Biomes – I | Biogeography

Terrestrial Biomes

Tropical Evergreen Rainforests Biome

• This biome extends upto 100 latitude on both sides of the equator.
• It covers the area of Amazon low land of south America, Congo basin of equatorial Africa and South Eastern Asian Islands extending from Sumatra to New Guinea.

Climate

• This area experiences high temperature throughout the year with range as little as 20C.
• However, the daily range of temperature is much higher than the annual range of temperature.
• This area gets heavy rainfall ranging between 150cm-250cm.
• It is distributed throughout the year.
• Rainfall occurs in the afternoon almost on daily basis.
• This also happens because of huge amount of water vapor reaching in the atmosphere due to high temperature.
• Hence, this area is considered to be an equable climate as both temperature and rainfall are high for whole of the year.

Natural Vegetation

• The combination of heat and moisture make this biome as perfect environment for a great variety of plants and animal species.
• The variety of plant species can be understood from the fact that one square kilometer may contain as many as about thousand of different types of plant species.
• Most of the trees have buttressed trunks, shallow roots and large dark evergreen leaves.
• The evergreen rainforest arranged in three levels.
  • The canopy or upper level where trees lies between about 20 metres to 50 metres. Most of them are hard wood trees like ebony, Mahogany, rose wood, sandalwood, cinchona, etc.
  • The second level of intermediary level where trees lies between about 10 meters to 20 meters. The most important plant of this group is palm trees. Apart from palm trees, epiphytic and parasitic plants are also found in this layer
  • The third or lower level lies from surface level to about 10 meters of heights. Under this category variety of plants are found namely orchids, ferns, mosses, herbs, bananas, pineapples etc. Because of tall and broad leaved dense plants, sunlight could not reach at the lowest level/surface. Because of poor photo-synthesis process at this level, number of plant species are very low.

Animal Life

• Like vegetation, evergreen rainforest is inhabited by numerous birds, mammals, insets etc.
• Some important animals of this biome are Jaguar, lemur, orangutan, elephant, etc. Macaw parrot, sloth and toucan are some of the important birds of this area.
• Most of the birds are colorful.
• The water bodies of the equatorial areas are also rich in animal life with alligators, tactless, fishes, frogs, Hippopotamus etc.
• Because of the impenetrability and high vegetation growth in the lower part, most of the insects, birds and animals resides on the branches of the trees.
• Generally, they do not come down to the ground.
• The productivity of the tropical rainforest biome is the highest of all biome types of the world.
• It may be pointed out that the rainforest biome represents only 13 percent of the total geographical area of the world but this biome accounts for the 40 percent of the total productivity of the world.

Tropical Deciduous Forests Biome (Monsoon Forests)

Location:

• Normally, the tropical deciduous forests are found in the regions of monsoon climate but there are some departures from this close relationship and near correspondence between the regions of monsoon cli­mate and deciduous forest.
• There are three major areas of tropical deciduous forest biome:
  • The Neotropics mainly West Indies,
  • Indo-Malaysian Zone (mainly in south and south-east Asia except equatorial ever­green rainforest areas) and
• Eastern Africa and northern Australia.
• Besides these major areas, some discontinuous localities of tropical deciduous forest biomes are also found in South Africa, Southern Bra­zil, South-Eastern U.S.A., Formosa (Taiwan), south­ern China and Japan.

Climate:

• This tropical deciduous forest biome is charac­terized by two distinct seasons viz. moist season and dry season.
• There are three main seasons in a year in India and surrounding monsoonal areas such as Paki­stan, Bangladesh etc. e.g.:
  • Dry warm summer season (March to June),
  • Warm humid summer season (July to October), and
  • Dry winter season (November to February).
• Average temperature of warm dry summer season ranges between 27°C and 32°C but the maxi­mum temperature ranges between 38°C and 48°C dur­ing May and June.
• Warm humid summer season records average temperature between 20°C and 30°C. The temperature during dry winter season ranges between 10°C and 27°C.
• The length of dry season is more important than the total amount of precipitation in affecting vegetation in the tropical deciduous forest biome.
• On an average, the mean annual rainfall is around 1500mm but there are much variations in temporal and spatial distribution of rainfall.
• Sometimes a few areas receive even less than 500 mm of mean annual rainfall.
• Even the temporal distribution of rainfall within a single year is highly variable because more than 80 percent of mean annual rainfall is re­ceived within 3 wet months of summer season (July, August and September).
• Thus, the rainy season records much surplus water whereas dry winter and summer seasons have marked deficit water because dry season receives less than 25 mm of rainfall per month.
• There is maximum evaporation during warm dry summer months which results in desiccation of soils and marked reduction in soil-water.
• This seasonal regime of annual monsoonal rainfall gives deciduous character to the vegetations which shed their leaves (mostly trees and shrubs) during the transitional period between winter and summer season.

