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PHTYTOGEOGRAPHY
PHYTOGEOGRAPHY
SUBMITTEDTO SUBMITTED BY
Dr . AGHIL SOORYA MANEESHA NATH N. M
DEPARTMENT OF BOTANY ROLL NO:- 11
STTERESA’SCOLLEGE 2nd MSc BOTANY
1
CONTENT
• Introduction
• Principles governing plant distribution
• Different types of distribution
• Theories of distribution
• Factors affecting plant distribution
2
INTRODUCTION
• It is the branch of biogeography that is concerned with the geographic distribution of
plant species and their influence on the earths surface .
• The term is derived from two words ‘phyto’ meaning plant and ‘geography’ meaning
distribution
• It covers all the aspects of plant distribution including the individual species and the
factors that regulate the composition of communities and flora.
• Phytogeography encompasses various fields such as environment, flora(taxa),
vegetation(plant community) and origin.
3
ASPECTS
• Phytogeography has two basic aspects
1. Ecological phytogeography:- It deals with the role of biotic and abiotic
interactions on plant distribution
2. Historical phytogeography :- It is related to historical reconstruction of the origin,
dispersal and extinction of taxa .
4
PHYTOGEOGRAPHY
• According to Campbell 1926 the main theme of plant geography is to discover the
similarities and diversities in the plants and floras of the present and past found in widely
separated parts of the earth
• Wulff 1943 states that phytogeography is the study of distribution of plant species in
their habitats and elucidation of origin and history of development of floras .
• According to Croizat 1952 phytogeography is the study of migration and evolution of
plants in time and space .
5
FATHER OF PHYTOGEOGRAHY
• One of the subjects earliest proponents was Prussian
naturalist Alexander von Humboldt who is often referred to
as the father of phytogeography .
• Humboldt advocated a quantitative approach to
phytogeography that has characterized modern plant
geography
6
Major Divisions of Phytogeography
• There are two major divisions of Phytogeography:
• (i) Descriptive or Static Phytogeography
• (ii) Interpretive or Dynamic Phytogeography
• Descriptive Phytogeography: This deals with the actual description of floristic or
vegetational groups found in different parts of the world.
• Early plant geographers described floras and attempted to divide earth into floristic and
botanical zones.
7
Major Divisions of Phytogeography
• Interpretive or Dynamic Phytogeography :This deals with the dynamics of migration and
evolution of plants and floras.
• It explains the reasons for varied distribution of plant species in different parts of the world.
• It is a borderline science involving synthesis and integration of data and concepts from
several specialized disciplines like ecology, physiology, genetics, taxonomy, evolution,
paleontology and geology.
• Good (1931I), Mason (1936), Cain (1944) and some others have pointed out the factors
involved in the distribution of plants
8
• Lowerence (1951) has suggested the modern principles of Phytogeography which
are classified into four groups:
• I. Principles concerning environment
• II. Principles concerning plant responses
• III. Principles concerning the migration of floras and climaxes
• IV. Principles concerning the perpetuation and evolution of floras and climaxes
9
PRINCIPLES CONCERNING ENVIRONMENT
• The distribution of plants is primarily controlled by climatic conditions.
• There has been variation in climate during geological history in the past which affected
migration of plants.
• The relations between land masses and seas have varied in the past. The large land
masses split up to form new land masses or continents which separated and reoriented.
Land bridges between continents acted as probable routes for migration of plant and
animal species.
10
• The land bridges became submerged in sea with the passage of time and the
possibility for migration of plants and animals from one continent to another
disappeared for ever.
• Soil conditions on plains and mountains of different land masses show secondary
control on distribution of vegetation. Halophytes, psammophytes, calcicols etc. have
developed because of edaphic conditions.
• Biotic factors also play important role in distribution and establishment of plant
species.
• The environment is holocentric, i.e., all environmental factors have combined effects
on the vegetation of a place.
