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INTRODUCTION:
Every species has its own peculiar and perhaps unique set of environmental requirements for survival. Unless these needs are met, extinction is certain. However, these requirements are not totally inflexible, for they may vary throughout the lifetime of the individual and the history of species. The environmental factors which influence the growth, distribution, abundance, behaviour and ultimate survival of organisms are of two basic types: non-living or abiotic environmental factors which determine the interactions between the population and the abiotic environment, and living or biotic environmental factors which include interactions between different populations and instinctive control mechanisms that are internal to the population itself (Clapham, Jr., 1973).
The abiotic environmental factors are of two basic kinds: 1. Physical abiotic factors such as temperature, light, pressure, geomagnetism, gravity, wind, precipitation, topography, habitat, etc. 2. Chemical abiotic factors such as pH, salinity (salt content), oxygen availability, nutrient availability, etc. In chapters 8, 9 and 10, some of these abiotic factors like wind, precipitation, pH, salinity, availability of oxygen and nutrients, habitat, etc., have already been discussed. In these three chapters, also have been discussed three fundamental media namely air, soil (land) and water which provide necessary conditions for sustaining life, serve as macrobabitats for different organisms and also act as abiotic environmental factors. In the present chapter will be discussed law of limiting factors and only some important abiotic factors such as temperature, light, etc.
NATURE OF RESPONSE OF ORGANISMS TO ABIOTIC ENVIRONMENTAL FACTORS
The environmental factors are not constant. They fluctuate temporally and geographically, and the stresses on animals result ing from these environmental alterations affect the complex biotic structure of our biosphere. Probably the first response of any organism to a change in the environment is physiological (Kendeigh, 1974). Different abiotic environmental factors influence organisms physiologically in various ways (Fry, 1947). These effects may be classified as follows:
1. Lethal. Causing death; for instance, extreme heat or cold, lack of moisture, and so forth.
2. Masking Modifying the effect of some other factor. Low relative humidity increases the rate of evaporation of moisture from body surfaces so that warm-blooded animals are able to survive at otherwise intolerably high air temperatures.
3. Directive. Producing an orienting response in relation to some environmental response so that the organism gets itself into favourable conditions
4. Controlling Influencing the rate at which some processes function, but not entering the reaction. Temperature, pressure, and viscosity, for instance, affect metabolism, secretion, and loco- is en motion. de simultan
5. Deficient. Some abiotic environmental factors curtail an avever,activity of an organism because some essential ingredient, such as empts alt, oxygen, or the like, is absent or at unfavourably low concen- thet ration. Further, the same abiotic environmental factor may produce different effects at different times and under different conditions. Temperature may be lethal, if extreme; masking, as when coldal ele reduces the demand of cold-blooded organisms for food; directive, by inducing a search for more favourable locations; or controlling quan as a modifier of the rate of metabolism.