Range Of Tolerance

With varying environmental factors, tolerance power of different organisms differ. The factor which is present in least amount becomes limiting for an organism. A factor that limits growth, development, reproduction or activity of an organism by its deficiency or excess is called limiting factor. Some examples are :

Plant growth is reduced in deserts due to less availability of water. In deep lakes, due to low phosphorus contents, growth of phytoplanktons is retarded. Low temperature limits plant growth at higher elevations. However, it should be noticed that ‘present in very less quantity’ may not be only limiting. Sometimes ‘too much presence’ may also become limiting factor and may lead to growth and distribution retardation. The organisms show a bell shaped curve to a range or gradient of single factor like light, temperature or nutrient concentration. In zone of stress, number of organisms become less. In optimum range, number is maximum. Range between ‘very low’ and ‘very high’ is tolerance range. This range is responsible for survival and growth of organisms. With wide range of tolerance, organisms are widely distributed. They are of restricted distribution in narrow tolerance range.

Fig. : The response of an organism to a range or gradient of an environmental factor, (temperature, light, nutrient)

ACCLIMATISATION

The gradual physiological adjustment to slowly changing new environmental conditions is called acclimatisation.It is development of a favourable morphological and physiological response to a change in environment. Change occurs due to change in season or some other reason. If the change in an environmental factor is beyond the tolerance range of an organism, the latter can migrate, come to resting stage, get acclimitised or die. Most of the acclimatisation changes are biochemical based. For example, as we go higher up in a mountain region, the rate of respiration increases with deeper breathing and increased intake of air to compensate for deficient oxygen in the atmosphere. People living at high altitudes have adapted to lower oxygen level of air by having higher lung capacity and larger number of red blood corpuscles.