Short Essay on The Medium of Biosphere

Two fundamentally different media exist in the biosphere; there are the media of air and water. All organisms must possess a medium and, once selected, it is unlikely to be changed. It is difficult to accept that in spite of all the different habitats to be found in our biosphere, from arctic wastes to tropical coral reefs, from arid, hot deserts to cold, acid oligotrophic swamps and from fresh water mountain streams to warm, polluted effluents, the medium in every instance must be either air or water.

This limited choice between media conveniently divides our habitats into two forms:

(a) Terrestrial habitats in which the medium is air.

(b) Aquatic habitats in which the medium is water; this habitat can be further subdivided into fresh water and salt water habitats.

The medium is the most basic requirement for all organisms. Plants and animals must possess a respiratory system which can function in one of other medium.

Those organisms which use a medium of water, or a water-based fluid, rely on gills or trans-tissue diffusion in order to obtain oxygen (for animals) or carbon dioxide (for plants). Terrestrial life forms use a medium of air and have a more complex pulmonary respiration system.

Apart from the vital respiratory role, the media are also responsible for the provision of buoyancy or support. In this capacity, water is approximately 850 times more buoyant than air, Table.

Accordingly, organisms which are confined to a medium of water can dispense with a rigid support mechanism. The ‘bones’ of a fish serve merely as a means of muscle attachment and provide little or no structural support. By comparison, a terrestrial animal requires a massively calcified bone structure to provide rigidity and onto which muscles can be attached.

Land plants attain a vertical posture by means of extra cellulose of lignin in their cell walls whereas water plants dispense with such features, and have instead air spaces which serve as buoyancy tanks.

The medium also has a major influence on the ease of movement. This is particularly relevant for animals and also assists the dispersal of plant fruits and seeds. A medium of air, because of its lower density, allows faster movement for less expenditure of energy than does a medium of water.

The Substratum

The substratum provides a surface or substance upon or within which an organism lives. It is sometimes difficult to distinguish where the medium ends and substratum begins.

For example, the earthworm spends its entire life history in the soil; the substratum is soil while the Tedium is water. Surrounding the body of the earthworm is a watery aid across which gaseous diffusion takes place. If the earthworm were artificially dried, then death would quickly result. Without the correct medium most organisms die, but if an earthworm was placed in a box containing damp polystyrene granules (a new substratum) then the worm could still live provided there was also a supply of nutrients. This illustrates a commonly observed feature; an organism can rarely change its medium while a change of substratum is quite common.

The most common source of substrata are roc that is soil. Other substrata include wood, hide, leaves, roots and water. Most substrata can be used by different organisms to provide a variety of living areas.

The function of the substratum has been defined by Andrewarl et. al., (1954) as providing: ” 1. A place of attachment (i. e., a home).

2. A source from which nourishment (food) can be obtained.

3. Provision of shelter. The Climate

Two climatic inputs, those of solar energy and precipitation, are fundamental requirements for life. From an ecological stand-point, climatic inputs can be considered an amalgam of favourable and unfavourable stimuli which promote or retard growth respectively.

The ecologist need not be concerned with the major atmospheric processes which determine the climate or an area. Instead, we must establish which critical climatological thresholds have relevance on the biosphere. In the past, climatologists have suggested that specific climatological values can be used as ‘deterministic switches’, the attainment of which causes a particular organic function to occur.

Well established critical values such as 0°C, 6°C, 500 mm precipitation per annum and 203 frost-free days per annum have all been variously proposed as cortical threshold values for plant growth. More recent workers have suggested that critical values should not be considered as absolute values at which specific events occur.

Instead, they should be used as maker points at which changes in ecosystem functions take place. Evans (1963) has provided examples to show how the quality, intensity and duration of climatic inputs can control the life history of plant species while Vernberg et. al., (1970) give similar evidence for animals species.