Dr. Shoeb Ahmad, Assistant Professor, Department of Zoology, AKI's Poona College of Arts, Science and Commerce, Camp, Pune -41100, Maharashtra (INDIA)

1. Animal Distribution
by
Dr. Shoeb Ahmad
Assistant Professor
Department of Zoology
AKI’s Poona College of Arts, Science and Commerce,
Camp, Pune 411001 (M.S.)

2. METHODS OF DISTRIBUTION
Basically two theories explain the spreading of animals all over
the globe:
1 Continental Drift hypothesis
2 Centre of Origin hypothesis
1 Continental Drift hypothesis: Proposed by Wagner, according
to him the earth was one whole mass when it was originated, but
about 135 million years ago (Cretaceous Period) the land mass
fragmented in to continents. With the fragmentation of land
mass animals were also distributed in their respective continents
2 Centre of Origin hypothesis: According to this theory the
individuals of a species spread out from the centre of their origin
because of their high reproductive capacity.

3. CLASSIFICATION OF ANIMAL DISTRIBUTION
There are three aspects of distribution of animals on earth, two of
which are distributed in space(land and water) and one in time.
In space:
(a) Geographical Distribution : Horizontal or Surfacial Distribution
(b) Bathymetric Distribution : Vertical or Attitudinal Distribution
In Time
Geologic Distribution : Durational Distribution
(a) Geographical Distribution :
There is no place on earth’s surface where life is not existing.
All over the earth’s surface organisms are distributed. The dense
tropical forest, high mountains, hot and drought place like
dessert, very cold polar regions life is existing. On these places
life is present but not in uniform one.

4. Dispersal of Animals
Animals dispersed because of following reasons:
Struggle for Existence – rapid multiplication, scarcity of food
and shelter and mate.
Changing in climatic condition : in search of favourable
climate animals migrated or become extinct . e.g. Horses and
elephant.

5. Barriers to Dispersal
1 Topographic Barriers: Alps (High and extensive Mountains) –
ranges the limit of distribution of some terrestrial forms. e.g. There
is marked differences b/w the species of northern and the southern
slope of the Alps.
2 Climatic Barriers: Temperature has a marked influence on limiting
the animal dispersal, particularly on cold blooded animals e.g.
Reptiles and amphibians are tropical and temperate in their
distribution.
3 Vegetative Barrier: The forest become barrier for larger terrestial
animals because they can’t penetrate the forest.

6. 4 Large water bodies: Extensive water bodies form barriers for
animals living in water. e.g. Fresh water fishes also prevented to
migration, although certain fish such as Salmon, Sturgeon and Smelt
(anadromous fishes) migrate from sea water to fresh water
annually. While in Eel (catadromous fish) reverse migration occurs.
5 Impurity and lack of salinity of sea water: An effective barrier to
the dispersal of certain kinds of marine animals starfishes,squids
(cepahlopods), sponges and corals because these animals require a
maximum purity and salininity of water.

7. Means for Dispersal
1. Land Bridges
2. Natural Rafts and Driftwood
3. Favouring Gales
1. Land Bridges : Panama (a country that links central and south
America; in map it shows like a bridge) opens up the avenue
for migration of many animals horses, wolves ans cats.
2. Natural Ratfs and Driftwood: Terrestrial animals also takes lift
upon drifting (floating) materials which enable them to
accomplished over water journeys.
e.g. Polar bear are also carried by floating shore ice.
Mass of driftwood creepers and leave and other natural rafts
also helps in dispersal of animals .
3. Favouring Gales : Many animals like insects, bird, and bats
dispersed by strong wind and they get their favorable
destinations. e.g. European birds transported to America by
way to Iceland and Green land

8. Bathymetric Distribution
Bathymetric distribution concern itself with the vertical range of
organisms in space i.e. from the highest Alpine peak to the abyssal
depth of the seal. A series of variations can be easily see in animals
from height to depth –
 Breathing: If animal is completely terrestrial and breath in air, it
can not live in water and those who are completely aquatic and
breath in water can not survive outside the water.
 Substratum: Animals differs from one another having or not
having substratum – Some animals are not able to walk they can
only swim, some animals have not complete walking capability
(Aves).

9.  Animals living in water have several limiting factors like – fresh
water or salty water, water pressure with depth.
On the basis of distribution of animals marine water has been
classified in to two zones :
(1) Pelagic Zone: It includes the water mass laying above the ocean
floor. Thus, the animals live here called Pelagic. Ex- Planktons
(microscopic) and necton (large swimming animals).
(2) Benthic Zone: It includes all the sea bottom. Thus, the animals
live in the bottom of the sea is called Benthonic.

10. Three Organic Realms
There are three realms are recognized for bathymetric as wells
for geographic standpoint
1. Geobiotic or Terrestrial
2. Limnobiotic or freshwater inhabiting (lakes and rivers)
3. Halobiotic or Marine or slatwater inhabiting (sea)
1. Geobiotic or Terrestrial – it ranges from alpine (high
mountains) to low land.
2. Limnobiotic or freshwater inhabiting (lakes and rivers) –
limited to terrestrial water such as pond lake and river and
contain very limited fauna.

11. 3. Halobiotic or Marine or saltwater inhabiting (sea)- It is very
important realm as we find here all the contrasting
characteristics abundantly that is why -
Halobiotic realm is divided in to four sub-realms
(a) Strand: It is tidal zone or transitional area between marine and
terrestrial realm. In this realm those animals are living those who
are capable to breath both in air and in water. Ex- molluscs.
(b) Flat Sea: it the area of sea up to 200 meter depth, rich both in
light and vegetation. Because of light, vegetation and food some
animals are limited to this area. Ex- Coral, Calcarious Sponges,
Zooids (Coelenterates), Sea Anemones etc

12. (c) Pelagic Realm: It includes the water mass laying above the
ocean floor. Thus, the animals live here called Pelagic. Ex-
Planktons (microscopic) and necton (large swimming animals).
(d) Abbysal Realm: Sea ares below depth of 200 meter, where
there is no substratum, low temperature, high pressure. Ex –
Large carnivorous marine animals, feed on dead organic
materials.

