The following points highlight the two major barriers to dispersal of animals. The barriers are: 1. Physical Barrier 2. Biological Barrier.
1. Physical Barrier:
The land is a barrier to aquatic animals and water is barrier to most land animals.
Large bodies of water:
This is the most effective barrier to almost all land animals including even the birds. The opposite banks of Amazon river are inhabited by different species of birds.
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Salt water:
It is a nearly absolute barrier to amphibians:
(i) The Pacific Islands are usually uninhabited by amphibians, except that they have been introduced by man.
(ii) In the Pacific coast many freshwater rivers follow almost parallel courses to the ocean. Each river has its own subspecies, even species. The expanse of salt water separating the mouths of rivers, even very small, is not crossed by fishes. If the streams are joined by flood in rainy season, the fishes move from one to the other river.
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(iii) The marine animals living in open sea—where the salinity is about 3.5 per cent—do not move to areas of low salinity like brackish and estuarine water, and vice versa.
Mountains and lowlands:
Mountains are described as islands in the sea of low lands. Mountains serve as barriers to low-land animals and the low lands are barrier to animals inhabiting mountain ranges.
The mountain goat and big horn sheep are restricted to mountains as they cannot cross low lands. The American opossums cannot cross high mountains. They thrive well in the low coastal range of California but could not invade Sierras. White-footed American mice are restricted to low lands.
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Forests and Prairies:
Extensive forests serve as barriers to the dispersal of grassland animals, prairies serve as barriers to forest animals. Here, adaptation to a particular type of food and also the distance are most important. Forest animals do not usually cross the grassland, while grassland animals do not cross large forests.
Temperature:
The temperature is an effective barrier to dispersal of animals.
(i) Animals in the surrounding areas are different from those living in deserts. Only those, specially adapted, cross the deserts and the desert animals do not move out of their habitat.
(ii) Arctic fox, polar bear, penguin and other animals of cold region are restricted within a definite temperature range and the same is true for animals of the tropical and subtropical zones.
(iii) Fishes can detect small differences in water temperature, as low as 0.1°C and possibly 0.03°C. The temperature difference sets a limit to the distribution of many fishes.
2. Biological Barrier:
The effectiveness of such barriers in the dispersal of animals is, however, somewhat doubtful. A few such facts may be mentioned.
Sedentary habit:
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A number of marine invertebrates, viz., cnidarians, echinoderms and others are sessile, but their free-living larvae can travel long distances. Certain vertebrates with powerful locomotors organs often keep them restricted within a limited area of distribution.
Homing instinct:
In birds and other vertebrates, homing instinct is an effective barrier and limits distribution. In the geese, the family groups migrate together resulting in a high degree interbreeding.