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TOPIC: TOLERANCE
CLAAS: SS2
CONTENT:
(a) Concept of minimum and maximum range of tolerance.
(b) Geographic range graph illustrating maximum and minimum range of tolerance
SUB-TOPIC 1: CONCEPT OF MINIMUM AND MAXIMUM RANGE OF TOLERANCE
Tolerance is the ability of an organism to withstand extreme variations in environmental conditions. Living organisms possess the ability to withstand minor unfavourable changes in the factors (biotic and abiotic) or their environment which affect their survival. Biotic components interact with abiotic components such as temperature, rainfall, high light intensity, salinity, and soil factor. Abiotic factors play a significant role in the distribution of living organisms. For example, the temperature can prevent a plant from growing and surviving in an environment. The limits within which organisms can withstand changes in the abiotic factors depend on what is prevalent in such an environment.
House Mouse
The ubiquitous house mouse, Mus musculus, had its origins in Eurasia. Its precise original distribution isn’t known, and the relationship between wild forms and commensals-mice that live in proximity to, and depend upon, people is a matter of dispute among zoologists. Today there are few places on Earth to which the house mouse has not been inadvertently introduced by people, who are more or less tolerant of its presence. Although capable of building up to pest proportions, destroying crops in the field, and depleting or contaminating stores of grain, as happens periodically in the notorious mouse plagues of Australia, house mice in other places seem able to live with their human hosts without causing much damage. In this, they differ from their larger and generally more dangerous and destructive cousins, the commensal brown rats and black rats.
Concepts of minimum and maximum range of tolerance
Organisms have a minimum and maximum limit to which they can tolerate their environment which influences their survival or success in such an environment. This range is known as their limit of tolerance. Most organisms cannot exist outside their limits of tolerance. They, therefore, escape from such an environment once these limits are exceeded or risk dying in such an environment.
Variation in Rooster Combs
The comb in domestic fowl, a naked, fleshy crest on the top of the heads of both adult male and female birds, is more developed in the male. The structure of the comb can be quite variable, ranging from a simple, single, erect or drooping, serrated appearance to more elaborate forms, depending on the variety of the domestic fowl.
The law of minimum tolerance was introduced in 1840 by Justice Liebig and the law states that “the growth of a plant will be limited by whichever requisite factor is most deficient in a local environment”.
This law was expanded in 1913 by V.E. Shelfore by applying it to animals and considering the fact that too much of anything could be as bad as too little of it. Hence, Shelford’s law of tolerance states “the distribution of species will be limited by the environmental factor for which the organism has the narrowest range of adaptability.
Since organisms can only live within certain minimum and maximum limits for each abiotic factor, the range between the upper and lower limit is 0oC and the maximum limit is 42oC. Their tolerance range is 0 – 42oC.
Based on the law of tolerance, an organism may have a wide range of tolerance for one abiotic factor and a narrow range for another factor. An organism with a wide range of tolerance for all factors is likely to have a wide distribution.
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