In this article we will discuss about:- 1. Location of Tropical Evergreen Rainforest Biome 2. Climate of Tropical Evergreen Rainforest Biome 3. Species Composition 4. Vertical Stratification 5. Animal Life.
Location of Tropical Evergreen Rainforest Biome:
Tropical evergreen rainforest biome provides optimum environmental conditions for the growth and development of plants and animals because it is characterized by high rainfall and temperature throughout the year. This biome is also called optimum biome because of uninterrupted supply of abundant moisture and water and heat throughout the year which ensures continuous and normal growth of plants. Normally, the evergreen rainforest biome extends between 10°N and 10°S latitudes.
The maximum development of this biome has taken place in Amazon Basin (South America), Congo Basin (Africa) and Indo-Malaysian Region (mainly in Java, Sumatra, Borneo, Malaysia and Guinea). This biome has developed, infact, in true equatorial climatic region though in some areas the spatial coverage of this biome far extends beyond the equator. For example, it extends upto Vera Cruz in Mexico (about 19°N) and upto 30° S in South America (fig. 45.1).
Climate of Tropical Evergreen Rainforest Biome:
The average annual rainfall of most of the areas of the tropical evergreen rainforest biome is about 2000 mm or even more. Except 2 or 3 months generally every month receives rainfall of at least 200mm. The aridity during 8 few months certainly increases with increasing distance from the equator. There is high temperature throughout the year. Mean annual temperature is around 20°C but the highest temperature of the year touches 30°C. The sun is more or less overhead throughout the year.
There is very little difference between the lengths of day and night throughout the year. Annual range of temperature is around but the daily range of temperature varies between 5°C and 10°C. Most of the rainfall is received through convectional mechanism which yields heavy downpour through cumulonimbus clouds daily normally between 2p.m. and 4 p.m.
The upper canopy of the tallest trees receives maximum sunlight whereas its amount decreases downward and it becomes minimum at the ground surface as there is complete darkness. This trend of progressively decreasing sunlight from the uppermost stratum to the ground stratum sets in keen competition among the various members of the vegetation community of the tropical evergreen rainforest biome for getting sunlight.
It may be pointed out that the uppermost canopy is also covered by climbers and creepers and therefore the leaves of the uppermost canopy receive only 25 percent of the total sunlight reaching the top surface of the uppermost canopy. The underlying areas of the tree canopy receive only 3 percent of total sunlight.
Wind speed also decreases downward from the uppermost canopy and it becomes almost zero at the ground surface but the moisture content in the air increases from the uppermost canopy downward to the ground surface because evaporation also decreases downward.
Almost absence of sunlight at the ground stratum results in the development of very few leaves and flowers of plants. The germination of seeds and growth of seedlings and saplings are exceedingly slow because of the paucity of sunlight on the forest floor. There is maximum interception of falling rains by the uppermost canopy of the forest cover and thus the intercepted rainwater reaches the ground through the leaves, branches and stems of trees and other plants in the form of aerial streamlets which allow maximum infiltration of rainwater in the ground surface.
The interception of rainwater by the uppermost forest canopy also allows much evaporation of intercepted rainwater because the rainwater is held at the outer surfaces of the leaves. Only one third of the total rainfall received at the uppermost forest canopy reaches the ground surface.
It is obvious that the vegetation has been controlled and determined by the typical characteristics of equatorial climate and in turn the vegetation has given birth to typical micro-climates in several strata of the vertical structure of the vegetation community of the tropical evergreen rainforest biome.
Species Composition of Tropical Evergreen Rainforest Biome:
The tropical evergreen rainforest biome accounts for the largest number of plant species. Though there is almost uniformity and similarity in the life- forms and the structure of the plants in all parts of this biome but there is much variation in the composition of plant species. There are numerous species of plants in the various parts of the tropical evergreen rainforest biome.
