Global Distribution of Animals
Mrs. Krupalini.V
Dear students welcome to our lecture series on geography. Today’s topic for the discussion is
Global distribution of animals. The most important questions about the distribution of animals
are, how populations of animals have come to be located where they are, and why those
populations do not exist in other places. So in order to answer these questions the topic can be
studied briefly under the following headings. They are…..
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Continental Drift Theory
Biogeographic zones
Patterns of Distribution
Means of dispersal
Barriers for Distribution
I.
Continental Drift Theory
Continental Drift theory was first proposed by Alfred Wegener. The theory explains that the
present day position of continents is not permanent, but the continents constantly drift and
change positions, suggesting that all continents in the past were placed together in a mass of
supercontinent. Wegener proposed that the earth’s continents had been drifting like rafts
following the breakup of a single great landmass called Pangaea (“all land”).
According to the theory of plate tectonics (tectonics means “deforming movement”), the earth’s
surface is composed of 6 to 10 rocky plates, about 100 km thick, that shift about on a more
flexible underlying layer.
During Permian, about 270 million years ago, a single large continent Pangaea started to
split into two land masses, the northern Laurasia and southern Gondwanaland, separated by the
sea of Tethys. The splitting perhaps caused what is known as the worst mass extinction in the
history of evolution.
During Jurassic, the continental masses began to break up leading to the formation of
Atlantic Ocean. Americas began to drift westward, Antarctica and Australia southwards and
India towards the northeast.
North and South Americas were connected together by a narrow corridor, through which
migrated reptiles and primitive mammals. Towards the end of some catastrophe struck the earth
resulting in another mass extinction and extermination of the most powerful of all animals that
ever existed-the dinosaurs.
During Eocene, North and South Americas got disconnected by the submergence of
panama connection, isolating the South American continents from the others for a very long
time. Almost at the same time North America also got disconnected from Europe for ever,
although a filter bridge may have existed between the two continents for some time.
During Miocene Indian plate had moved sufficiently northwards to collide with the
Eurasian plate, obliterating the Sea of Tethys and starting a process of mountain-building that
continued into the Pliocene. In the beginning, the rising Himalayas did not produce an effective
barrier between the Palaearctic and Oriental Regions but they gradually rose to great heights of
today and effectively checked the movement of animals between the two regions.
The continental drift explains puzzling distributions of animals, such as the similarity of
invertebrate fossils in Africa and South America, as well as certain similarities in present-day
faunas at the same latitudes on the two continents.
However, the continents have been separated for all of the Cenozoic era and probably for much
of the Mesozoic era as well, much too long to explain the distributions of some modern
organisms such as placental mammals. Continental drift theory is, nevertheless, enormously
useful in explaining interconnections between flora and fauna of the past.
The present distribution of marsupial mammals is an excellent example of the influence of
continental breakup. Marsupials appeared in the Middle Cretaceous period, about 100 million
years BP, probably in South America.
Because South America was at that time connected to Australia through Antarctica, the
marsupials spread through all three continents. They also moved into North America, but there
they encountered placental mammals, which had dispersed to that continent from Asia.
Marsupials evidently could not coexist with placentals, and so became extinct in North America.
About 50 million years BP, Australia drifted apart from Antarctica, barring entrance to
placentals. Australia remained in isolation, allowing marsupials to diversify into the present rich
and varied fauna.
II.
Biogeographic zones:
Except for the earthworm and the ant, there are no animals which are found nearly everywhere in
the world. Most are only found in certain areas, and some have a very narrow range. The science
of zoogeography includes the study of the facts regarding animal distribution and an effort to
explain those facts. It has sometimes been said that geographic distribution is distinctive
evidence in favor of evolution.
There are six biogeographic areas, each characterized by certain unique animals. These six
divisions were originally based on mammalian distribution, but they have been found useful for a
number of other types of animals.
(1) Nearctic region: It includes North America, from the Mexican highlands on the south to the
Arctic islands and Greenland in the north. Characteristic animals include: prong-horned antelope,
mountain goat, caribou, muskrat, rattlesnake, bison, catfish, turkey, and raccoon.
Fascinatingly many North American animals are closely related to those found in eastern Asia,
and some species are identical. Because of this, the two regions have also been grouped together
as the Holarctic Region.
(2) Palearctic region: This area includes Eurasia (Europe and Asia), and extends on the south to
North Africa, Afghanistan, Persia, and the Himalayas. Characteristic animals are: wild boar,
fallow deer, roe deer, hedgehog, antelope, wolves, bears, moles, rodents, thrushes, magpie,
salamanders, frogs, and toads.
(3) Neotropical region: It included South America, Central America, the Mexican lowlands, and
the Caribbean islands. The main animals are: alpaca, llama, arboreal sloth, armadillo, guinea pig,
anteater, vampire bat, most hummingbirds, jaguar, and New World monkeys.
