The life containing and life supporting environment of the world is restricted to a very irregular layer (5 to 20 km thick) around the globe.
This thin veil of living material on the earth is called ecosphere or biosphere.
Thus, the biosphere is that part of earth in which life exists (Hutchinson, 1970) Biosphere is synonymous with biota and refers to the sum of all living organisms on the earth Biosphere may be divided into para-biosphere and eubiosphere.
The parabiosphere is that part of biosphere where environmental conditions are not entirely hospitable and this includes such broad areas as higher altitude, the polar regions, the deepest ocean troughs the most extreme deserts and certain localized regions as volcanoes, geysers and heavily polluted areas of land and water.
The remaining portion (The eubiosphere) is composed of three chief media-air, water and earth or land and accordingly it has been divided into three subdivisions:
(iii) Lithosphere or pedosphere
In ecology we study the reciprocal relationship between an organism or a group of organisms and its environment. The environment literally means the surrounding. The environment is the aggregate of all those things and set of conditions which directly or indirectly influence not only the development or growth and quality of life of individual organisms but also the communities at a particular place.
It is comprised of a number of factors which interact with one another and also influence the responses of the organisms. Any external force, substance or condition affecting the organisms in any way is referred to as environmental factor. Soil, moisture, wind and temperature are thus factors and the environment thus is sum total of all such factors. The natural place where organisms or communities of organisms live is called habitat.
The habitat implies a particular set of environmental factors in a given locality and is, therefore, generally used in a more concrete sense than the environment, as for example, water is the habitat of aquatic organisms and land is a habitat for numerous terrestrial organisms.
Aquatic habitat comprises three major categories namely, fresh water habitat, marine habitat and brackish habitat. Biosphere can be divided into many major categories of land masses called biomes. Biomes are subdivisions of earth biota. They are distinct large areas of earth with relatively homogeneous climate and flora and fauna as for example, deserts, forests, prairies etc. Biomes are subdivided into small units, each with its own particular set of physical conditions. These small units are called Zones.
For example, a forest biome can be divided into ground zone and canopy zone, and likewise desert biome can be divided into surface and subterranean zones. The ocean can be regarded as single biome (the marine biome) which can be divided into surface, abyssal and intertidal or littoral zones.
Within each biome there may be numerous habitats, each characterised by a particular set of conditions and well adapted community of organisms. Within a particular habitat an individual is generally confined to restricted situation which is called ecological niche.
The environmental or the habitat factors influence the characters and composition of individual plants and plant communities. Any feature of an organism or its part which enables the organism to exist under conditions of its habitat is called an adaptation. An organism accumulates many adaptive features in it.
Such features may ensure a degree of success either by allowing the organism to make full use of the amounts of nutrient, water, heat and light available to it or by providing a significant amount of protection against some un-favourable or adverse factors, such as very high or very low temperature, drought, parasitism and so on. The adaptive features of organism may be hereditary (i.e., they may be genetically controlled) or they may be induced by the habitat factors.