active (running, swimming, or flying) or passive (carried by rivers, ocean currents, objects floating in the ocean, or wind). Passive dispersal mainly characterizes such small animals as marine plankton and insects, but sometimes it is significant for larger animals as well. For example, a flock of white herons was transported by a storm from Africa to America, where the birds then settled. Small reptiles, such as snakes and geckos, sometimes are dispersed on floating tree trunks. Some small animals are dispersed by larger ones; thus, birds transport not only parasites but sometimes also mollusks and the eggs of freshwater animals.