All animals do not spread across the sea or other barriers with the same speed, some spread faster and others slowly and some do not cross the barriers at all. Therefore, distribution also depends on the animal’s body size, psychology, reproductive rate, locomotory organs, physiological endurance and some means to carry them to long distances across barriers. Some means of dispersal are discussed below.
1. Land bridges. They are land connections between two large land masses which are separated by sea that may have existed in the past and facilitated movement of animals across them. The theory of land bridges was formulated to explain away discontinuous distribution of animals in continents that are presently separated by thousands of kilometres of oceans, For instance, ostriches and lungfishes occur in South America, Africa and Australia and few species of marsupials are found in South America apart from their home in Australia and alligators exist in America and China. They are of two types, namely, Corridor bridges and Filter bridges.
1. Corridor bridges. They are land connections of continent size stretching across oceans and connecting two continents. When a single continental mass called Gondwanaland existed in Mesozoic, all southern continents, namely, South America, Africa, Australia and Antarctica were connected by huge corridors across which animals could freely migrate. The continuity of southern land masses can account for the distribution of marsupials, lungfishes, flightless birds, side-neck turtles and crocodiles if we presume that the bridges existed after the origin of these groups and that Antarctica was a habitable continent at that time.
South Atlantic Corridor. This bridge is supposed to have connected South America with Africa and also included islands of St. Helena, Tristan da Cunha and Ascension and facilitated spread of lungfish, characin fish, pipid toads, ostriches, porcupines and monkeys which are common in both the continents today.
Lemuria Corridor. The continent of Lemuria is believed to have existed till early Coenozoic and connected Africa, Madagascar and India across which lemurs and their relative lorises spread.
Antarctica Corridor. This would have been the largest land bridge that connected South America with Australia through Antarctica during Mesozoic and its position was more northerly and habitable.
Panama Corridor. During end Cretaceous and early Palaeocene North America and South America were connected by a narrow corridor, through which migrated to the south marsupials and early placentals such as Condylartha which were ancestors of modern ungulates. After Palaeocene this corridor submerged, disconnecting the two continents and isolating the South American fauna which then got a chance to diversify. The two continents again joined together sometime in Pliocene facilitating advent of New World Monkeys, rodents and placental carnivores.
Arctic Corridor. During Mesozoic Nearctic and Palaearctic Regions were connected by a broad corridor that later narrowed but continued until late Eocene, allowing free exchange of marsupials and insectivores and also freshwater fishes and amphibian.
Land bridge theory does not explain with certainty the distribution of late Mesozoic and Coenozoic animals and can perhaps be used to explain invertebrates, fish, amphibians and primitive reptiles as these animals existed when the super continents, Laurasia and Gondwanaland existed and animals were uniformly distributed over all continents.
2. Filter bridges.Filter bridges are series of islands between two land masses that allow some animals to spread across but stop others. The animals could spread by “island hopping”, crossing small stretches of sea by swimming, flying, rafting or through wind. Such a filter bridge now exists between the Oriental and Australian Regions through Wallacea. The two regions are separated by a chain of islands called Malaya Archipelago and it is difficult to determine which island belongs to which region. Two lines, namely, Wallace’s line and Weber’s line were suggested as boundaries and they enclose an area called Wallacea that contains large number of islands that serves as Filter Bridge between the Oriental and Australian regions.
Bering Strait contains a chain of islands that served as Filter Bridge during Oligocene and Miocene and then after Pleistocene between eastern Asia and Alaskan side of North America.
During late Cretaceous Africa got disconnected from Eurasia, was surrounding by sea on all sides and perhaps had a filter bridge in the north which lasted until late Oligocene. Arboreal primates such as lemurs and other prosimians and insectivores arrived in Africa from the Palaearctic through this route and later evolved into old world monkeys and apes.
2. Sweepstakes. Rafts, driftwood, icebergs and other floating objects in the sea can carry small animals, their eggs and other stages to long distances. But this is a one-way transport, uncertain and enormously dangerous. Natural rafts are made when trees are uprooted by storms, cyclones or hurricanes to make small floating islands on which insects, snails, reptiles of all kinds, rats, insectivores and other small animals can make a journey to islands thousands of miles away. Rats are known to be accomplished rafters and hence can boast of worldwide distribution.
3. Winds and storms. Wind is used by many plants for dispersal of their seeds for which they possess specialised aerodynamic structures to keep them airborne and drift to long distances. Flying insects can also be carried by air current to long distances across oceans. Insect nets tied to airplanes have collected insects at altitudes of 15,000 to 20,000 above sea level. Birds and bats can make use of favourable air currents to cross long stretches of sea.
4. Through birds. Birds being gifted with the power to fly can cross long stretches of sea to travel from one continent to another and to remote islands in the sea. Water birds can carry eggs of snails and seeds of plants on their legs and transport them across the sea. Parasites such as lice and mites can travel on their bodies hidden under the feathers and helminths in the intestine only to be released in remote uninhabited areas elsewhere.
5. Human agency. Rats, cockroaches, houseflies and grain feeding insects have been constant companions of man in habitation as well as in travel. They are quite common in cargo ships and have spread to all places visited by man. Pets such as dogs, cats, rabbits, sheep and goats have travelled with man to all parts of the world and have sometimes become wild as in Australia where otherwise placental mammals were unknown.