DISTRIBUTION OF ANIMALS
(Dr. Girish Chandra)
Zoogeography deals with the study of distribution of land animals and those living in freshwater. Marine animals cannot be confined to any one region as the seas are interconnected and the only possible barrier for them can be climate. Hence most of the animal distribution studies are based on the land vertebrate fauna that also includes freshwater fishes and amphibians, which due to osmotic problems cannot cross salt water to reach from one land mass to another, although land makes only about 29% of the earth’s surface and land vertebrates only 2% of the entire animal kingdom. Theoretically zoogeography is the study of all animals that includes invertebrates of which insects constitute the largest group. Earlier studies involved almost exclusively the bird fauna, which being gifted fliers can cross all kinds of barriers and some of them do reach from one end of the globe to another.
Fascination for zoogeographical studies arose from the Darwinian philosophy that animals lived where evolution made them most adapted to live in. But a cursory look reveals that elephants, zebras, giraffes, lions and a large number of African animals can live equally comfortably in South America and so can do the tropical American monkeys, jaguars, Llamas, sloths, armadillos, anacondas and a large number of birds in Africa but they are limited to their regions. Alligators can live almost in any tropical habitat but are confined to the New World and China, whereas crocodiles occur in the tropics of all continents. Given the opportunity, marsupials will do well in any other continent but are restricted to Australia and one of them, opossum, does exist in the New World. No placental mammal existed in Australia and New Zealand until recently when we carried our pets and sheep along and some of them escaped and became wild to disturb the ecological balance that existed for millions of years. Lungfishes and ostriches inhabit widely separated continents of South America, Africa and Australia. Camels are found in the deserts of Middle East to Mongolia and India but one species reaches as far away as South America. Similarly, four species of tapirs live in South America but one species has gone to far eastern Sumatra and nowhere in between where climate is quite suitable for it to live. Limbless amphibians also show similar distribution. Such peculiarities in the distribution of animals triggered more detailed studies to find out why animals could not be distributed in all places where climate and other environmental conditions are conducive.
PRINCIPLES OF ANIMAL DISTRIBUTION
Species of animals and plants show some well-known patterns of distribution, which have resulted from adaptations that they have acquired during evolution. Jordan and Kellog proposed three laws that govern distribution and postulated that every species should be found everywhere unless:
- It was unable to reach the place where it is absent due to barriers.
- It was unable to stay and adapt there because of unfavourable conditions.
- It became modifies into another species due to directional selection.
Other principles of distribution are as follows:
- Species are distributed in areas where they are most adapted.
- Different barriers act differently for different species depending on their modes of locomotion and dispersal.
- Poikilotherms generally spread slowly as compared to homeotherms (birds and mammals).
- Food, temperature, water etc. restrict animals to an area even without barriers. For example, penguins are restricted to Antarctica, giant panda to bamboo forests in China and Koala to eucalyptus forests of Australia.
- Parasites generally disperse with their hosts and predators along with prey.
- Majority of birds do not face any physical barriers due to their ability to fly over them.
- Low temperature prevents tropical animals to migrate northwards and polar and temperate animals to migrate southwards.
- For aquatic animals land is the barrier while for terrestrial animals sea, rivers and lakes are physical barriers.
- Oceans act as barriers for freshwater fishes and amphibians due to osmotic problems.