Water temperature (°C)

An organism can survive and function in conditions outside its optimal range, but its performance is greatly reduced. It cannot survive under conditions that fall outside its tolerance limits. An organism's range may be determined by the levels of one or more factors, such as pH, temperature, or salinity.


Some organisms can adjust their tolerance to abiotic factors through the process of acclimation (AK-luh-MAY-shuhn). For example, goldfish raised at different temperatures have somewhat different tolerance curves, as shown in Figure 18-5. Be sure not to confuse acclimation with adaptation. Acclimation occurs within the lifetime of an individual organism. Adaptation is genetic change in a species or population that occurs from generation to generation over time.

Control of Internal Conditions

Environments fluctuate in temperature, light, moisture, salinity, and other chemical factors. There are two ways for organisms to deal with some of these changes in their environment. Conformers are organisms that do not regulate their internal conditions; they change as their external environment changes. The internal conditions of a conformer remain within the optimal range only as long as environmental conditions remain within that range. In contrast, regulators are organisms that use energy to control some of their internal conditions. Regulators can keep an internal condition within the optimal range over a wide variety of environmental conditions.

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