2.4 Ecological Tolerance

Range of Tolerance

Every species has an ecological tolerance range for abiotic factors like temperature, pH, salinity, and water availability. Within this range are a zone of physiological stress (survivable but stressful) and an optimal range (best growth/reproduction). Beyond the tolerance limits lie zones of intolerance, where the organism cannot survive.

Limiting Factors

A limiting factor is the environmental factor closest to an organism's tolerance limit, restricting population growth even if other conditions are ideal. This follows the law of tolerance: too little or too much of any factor can limit a species. Limiting factors help explain species distribution and abundance.

Environmental Change & Species Response

When conditions shift outside a species' tolerance range, populations may decline, migrate, or go extinct. Species with narrow tolerance ranges are more vulnerable to environmental change than those with wide tolerance ranges, making tolerance range a key predictor of a species' sensitivity to disturbances like climate change or pollution.

Key terms

Ecological tolerance

The range of abiotic conditions (e.g., temperature, pH) a species can survive and reproduce within.

Optimal range

The specific conditions within a tolerance range where a species grows and reproduces best.

Zone of physiological stress

Conditions where an organism survives but experiences stress, outside the optimal range but within tolerance limits.

Zone of intolerance

Conditions beyond a species' tolerance limits where it cannot survive.

Limiting factor

The environmental factor closest to a species' tolerance limit that restricts its population growth.

Try an AP-style question

What is an organism's range of tolerance?

1. Its geographic range and nothing else
2. The conditions in which it can survive
3. The number of offspring it produces
4. The total length of its lifespan