patterns of plant distribution Presentation1.pptx

The different patterns of distribution that one can observe on the earth's surface are partly due to a combination of physical factors, which provide an envelope within which the life processes function. Spatially, for instance, vegetation is gregarious, i.e. it grows together in groups of various sizes and shapes.

1. PATTERNS OF PLANT DISTRIBUTION The global distribution of plant species is a naturally regulated process, controlled by several factors, such as evolutionary history, migration, ecological amplitude, etc. Evolutionary history Some of the existing plant species are ancient, while the great majority are recent in origin. Species differentiation among plants involves three major processes, namely natural hybridization between related species, mutation within species, and natural selection from populations. In natural selection, only the fittest individuals which find the habitat conditions most favourable and well within their ecological amplitude are selected. In other words, nature selects only the fittest individuals which are best adapted for the existing environmental conditions and gradually eliminates the others. Thus, natural selection is a major factor in the evolution and distribution of species

2. (b) Migration Migration is the direct factor responsible for the dispersal and distribution of plant species. In this case, newly evolved and already existing species migrate to new areas, simultaneously undergoing changes. Plant migration is accomplished by the far and wide transport and dispersal of migrules , such as spores, seeds, fruits, germules , propagules, etc. This is followed by the establishment of migrules in new areas, leading to the spreading and distribution of species. The agents of plant migration include wind, water, insects, birds, some mammals, including man, etc. The hurdles or obstacles on the way of plant migration are called migration barriers . They often prevent migration and thereby restrict the range or extent of distribution. The common barriers in the migration and dispersal of plant species include environmental barriers, biotic barriers and geographic barriers. Environmental barriers These are the environmental factors which inhibit migration and thereby lower the range of distribution. Climate is the commonest environmental barrier. Extremes of climate, unfavourable or harmful climate, unsuitable changes in climate, etc. often act as barriers of plant migration

3. unfavourable climate, or an unfavourable change in the climate of a particular area, forces the species for migration to areas of favourable climate for survival. The failure of some species to migrate pushes them to gradual extinction. (ii) Biotic factors A variety of biotic factors act as barriers of plant migration. Many animals selectively feed on plant migrules , adversely affecting their dispersal and distribution. Also, there would be many competing organisms which interfere with the dispersal, growth and establishment of new arrivals. There is often the absence of symbionts, such as mycorrhizae, actinorrhizae , pollinating insects, etc., along the migratory routes. This probably affects the growth and multiplication of the migrating species.

4. (iii) Geographic barriers Geographic barriers which prevent the extensive migration and the far and wide distribution of plant species include vast and deep oceans, extensive deserts, high-altitude mountain ranges, etc. Oceans are perhaps the most effective barriers which cannot be crossed by most island-originated floras. This might be the reason why there is high preponderance of endemic species in isolated islands. Angiosperms have mostly migrated from the southern to the northern hemisphere through several land connections, called gates of angiospermae . Great movements of floras have taken place in the distant past and are still continuing. Pollen analysis of the peat deposits of Pleistocene and recent epochs have adduced sufficient evidences in support this.

5. (c) Ecological amplitude Ecological amplitude is the range of tolerance of a species towards the environmental conditions of a particular habitat. Every species has a definite range of tolerance towards the environmental conditions in which it can survive successfully. T hus , the successful existence of a species in a particular habitat indicates that the environmental conditions of that habitat are within the tolerance range or ecological amplitude of that species. Ecological amplitude is genetically determined and hence different species will have different ecological amplitudes. Ecological amplitude is a crucial factor in the distribution of plants. Species having high ecological amplitude may be widely distributed in different geographical regions or different environmental conditions. This is because a wide range of environmental conditions fall within their ecological amplitude. On the other hand, species with low ecological amplitude enjoy only restricted distribution because they cannot tolerate a wide range of environmental conditions..

6. Within a species, there may be several genetically different groups of populations and each of them is adjusted only to a particular set of ecological conditions. They are called ecotypes. Patterns of plant distribution Plant distribution is a highly varied phenomenon. Based on it, three categories of species can be recognized, namely native species, exotic species and introduced species. Natives are the species distributed exclusively in the region of their origin. Exotics are the species distributed outside the region of their origin. They reach new areas through migration. Introduced species are the species willfully introduced to new areas by man. Based on the area inhabited by plant species and their ecotypes , plants are classified into wides and endemics . Wides are distributed in large tracts either in a continuous or in a discontinuous range. On the other hand, endemics are restricted only to a particular habitat or area and nowhere else in the world.

7. Thus, plants have three major types of distribution, namely continuous, discontinuous and endemic. Continuous distribution This is the distribution in which a taxon is found throughout the world without any interruption in any climatic zone. There are five main types of continuous distribution, namely cosmopolitan, circum -polar, circum -boreal, circum -austral and pantropical.

