The existence, abundance and distribution of a species in an ecosystem are determined by the range of tolerance to environmental variables. Lets learn more about Shelford’s law of tolerance.
The existence, abundance and distribution of a species in an ecosystem are determined by the range of tolerance to environmental variables. Organism growth and survivorship are maximal at optimum value of the environmental factor or resource and decrease below (suboptimal) and above (supraoptimal) values. Therefore, an arbitrary optimum range, within which the organism’s performance is close to the maximum value, is often defined. Tolerance characterizes organism’s response to stress and is the organism’s ability to grow and survive in suboptimal and supraoptimal conditions. In the real world, environmental conditions exist as gradients. The performance of an organism changes along the gradient.
According to Shelford’s law of tolerance (formulated in 1913 by Victor E. Shelford), there are upper and lower threshold values on the environmental gradient beyond which the species cannot survive. These threshold values are the upper limit of tolerance and the lower limit of tolerance. The whole range over which the species is able to survive is the range of tolerance. Tolerance ranges differ for one species to another. Organisms may have a wide range of tolerance for one factor and a narrow range for another. When conditions are not optimal for a species with respect to one ecological factor, the limits of tolerance may be reduced for other ecological factors. For example, when soil nitrogen is limiting, the resistance of grass to drought is reduced. The prefix steno – meaning ‘narrow’ and eury – meaning ‘wide’ have been used for comparing the relative narrow and wide range of tolerance, respectively of an organism to a given environmental factor. For example, stenothermal organisms can tolerate only a narrow range of temperature whereas eurythermal organisms have wide range of temperature tolerance.
Figure 1: Shelford’s Law of tolerance diagram explains the Response of an organism to a range or gradient of an environment factor.
Similarly, Stenohydric and euryhydric refer to narrow and wide tolerance, respectively, to water. Stenohaline and euryhaline refer to narrow and wide tolerance, respectively, to salinity. Stenophagic and euryphagic refer to narrow and wide tolerance, respectively, to food. Stenoecious and euryoecious refer to narrow and wide tolerance, respectively, to habitat selection.
These terms apply not only to the organism level but equally well to the community level. For example, warm-water coral reefs occupy shallow sunlit, warm, and alkaline waters in order to grow and calcify at the high rates necessary to build and maintain their calcium carbonate structures. They are very stenothermal, in that they prosper only within a very narrow range of temperature. A prolonged 2°C drop in temperature is a stress full condition causing bleaching or loss of symbiotic algae.
Species differ from each other in terms of their demands (requirements) from their environments and consequently also in the extent to which they can tolerate the fluctuations in their environmental conditions. This range of demands and consequent range of tolerance of a species is known as its ecological amplitude. In other words, it is the range of environmental factors that a particular species can tolerate.
FAQs on Shelford's Law of Tolerance
According to Shelford’s law of tolerance (formulated in 1913 by Victor E. Shelford), there are upper and lower threshold values on the environmental gradient beyond which the species cannot survive.
The term “tolerance” can refer to two different things: (1) an organism’s capacity to survive in unfavourable conditions, and (2) the maximum allowable concentration of a chemical in food.
Four basic models of tolerance: religious, medical, anthropological, and psychological.