In this article we will discuss about:- 1. Location of Savanna Biome 2. Climate of Savanna Biome 3. Vegetation Community 4. Animal Community 5. Man and Savanna Biome.
Location of Savanna Biome:
The word savanna has been used for different meanings by various scientists e.g., the word ‘Savanna Region’ has been used by the climatologists to indicate a particular type of climate i.e., tropical wet and dry climate (Aw climate of Koeppen) as savanna climate, while the botanists have used the word savanna for a typical type of vegetation community of the tropical regions characterized by the dominance of grasses.
Normally, the savanna biome refers to that vegetation community of the tropical areas which is characterized by the dominance of ground cover by partially xeromorphic herbaceous plants, upper stratum of scattered trees and middle layer of sparse shrubs.
The savanna biome extends in both the hemispheres between 10°-20° latitudes and includes Llanos of Columbia and Venezuela; South-Central Brazil, Guiana, Paraguay (all in South America); hilly areas of the Central America; Central and East Africa (maximum extent in Sudan); Northern Australia and some areas of India (the savanna of India is not the original and natural vegetation cover rather it has developed due to human interference with the original forest cover resulting into the development of widespread man induced grasslands).
There is no unanimity of views of the scientists about the origin and evolution of savanna grassland biome. According to the majority of the scientists the savanna biome is the result of interference and modifications in the natural environments of the regions now considered as savanna region by man himself (human activities like deforestation, frequent forest fires, overgrazing etc., are considered to be the main factors for the evolution of savanna biome).
There are clear-cut evidences to demonstrate that the savanna regions of India have certainly originated and developed because of deforestation of the original forests by man because Indian savanna areas are found within and around deciduous forest covers. Unlike other main savanna areas of the world Indian savanna areas are dominated by shrubs instead of grasses.
It may be concluded that the savanna biome is the outcome of a set of complex factors such as characteristic features of climate, geomorphic history, natural fires, the evolution of grazing animals and their consequent impact on natural original vegetation and above all the presence of man and his various activities.
Climate of Savanna Biome:
The savanna climate is characterized by distinct wet and dry seasons, mean high temperature throughout the year and abundant insolation. Mean annual rainfall ranges between 250-500mm on the desert fringes of the savanna and 1300-2000mm on its border with the equatorial climate. Temperature does not fall below 20°C in any month of the year.
There are three seasons on the basis of the combinations of temperature and humidity (though on an average there are only two seasons as referred to in the beginning but the dry season is further divided into warm dry season and cold dry season on the basis of temperature) viz.:
(i) Cold dry season is characterized by high day-temperature ranging between 26°C-32°C, but relatively low temperature during nights, usually 21°C;
(ii) Warm dry season is characterized by almost vertical sun’s rays, high temperature ranging between 32°C-38°C due to abundant insolation, and
(iii) Warm wet season receives between 80 to 90 percent of the total annual rainfall.
It may be pointed out that there is much pronounced variation in the spatial distribution of mean annual rainfall in the different parts of the savanna biome of the world mainly because of two major factors viz.:
(i) Distance from the equator, and
(ii) The nature of topographic features.
For example, the savanna region of Brazil, locally called as Cerrado, having the average absolute relief of 1300m AMSL, records mean annual temperature and mean annual rainfall of 20°C-26°C and 750 mm-2000mm respectively.
The Llanos of Columbia is characterized by mean annual rainfall of 2000mm-4000mm (near Andes Mountain) and mean annual temperature of 22°C and the maximum temperature of 32°C. The Indian Savanna is characterized by highest temperature (being 45°C-48°C in May and June) and lowest temperature (being 5°C or even less during the month of January) of all the Savanna regions of the world and mean annual rainfall well below 1500 mm, 80 to 90 percent of which is received during a brief period of 3 months (15th June to 15th September).
Vegetation Community of Savanna Biome:
Though the general characteristics of typical Savanna vegetation are trees and grasses but the Savanna biome is, no doubt, dominated by grasses.
The Savanna vegetation community has developed layered structure wherein three distinct layers have clearly developed:
(i) Ground layer (stratum) is dominated by various types of grasses and herbaceous plants. The grasses, the most dominant vegetative member of the Savanna biome, are generally coarse, stiff and hard and of course perennials having the height of 80 cm but very long grasses reach up to 350cm (3.5m) in height.
The African elephant grass attains the enormous height of 500cm (5m). The leaves of these grasses are almost flat which are shed during dry season but they are regenerated during wet season. The Savanna grasses are usually tufted in structure and form. It may be pointed out that not all the grounds are continuously covered by Savanna grasses; rather there are frequent open patches which are devoid of grasses.
The root systems of the Savanna grasses consist of lateral dense network of fine branches which penetrate upto the depth of 2.5m in the soil cover. The important genera of the Savanna grasses are Hyparrhenia (elephant grass), Panicum, Pennisetum, Andropogon and African species Imperata cylindrica. The grasses bear deserted look during dry warm summer season but they become lush green again during humid summer season.
(ii) Middle layer consists of shrubs and very stunted woody plants.
