
Shelford's law of tolerance states that an organism's survival and reproduction are limited by environmental factors exceeding its upper or lower tolerance thresholds.Victor Shelford formulated the law of tolerance in 1913: the presence and prosperity of a species in an ecosystem is determined by how well the values of environmental factors match its range of endurance. For each factor (temperature, humidity, light, salt concentration), there is a zone of tolerance — the interval between minimum and maximum values in which life is possible. Within this zone lies the optimum point, where conditions are ideal, and toward the edges, viability declines. If even one factor exceeds the tolerance limits, the species disappears, even if everything else is normal. This principle supplemented Liebig's law of the minimum, which only addressed the deficiency of substances. Shelford showed that excess is also harmful: one can overwater a plant or overfeed it with fertilizers.
Victor Shelford, an American zoologist and ecologist, published the work "Animal Communities in Temperate America" in 1913, where he outlined the concept of tolerance. He studied the distribution of insects and fish in relation to temperature, humidity, and other factors and noticed that species disappear not only with a deficiency but also with an excess of a resource. His ideas advanced the concepts of physiological ecology and became the foundation for the theory of the ecological niche. Later, the law was supplemented with the notion of a species' "ecological amplitude" and used in predicting the consequences of environmental pollution.
The law is applied in agriculture to determine the cultivation boundaries of crops, in conservation biology to assess species' vulnerability to climate change, and in toxicology to establish safe doses of substances. Limits of applicability: the law describes stationary conditions and does not account for acclimation (gradual habituation) and adaptation; the interaction of factors can shift tolerance limits (for example, high humidity makes heat less tolerable). For species with a wide ecological amplitude (eurybionts), the limits are blurred.
The law is used to predict how climate change will affect crops or the spread of pests. For example, knowing the temperature limits of the Colorado potato beetle, one can estimate how far north it will advance.
Brine shrimp live in salty lakes where other animals perish. Their tolerance limit for salt is so high that they survive in water almost saturated with salt — where crystals are already precipitating to the bottom.
| Φ_eff | variable | effective flux — flux weber |
| Φ_min | variable | minimum threshold flux — flux weber |







