IB Biology Topic B4.1.4: Range of Tolerance of a Limiting Factor Notes

A Limiting Factor Sets the Edge of Where a Species Can Live

1. Every organism depends on abiotic variables such as temperature, water, or light to carry out its life processes.
2. A limiting factor is the variable that is closest to its tolerance limit and so restricts a process or the distribution of a species.
3. When conditions push that one variable too far, the species cannot survive there no matter how favourable the other variables are.

Performance Follows a Bell-Shaped Zone of Tolerance

1. The zone of tolerance is the range of an abiotic variable over which a species can survive.
2. In the middle sits the zone of optimum, where the species thrives and abundance peaks.
3. On either side of the optimum lie zones of physiological stress, where the organism survives but performs poorly.
   1. Here growth slows and reproduction becomes difficult, so abundance falls.
4. The edges of the survivable range are the limits of tolerance, beyond which the species cannot persist.
5. Past those limits are the zones of intolerance, where the variable is too extreme for the organism to survive at all.

Abundance Rises and Falls Across a Single Abiotic Variable

1. To see the pattern clearly, hold everything else constant and change one variable at a time.
2. As the variable rises from a low extreme, performance climbs out of a zone of intolerance and into the survivable range.
3. Performance keeps rising through the stress zone until it peaks at the optimum.
4. Past the optimum it falls again through the upper stress zone and drops to zero at the upper limit of tolerance.

The Factor Nearest Its Limit Is the One That Bites

1. At any moment a species is exposed to many abiotic variables at once.
2. The limiting factor is whichever variable currently sits closest to a limit of tolerance.
3. That single variable caps performance and can exclude the species from a habitat even when the other conditions are ideal.

1. Use these prompts to retrieve the key ideas from memory before moving on.
2. Answer each in one or two sentences, then check back against the text.