(iii) Top or canopy layer is formed by trees of various sorts. The general characteristics of trees depend on the availability of water and moisture and therefore there is a great taxonomic variety of Savanna trees which are usually 6.12m in height. The Savanna trees have developed various unique characteristics to cope with the dry conditions of this biome.
For example, there are a few species of trees which have developed such mechanisms which help them to reduce evapotranspiration from their leaves during warm dry season and enable them to remain green even during dry season of deficient water supply. On the other hand, there are such tree species which cannot withstand dry conditions and therefore they shed their leaves and bear the characteristics of deciduous trees.
The roots of the Savanna trees have also developed according to the environmental conditions as they are very large which can penetrate into the soil and ground up to the depths from 5m to 20m so they can obtain water from groundwater even during dry season when the groundwater table falls considerably.
The smaller plants and many herbaceous plants have special kinds of root systems characterized by root tubers and swellings so that they may preserve water which may be used by the plants during dry season, because the roots of these plants seldom reach the depth of more than 20cm in the soils and the coarse soils up to this depth become dry during dry season.
The trees form flattened crown or canopy but they are very sparsely distributed. Several branches come out from the stems which are mixed up with the middle layer. Some of the Savanna trees are fire resistant (pyrophytic) as they have thick bark and thick bud-scales. The Savanna biome is characterized by the monotony of tree species as there are very few tree species per unit area as compared to the tropical rainforest and tropical monsoon deciduous forest biomes.
For example, baobab is the only significant tree from Tanzania to Senegal and the Savannas of Ivory Coast and Sudan are dominated by palm trees. The important tree species are Isoberlinia, the baobab and dom palm in African Savannas; species of Eucalyptus such as E. Marginata and E. Calophylla in Australia; pine trees in Honduras etc.
The net primary productivity ranges from one place to another place depending on the nature of tree densities. The mean net primary productivity of the Savanna biome is 900 dry grams per square metre per year but there is great spatial variation in the productivity as it ranges from 1500 dry grams per square metre per year in the closed savanna (dominated by trees and shrubs) to a minimum of 200 dry grams per square metre per year in the desert scrub Savanna.
On the basis of the proportion of trees and grassland and the structure of the vegetation the Savanna biome may be divided into the following four types:
(i) Woodland savanna is dominated by trees and shrubs which form dense upper canopy. This Savanna is, thus, also called as closed savanna. Inspite of comparatively closed upper tree canopy of the topmost layer, enough sunlight reaches the ground surface to support ground cover of herbaceous plants. There is more or less general absence of epiphytes but some climbers having their roots in the ground are present.
(ii) Tree savanna represents relatively open vegetation cover in terms of trees, and shrubs which are sparsely distributed. The ground cover is dominated by grasses. No tree conopy is developed.
(iii) Shrub savanna is represented by treeless vegetation which is dominated by grasses at the ground layer and shrubs at the second layer. Infact, shrub Savanna is two layered vegetation where the topmost layer is formed of shrubs and the ground cover consists of grasses.
(iv) Grass savanna is characterized by general absence of trees and shrubs and over dominance of dense grasses. The grass cover is not continuous; rather it is separated by intervening patches of grassless areas.
The frequent fires, both natural and anthropogenic (intentional annual burning of grasses by man), are common features of all the aforesaid Savanna biomes. Though many organic materials are destroyed due to annual burning of grasses by man, regular fires in Savanna grasslands are very important ecological processes because these favour regeneration of grass every year, mineralization of leaf litter and regulation of fauna.
‘Thus, fire appears to be a normal part of the Savanna biome and one of the major factors in its nature Savanna (is) a delicate balance of the outcome of climate, soils, vegetation, animals and fire, with fire as the key agent whereby men have created the biome; as it now stands this biome in Africa cannot be regarded as climatic climax but as a product of human activity’.
No doubt, frequent burning of grasses by man has been responsible for the evolution of a few fire- resistant species of trees and grasses such as Imperata spp (a type of grass).
Animal Community of Savanna Biome:
It may be pointed out that animal communities of different Savanna areas of the continents show a wide range of species diversity because of the fact that:
(i) Different Savanna areas have developed differently in different environmental conditions during various stages of evolution, and
(ii) The degree of human interference has greatly varied in different Savanna regions.
The availability of food during the different seasons depends on the environmental conditions. Since there is maximum growth and development of vegetation during wet summer season and almost barren ground during dry summer season and hence there is abundance of food during wet season but there is marked scarcity of food during dry season.
This seasonal regime of the availability of animal food has largely affected animal community in the Savanna biome. Secondly, hunting of animals by man has also adversely affected them. Inspite of these limiting factors the Savannas are capable of supporting a very diverse fauna.
The African Savanna accounts for the largest number and the greatest variety of grazing vertebrate mammals in the world. For example, the East African Savanna carries 40 species of very large herbivorous mammals such as African buffalo, zebra, giraffe, elephants, many types of antelopes, hippopotamus etc. of which even 16 species graze together in the same habitat. On the other hand, the South American and Australian Savannas do not have large number of grazing mammals similar to the African Savanna but great variety of birds like those of the African Savanna is invariably found.
