Types and areas of natural distribution, factors affecting distribution, main habitat and vegetation types of the world
Types and areas of natural distribution:-
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.
Factors affecting distribution:- Plants can be herbs, shrubs, trees, creeper, climber. Plants are the most important renewable resources and are not equally distributed over the earth.
The following factors affect the growth and distribution of plants:
i. Land:-
- Plants are different in different lands such as Mountainous, Plateau, and Plain.
- Pains are suitable for Agriculture.
ii. Soil:-
- Sandy soil of desert support cactus and thorny bushes.
- Wet, Marshy and Deltaic soil are suitable for Mangrove and deltaic vegetation.
- Hill slopes with some depth are suitable for Conical Trees.
iii. Temperature:- Temperature cause the growth and germination of plants.
- Tropical Wet Forest with broad-leaved evergreen trees near the equator.
- Dry forest and scattered trees between 10 to 23-degree latitudes.
- Desert vegetation largely comprises herbs and shrubs near 30 degrees latitudes.
- In the north of the desert, there is temperate grassland or prairie.
- In the north of the grassland, there is a deciduous temperate forest.
- Boreal forest in subarctic in the south of Arctic circle.
- Arctic tundra.
iv. Precipitation:-
- The heavy rainfall region has more dense vegetation and a variety of wildlife.
- Control the growth and diversity of plants; dense forests are found on the equator. Diversity decrease from the equator to the poles as rainfall and temperature decrease from the equator to the poles.
v. Photoperiod:- It influences by latitude, altitude and seasons.
- Variation in the duration of Sunlight at the different places is due to differences in latitude, altitude, season, and duration of the day.
- Larger the day duration larger the trees grow.
vi. Hazards:- Such as fire, landslides.
vii. Human activities:- Such as human settlement, urbanization, pollution, war, deforestation, etc.
Main habitat:- The area where a particular organism lives naturally is called its habitat. The five major habitats are:
i. Forests
ii. Grasslands
iii. Deserts
iv. Mountains
v. Polar regions
vi. Aquatic habitat:- Oceans and freshwater together form the aquatic habitat.
Vegetation types of the world:-
i. Forest biomes:- Which have a dense, closed canopy of trees. If you stand and look upwards in a forest, you see few large gaps between the crowns of the trees, which tend to overlap and interlock with one another. Forest biomes include tropical rainforest, temperate evergreen forest, temperate deciduous forest, and cold climate conifer forest (also called "boreal conifer" and "taiga").
ii. Woodland biomes:- These arc rather like forest but with a more open canopy, with significant gaps between individual trees so that their crowns often do not touch. These include Mediterranean woodland, tropical dry woodland, and boreal woodland. A typical sort of definition of woodland would be that less than 70% of the canopy above is trees, with the rest being open sky.
iii. Shrub or scrub biomes:- They have low woody plants, usually with a rather gnarled appearance and multiple stems instead of a single trunk. They include temperate semi-arid scrub, tropical semi-arid scrub, Mediterranean scrub (garrigue).
iv. Grasslands:- They look rather like a lawn or meadow, both of which are human creations, except that these are natural, not cultivated. For example, in the category of grasslands there are the savannas in the tropics, steppe or prairie in the temperate zones, and grassy tundra in very cold climates. Sometimes there may be an open scattering of trees or shrubs.
v. Desert biomes:- They are distinguished mainly by lack of vegetation, with differing degrees of openness, or even no vegetation at all. Semi-desert is a sort of transitional open scrub or open grassland, whereas "true" desert has almost no vegetation. People tend to imagine that most deserts are sandy, in fact, more often they are covered by stones or bare rock.