Practice question

A tolerance curve usually has an optimum near its centre, where physiological performance, growth, and reproduction are greatest. Moving toward either the lower or upper limit enters a zone of physiological stress. Organisms can remain alive there, but energy must be diverted to homeostasis, leaving less for growth and reproduction; abundance consequently tends to be low. Beyond the tolerance limits lies the zone of intolerance, where individuals cannot survive for long and the species is absent. The “zone of extinction” is not the standard segment of a Shelford tolerance curve, while an optimal zone would show vigorous reproduction rather than reduced reproduction. Stress can arise from temperature, salinity, pH, oxygen, moisture, or any other critical factor, and the width of the curve differs among species and life stages. Reproductive stages are often more sensitive than adults, so a population may contain temporary survivors without being able to replace itself. Interacting stresses can further narrow the effective range. The zone-of-stress concept therefore distinguishes mere survival from ecological success: persistence of individual organisms does not necessarily imply a self-sustaining population under those conditions.