PRINCIPLES CONCERNING ENVIRONMENT
11
PRINCIPLES CONCERNING
PLANT RESPONSES
• Range of distribution of plants is limited by their tolerances. Each plant species has a range
of climatic and edaphic conditions. Therefore, tolerance of a large taxon is the sum of
tolerances of its constituent species.
• Tolerances have a Genetic basis. The response of plants to environment is governed by
their genetic makeup.
• Many of the crops through breeding and genetic changes have been made to grow in
wider range of environmental conditions. In nature, hybrid plants have been found to have
wider range of tolerances than their parents.
12
PRINCIPLES CONCERNING
PLANT RESPONSES
• Different ontogenetic phases have different tolerances.
• Different developmental stages of plants show different degree of tolerances, as for
example seeds and mature plants are more tolerant to temperature and moisture
variations than their seedlings.
13
PRINCIPLES CONCERNINGTHE
MIGRATION OF FLORAS AND CLIMAXES
• Large scale migrations have taken place. The fossils and palaeoecological evidences reveal
that large scale migrations of plants and animals have taken place during Mesozoic era
andTertiary periods.
• Migration resulted from transport and establishment. In the process of migration plants
are dispersed to new habitats through their propagules such as spores, seeds, bulbils etc.,
and there they are established if environmental conditions are favourable. Plants grow
and reproduce there and progeny perpetuates through ecological adjustments
14
PRINCIPLESCONCERNINGTHE
PERPUTATION AND EVOLUTION OF
FLORAS AND CLIMAXES
• Perpetuation depends first upon migration and secondly upon the ability of species to
transmit the favorable variations to the progenies.
• Evolution of floras and climaxes depends upon migration, evolution of species and
environmental selections.
15
Distribution
• On the basis of area of the earth surface occupied by the plants, the various taxa are
categorized as under:
1.Continous distribution
2. Wides
3.Endemics
4. Discontinuous species
16
CONTINOUS DISTRIBUTION
• Some taxa are uniformly distributed in the area and give rise to continuous distribution .
• Four main types :
• 1. the cosmopolitan
• 2. the circumpolar
• 3. the circumboreal
• 4. the pantropic
17
• Cosmopolitan
• These taxa are distributed all over the globe.They occur in all six widely inhabited continents.
• eg:- many weeds and lower groups of cryptograms
• Circumpolar
• Taxa distributed around the north or south pole
• eg:- Saxifraga oppositifolia
• Circumboreal
• Taxa distributed around the boreal zone .
• eg:- Ribes (gooseberries)
• Pantropic
• Taxa occurring practically throughout the tropics or at least widespread in the tropical region in
Asia ,Africa and America.
• eg :- Palm family
18
WIDES
• Plants widely distributed over the earth in definite climatic zones and the different
continents are referred to as wides.
• Cosmopolitan is applied for wides but in fact, no plant is cosmopolitan in real sense of
the term.
• Taraxacum officinale and Chaenopodium album are the common examples of the
wides. Plants of tropical regions are called Pantropical.
• The plants of very cold climate may not only be found in the arctic regions but also in
alpine zone of mountains in tropical and subtropical regions. These are called arctic-
alpine plants.
19
ENDEMICS
• A taxon whose distribution is confined to a given area is said to be endemic to that
area
• The taxon may be of any rank, although it is usually at a family level or below, and its
range of distribution may be wide, spanning an entire continent, or very narrow
covering only a few square meters.
• The concept of endemism is important because in the past the formulation of
biogeographic regions was based on it.
20
• Now the endemic species have been grouped into the following categories:
• (i) Relics or Paleoendemics: They are the survivors of once widely distributed ancestral
forms, for example, Ginkgo biloba (restricted to China and Japan), Sequoia sempervirens
(confined to coastal valleys of California, U.S.A.).
• These species are called Paleoendemics or epibionts. A great majority of the endemic
species belonging to this type have many fossil relatives.