For example, 6000 to 7000 species of flowering plants in the western Africa (Congo Basin); 20,000 species of flowering plants but quite different from the western Africa in Malaysia; 40,000 species of flowering plants in Brazil and 2000 species of flowering plants in Panama Canal Zone have been identified. It well be virtually impossible to present all the species of plants by names if all the species of all plants of the tropical evergreen rainforest biome become known to the botanists.
Tree is the most significant member of the tropical evergreen forests. The number of tree species is so large and their diversity is so great that one hectare of land in the equatorial region accounts for 40 to 100 species of trees. It may be pointed out that tree species account for 70 percent of the total plant species of the tropical evergreen rainforests. Creepers or climbers are the second important floral members of the rainforests.
These belong to the category of vines and range in form from fine string-like stems to massive cable-like forms. These creepers are so circuitous and highly irregular in form that it becomes difficult to find out their actual lengths. ‘They ramble through the forest, scaling the highest emergent trees and frequently looping down to the ground and then ascending further sections of the forests. In the constant struggle for light, climbers have developed this mechanism for reaching the high insolation areas with the investment required for a large supporting biomass’.
The climbers, thus, so greatly bind several trees and plants together that the accessibility in the forest cover becomes almost zero. This typical structural form of the tropical evergreen rainforest has also affected the form of animal life. It may be pointed out that the tropical rainforests account for 90 percent of all climbing species.
Climbers are divided into two major groups as presented by P.W. Richards (1952) e.g.:
(i) Climbers of the lower strata of the forests include herbaceous plants, and
(ii) Long woody climbers known as lianas are found in all strata (right from the ground stratum to the uppermost stratum of forest canopy) of the forests. Lianas are the most significant members of the climbers or creepers. The lianas are characterized by thickly woody stems of 20 cm diameter or even more, longer lengths upto 240m or even more, and large crowns of numerous leaves sometimes resembling the crowns of trees.
(iii) A third type of tropical climbers has been also identified i.e., the epiphytes which do not have their roots on the ground surface, rather these are evolved on the trunks, stems, branches and leaves of trees, shrubs and herbs, climbers etc.
The epiphytes live in almost all the strata or layers of the forests and they do not require climbing ability as they grow upward and reach the uppermost canopy of the forest in search of sunlight. These epiphytes provide certain habitats to micro-organisms such as planarians, earthworms, snails, woodlice, millipedes, centipedes, termites, ants, grasshoppers, earwigs, scorpions, snakes, tree frogs, lizards and a host of insect larvae.
Epiphytes are further divided into 4 subtypes on the basis of their morphology as presented by S.A. Cain and G.M. de Oliverio Castro (1959) e.g.:
(i) Holo- Epiphytes, the roots of which never reach the ground surface;
(ii) Hemi-Epiphyles are first evolved on the branches, trunks and stems of trees and grow upward but ultimately their roots reach the ground surface and are thus estalished in the ground and take the form of climbers;
(iii) Pseudo-Epiphytes first evolve in the ground and grow upward to establish themselves on the branches and stems of trees. After the pseudoepiphytes are established on the trees, their roots disappear but their upper parts are maintained;
(iv) Semi-Parasite Epiphytes are those climbers which get their food from other autotrophic plants.
Epiphytes are also divided on the basis of their forms e.g.:
(i) Macro-Epiphytes which include several types of ferns and shrubs of angiosperms such as Orchidaceae, Bromeliaceae, Asclepiadaceae etc., and
(ii) Micro-Epiphytes which include moss, lichens, algae etc.
It may be pointed out that the aforesaid two major members of the tropical rainforest biome viz. trees and climbers are autotrophic green plants which manufacture their own food through the process of photoshynthesis but there are other numerous parasite and saprophyte plants in this biome.
Vertical Stratification of Tropical Evergreen Rainforest Biome:
The vertical stratification of vegetation community means the composition of plant assemblages from the soil surface or ground surface to the uppermost forest canopy. Thus, the different strata of the vertical profile of plant community in a given region represent different layers of habitat of different plant groups. ‘Stratification results from competition between species for favoruable locations which, in turn, exerts control over micro-climate and other factors affecting the habitats of plants and animals’.
