The following points highlight the two main factors which stimulate, affect and control the dispersal of animals. The factors are: 1. Physical Factors 2. Biotic Factors.
1. Physical Factors:
Environmental factors include land and water distribution, sediments, physical barriers, physical events, distance etc. Lands and waters are barriers for the dispersal and migration of aquatic and terrestrial or land animals respectively. Water bodies such as large lakes, inland seas, and oceans are effective barriers for the dispersal and migration of land animals because most of the land animals lack in swimming mechanism.
Even those land animals which have the swimming mechanisms cannot cross over the wide expansion of seas and oceans because of large distances. Thus, the water bodies completely deter the dispersal of non-swimming animals, slow down the rate of dispersal of land animals having swimming mechanisms over smaller water bodies like lakes and large water bodies like seas and oceans completely make the dispersal of land animals (whether having swimming mechanisms or not) impossible.
If such animals (having swimming mechanisms) venture in the seas and oceans, they die soon after they become tired. There is a genetic tendency among the lemmings of Norway to move westward every year (lemmings have inherited the habit of seasonal westward migration from their ancestors who used to migrate westward as North America was united with Eurasia before Jurassic period) and in doing so they die in the Atlantic Ocean (Norwegian Sea) every year.
Several cases of water barriers have been reported e.g.:
(i) H. Wolda (1963) has reported from his studies of ‘natural populations of polymorphic land snails’ that about half of the total population of 300 snails of the species of Capaea nemoralis died while crossing a narrow ditch full of water (2.4 km wide);
(ii) K. Zimmerman (1935) has reported through his study of ‘race analysis of the middle European vole’ that river Elbe is an effective barrier to voles because they cannot cross over the rivers;
(iii) K.R. Kelson (1951) has reported that the families of rodents living on either side of the Colorado River do not have any contact and therefore they are also not related to each other.
The properties of oceanic water like salinity, density, temperature etc. also affect animal dispersal. For example, the animals of freshwater habitats (and having swimming mechanisms) cannot swim across saline sea water. If water bodies hinder, deter and taboo the dispersal of land animals, these facilitate the dispersal, migration and spreading of aquatic animals but most of the aquatic animals cannot cross over the land surface because they do not have walking mechanisms.
Topographic factors such as high mountains, highly rugged and ravinated terrains, long, wide and deep valleys, extensive swamps and marshes, deserts whether warm or cold are effective barriers in the dispersal, migration and spreading of animals from their native places to other areas.
There is wide range of variation between the animals of the northern and the southern sides of the Himalayas and the eastern and the western sides of the Rockies and the Andes because these lofty Alpine mountains are effective barriers which stop free exchange of animals of either side of these mountains but the natural passes or man-made passes (constructed for the highways) enable the animals of one side of a mountain barrier to migrate into the other side provided that there is no large-scale climatic difference between two sides of the mountain concerned.
Dispersal and migration of animals are also motivated by the typical characteristics of vegetation of a particular habitat. For example, a few insects such as western tent caterpillars lay their eggs on tall trees. The animals of such habits do not migrate to those areas where tall trees are not present.
Adverse and severe climates are also hurdles in the dispersal and migration of animals except seasonal migration. Distance factor is also important in the dispersal and migration of animals.
If topographic and climatic uniformity is found over larger areas, gradual dispersal takes place without hindrance for longer distances but if the region is characterized by topographic and climatic heterogeneity or if there are frequent changes in topographic characteristics and climatic conditions after short distances, dispersal is limited to only short distances. It is obvious that distance factor does not play an independent role rather it influences animal dispersal together with topographic and climatic factors.
Inhospitable environments also deter if not stop the dispersal and migration of animals. Such inhospitable habitats and environments are the plant-less areas of soft deposits such as sands over vast areas in the form of deserts, toxic deposits derived from the recent volcanic eruption and lava flows, vast cold deserts having permanent continental glaciers and ice sheets because such habitats do not have the necessary conditions to become suitable habitats as these lack in all of the basic requirements of animals e.g., favourable climatic conditions, availability of food, suitable places to live, to breed and to have sexual contacts.
It may be pointed out that under exceptional circumstances the animals tide over the barriers of inhospitable environments and become adapted to them while migrating through them. It may be remembered that the topographic and climatic barriers are not permanent, rather they change over geological time. The topographic characteristics are controlled by endogenetic movements and high mountains are continuously eroded and denuded and therefore are lowered down in height under prolonged period of crustal stability and thus topographic barriers of same geological period are eliminated but new barriers are created due to fresh orogeny.
Similarly, previous oceans are removed due to collision of convergent plates whereas new seas and oceans are created due to divergent plates. The geological and geophysical evidences have proved that the Atlantic Ocean has been closed and opened twice during the past geological time- scale.
