No two ecosystems in locations around Earth can be said to have the exact assortment of species forming habitats in them. However, terrestrial ecosystems and biological communities of organisms can be classified into categories on the basis of climatic features and the predominant plant forms appearing in these locations. On the basis of these categories, the different types of biomes of the world are classified. The presence of similar vegetation or the complete absence of it in similar climatic conditions mark many terrestrial ecosystems around the world that fall under similar biome types among the biomes of the world.
This method of distinguishing biomes of the world is utilized because climate majorly influences plant growth in terrestrial ecosystems. Climatic features in terms of commonalities in the biomes of the world causes particular types of plant growth that are restricted to locations with particular climatic features. There are also other similarities in terms of other features such as in animal communities and in biological productivity for example that can however, not be as marked as similarities in plant growth. Physical features can be another determinant, such as the influence of elevation for example, which can lead to Alpine biomes in mountainous regions with a high degree of elevation from sea level.
Walter’s System of Classification of Different Biomes
Climatic features might also determine the boundaries of terrestrial biomes, especially in terms of differences in temperature and precipitation. There is a dramatic difference for example, in the vegetation found in temperate seasonal forests as against the vegetation found in subtropical desert biomes. Temperature and precipitation can act as the most important variables determining species distribution especially in the case of plants. These two factors are the ones most influential in determining plant distribution and species make up in different biomes and also even within biomes.
There can thus be many biomes distributed through Earth and in a general manner of classification many climate classification systems can be referred to. One that can be mentioned in particular is the climate zone system developed by Heinrich Walter that can be useful in classifying the biomes of the world. This system is useful as it matches the boundaries of biomes with the differences in the distribution of vegetation types. In another scheme for classifying the biomes of the world, Robert H. Whittaker first classified biomes according to the distribution of vegetation types, and then plotted a climate diagram which demarcated the approximate boundaries of the biomes of the world taking into account temperature and precipitation as variables. Whittaker’s results were very similar to Walter’s system of the biomes of the world.
Walter identified nine biome types whose boundaries correspond to variations in vegetation types in terms of temperature and precipitation (Ricklefs, 2008) as –
- Tropical Rainforest Biomes – This is the only biome type among the biomes of the world that is mostly present in the equatorial region and is mostly moist in terms of climate with a lack of abrupt seasonality in terms of temperature. The vegetation here can be characterized as evergreen tropical rainforest.
- Tropical Seasonal Forest / Savanna Biomes – In this biome type among biomes of the world, the summers are wet and go through a rainy season whereas winter tends to be a dry season. The vegetation here shows seasonal variance and can occur as scrubs or savanna.
- Subtropical Desert Biomes – Also called hot deserts, rainfall is scant and highly seasonal, with an arid climate. The vegetation in this type of biome comprises desert vegetation with a considerable amount of surface that is exposed with very little (if any) vegetation.
- Woodland / Shrubland Biomes – Areas falling under this biome have a Mediterranean climate, with a moist rainy winter season and a dry summer season that can be like a drought. The vegetation in these regions comprise mostly of drought-adapted Sclerophyllous plants that can also be frost sensitive woodlands and shrublands.
- Temperate Rainforest Biomes – In this type of biome the maximum rainfall occurs during the summer season while occasional frost can also be observed. The vegetation comprises temperate evergreen forests that can be frost sensitive.
- Temperate Seasonal Forest Biomes – The climate in this type of biome is relatively moderate with freezing temperatures in the winter season. The vegetation in this type of biome is usually frost resistant and comprises of deciduous temperate forests.
- Temperate Grassland / Desert Biomes – The climate in this type of biome chiefly comprises continental cold deserts that are arid with warm or hot summers and very cold winters. The vegetation here can mostly occur as temperate desert vegetation or as grasslands.
- Boreal Forest Biomes – In this biome the climate is cold temperate where summers are cool whereas winters can be long and extremely cold. The vegetation tends to be evergreen forests with frost resistant needle leaved forests.
- Tundra Biomes – The climate in this biome can comprise of very short and cool summers while winters are long and extremely cold. The vegetation here if it appears comes as low evergreen vegetation and trees can be a rare sight.
Fig 1: The Various Biomes of the World
Source: M.A., 2005
A Broad Categorization
The biomes of the world can also be broadly categorized purely in terms of vegetation as forest and woodland biomes, grassland biomes, desert biomes, tundra biomes and alpine biomes. These under Walter’s system of classification are further classified into climate types under the different biomes of the world. These in turn can help us in understanding Walter’s system of the classification of biomes of the world better.
Forest and woodland biomes predominantly consists of areas with trees whose height and spacing in terms of clusters are closely determined by temperature and rainfall. Forest biomes can be tropical forests nearby the equator where trees occur in dense clusters, as subtropical forests where temperatures are fairly high at most times of the year with seasonal rainfall, as temperate forests which can be warm or cool temperate forests which depend on temperature but receive sufficient rainfall to be counted as tropical or seasonal temperate forests, as boreal forests where summers are warm and wet while winters are cold and dry, and as woodlands where precipitation is moderate and often seasonal and often occur at regions in transition between forests and grasslands or shrublands and can occur in varied temperature variations where vegetation is adapted to drought conditions.
Grassland biomes occur in areas of the world that are moderately dry and are mainly divided into savanna grasslands and temperate grasslands. Savanna grasslands cover most of the African continent and can also be found in India, South America and Australia. Here summers are hot and wet and winters are warm and dry. Temperate grasslands occur as the pampas of Argentina and Uruguay, the steppes of Russia, the prairies of North America, the veldts of South Africa and the puszta of Hungary. The grasses are taller in the wetter regions and shorter in the drier regions.
Desert biomes are characterized by dry and arid climates that can be hot or cold. The precipitation here can be less than about 250 mm of rainfall annually. Desert biomes can comprise of hot deserts such as the Sahara Desert and the Thar Desert found in the tropics or the lower latitudes whereas the cold deserts such as in Mongolia are found in the higher latitudes or at very high elevations such as in Ladakh. The vegetation tends to be scarce and widely spaced.
Tundra biomes are found in the very cold regions by the polar regions in the northern hemisphere and are bounded by the boreal forest biomes towards their southern portions. The climate is extremely cold and desert-like and are followed northwards by the polar cold deserts that are more or less permanently under ice cover. In the southern hemisphere most of Antarctica forms a polar cold desert permanently under ice cover. However, tundra biomes unlike most of Antarctica are able to sustain life as summer temperatures can usually go up to 12oC. Soil here usually occurs as permafrost.
Alpine biomes are curiously uncategorized in Walter’s system of classification of biomes of the world and here we shall attempt to summarize this biome. As one travels up a mountain range it is possible for one to experience a number of different biomes as well and different types of biodiversity. While one can witness deciduous forests, grasslands, cold deserts, etc as biomes at various levels of the mountain range, at very high altitudes in many mountain ranges of the world are alpine biomes. Alpine biomes are located at very high altitudes in mountain ranges just below the snowline, where the amount of carbon dioxide is significantly lower and winters can be freezing depending on altitude, latitude and precipitation. The climate here is too cold to have trees and mostly there are small leaved shrubs and heaths that grow close to the ground (Education Services Australia, 2013).
Given these different types of vegetation growth, mixing them with different levels of temperature, precipitation and altitude regions produces different types of biomes of the world that can be classified in the manner in which Walter has classified them. Thus different mixes of temperature, precipitation and certain physical features produces different types of vegetation growth that can classified into the different biomes of the world. Climate thus plays a critical role in terrestrial biomes whereby different types of vegetation growth are produced.