13. Geological Distribution: Durational Distribution
Geologist divided earth’s past history into number of major
divisions called Eras
Eras : Name of duration of time that indicate the degree of
evolutionary advancement of life

14. Archeozoic and Proterozoic Era
•Called as eras of age of unicellular life .
•Animals having low organization.
•Also called as ‘Age of Primitive Marine Invertebrates’
•Ex- Sponges, Marine Algae
Paleozoic Era (Age of Fishes)
Cambrian Period: Vertebrates like fishes evolved from invertebrates.
Ordovician Period: Cephalopods, Tribolites , and fishes evolved.
Silurian and Devonian Period: Scorpion, Amphibian and terrestrial
vertebrates evolved.
Mississippian, Pennsylvanian, Permian Period : Shark were dominant
fishes during Mississippian period. Insects, Snails & Amphibians were
evolved

15. Mesozoic Era (Age of Reptiles)
Triassic and Jurassic Period
•Fishes, Amphibians, reptiles, bird and mammals were evolved, but
reptiles were more dominant .
•Flying reptile and reptile like birdswere recorded in this period.
Cretaceous Period
• Rise of flowering plants.
•Mammals were numerous, but small .
•Birds were modern with teeth

16. Cenozoic Era (Age of mammals)
Paleocene Epoch
Dinosaurs were apparently extinct and had been replaced by
mammals.
Eocene Epoch: Warm blooded mammals evolved.
Oligocene, Miocene and Pliocene Epoch
•Glacial age
•Evolution of mammals, first man like apes.
•Placental mammals diversified.
•Formation of new land bridges
•Many migrations occurred all over the world.
Pleistocence and Psychozoic Epoch (Age of Man)
Some animals evolved in warmer interglacial areas .
Age of man began with the greater perfection of man’s mentality

17. Patterns of Animal Distribution
Animals are not uniformly distributed on land and
water. They are restricted to certain areas by several
factors such as climate, food, shelter etc.
On the basis of above said restriction factors animals
distribution is classified into four categories –
(i) Continuous Distribution
(ii) Discontinuous Distribution
(iii)Bipolar Distribution
(iv)Isolated Distribution

18. (i) Continuous Distribution
 Wide range of animals are distributed throughout the world in all
climatic zones or in all the confinements is called Continuous
Distribution and such type of animals are called Eurytopic Animals.
 Eurytopic animals are adapted to a wide variety of environmental
conditions.
 They are not specific to any particular type of food.
 Many have special power to cross barriers by flight or rafting or
swimming.
Ex- Rats, Bats, Hawks, Cockroach, Flies, lizards, Mosquitoes, Snakes and
man
 Many of eurytopic animals have continuous distribution because they
are companions of man and have travelled to long distance with him

19. Continuous distribution has following types –
(A) Cosmopolitan Distribution: Animals or Species occurring in all
climatic zone. Ex- Falco pereginus (Hawk)
(B) Circumpolar Distribution : Animals or species found in a
particular pole of the globe. Ex- Polar Bear
(C) Circumboreal and Circumanstral Distribution: Animal or species
which are distributed in a near continuous belt in the
temperature region of northern or southern hemisphere are said
to have circumboreal* and circumanstral* distribution
respectively
*Circumboreal – Throughout Northern Hemisphere
*Circumanstral - Throughout Southern Hemisphere

20. (ii) Discontinuous Distribution
When continuity of distribution of a species is
broken by uninhabited area which are sometime
very large stretches of oceans.
OR
A species distributed in two or more widely
separated geographical areas is called
discontinuous distribution

21. There are four reasons for why animals are distributed in widely
separated areas –
(1) Animals reach distant areas by different routes – insects, snail,
and rats by rafting, turtles by swimming, and birds by favouring
gales (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 the
isolating animals to long distance
(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

22. Examples of Discontinuous Distribution
Peripatus has 75 species distributed in Southeast Asia, East Indies,
Australia, New Zealand, Africa, and Central America.
Alligators occur in America (Alligator mississipiensis) and Asia
(Alligator sinensis).
Two Species of elephants are surviving today – Loxodonta
africana in south Africa while another elephant , Elephas maximus
found in India, Burma, Thailand and Sri Lanka

23. (iii) Bipolar Distribution
Species that can survive only in cold arctic climate can not migrate to
the warmer areas and hence restricted to the polar regions
Ex – Polar Bear, Arctic For, Lemmings are restricted in northern pole
while Penguins are restricted to Antarctica regions
(iv) Isolated Distribution
 Some animals found only in certain isolated areas, such forms
are said to have isolated distribution.
Ex. Sphenodon (living fossil) - Reptile - New Zealand
Prototheria – Egg laying Mammal (Playpus [Duckbill]) – Australia
Metatheria – Pouched Mammals (Kangaroo) – Australia

24. Factor Affecting Animal Distribution
Animals spread out in the biosphere through migration. Migration
and dispersal of animals are controlled by many factors or barriers
(1) Physical Barrier: It includes mountains, rivers, lakes, saes,
vegetations or forests and long distance.
(2) Climatic Barrier: Temperature, moisture, light and pH.
(3) Biological Barrier: Food, predators and enemies