Continuous and discontinuous distribution of plants and geographical barriers
Distribution of vegetation:-
a. Continuous distribution
b. Discontinuous distribution
a. Continuous distribution:- Some taxa are uniformly distributed in the area and give rise to continuous distribution. The area of a taxon or of a vegetation feature is never really continuous. It is of four main types:
i. Cosmopolitan:- These taxa are distributed all over the globe. They occur in all six widely inhabited continents, Eg.- many weeds and lower groups of cryptogams.
ii. Circumpolar:- These taxa are distributed around the North or South Pole Eg.- Saxifraga oppositaefolia, Eutrema edwardsii.
iii. Circumboreal:- These taxa are distributed around the boreal zone Eg.- Ribes (Gooseberries) and Danthonia (poultry grass).
iv. Pantropic:- These taxa occurring practically throughout the tropics and sub-tropics or at least widespread in the tropical region of Asia, Africa and America, Eg.- Palm family (Arecaceae).
b. Discontinuous distribution:- Most taxa may have isolated patches of areas widely separated apart. These constitute discontinuous distribution pattern. The plants are separated by wider gaps which cannot be overcome or crossed by the propagules. The main causes of discontinuity are environmental (due to particular topographic, climatic, edaphic or biotic characteristics of tracts separating the areas). Following are the familiar discontinuous types:
i. Arctic – Alpine:- Taxa are distributed in arctic region and in mountainous system of temperate zone. Eg.- Salix herbacea, Saxifraga oppositaefoilia.
ii. North Atlantic:- Taxa are distributed in North America and Europe and also some times locally in Asia. Eg.- Lycopodium inundatum and Spiranthes sp.
iii. North Pacific:- Taxa are distributed chiefly in North America and Eastern Asia, Eg.- Torreya (Torrey pine) Symplocarpus foetida.
iv. North—South American:- Taxa are distributed in North and South America but lacking continuity in between, Eg.- Sarraceniaceae (pitcher-plant family)
v. Europe – Asian:- Taxa are distributed in Europe and Asia but lacking continuity between. Eg.- Leontice altaica, Cimicifuga foetida.
vi. Mediterranean:- Taxa are distributed at European and African shore of Mediterranean sea. Eg.- Platanus
vii. Tropical:- Taxa are distributed in two or more separate tropical regions. Eg.- Buddleia
viii. South pacific:- Taxa are distributed at least in South America and New Zealand. Eg.- Jovellana
ix. South Atlantic:- Taxa are distributed at least in South America and Africa. Eg.- Asclepias
x. Antarctic:- Taxa are distributed on Arctic mainland and parts of South America, New Zealand and Austral islands. Eg.- Nothofagus
xi. Gondwana type:- Taxa are distributed in more than one continent which tends to embrace Africa, Madagascar, India and Australia. Eg.- Some sedges.
Geographical barriers:- They include the formation of a mountain range, the movement of a glacier, and the division of a large lake into several smaller lakes.
> Ecologists who study biogeography examine patterns of species distribution.
> No species exists everywhere; for example, the Venus flytrap is endemic to a small area in North and South Carolina.
> Endemic species:- It is one which is naturally found only in a specific geographic area that is usually restricted in size.
> Generalized species:- Species which live in a wide variety of geographic areas. The raccoon, for example, is native to most of North and Central America.
> Species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution. Therefore, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century.
> Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers.
> Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and animals. Approximately 3/4 of living plant and mammal species are endemic species found solely in Australia.
> Sometimes ecologists discover unique patterns of species distribution by determining where species are not found. Hawaii, for example, has no native land species of reptiles or amphibians, and has only one native terrestrial mammal, the hoary bat. Most of New Guinea, as another example, lacks placental mammals.
> Plants can be endemic or generalized.
> Endemic plants are found only on specific regions of the Earth, while generalists are found on many regions.
> Isolated land masses—such as Australia, Hawaii, and Madagascar—often have large numbers of endemic plant species.
> Some of these plants are endangered due to human activity. The forest gardenia (Gardenia brighamii), for instance, is endemic to Hawaii, only an estimated 15–20 trees are thought to exist.