Similarly, the climate of given habitat changes over time. All these mean that no part of the globe is permanent barrier in the dispersal and migration of animals. Since the dimension and location of topographic and climatic barriers change through time, animal dispersal and migration become possible over larger areas and thus the distributional pattern of animals becomes highly diversified.
2. Biotic Factors:
Animals have certain intrinsic characteristics which motivate and stimulate them to change their places. Such intrinsic characteristics of animals is called innate ability which is also called vagility (the ability to move about and thus to succeed in the struggle for existence). The innate ability of animals includes those intrinsic properties of animals which enable them to move and migrate.
Such intrinsic properties of animals include the ability and habit of burrowing, creeping and crawling, hopping, climbing, digging, running, leaping, swimming, flying etc. These qualities of animals not only determine the type of dispersal and migration but also determine the nature and rate of dispersal.
When the animals become mobile and move out from their places motivated and stimulated by their intrinsic properties, the resultant dispersal is called active dispersal which is exclusively dependent on the degree of mobility of the animals. Besides, several inherited qualities and social behaviour also help in the active dispersal of animals. Sometimes the animal dispersal is effected by external carriers. Such dispersal is called passive dispersal.
(i) Passive Dispersal:
Passive dispersal of animals occurs when they are transported by other animals including man or by means of various types of transport developed by man (e.g., two wheelers like bicycles, scooters and motor cycles; four wheelers like cars and jeeps, buses and trucks; rails; ships and aeroplanes) or by natural means of transport such as rafts, ocean currents, winds etc.
The carriers which help in the dispersal (passive dispersal) of animals are divided into 4 groups as given below:
(a) Aeolian Carriers:
Aeolian carriers include airmasses and air currents which transport micro-organisms from one place to another place without the willingness of the organisms because gusty winds lift them and carry them to various destinations. The dispersal of microorganisms by wind is called anemochore dispersal.
Several studies have been carried out about the dispersal of insects by winds e.g.:
(i) P.I. Darlington (1957) has reported that the force and insect-carrying power of winds Increases approximately with the square of the velocity of wind, and
(ii) According to S.C. Kendeigh (1961) about 12.5 million insects are carried away per hour during summer season in the southern England etc.
Some animals carried by wind aloft are attached to the aircrafts because of their (of such animals) inbuilt trapping devices and are carried away to far off places. This type of trapped dispersal takes place in Diptera, Hymenoptera, Homoptera and other arthropod insects such as spiders and mites (are the animals which have wings of uniform structure-cicads).
Some animals have the ability and their natural habit to hop in the air for wind-assisted gliding. Such animals include caterpillars, squirrels, lemurs, marsupials, lizards, frogs, snakes etc. These animals are picked up by atmospheric storms while they are in the air during hopping and glidding and thus are carried away over various sorts of physical barriers. Such aeolian dispersal is called chance dispersal. Hurricanes, typhoons, tornadoes and temperate cyclones are effective carriers of animals which have gliding habits.
(b) Aquatic Carriers:
Aquatic carriers are mainly the sea waves, tidal currents and oceanic currents which carry smaller as well as larger animals (whether they have swimming mechanisms or not) to distant places. It may be pointed out that many of the animals become dead in transit. Such dispersal of animals is called as hydrochore dispersal.
According to C.H. Lindroth (1957) the spread of staphylinid beetle in the north-western Europe, Spitzbergen, Iceland and Greenland has been possible through several branches of the Gulf Stream. Ocean currents have carried several types of animals to many isolated islands.
It may be pointed out that long distance transport of animals through oceanic water by ocean currents depends on a variety of intrinsic properties of the animals e.g. ability to withstand longer duration of immersion in the oceanic water, the characteristics of body surface of the animals in terms of the ability of water permeability, respiration ability, temperature and salinity of the oceanic water etc. Some aquatic animals are also transported by floating rafts and debris.
(c) Organismic Carriers:
Organismic carriers are those animals which are symbiotic in character. In other words, the animals which depend on each other and live together are called symbiotic animals. The dispersal caused by such animals is called biochore dispersal. Mostly parasites are dispersed by active and mobile animals.
The biochore dispersal by animal is caused in two ways e.g.:
(i) Ectozoic biochore dispersal involves external transport of animals by carrier animals. This so happens when small animals are attached and stuck to the bodies of the carrier animals and thus these are transported and carried to various destinations which are visited by the carrier animals. Birds also help in ectozoic dispersal because these carry the eggs of other animals.
Sometimes birds also carry insects, worms, snails etc. inadvertently. Wading animals like ducks, rails and terns transport smaller aquatic organisms to other places. When dolphins move in groups, enormous sea waves are generated. These dolphin-generated sea waves cause mass movement offish shoals in the direction of wave propagation.
