Tolerance is the ability of living organisms to withstand or tolerate little unfavourable changes in the environment which affect their survival.
Living organisms can only live in a particular habitat if they can tolerate the ranges of the abiotic factors that operate in it. Due to changes in environmental factors, some of these conditions are sometimes unfavourable. Too little or too much of certain environmental factors such as light, heat, cold, acidity and alkalinity might produce unfavourable conditions.
Tolerance range is defined as the range between the minimum and maximum limits to which organisms can tolerate certain changes in their environment so as to survive. Organisms can only live within certain minimum and maximum limits for each abiotic factors. Death occurs beyond this range. For example, for most animals, the minimum temperature limit is 0°C while the maximum limit is 42°C. Their tolerance range is 0 – 42°C. Below 0°C or above 42°C, the organisms may die.
Again, one factor may affect the tolerance range than other factors. For example, when the oxygen level of water is low, lobsters can only tolerate temperature up to 29°C but at a higher oxygen level, they can tolerate temperature up to 32°C. Also, while some plants can withstand long period of drought, many others cannot.
Geographic range refers to the areas where a species of organism can only be found within the minimum and maximum limits of its tolerance.
Different abiotic factors like rainfall, temperature, light intensity, availability of food, relative humidity, day length, wind, etc are often responsible for the geographical boundaries of species of organisms. For example, the geographic range of the tropical rainforest is within the equator as a result of high rainfall and high temperatures, whereas tropical rainforest cannot be found at the Northern and Southern poles because of low rainfall and temperature.