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In this article we will discuss about the six main types of dispersal of animals.
(1) Gradual Dispersal:
Gradual dispersal is secular in nature because it involves longer period of time and very slow rate of migration but such dispersal covers larger areas and results in wide-spread distribution of animals over time.
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(2) Rapid Dispersal:
Rapid dispersal involves mass exodus of animals from one area to the other area but the animals involved in such type of dispersal are unable to colonize in the areas where they are rapidly migrated or dispersed because either they are not allowed by man or by the local environmental conditions to settle down in new habitats. Such rapid dispersal involves certain animals only e.g., lemmings, locusts, butterflies, several varieties of moths, dragonflies etc.
(3) Seasonal Dispersal:
Seasonal dispersal involves those seasonal migratory animals which leave their habitats due to seasonal extreme weather conditions every year and migrate to those areas which have favourable seasonal weather conditions. These migratory animals again return to their native places when the season becomes favourable for them.
Many arctic birds leave their places during winter season every year because of the severity of cold and come to subtropical and tropical areas and they again return back to their respective native places during next summer season. The migratory birds, inspite of dangers of atmospheric storms, droughts and of man himself, millions in numbers, undertake bi-annual journeys covering the whole of continents and oceans.
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Some of the Greenland wheatears birds fly directly from Greenland to Spain during winter seasons covering a distance of 3,200 km. Similarly, blackpoll warbler (a small bird like sparrow) flies directly from New England of the U.S.A. during winter season to Venezuela of South America. Arctic terns cover the longest distance among all the seasonal migratory birds as these cover the total distance of over 35,000 km each year.
Some of the Siberian arctic terns have been located during summer season in Australia (there is winter season in the northern hemisphere). Many Siberian birds visit India each year during winter season and they return back to their native places during summer season the same year.
(4) Forced Dispersal:
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Forced dispersal occurs when animals are forced to leave their native places due to sudden and catastrophic events having adverse effects on animals e.g., prolonged droughts, recurrent floods, forest fires, volcanic eruptions etc.
All such severe events cause scarcity of food leading to famines. Consequently, the animals of the affected locality migrate to other such areas where they can get plentiful food. Many animals migrated from Siberia because of severe forest fire which broke out there in 1915.
(5) Sometimes many animals migrate from their native areas because they do not find sufficient and suitable breeding places due to phenomenal increase in the populations of animals.
(6) Anthropogenic Dispersal:
Anthropogenic dispersal recently man has largely been involved in the dispersal of animals through his deliberate efforts. The dispersal of animals to very distant places by man has introduced diversification in the regional fauna on the one hand, while it has caused several environmental and ecological problems on the other hand.
For example, the arrival of European deers and rabbits in New Zealand has resulted into large-scale destruction of native natural vegetation due to mass grazing. This has led to accelerated rate of soil erosion and thus the loss of agricultural land and fertile soils.
Besides, dispersal of animals is also caused due to continental drift and climatic changes.