The Australian Savanna is dominated by marsupials (typical mammals of South American and Australian origin having pouch in their bodies to keep and feed their offsprings). There are at least 50 species of kangaroo in the Australian Savanna which greatly vary in size ranging from very large red kangaroo (1.5m tall) to very small species of wallaby (only 30 cm in height).
The large grazing mammals of the South American Savannas include deer and guanaco. Besides, toucans, parrots, nightjars, kingfishers, doves, finches, parakeets, wood peckers are also found in large number in the South American Savannas.
It may be pointed out that relatively less denser cover of vegetation in the Savanna biome provides maximum mobility to the animals and thus the Savanna grasslands have been responsible for the origin and evolution of great number of large mammals (like elephant, giraffe, zebra, ganda, hippopotamus, antelopes etc.) and birds such as courses, bustards, game birds, ostrich, and several non-flying birds like emu.
There is complete correlation and correspondence between the structure and seasonal regime of the Savanna vegetation and invertebrate animals. The invertebrate animals include insects (such as flies – diptera, locusts, grasshoppers, termites-Isopetra, ants and arthropods (like spiders, scorpions etc.) which are found profusely in the various parts of the Savanna regions.
The density of oligochaete worms, spiders and insects in the Guinea Savanna of tall grasses of the western Africa is 50,000 to 60,000 per 300 square metres of area during dry season but the density of these organisms increases to 1,00,000 during wet season because of regeneration of dense cover of green grasses.
The rainy season is characterized by the dominance of smaller animals (such as springtails, ants, earwigs, cockroaches, small crickets, carabid beetles etc.) whereas the larger invertebrates dominate during dry season like locusts, grasshoppers, mantids and crickets.
It may be pointed out that inspite of large number and great variety of animals of invertebrate and vertebrate categories (ranging from micro-organism-like insects to very large bodied animals like giraffe and elephants) there is no competition for food among the animals in the Savanna biome because of the fact that the animals of this biome have developed typical feeding habits and mechanisms according to the characteristics of the vegetation.
For example, giraffe uses the top layers of the trees and shrubs through his exceptionally long neck, zebra lives on the leaves of shrubs and the heads of tall grasses, wild-beasts graze the grasses of medium height whereas the gazelles (deer family) depend on short grasses. It appears that there is close correspondence between the vertical stratification of the vegetation community and feeding habits of the animals of the Savanna region.
Thus, the Savanna biome is characterized by the development of grazing succession which enables the animals of various species and sizes to live in the same habitat without having much competition among themselves for food.
There is also wide range of variation in the seasonal mobility of the ungulate animals (animals having hoofs) and thus the seasonal variability of the animal mobility has also discouraged competition among the animals for food.
Based on seasonal characteristics of mobility A.F. Lamprary (1964) has divided the animals of the Savanna biome into the following 5 categories:
(i) Animals with little or no seasonal movement, e.g., giraffe. Grant’s gazelle, hartebeest etc.
(ii) Animals having partial movement during dry season, e.g., impala
(iii) Animals having partial movement during wet season e.g., warthog, dikdik, waterbuck, rhino etc.
(iv) Animals migrating during dry season, e.g., buffalo, zebra, wild beest, eland, elephant etc.
(v) Animals used to passage migration, e.g., buffalo, zebra, elephant etc.
The East African Savanna is the richest of all the other Savannas in terms of total animal population.
‘Where a rich fauna still exists, as in East and Central Africa, it may achieve a yearlong vertebrate biomass of 100 x 105 kilogram per hectare live weight’. The average net primary productivity (NPP) of the Savanna biome is 900 dry gram per square meter per year whereas the total Net Primary Productivity of all the Savannas of the world is 13.5 x 109 tons per year.
The termites are very significant animals of the Savanna biome because they help in decomposing the organic matter and in recycling the nutrients. According to an estimate the biomass of termites in Ivory Coast is 12kg per hectare and these consume 30kg of cellulose per hectare per year and rearrange several dozen tons soils every year.
Man and the Savanna Biome:
The impact of man in the Savanna biome right from the evolution of human races in the various parts of the present-day Savannas to the present-day technologically advanced society has been so immense that the very nature and the characteristics of Savanna grasslands are the outcome of the continued man’s interferences with the original natural environmental conditions particularly natural vegetation and related micro-climates.
The regular burning of vegetation generates lush green grasses during the wet season which support large number and variety of grazing animals but simultaneously this routine annual practice reduces the number of large animals feeding on the leaves of trees because frequent fires are not conducive for luxuriant growth of trees.
The rapidly increasing human population for the last 50 years or so has put enormous strain on the natural Savanna grasslands because a vast area of the original grasslands has been converted into agricultural fields to grow more food crops to feed the teeming millions. The rapid rate of expansion in the agricultural lands under the new scheme of green revolution has further been responsible for the shrinkage of natural Savanna grasslands.
Furthermore, enormous increase in the number of domesticated animals has greatly damaged the grasslands. In nut shell, the impact of human activities has resulted in the shrinking of the areas of grasslands and reduction of natural vegetation which have caused shortage of food supply to the animals. All these have ultimately adversely affected the animal communities. Consequently, the number of animal species and their total population are gradually decreasing.