Tolerance range describe two laws,

  • Liebig's law of minimum.
  • Shelford's law of tolerance.
Liebig shows plants require certain kinds and quantities of nutrients for their growth. If one of these substances is to be absent the plant will die. If it is minimum quantities only, the growth of the plant will be minimum.
(Not only substance but also other environmental factors affect the growth of plants.)

law of minimum - "The functioning of an organism is controlled or limited by that essential environmental factor or combination of factors present in least favorable amount"


Law of tolerance 

Environmental factors could be limiting at the minimum as well as the maximum quantity.

"Absence or failure of an organism can be controlled by qualitative or quantitative deficiency or excess with respect to any one of the several factors, which may approach the limits of tolerance for that organism."

This law of tolerance is illustrated by a bell-shaped curve.
X-axis - Environmental variable or environment dimension 
  • pH, salinity, temperature..etc
Y-axis - The growth of the population

The curve shows us the response of the environmental variable on the population size. How well organisms are able to tolerate environmental variables. 
  • Different temperatures and survive.
Each ecological factor to which an organism respond has a maximum and a minimum limiting effect known as the upper limit of tolerance and lower limits of tolerance respectively.                                                                      
Between these two limits lies a gradient of the environment variable knows as the tolerance range.
Within this range, organisms will survive.
Within the tolerance range lies the optimum range.

The diagrammatic representation of Shelford's law of tolerance



Rise from the upper limit or decrease from the lower limit of the tolerance lead to a condition of physiological stress for that organism.
further rise or decline can cause the death of the organism as they both reach the zone of intolerance.

Physiological optimum lies within the optimum range where the bell shape curve reaches the maximum size of the population. At this point, the organism in the population performs best with particular environmental factors.

Tolerance ranges are not necessarily fixed. The maximum and minimum of tolerance for a species vary seasonally, geographically, and also depend on the life cycle. The range of the optimum may differ for different processes within the same organism.

Subsidiary principles to the law of tolerance stated,
  1. Organism may have a wide range of tolerance for one factor and a narrow range for another.
  • The organism that has a wide range of tolerance prefix "eury"
  • The organism that has a narrow range of tolerance prefix "steno"
      2. Organisms with a wide range of tolerance for all factors are likely to be most widely distributed. When conditions are not optimum for a species with respect to one ecological factor, the limits of tolerance may be reduced with respect to other ecological factors.

      3. Organisms are not actually living in the optimum range with regard to a particular physical factor.

      4. The period of reproduction is usually a critical period when environmental factors are most likely to be limiting. The limits of tolerance for individuals in the reproductive states are narrower than individuals in the non-reproducing states. 

Relative limits of tolerance of stenothermal and eurythermal organisms