Abstract
The term tolerance range is normally used in reference to an abiotic aspect of the environment (temperature, salinity, pH, oxygen levels, toxin concentrations, etc.) and indicates the critical maximum and minimum limits of that abiotic variable that the organism can withstand. Species may exhibit very wide or relatively narrow tolerance ranges, and may have very high or low tolerance limits, depending on their biology and the environments to which they are adapted. In general, tolerance limits are reached when organisms can no longer successfully regulate their internal state and when cellular components no longer function correctly as the result of environmental change. Behavioral activities, however, may help organisms effectively avoid some environmental extremes. Organisms may alter their tolerance range in two ways: through phenotypical plasticity or evolutionary change. Plasticity in this context is usually called acclimation, and can occur to varying degrees and through a number of physiological and cellular mechanisms. Evolutionary changes in tolerance range may be similarly variable in pattern, and obviously populations that fail to adapt adequately to changes in their abiotic environment will go extinct. Organisms that are exposed to stressful levels of multiple abiotic variables at once may experience reduced tolerance ranges for some or all of these variables.