There are five layers or strata from the ground surface to the uppermost canopy of the tropical evergreen rainforest biome out of which three upper layers consist of trees:
(i) First or top layer (stratum) represents the uppermost canopy of the tallest trees of the forests. The top surface of the uppermost stratum is like an umbrella but the level of the top surface is not uniform, rather it is discontinuous and wavy in character. This layer receives maximum amount of sunlight and intercepts the rain drops. The height of the topmost layer ranges between 30m and 60m. This layer is also called as dominant layer.
(ii) Second layer is formed below the first and uppermost layer of the forest canopy at the height of 25m to 30m. It is also called as the codominant layer or the second dominant layer. The upper crown of this layer is mopshaped.
(iii) Third layer is formed of lower and smaller trees, the crown of which is at the height of 15-20m from the ground surface. The trees of this layer have a typical characteristic of their leaves in that they (leaves) are much larger than the leaves of the aforesaid first two layers because of the fact that these larger leaves can trap more sunlight which is very low in this layer.
(iv) Fourth layer represents the shrub layer which is below the aforesaid three layers. This layer is not continuous rather it is fragmented and sporadic in nature. This layer also includes the saplings but these are not permanent members of this layer because after growing they reach the upper layers. This herbaceous or shrub layer also includes some pygmy (dwarf or stunted plants and trees) trees but of less than 5-m height. The crown of this layer is usually 5m from the ground surface.
(v) Fifth or lower ground layer represents the plants growing at the ground surface but seldom gains height of more than a metre or two. This layer is dominated by herbaceous plants and fern. These herbaceous plants do not form dense cover and have fewer species because the absence of light does not allow much growth of herbaceous plants.
Animal Life of Tropical Evergreen Rainforest Biome:
There are some unique characteristics of animals of the tropical evergreen rainforest biome which are not observed in other biomes e.g.:
(i) There is regular growth of plants throughout the year and therefore there is regular and constant supply of abundant food for the animals, with the result they have not to migrate for food. In other words, the animals of the tropical evergreen rainforest biome are least mobile,
(ii) The forest is full of animal activities throughout 24 hours i.e., throughout day and night because animals of different species live in the various vertical strata of the forests.
Some animals are active during day time while others are active during nights. It is very unique but interesting feature of the rainforests that there is always hue and cry which makes this biome alive.
P.W. Richards (1970) has presented a very stylish and poetic description of the diurnal rhythmic activities of different animals living in different strata of the rainforest of South America. During day time the biological environment of the lowest stratum or the ground layer is made lively through the activities of several species of deers like brocket deer, coatimundis (a South American herbivorous animal), and agoutis while howler monkeys jump from one branch to the other branch of small trees.
The upper strata are made lively through the activities of curassows (a type of South American bird), tinamous (a type of bird) and numerous colourful humming birds and numerous butterflies make the environment of the lower strata very soothing and charming while numerous lizards and other climbing animals move vertically along the stems and branches of trees of different vertical strata.
Several animals like silky ant-eaters, woolly oppossum, kinkajous, armadillos, and several predators ranging from jaguars to owls are busy during nights for getting their food. The mornings and evenings are always made lively and vibrant by the noise of several animals and birds like howler monkeys, parrots, parakeets, tree frogs and numerous types of opthoptera. Thus it is obvious that the animals make the tropical evergreen rainforests always alive day and night.
It may be pointed out that the vertical stratification of the rainforest biome has largely affected the life-forms of animals living in the different layers (strata) of the forests. Most of the animals are arboreal (tree living) and thus they have been provided additional features by the nature to climb the trees like claws, adhesive pads, fingers, toes and several clinging mechanisms.