• They are also called living fossils. Because of little variability the endemics are adapted
only to a particular environment and even if they reach new areas, they fail to establish
themselves in new environment.
21
• (ii) Neo endemics:
• The other endemics may be modern species which have had not enough time for
occupying a large area through migration.They are called neo endemics.
• There are several such genera which are widely endemic or few species of which are
endemic.
• Neo endemics show good variability and have many biotypes, grow in diverse
habitats and have wide tolerance for habitats.
22
• (iii) Pseudo endemics:
• These endemics arise due to mutation in existing population at a particular place.
• These pseudo endemics or mutants may or may not persist for long in the particular
area where they originate.
• Endemism results from the failure on the part of species to disseminate its seeds fruits
spores or propagules because of existence of great barriers like mountains, oceans and
large deserts.
• The oceanic islands which are isolated from rest of the world by large expanses of
water abound in endemic species and water barrier checks the migration of those
species outside their original habitat.
23
DISCONTINOUS DISTRIBUTION
• When plants occur at two or more distant places of the world which are separated by
overland’s or oceans hundreds or thousands of kilometers apart. Such a distribution is
called discontinuous or disjunct distribution.
• Familiar discontinuous ranges are:-
1. Artic-Alpine 6. Mediterranean 11. Gondwana type
2.North Atlantic 7. Tropical
3.North Pacific 8. South- Pacific
4.North-South American 9. South – Atlantic
5.Europe-Asian 10. Antarctic
24
• Artic-Alpine :
• Taxa distributed in arctic region and in mountainous system of temperate zone.
• North Atlantic:
• Taxa distributed in North America and Europe and also some times locally in Asia .
• North Pacific:
• Taxa distributed chiefly in North America and Eastern Asia
• North-South American :
• Taxa distributed in North- South America but lacking continuity in between
• Europe-Asian :
• Taxa distributed in Europe and Asia but lacking continuity in between
• Mediterranean :
• Taxa distributed at European and African shore of Mediterranean sea
25
• Tropical:
• Taxa distributed in two or more separate tropical regions.
• South Pacific :
• Taxa distributed at least in South America and New Zealand
• South Atlantic :
• Taxa distributed at least in South America andAfrica
• Antarctic :
• Taxa distributed on Antarctic mainland and parts of South America , New Zealand and Australia
islands
• Gondwana type:
• Taxa distributed in more than one continent which tends to embrace Africa, Madagascar , India,
Australia
26
• The significant phytogeographical causes for discontinuous distribution are as
follows:
• (i) The species might have evolved at more than one place and they failed to migrate
outside their original habitats because of barriers.
• (ii) The species which were once widely distributed in the past disappeared from certain
areas and are now surviving in some distant pockets.
• (iii) The climate may also be a factor for discontinuity in distribution of species. Plants
having specific climatic requirements are found in widely separated areas with similar
environmental conditions.
• For example, plants of arctic regions are also found in alpine zone of high mountains in
tropics and subtropics.
27
Theories of Discontinuous Distribution:
• Theory of Land Bridge:
• According to this theory, land bridges occurring in between the separated continents
are believed to have helped in the migration of various taxa from one continent to the
other.
• Uniform distribution of plants and animals in different parts of the world during
Paleozoic era is believed to have been due to those land bridges.
• With the passage of time the land bridges became submerged in sea and the
connections between the various continents snapped beyond the dispersal capacity of
organisms resulting thereby the discontinuity in the distribution
28
• Theory of Continental drift:
• The theory of continental drift was propounded by Wegner (1912 1924) According to
him the whole land- mass of the world was a single super continent during Paleozoic
era. He named it as Pangaea.
• That super continent was surrounded by sea on all the sides which was named
Panthalassa. During Mesozoic, Pangaea split up into two large landmasses; Laurasia in
the north and Gondwanaland in south.
• About 135 million years ago reorientation of continents began. The continents were
drifted apart by the oceans.This is called Continental Drift.
