The law of tolerance, usually called Shelford’s law of tolerance, states that an organism’s needs must fall within a range of acceptable limits, or the organism will struggle to survive. For example, an animal may require between 10 and 20 grams of food per year to survive. The essential point of the law is that organisms can adapt to varying conditions, within rather rigid limits.
The law of tolerance can be applied to all aspects of an organism’s life. For example, a heliothermic animal may require a minimum amount of hours spent basking in the sun to thrive. Above this minimum amount, the organism can adjust its behavior and physiology to adapt to the increase in available sunlight. However, if the amount of sunlight increases beyond the maximum established by the law of tolerance, the organism may fail to thrive.
Available food, reproductive partners, oxygen or any number of other requirements that an organism needs are subject to the law of tolerance. If the law of tolerance was not an accurate description of the real world, organisms may have exact requirements for life. For example, an animal may require exactly 10.75 grams of food per year, instead of 10 grams or 11 grams of food.