Some animals have developed the ability to glide in the air e.g. fox, tree frogs, squirrels, tree snakes etc. The ground animals have to pass through the dense and thick covers of trees, shrubs and climbers and thus they have acquired special qualities to make their way through the forests viz. (i) Mammals have generally larger and sturdy bodies so that they can move by pushing thickets of plants away. Such animals include chimpanzee, gorilla, bison, African elephant, ocapi, leopard, numerous genera of pigs etc.
Some ground animals are very small in size but are very much mobile and clever so that they can pass through dense vegetation. The third category of ground animals includes hidden small organisms belonging to the group of cryptozoic animals. Such organisms live beneath stones, logs, dead branches of trees, parts of trees, litters of leaves etc.
The number, density and diversity of animals in the tropical evergreen rainforest biome increase from the ground layer towards increasing strata (layers) upward because of the fact that food supply also increases upward from the ground stratum due to increasing trend of sunlight in the same direction. It may be remembered that the green plants manufacture their food through the process of photosynthesis which requires sufficient amount of sunlight.
It is obvious that the vertical stratification of the rainforest has given birth to well organized vertical stratification of animals. J.L. Harison (1962) has identified the following strata of animals and their important members from the uppermost stratum to the ground stratum:
(i) Upper air animal community:
The upper surface of the canopy of the topmost stratum of the rainforest is dominated by insectivorous birds and bats but a few species are also carnivores. These birds belong to the category of fast flying species such as Asian falconet, swifts, seviftlet etc.
(ii) Main canopy animal community includes those birds and fruit bats which live in the canopy of the tallest trees forming the uppermost stratum. The important animals of the topmost canopy of the Amazonian rainforest are toucans, parakeets, barbets, cotingas, curassows, bill birds etc. A few small mammals, such as squirrels, herbivorous monkeys etc., are also found in the topmost stratum.
(iii) Middle-zone flying animal community includes mostly flying birds and insectivorous bats.
(iv) Middle-zone climbing animal community:
The animals of this zone have various climbing mechanisms and they reach the topmost stratum through the stems and branches of trees and climb down to the ground stratum. These belong to both categories of carnivorous and herbivorous animals. Squirrels and civets are the important members of rodents which belong to the category of mammals. The binding, circuitous and criss-crossing networks of various types of climbers facilitate easy movement of these climbing animals through all the vertical strata. These creepers also provide ideal habitats for these animals.
(v) Large ground animal community includes mostly animals but there are also some birds in the ground stratum. These animals have large but sturdy bodies and lack in climbing qualities and mechanisms. The herbivorous large animals are few in number, important being mouse deer and cassowaries. Numerous members of pig family feed on roots, tubers and bulbs.
It may be pointed out that the largest animals of the tropical rainforests are smaller in size than their counterparts in other open forests and grasslands because here the ground animals have to pass through dense vegetation cover and therefore they have been provided relatively smaller bodies but with great power and strength so that they can make their ways through dense forest. For example, the forest elephants of the rainforest biome are much smaller in size than the bush elephants and savanna elephants.
(vi) Small ground animal community includes small animals and micro-organisms. Most of these animals are insectivorous such as Argus pheasant, peacocks and numerous types of fowl such as Guinea fowl.
The primary 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 40 percent of the total net primary productivity of the world. The average net primary production of this biome is 5000 dry grams per square metre per year. Wood constitutes the largest share of the total biomass and net primary production.
There is maximum competition among the various members of vegetation communities of the tropical evergreen rainforest biome to get light. Thus it is obvious that the plant species of almost similar characteristics and features can survive in this competition. This is the reason that inspite of largest number of plant species in this biome they have more or less similar life-forms.
Man has also started to damage this biologically rich ecosystem through his rapacious economic activities. A sizeable portion of Amazonian rainforests has already been damaged through mining activity and industrial and agricultural expansion.
The construction of large dams and reservoirs on Amazon and its tributaries at the cost of rich forest cover has upset the ecological balance of the area. The powerful elite society and even the local Brazilean government is not paying any attention to the mass movement of the local aborigins against mass felling of trees.