• The occurrence of Dinosaurs and many fossil plants lend support to the existence of
Laurasia and Gondwanaland.
• With the separation of continents the distribution areas of several plant and animal
species got separated and gave rise to discontinuous distribution areas.
29
Factors Affecting Distribution of Species
• Several factors are known to affect the geographical distribution of plant species, some of
which are as follows:
• 1. Geological history and distribution
• 2. Migration
• 3. Ecological amplitude.
30
Geological history and distribution
• The place where a species first originated is called its centre of origin. Evolution of species is a
slow but continuous process. Some of the species in present day flora are quite old while a great
majority of them are recent in origin.
• The process of species differentiation involves:
• (i) Hybridization between the related species as well as mutation
• (ii)The natural selection of the hybrid and mutant populations.
• In the selection process not all the hybrids and mutants are selected by nature and only the
fittest individuals which find the habitat conditions within their ecological amplitudes are
selected and the individuals least fit are eliminated. Changing climate has also played important
role in the origin of new species. In the course of evolution several old species became extinct,
some of which can be found even today as fossils. The fossils provide direct evidence for the
existence of various taxa in the past.
31
MIGRATION
• The newly evolved species starts migration to new areas and side by side it undergoes further
evolutionary changes. The dispersal of propagules is brought about by several agencies like wind,
water, glaciers, insects, animals, even man.
• Migration may be adversely affected and sometimes even totally stopped by some factors called
migration barriers. Barriers in the dispersal of species may be classified as ecological or
environmental and geographical.
• The climate, an ecological barrier, plays important role in distribution and establishment of
species. Unsuitable climatic condition or change of climate in particular area forces the species to
migrate from one place to another and the failure of some species to migration leads them to
gradual extinction.
32
• Besides climate, there are geographical barriers, as for example, high mountains, vast oceans or
deserts.
• The fresh water plants, for example, cannot be dispersed across oceans if their propagules are
suitable only for fresh water dispersal and similarly propagules of land plants from one country
cannot reach other country separated by vast oceans and mountains.
• Species are called natives of the place of occurrence if they originated there. Outside the area
of its origin, the species is referred to as exotic. Exotic species reach new area through
migration. If any species is introduced intentionally in new area by man then it is called
introduced species.
33
Ecological amplitudes and distribution
• Environmental conditions not only influence the life and development of plants but also
determine the presence or absence, vigor or weakness and relative success or failure of various
plants in a particular habitat.
• Each plant species of a community has a definite range of tolerance towards physical and
biological environment of the habitat.This is referred to as ecological amplitude.
• The ecological amplitude is governed by genetic set up of the species concerned and thus
different species have different ecological amplitudes which may sometimes overlap only in
certain respects.
• Further, some species may occur at different geographical regions as and when the conditions
fall within their ecological amplitudes. As for example, some plants of temperate region say
conifers, may also be found in alpine zone of high mountains in tropical and subtropical regions.
34
• The other consideration in ecological amplitude as a factor in plant distribution is its change
with time. In sexually reproducing plants the hybridization between related species results in
offspring’s with new genetic composition.
• With the change of environment the plant species also make adjustments with new
environment by shifts in their ecological amplitudes facilitated by changes in the genotype.
• Within a species there may occur several genetically different groups of individuals
(populations) which are adjusted to particular set of ecological conditions.
• These populations are called ecotypes or ecological races or ecological populations.
• In Euphorbia thymifolia, for example, there are two major populations-one is calcium loving or
calcicole and the other type is calcium hating or calcifuge.
35
REFERENCES
• Amit Pandey , Pandey K., Shukla J. P., “Environmental Biology and Ecology” Narendra Publishing
House- 2009. Page no. 178- 205.