8. Cosmopolitan distribution This is the distribution in which a plant species or a taxon is found in all climatic zones. The families Graminae , Asteraceae , Cyperaceae , Caryophyllaceae , Fabaceae , Orchidaceae , Liliaceae , etc. are cosmopolitan in distribution. Some common species which enjoy cosmopolitan distribution are Phagmitis communis , Chenopodium album, Poa annua , Urtica diocica , etc .(ii) Circum -polar distribution This is the distribution exclusively restricted to the polar region e.g., Saxifaga oppositaefolia , Corex lapponica , Ranunculus nivalis . (iii) Circum -boreal distribution This is the distribution in which the species inhabit an almost continuous belt in the temperate regions of northern hemisphere. e.g., Alnus , Draba , Acer.

9. (iv) Circum -austral distribution This is the distribution in which the species inhabit a continuous belt in the temperate regions of the southern hemisphere. e.g., the genus Danthonia . (v) Pan-tropical distribution This is the distribution throughout the tropical region. e.g., the family Palmaceae , and the genera Bauhinia, Dalbergia , Dioscorea , Phyllanthus , Corcharus , Cassia, Ocimum , Eugenia, etc.

10. (b) Discontinuous or disjunct distribution Discontinuous distribution is the distribution of the members of a taxon in widely distant, geographically unconnected and climatically different regions that are separated from each other by extensive migration barriers, such as oceans or seas. Discontinuous distribution is exhibited by families, genera and species. Examples are the families Winteraceae and Staphylaceae , the genus Magnolia (found in India, Sri Lanka, S. America, China, etc.), and the species Sida rhombifolia (found in India and Malaysia) and Cleome monophylla (found in India and Africa).

11. Hypotheses about discontinuous distribution Various hypotheses have been advanced to account for the phenomenon of discontinuous distribution among plants. The most popular ones are long-distance dispersal hypothesis, migration hypothesis, land bridge hypothesis and continental drift hypothesis . Long-distance dispersal hypothesis This hypothesis was originally proposed by Charles Darwin and later on modified by Wallace and Schimper . It holds that the germules of some plants are transported from the places of their origin to very far away regions by wind and birds, without getting dropped anywhere in the intervening regions. In the new regions, the germules germinate, grow, reproduce and multiply and establish. This hypothesis failed to get scientific support and hence was rejected.

12. (ii) Migration hypothesis Migration hypothesis holds that successful long-distance migration leads to the extensive spreading and distribution of plants all over the world. The total destruction of the migrants in the intervening areas by natural calamities results in their discontinuous distribution. This hypothesis does not prove to be sound because very long distance migration is practically impossible due to the existence of highly formidable migration barriers.

13. iii) Land bridge hypothesis This hypothesis is based on the presence of intercontinental land-bridges in the geological past. Its major tenets are the following. Initially some continents were connected together by land-bridges. (ii) These land-bridges allowed the exchange of plants and animals between the connected regions. Thus, some species got distributed different continents. (iii) In course of time, the land-bridges sank deep in the sea, resulting in discontinuity in the distribution of some species. A major short-coming of this hypothesis is that it upholds the sinking of landbridges . In reality, genuine land-bridges never sink, but float in the sea. Simpson has classified land-bridges into three kinds, namely corridors, filter bridges and sweepstake bridges.

14. Corridors Corridor is a broad intercontinental land connection which permits extensive exchange of flora and fauna between the connected continents. It forms an open and free migration route that permits more or less uninhibited faunal interchange. Through corridors, many or most of the animals of a faunal region can migrate to another. Migration along corridors is thus unobstructed and rapid. Land bridges are typical examples of corridors. A dispersal corridor has long existed between Western Europe and China via Central Asia.

15. Filter bridges Filter bridges are relatively narrow and geologically short-living land-brides which permit only the selective exchange of flora and fauna between the connected regions. So, they "filter out" some organisms from passing through them and allow the passage of others. Filter bridges were fairly common. They are believed to have existed between Australia and Asia, between N.America and S.America , etc.

16. Sweepstake bridges Sweepstake bridges are the 'bridges' without actual land connection. In this case, the gap between adjacent land masses is bridged by uprooted trees and similar objects. Sweepstake bridges allow the passage of small animals and plant migrules , especially seeds. Sweepstake route is a possible dispersal route of faunal interchange. It may not be used by most animals, but only by some as a matter of chance. The colonization of Madagascar is believed to have taken place mainly through sweepstake route.

17. iv) Continental drift hypothesis Continental drift hypothesis was first formally advanced by Wegner (1910). Hence, it is often called Wagner's continental drift hypothesis. At some geological past, all continents existed as a single landmass, called Pangaea, surrounded all around by a common sea, called Panthalassa. (ii) Nearly 200 million years ago, Pangaea split up into two land masses, Laurasia and Gondwanaland. The former moved northward, and the latter moved southward. (iii) Again, Laurasia broke up into Greenland and North America, and Gondwanaland into Africa, Australia, S.America , New Zealand, etc. (iv) Further splitting, drifting and re-orientation occurred later, forming the continents and the islands of the present day. (v) Breaking of the landmasses into pieces and the movement (drifting) of these pieces in different directions caused the discontinuity of landmasses and the consequent discontinuous distribution of plants. There are convincing evidences in support of continental drift. So the continental drift hypothesis of discontinuous distribution appears to be acceptable in the present state of information.