(4) Ethiopian region: This large region includes Africa (including the Sahara desert), southern
part of the Arabian Peninsula, Madagascar, and other islands off the coast of Africa. The typical
animals includes: African elephant, rhinoceros, lion, chimpanzee, gorilla, hippopotamus, zebra,
giraffe, and a variety of antelopes, secretary bird, and guinea fowl.
Only a few kinds of sheep, goats, and pigs are to be found. Madagascar has a variety of unique
animals.
(5) Oriental region: This are covers India, and all Southeast Asia and island nations east of it
(Malay, Sumatra, Borneo, Java, Celebes, and the Philippines).
Characteristic animals are: Indian elephant, Indian rhinoceros, Old World monkeys, baboons,
orangutan, peacock, jungle fowl, cobras, and certain lizards, frogs, and toads. This region is
sometimes referred to as the Paleotropical Realm.
(6) Australian region. This area includes New Guinea, Australia, Tasmania, New Zealand, and
Pacific islands. Characteristic animals are: most marsupials (pouch animals), all egg laying
mammals, emu, cassowary, lyre birds, birds of paradise, cockatoos, tree frogs, tuataras, kiwis,
and Australian lungfish. But one exceptional case is bats and rat--both of which were introduced
by man.
III.
Patterns of Distribution
Animals are not uniformly distributed on land and in freshwater. They are restricted to certain
places by several factors such as climate, food, shelter, flora and fauna etc. Generally distribution
can be classified into three categories, namely, Continuous, Discontinuous and Bipolar
distribution.
Discontinuous distribution
1. Continuous distribution: Continuous distribution refers to animals being able to adapt to a
changing environment. They are not dependant on a specific habitat/climate or food for their
survival. Such animals are called Eurytopic organism. Eurytopic organism are adapted to a wide
variety of environmental conditions and are not specific to any particular type of food. They may
have special power to cross barriers either by flight, rafting or swimming and can adapt to new
environmental conditions.
Wide ranging animals include rats, bats, hawks, cuckoos, cockroaches, flies, mosquitoes, lizards,
snakes and man. Many of the eurytopic animals have continuous distribution because they are
companions of man and have travelled to long distances with him.
2. Discontinuous distribution: Unlike continuous distribution, discontinuous distribution does
not have uniform distribution of organisms. Its uniformity is broken by large land or water
masses that are mostly uninhabited. One reason for this occurrence is, if the land creates barriers
because of disruption in the earth’s crust, which makes the animals on that land mass isolated
from each other. Another reason is that some large land masses may have been connected at
some point by a natural bridge, which eventually broke off and lead to isolation.
There are primarily four reasons why animals are distributed in widely separated areas.
1. Animals reach distant areas by draw routes, as insects, snails and rats by rafting, turtles
by floating and swimming and birds blown by storms.
2. The species was earlier distributed continuously but the land masses in the
intermediate areas submerged, breaking the species into widely separated populations.
3. Continental drift separated the continents and carried them to long distances, isolating
the animals from other areas.
4. A widely distributed species can become extinct in the intermediate areas due to
change in the climate to which the species is unable to adapt.
Examples for this kind of distribution include; Peripatus (Phylum Onychophora) has 75
species distributed in Southeast Asia, East Indies, Australia, New Zealand, Africa, South and
Central America.
The three surviving genera of lungfishes occur in three different continents; Protopterus
in Africa, Lepidosiren in South America and Neoceratodus in Australia.
Similarly Ostriches are distributed in the southern hemisphere; Rhea americana in
Argentina and Brazil, Struthio camelus in Africa, Emu in Australia, Cassowaries in North
Queensland and New Guinea and Kiwi in forests of New Zealand.
Marsupials occur in Australia but one family Didelphidae occurs in South America that
includes opossum (Didelphis), water opossum (Chironectes) and opossum rat (Coenolestes).
Of the two elephant species surviving today, Loxodonta africana occurs in Africa while
another species, Elephas maximus is found in India, Burma, Thailand and Sri Lanka.
3. Bipolar distribution: Bipolar distribution means the interrupted distribution of animals,
whereby the same species, genus, or family lives in the temperate latitudes of the northern and
southern hemispheres, but they are absent in the tropics.
The occurrence of bipolar distribution is caused by the conditions that prevailed in the ice age,
when the temperature in the tropics dropped and northern organisms were able to spread to the
tropics and reach the southern hemisphere. Later when the temperature rose, these organisms
died out in the tropics, thereby giving rise to an interrupted (or noncontinuous) range.
Some species because they are adapted to the cold arctic climate cannot migrate to the warmer
areas and hence restricted to the Polar Regions. Polar bears, arctic fox, lemmings, coyote and
reindeers are found in the northern Polar Regions while penguins are restricted to the Antarctica
region.