• Arumugam. N., Kumaresan.V., “Plant Ecology and Phytogeography” Saras Publication, 2010
• http://www.biologydiscussion.com/ecology/phytogeography-climate-vegetation-and-botanical-
zones-of-india/6925
36
THANK YOU
37

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Phytogeography........................pptx

  • 1. PHTYTOGEOGRAPHY PHYTOGEOGRAPHY SUBMITTEDTO SUBMITTED BY Dr . AGHIL SOORYA MANEESHA NATH N. M DEPARTMENT OF BOTANY ROLL NO:- 11 STTERESA’SCOLLEGE 2nd MSc BOTANY 1
  • 2. CONTENT • Introduction • Principles governing plant distribution • Different types of distribution • Theories of distribution • Factors affecting plant distribution 2
  • 3. INTRODUCTION • It is the branch of biogeography that is concerned with the geographic distribution of plant species and their influence on the earths surface . • The term is derived from two words ‘phyto’ meaning plant and ‘geography’ meaning distribution • It covers all the aspects of plant distribution including the individual species and the factors that regulate the composition of communities and flora. • Phytogeography encompasses various fields such as environment, flora(taxa), vegetation(plant community) and origin. 3
  • 4. ASPECTS • Phytogeography has two basic aspects 1. Ecological phytogeography:- It deals with the role of biotic and abiotic interactions on plant distribution 2. Historical phytogeography :- It is related to historical reconstruction of the origin, dispersal and extinction of taxa . 4
  • 5. PHYTOGEOGRAPHY • According to Campbell 1926 the main theme of plant geography is to discover the similarities and diversities in the plants and floras of the present and past found in widely separated parts of the earth • Wulff 1943 states that phytogeography is the study of distribution of plant species in their habitats and elucidation of origin and history of development of floras . • According to Croizat 1952 phytogeography is the study of migration and evolution of plants in time and space . 5
  • 6. FATHER OF PHYTOGEOGRAHY • One of the subjects earliest proponents was Prussian naturalist Alexander von Humboldt who is often referred to as the father of phytogeography . • Humboldt advocated a quantitative approach to phytogeography that has characterized modern plant geography 6
  • 7. Major Divisions of Phytogeography • There are two major divisions of Phytogeography: • (i) Descriptive or Static Phytogeography • (ii) Interpretive or Dynamic Phytogeography • Descriptive Phytogeography: This deals with the actual description of floristic or vegetational groups found in different parts of the world. • Early plant geographers described floras and attempted to divide earth into floristic and botanical zones. 7
  • 8. Major Divisions of Phytogeography • Interpretive or Dynamic Phytogeography :This deals with the dynamics of migration and evolution of plants and floras. • It explains the reasons for varied distribution of plant species in different parts of the world. • It is a borderline science involving synthesis and integration of data and concepts from several specialized disciplines like ecology, physiology, genetics, taxonomy, evolution, paleontology and geology. • Good (1931I), Mason (1936), Cain (1944) and some others have pointed out the factors involved in the distribution of plants 8
  • 9. • Lowerence (1951) has suggested the modern principles of Phytogeography which are classified into four groups: • I. Principles concerning environment • II. Principles concerning plant responses • III. Principles concerning the migration of floras and climaxes • IV. Principles concerning the perpetuation and evolution of floras and climaxes 9
  • 10. PRINCIPLES CONCERNING ENVIRONMENT • The distribution of plants is primarily controlled by climatic conditions. • There has been variation in climate during geological history in the past which affected migration of plants. • The relations between land masses and seas have varied in the past. The large land masses split up to form new land masses or continents which separated and reoriented. Land bridges between continents acted as probable routes for migration of plant and animal species. 10
  • 11. • The land bridges became submerged in sea with the passage of time and the possibility for migration of plants and animals from one continent to another disappeared for ever. • Soil conditions on plains and mountains of different land masses show secondary control on distribution of vegetation. Halophytes, psammophytes, calcicols etc. have developed because of edaphic conditions. • Biotic factors also play important role in distribution and establishment of plant species. • The environment is holocentric, i.e., all environmental factors have combined effects on the vegetation of a place. PRINCIPLES CONCERNING ENVIRONMENT 11
