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Tolerance range
Tolerance of steelhead trout
Optimum range
Upper limit
Lower limit
Number of Organisms
Limiting factors
Any abiotic factor or biotic factor that restricts the numbers, reproduction, or distribu-
tion of organisms is called a
limiting factor.
Abiotic limiting factors include sunlight,
climate, temperature, water, nutrients, fire, soil chemistry, and space. Biotic limiting
factors include living things, such as other plant and animal species. Factors that restrict
the growth of one population might enable another to thrive. In a desert oasis, water is
a limiting factor for all the organisms. Temperature might also be a limiting factor.
Desert species must be able to withstand extreme temperatures.
Range of tolerance
For any environmental factor, an upper limit and lower limit define the conditions in
which an organism can survive. For example, steelhead trout live in cool, coastal rivers
and streams from California to Alaska. The ideal range of water temperature for steel-
head trout is between 13°C and 21°C, as illustrated in
Figure 6
. However, steelhead trout
can survive water temperatures from 9°C to 25°C. At these temperatures, steelhead trout
experience physiological stress, such as inability to grow or reproduce. They will die if
the water temperature goes beyond the upper or lower limits.
The ability of any organism to survive when subjected to abiotic factors or biotic factors
is called
tolerance.
Consider
Figure 6
again. Steelhead trout tolerate a specific range of
temperatures. That is, the range of tolerance of water temperature for steelhead is 9°C to
25°C. Notice that the greatest number of steelhead live in the optimum range in which
the temperature is best for survival. Between the optimum range and the tolerance
limits lies the zone of physiological stress. At these temperatures, there are fewer fish.
Beyond the upper tolerance limit of 25°C and the lower tolerance limit of 9°C, there are
no steelhead trout. Therefore, water temperature is a limiting factor for steelhead when
water temperature is outside the range of tolerance.
Figure 6
Steelhead trout are limited by the temperature of the water in which they live.
Infer
which other abiotic factors might limit the survival of steelhead trout.
Lesson 1 • Organisms and Their Relationships
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