Natural Vegetation:

• The number of plant species is less in the tropi­cal deciduous forest biome than the tropical evergreen rainforest biome.
• Since the density of plants is also lower in this biome than the rainforest biome and hence there is comparatively less competition among the plants for getting sunlight.
• The height of most of the trees ranges between 12m and 30m.
• There are four strata or layers in the vertical structure of the tropical deciduous forests. The uppermost and the second strata consist of trees; the third stratum is formed by shrubs whereas the last and the fourth stratum or the ground stratum represents herbaceous plants.
• Most of the trees are deciduous but the shrubs of the third stratum are evergreen.
• The trees are charac­terized by thick girth of stems, thick, rough and coarse bark and large hydromorphic leaves or small, hard xeromorphic leaves.
• The large hydromorphic leaves enable the trees to trap more and more rainfall during wet seasons but these large leaves are shed in dry periods to conserve moisture whereas small and hard xeromorphic leaves enable the trees to withstand dry weather and water deficiencies.
• Though there are numerous climbers mainly lianas and epiphytes but their numbers are far less in the tropical deciduous forest biome than the tropical ever­green rainforest biome.
• Sal (Shorea robusta) and teak trees of Indian deciduous forests form forest canopy but its shape is not like umbrella or cauliflower as is the case with the forest canopy of the rainforest biome.
• Bamboo is another important member of the Indian deciduous forests.
• Besides, there are numerous trees, climbers, shrubs and grasses which have spatial variations from one region of the tropical deciduous forests to the other region.

Animal Life:

• It is a significant ecological principle that more is the development of stratification of the vertical structure of the vegetation community of a biome and greater the number of plant species, the more will be the number of animal species, their total population and species diversity.
• This ecological prin­ciple holds good in the case of tropical evergreen rainforest biome.
• But there are comparatively lesser number of animals species in the monsoon deciduous forest biome than the rainforest biome because of comparatively less developed verti­cal strata and hence less diversification of animal species.
• The seasonal character of vegetation commu­nity in terms of dense vegetation cover, full develop­ment of leaves and their evergreen form during rainy months (wet summer season, July to September) and shedding of leaves, drying of herbaceous plants etc. during dry season mainly during dry warm season has affected and determined the seasonal behaviour of animal communities particularly breeding and migrat­ing behaviour.
• In other words, the seasonal character of the monsoon deciduous forests has been responsible for the seasonal regime in the breeding and reproduc­tion and migration of animals.
• For example, birds in east Africa breed twice during the two different seasons of a year. Indian dogs generally breed once a year mainly at the end of wet monsoon season (during October-November).
• The animals of the tropical and sub-tropical dry deciduous forest biome range from very small animals (micro-organisms) to very large-bodied animals like elephants, horses, hippopotamus, ganda or rhinos, li­ons, forest buffalo together with a large population of birds of several species.
• This biome represents the largest number of domesticated mammals because of the development of agriculture.
• This biome also car­ries the largest number of human population of the world.

Man and Monsoon Deciduous Forest Biome:

• The tropical and sub-tropical monsoon decidu­ous forest biome is one of the most disturbed ecosys­tems of the world.
• The forests have been adversely affected by both natural and anthropogenic processes.
• There are numerous cases of frequent forest fires every year kindled by either natural processes such as light­ning or anthropogenic factors such as inadvertent ac­tions of man (throwing of burning ends of ‘bidi’ or cigarettes by the herdsmen in the forest) or advertent and intentional actions of man (such as clearing of forests through deliberate burning for agricultural pur­poses under jhum cultivation which is very much preva­lent in India) and large-scale grazing.
• The forests of the monsoon deciduous forest biome have been so rapidly destroyed within the last 50 years or so through the rapacious utilization of forest resources for commer­cial and industrial purposes and large-scale clearance through mass felling of trees for agricultural land that the vegetation cover has shrunk to a very critical size.
• The rapid rate of deforestation has led to the initiation of several ecological and geological prob­lems.
• Several species of precious animals have now become endangered species because of destruction of their natural habitats.
• For example, lions and tigers and even elephants have become endangered species in India partly because of deforestation and partly by mass hunting of these animals.
• Similarly, Indian rhi­nos are facing extinction because of their mass killing.
• The rapid rate of deforestation in the monsoon lands mainly in India has caused accelerated rate of soil loss through rill and gully erosion, siltation of river beds and consequent recurrent severe floods in the alluvial rivers.