  • 12. PRINCIPLES CONCERNING PLANT RESPONSES • Range of distribution of plants is limited by their tolerances. Each plant species has a range of climatic and edaphic conditions. Therefore, tolerance of a large taxon is the sum of tolerances of its constituent species. • Tolerances have a Genetic basis. The response of plants to environment is governed by their genetic makeup. • Many of the crops through breeding and genetic changes have been made to grow in wider range of environmental conditions. In nature, hybrid plants have been found to have wider range of tolerances than their parents. 12
  • 13. PRINCIPLES CONCERNING PLANT RESPONSES • Different ontogenetic phases have different tolerances. • Different developmental stages of plants show different degree of tolerances, as for example seeds and mature plants are more tolerant to temperature and moisture variations than their seedlings. 13
  • 14. PRINCIPLES CONCERNINGTHE MIGRATION OF FLORAS AND CLIMAXES • Large scale migrations have taken place. The fossils and palaeoecological evidences reveal that large scale migrations of plants and animals have taken place during Mesozoic era andTertiary periods. • Migration resulted from transport and establishment. In the process of migration plants are dispersed to new habitats through their propagules such as spores, seeds, bulbils etc., and there they are established if environmental conditions are favourable. Plants grow and reproduce there and progeny perpetuates through ecological adjustments 14
  • 15. PRINCIPLESCONCERNINGTHE PERPUTATION AND EVOLUTION OF FLORAS AND CLIMAXES • Perpetuation depends first upon migration and secondly upon the ability of species to transmit the favorable variations to the progenies. • Evolution of floras and climaxes depends upon migration, evolution of species and environmental selections. 15
  • 16. Distribution • On the basis of area of the earth surface occupied by the plants, the various taxa are categorized as under: 1.Continous distribution 2. Wides 3.Endemics 4. Discontinuous species 16
  • 17. CONTINOUS DISTRIBUTION • Some taxa are uniformly distributed in the area and give rise to continuous distribution . • Four main types : • 1. the cosmopolitan • 2. the circumpolar • 3. the circumboreal • 4. the pantropic 17
  • 18. • Cosmopolitan • These taxa are distributed all over the globe.They occur in all six widely inhabited continents. • eg:- many weeds and lower groups of cryptograms • Circumpolar • Taxa distributed around the north or south pole • eg:- Saxifraga oppositifolia • Circumboreal • Taxa distributed around the boreal zone . • eg:- Ribes (gooseberries) • Pantropic • Taxa occurring practically throughout the tropics or at least widespread in the tropical region in Asia ,Africa and America. • eg :- Palm family 18
  • 19. WIDES • Plants widely distributed over the earth in definite climatic zones and the different continents are referred to as wides. • Cosmopolitan is applied for wides but in fact, no plant is cosmopolitan in real sense of the term. • Taraxacum officinale and Chaenopodium album are the common examples of the wides. Plants of tropical regions are called Pantropical. • The plants of very cold climate may not only be found in the arctic regions but also in alpine zone of mountains in tropical and subtropical regions. These are called arctic- alpine plants. 19
  • 20. ENDEMICS • A taxon whose distribution is confined to a given area is said to be endemic to that area • The taxon may be of any rank, although it is usually at a family level or below, and its range of distribution may be wide, spanning an entire continent, or very narrow covering only a few square meters. • The concept of endemism is important because in the past the formulation of biogeographic regions was based on it. 20
  • 21. • Now the endemic species have been grouped into the following categories: • (i) Relics or Paleoendemics: They are the survivors of once widely distributed ancestral forms, for example, Ginkgo biloba (restricted to China and Japan), Sequoia sempervirens (confined to coastal valleys of California, U.S.A.). • These species are called Paleoendemics or epibionts. A great majority of the endemic species belonging to this type have many fossil relatives. • They are also called living fossils. Because of little variability the endemics are adapted only to a particular environment and even if they reach new areas, they fail to establish themselves in new environment. 21
