Here is your Short Essay on Shelford’s Law of Tolerance !
Organisms may be limited in their growth and their occurrence not only by too little of an element or too low an intensity of a actor but also by too much of the element or too nigh intensity of the factors.
For example, carbon dioxide is necessary for the growth of all green plants, small increase in concentration of carbon dioxide in the atmosphere will, under certain circumstances; increase the rate of plant growth, but very considerable increases become toxic. Likewise, small additions of arsenic to the human diet actually have a tonic effect, further increase in the dosage, however, soon proves fatal.
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The idea that factors could be limiting at their maximum as well as minimum quantities was incorporated in law of tolerance formulated by V. E. Shelford in 1913. This law postulates that each ecological factor to which an organism responds has maximum and minimum limiting effects between which lies a range or gradient that is now known as the limits of tolerance.
Between the lower and upper limits of tolerance lies a broad middle sector of a gradient which is called the zone of compatibility, the zone of tolerance, the biogenetic zone or the zone of capacity adaptation.
The region at either end of the zone of compatibility is called the lethal zone or the zone of resistance or zone of intolerance. The zone of compatibility too includes a broad range of optimum and narrow zones of physiological stresses in between the range of optimum and lethal zones.
Upper and lower limits of tolerance are intensity levels of a Factor at which only half of the organisms can survive. These limits are sometimes difficult to determine, as for example with low temperature, organisms may pass into an inactive, dormant, or hibernating state from which they may again become functional when the temperature rises above a threshold at high temperatures, there may be similar inactivation or aestivation before the lethal level is attained. Even without dormancy occurring, there are normally zones of physiological stresses before the limits of tolerance are reached.
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The species as a whole is limited in its in activities more by conditions that produce physiological discomforts or stresses than it is by the limit of tolerance themselves. Death verges on the limits of tolerance, and the existence of the species would be seriously jeopardized if it was frequently exposed to these extreme conditions.
Therefore, in retreat before conditions of physiological stress there is a margin of safety, and the species adjusts its activities so that limits of tolerance are avoided. There is a variation in hardiness of individuals within a species, so that some hardy individuals find existence possible under conditions that disrupt other individuals. The population level of a species becomes reduced before the limits of its range are actually reached.
Further, species vary in their limits of tolerance to the same factor. For example, the Atlantic salmon spends most of its adult life in the sea, but goes annually into fresh-water streams to breed. Most other marine fishes are killed quickly when placed in freshwater, as are fresh-water fishes when placed in salt water. The following terms are used to indicate the relative extent to which organisms can tolerate variations in environmental factors.
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The relationship between populations and environmental factors can be shown in the shapes of the tolerance curves for the specific variables shows two such curves, one in which the optimum is very broad and one in which it is quite definite. The prefix steno-means that the species, population, or individual has a narrow range of tolerance and the prefix eury-indicates that it has a wide range.
Moreover, a plant or animal may have a wide range of tolerance for one factor in the environment, but a relatively narrow range of tolerance for another condition. Thus, we find that some species of fresh-water fishes are eurythermal but they are stenohaline.