The limits of an organism's resilience to an environmental factor. Refers to the ability of a species to adapt to changing environmental conditions. Expressed by special terms with the prefix steno (narrow) or eury (broad). For example, a species is stenothermic or eurythermic (in terms of temperature), stenohaline or euryhaline (in terms of salinity), stenophagous or euryphagous (in terms of diet).

Ecological valence, or ecological plasticity or ecological tolerance, is the ability of an organism to exist within a range of values of an environmental factor. This range is also called ecological amplitude. Plasticity is determined by the response rate. Species with a wide amplitude are called eurybionts. They have the ability to inhabit habitats with different conditions. Species that live within a narrow range of factors are called stenobionts. Their ability to occupy habitats is severely limited.