  • 22. • (ii) Neo endemics: • The other endemics may be modern species which have had not enough time for occupying a large area through migration.They are called neo endemics. • There are several such genera which are widely endemic or few species of which are endemic. • Neo endemics show good variability and have many biotypes, grow in diverse habitats and have wide tolerance for habitats. 22
  • 23. • (iii) Pseudo endemics: • These endemics arise due to mutation in existing population at a particular place. • These pseudo endemics or mutants may or may not persist for long in the particular area where they originate. • Endemism results from the failure on the part of species to disseminate its seeds fruits spores or propagules because of existence of great barriers like mountains, oceans and large deserts. • The oceanic islands which are isolated from rest of the world by large expanses of water abound in endemic species and water barrier checks the migration of those species outside their original habitat. 23
  • 24. DISCONTINOUS DISTRIBUTION • When plants occur at two or more distant places of the world which are separated by overland’s or oceans hundreds or thousands of kilometers apart. Such a distribution is called discontinuous or disjunct distribution. • Familiar discontinuous ranges are:- 1. Artic-Alpine 6. Mediterranean 11. Gondwana type 2.North Atlantic 7. Tropical 3.North Pacific 8. South- Pacific 4.North-South American 9. South – Atlantic 5.Europe-Asian 10. Antarctic 24
  • 25. • Artic-Alpine : • Taxa distributed in arctic region and in mountainous system of temperate zone. • North Atlantic: • Taxa distributed in North America and Europe and also some times locally in Asia . • North Pacific: • Taxa distributed chiefly in North America and Eastern Asia • North-South American : • Taxa distributed in North- South America but lacking continuity in between • Europe-Asian : • Taxa distributed in Europe and Asia but lacking continuity in between • Mediterranean : • Taxa distributed at European and African shore of Mediterranean sea 25
  • 26. • Tropical: • Taxa distributed in two or more separate tropical regions. • South Pacific : • Taxa distributed at least in South America and New Zealand • South Atlantic : • Taxa distributed at least in South America andAfrica • Antarctic : • Taxa distributed on Antarctic mainland and parts of South America , New Zealand and Australia islands • Gondwana type: • Taxa distributed in more than one continent which tends to embrace Africa, Madagascar , India, Australia 26
  • 27. • The significant phytogeographical causes for discontinuous distribution are as follows: • (i) The species might have evolved at more than one place and they failed to migrate outside their original habitats because of barriers. • (ii) The species which were once widely distributed in the past disappeared from certain areas and are now surviving in some distant pockets. • (iii) The climate may also be a factor for discontinuity in distribution of species. Plants having specific climatic requirements are found in widely separated areas with similar environmental conditions. • For example, plants of arctic regions are also found in alpine zone of high mountains in tropics and subtropics. 27
  • 28. Theories of Discontinuous Distribution: • Theory of Land Bridge: • According to this theory, land bridges occurring in between the separated continents are believed to have helped in the migration of various taxa from one continent to the other. • Uniform distribution of plants and animals in different parts of the world during Paleozoic era is believed to have been due to those land bridges. • With the passage of time the land bridges became submerged in sea and the connections between the various continents snapped beyond the dispersal capacity of organisms resulting thereby the discontinuity in the distribution 28
  • 29. • Theory of Continental drift: • The theory of continental drift was propounded by Wegner (1912 1924) According to him the whole land- mass of the world was a single super continent during Paleozoic era. He named it as Pangaea. • That super continent was surrounded by sea on all the sides which was named Panthalassa. During Mesozoic, Pangaea split up into two large landmasses; Laurasia in the north and Gondwanaland in south. • About 135 million years ago reorientation of continents began. The continents were drifted apart by the oceans.This is called Continental Drift. • The occurrence of Dinosaurs and many fossil plants lend support to the existence of Laurasia and Gondwanaland. • With the separation of continents the distribution areas of several plant and animal species got separated and gave rise to discontinuous distribution areas. 29