A bipolar distribution is particularly characteristic of marine organisms: several marine
invertebrates (for example, mussels), marine fish (the anchovy, Pacific saury, basking shark, and
porbeagle), some mammals (southern whale, gray dolphin), and many species of marine algae
(kelp and fucus).
IV.
Means of dispersal:
All the animals do not spread across the sea or other barriers with the same speed, some
spread faster and others slowly and some animals do not cross the barriers at all. Therefore, the
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.
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 discontinuous distribution of animals in
continents that are presently separated by thousands of kilometers of oceans.
They are of two types, namely, Corridor bridges and Filter bridges.
1. Corridor bridges: Corridor bridges 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.
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 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 the 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 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 assurance the distribution of late Mesozoic and
Coenozoic animals and perhaps be used to explain the distribution of invertebrates, fish,
amphibians and primitive reptiles.
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 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.
V.
Barriers for Distribution
Abiotic Factors
The most familiar limitations to the geographic distribution of animals include overwhelming
physical barriers, such as mountains, oceans, rivers, deserts, and expansive human development.
These obstacles directly prevent species from dispersal, especially in the cases of oceans for
terrestrial species, land masses for marine species, and transitions between freshwater and marine
systems for many aquatic organisms.
Some species are also limited by physical barriers that emerge periodically, such as in areas
characterized by frequent flooding, fires, or volcanic eruptions.
Although species of animals are often capable of physically cross the distances separating
ecosystems, gradients in such factors as temperature, soil and water chemistry, and precipitation
present physiological barriers to dispersal.
For example, the flat topographies of many deserts are not effective physical barriers to
dispersal, but their extremely dry conditions prevent organisms with high water demands from
colonizing deserts or even dispersing across them to reach suitable habitats on the other side.
There are four main factors due to which animals and plants are prevented from spreading to
every possible area.
Climate: Animals are adapted to a combination of temperature and humidity that is affected by
rainfall. Lower temperature prevents majority of reptiles from migrating northwards into the
temperate areas.
Polar bear, penguins and a large number of mountain inhabiting species are adapted to cold
climate and cannot come down to tropics and subtropics. Amphibians need high humidity not
only for their survival but also for reproduction and hence cannot venture into areas of low
rainfall.
Majority of animals cannot cross or survive in deserts due to extremely low moisture and high
temperature. That makes deserts effective physical barriers.
Fishes, although adapted to live in aquatic environment, are clearly restricted to either marine or
freshwater habitats apparently due to osmotic problems.
Very few migratory fishes can make use of both environments such as species of salmons and
eels that migrate thousands of kilometres for reproduction. Low temperature of mountains
prevents certain animals such as parrots from spreading to these areas.
Vegetation: Like animals plants are also sensitive to temperature and rainfall, by this plants
affect dispersal of animals because the animals are depend on vegetation for food. Tropical areas
support broad-leaved dense forests whereas in temperate areas only cold tolerant conifers can
survive, each type supports its distinctive fauna.
Desert vegetation can support a few animals. Some animals can feed on many types of
vegetation and hence can spread to larger areas but others are selective, so they would not accept
anything apart from their specific diet.
For example, giant panda feeds on bamboo shoots in China and Koala can live only on
eucalyptus leaves in Australia. Such animals cannot survive outside their habitats.
Other animals: Different animals at different tropic levels make food chains which are
interlinked in a complex food web. Such interactions among animals often restrict a particular
species to migrate alone to other areas.
Interaction between predator and prey, parasite and host and among commensals and competitors
create problems in an ecosystem. Any immigrating exotic species can upset the balance in the
native population.
Dingo dogs, placental cats and foxes are in a danger of eliminating native carnivores in
Australia. When two species have similar ecological requirements, they become competitors and
one of the species is generally eliminated and restricted to a very small area.
Parasites generally have specific hosts and hence must migrate together to new areas. Predators
and prey also show similar interactions.
Physical barriers: Barriers such as mountains, deserts, rivers and oceans physically prevent
animals from entering in to new areas even when environment is favorable to their survival. For
land animals water is a barrier and for aquatic animals land.
Fresh water fishes and amphibians cannot cross seas, but amphibious reptiles such as tortoises,
lizards and snakes, owing to their thick and impermeable skin have crossed seas to reach distant
islands.
Climate and scarcity of vegetation makes deserts and mountains effective barriers rather than
inability of animals to walk over them. Generally, rivers and lakes do not form effective barriers
for most of the vertebrate species if they are good swimmers and usually they are, and rivers
form a network of highways for migrating freshwater fishes. Distribution of land animals is
therefore restricted by one of the environmental limiting factors.
Summary
A closer look at the science of biogeography reveals the patterns of the distributions of animals
across the globe. The species of animals differ in their requirement for different factors to
survive. Positive factors encourage the growth of different species while negative factors
decrease the growth of species in the area. Species of animals possesses characteristics that allow
it to survive in certain conditions. This is why the distributions of animals vary from one area to
another.
Thank you.