(ii) Endozoic biochore dispersal involves internal transport of parasitic organisms such as viruses. Such organisms are hidden in the stomach or are mixed with bloods, sputum and stool of animals and these organisms are carried away by the animals to various destinations. Several types of worms such as tape worms, ring worms etc. live in the stomachs of human beings as parasites and are carried away to far off places in the case of journeys to distant overseas countries undertaken by man.
(d) Anthropogenous Carriers:
Anthropogenous carriers are the most powerful and effective means of passive dispersal and migration of animals involving the whole globe.
The dispersal caused and effected by man is called anthropochore dispersal which is of two types e.g.:
(i) Deliberate dispersal and
(ii) Accidental or inadvertent dispersal.
The deliberate transport of birds, rabbits and deers from Europe to New Zealand and the transport of several birds from Europe to North America are the examples of deliberate anthropochore dispersal. Accidental transport of animals involves the carriage of animals attached with human bodies, his luggages and means of transport.
For example, brown rats are now widely distributed throughout the world because of their undesired transport to various parts of the world through ships. The deliberate transport of animals by man from their native places to other destinations has in some cases changed the natural distributional pattern of the concerned animals.
For example, transport of lions from Africa to Great Britain has resulted into phenomenal increase in their numbers to such an extent that the density of lions per hectare of national parks in Great Britain has become much higher than their density in their native places in Africa.
The favourable environmental conditions of the regions where animals are transported by man whether by deliberate action or by accident, incourage rapid growth of populations of the transported animals and thus the spreading of concerned animals is accelerated.
For example, one pair of European rabbits transported to Australia is capable of producing 90,00,000 off-springs within a period of only three years. European starling was brought to North America in 1989 and since then starlings have spread over larger area of the U.S.A. and Canada. The dispersal of African snails to various parts of the world is the example of effective anthropogenic dispersal of animals.
(ii) Active Dispersal:
Active dispersal of animals takes place because of motivation of their intrinsic properties such as the properties of digging, burrowing, creeping and walking, hopping and running, floating, flying, climbing etc. These innate (inborn or inherent) abilities of animals motivate them to become mobile and move out of their places.
Natural dispersal of animals is a slow and gradual process which covers major part of the world. The freshwater fishes and land invertebrates were originated in the tropical areas of the old world from where they migrated to Europe, Africa and Siberia (Asia).
These animals crossed through Bering Strait to reach North America from where they spread in Canada and the U.S.A. and migrated further southward through central American bridge to South America. The migration of these animals from Asia to Australia was not smooth because of the presence of water gap.
Mesosaurs (a permian reptile), glossopteris (permian plant) and cynognathus (a reptile of late triassic period) were originated in South America from where cynognathus and mesosaurs migrated to Africa and glossopteris plants migrated to Africa, Antarctica, India and Australia. Lystrosaurs (late triassic reptile) was originated in Africa from where it was dispersed to India and Australia.
Fig. 44.1 depicts major routes of widespread dispersal of plants and animals at global scale. Deer, saiga, sheep, bison, musk oxen, mastodons, mammoths, and man migrated from Asia to North America via Bering Bridge whereas beavers, opossums, racoons, horses etc., migrated from North America to Asia via–Bering Bridge. Racoons, cats, horses, tapirs, llamas, camels etc. migrated from North America to South America viz.
Central American Bridge whereas capybaras, porcupines, armadillos, glyptodonts, ground sloths etc. was dispersed from South America to North America via Central American Bridge. Butterflies, moths, beetles, grasshoppers, locusts, bugs etc. were dispersed from Africa to Europe and Asia whereas these animals were transported to South America by ships.
It may be pointed out that the fate of dispersal of animals is not always positive. The dispersal becomes unsuccessful when the animals dispersed to new habitats are unable to adapt to new environmental conditions and ultimately perish. On the other hand, the successful spread of animals becomes possible when dispersed animals become able to adapt to new environmental conditions and establish themselves in new habitats and start fresh colonization.
There may be many alternative possibilities of dispersed animals from old habitat to new habitats:
(i) The migrating or invading species of animals after reaching new habitats having different environmental conditions may be able to compete with the original occupants of the concerned habitat and finally occupy the habitat.
(ii) The migrating or invading species of animals may not be able to compete with the original occupants of the invaded habitat or may not be able to adapt to new environmental conditions of the invaded habitat. Consequently, the migrating animals are eliminated.
(iii) It may be possible that neither the immigrants (migrating or invading) nor the original occupants of the concerned habitat are eliminated completely, rather both live in the same habitat.
(iv) The migrating or invading animals may live in the open and vacant spaces of the new habitat and thus may form their own colonies through the process of gradual colonization.
(v) A few of the species of migrating or invading animals may establish sexual contacts with the original occupants and thus can produce hybrid species through interbreeding.
(vi) If the environmental conditions of the habitat to which the animals have been migrated or dispersed are favourable, the migrating newcomers (animals) start colonization of the new habitat.