  • 30. Factors Affecting Distribution of Species • Several factors are known to affect the geographical distribution of plant species, some of which are as follows: • 1. Geological history and distribution • 2. Migration • 3. Ecological amplitude. 30
  • 31. Geological history and distribution • The place where a species first originated is called its centre of origin. Evolution of species is a slow but continuous process. Some of the species in present day flora are quite old while a great majority of them are recent in origin. • The process of species differentiation involves: • (i) Hybridization between the related species as well as mutation • (ii)The natural selection of the hybrid and mutant populations. • In the selection process not all the hybrids and mutants are selected by nature and only the fittest individuals which find the habitat conditions within their ecological amplitudes are selected and the individuals least fit are eliminated. Changing climate has also played important role in the origin of new species. In the course of evolution several old species became extinct, some of which can be found even today as fossils. The fossils provide direct evidence for the existence of various taxa in the past. 31
  • 32. MIGRATION • The newly evolved species starts migration to new areas and side by side it undergoes further evolutionary changes. The dispersal of propagules is brought about by several agencies like wind, water, glaciers, insects, animals, even man. • Migration may be adversely affected and sometimes even totally stopped by some factors called migration barriers. Barriers in the dispersal of species may be classified as ecological or environmental and geographical. • The climate, an ecological barrier, plays important role in distribution and establishment of species. Unsuitable climatic condition or change of climate in particular area forces the species to migrate from one place to another and the failure of some species to migration leads them to gradual extinction. 32
  • 33. • Besides climate, there are geographical barriers, as for example, high mountains, vast oceans or deserts. • The fresh water plants, for example, cannot be dispersed across oceans if their propagules are suitable only for fresh water dispersal and similarly propagules of land plants from one country cannot reach other country separated by vast oceans and mountains. • Species are called natives of the place of occurrence if they originated there. Outside the area of its origin, the species is referred to as exotic. Exotic species reach new area through migration. If any species is introduced intentionally in new area by man then it is called introduced species. 33
  • 34. Ecological amplitudes and distribution • Environmental conditions not only influence the life and development of plants but also determine the presence or absence, vigor or weakness and relative success or failure of various plants in a particular habitat. • Each plant species of a community has a definite range of tolerance towards physical and biological environment of the habitat.This is referred to as ecological amplitude. • The ecological amplitude is governed by genetic set up of the species concerned and thus different species have different ecological amplitudes which may sometimes overlap only in certain respects. • Further, some species may occur at different geographical regions as and when the conditions fall within their ecological amplitudes. As for example, some plants of temperate region say conifers, may also be found in alpine zone of high mountains in tropical and subtropical regions. 34
  • 35. • The other consideration in ecological amplitude as a factor in plant distribution is its change with time. In sexually reproducing plants the hybridization between related species results in offspring’s with new genetic composition. • With the change of environment the plant species also make adjustments with new environment by shifts in their ecological amplitudes facilitated by changes in the genotype. • Within a species there may occur several genetically different groups of individuals (populations) which are adjusted to particular set of ecological conditions. • These populations are called ecotypes or ecological races or ecological populations. • In Euphorbia thymifolia, for example, there are two major populations-one is calcium loving or calcicole and the other type is calcium hating or calcifuge. 35
  • 36. REFERENCES • Amit Pandey , Pandey K., Shukla J. P., “Environmental Biology and Ecology” Narendra Publishing House- 2009. Page no. 178- 205. • Arumugam. N., Kumaresan.V., “Plant Ecology and Phytogeography” Saras Publication, 2010 • http://www.biologydiscussion.com/ecology/phytogeography-climate-vegetation-and-botanical- zones-of-india/6925 36