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Ecological
Tolerance

Understand how organisms survive within ranges of environmental conditions and what happens when conditions push beyond their limits.

AP Environmental Science

What is Ecological Tolerance?

Every organism can survive only within a certain range of environmental conditions: temperature, salinity, pH, moisture, light, and more. This range is called the organism's tolerance range.

Key Terminology

Optimum Range
The ideal conditions where an organism thrives and reproduces most successfully.
Zone of Stress
Conditions outside the optimum where organisms survive but with reduced fitness or growth.
Zone of Intolerance
Conditions beyond tolerance limits where the organism cannot survive.

The Tolerance Curve

Environmental Variable (e.g., Temperature) Population / Performance Zone of Intolerance Zone of Stress Optimum Range Zone of Stress Zone of Intolerance Lower Limit Upper Limit Peak Performance

Limiting Factors and Tolerance

A limiting factor is any environmental condition that restricts the growth, abundance, or distribution of an organism. The factor for which the organism has the narrowest tolerance range is the most critical.

Abiotic Factors
Temperature, water, sunlight, pH, salinity, dissolved oxygen, nutrients
Biotic Factors
Predation, competition, disease, symbiosis, food availability

Competition restricts occupied habitat along a tidal gradient

Two matched rocky-shore views show upper small barnacles in both states, crowded larger barnacles lower on the left and smaller barnacles occupying the lower rock after the illustrative competitor removal at right.In the barnacle comparison, competition restricts Chthamalus to the upper intertidal. Removing its competitor permits occupancy farther down the shore while the upper desiccation limit remains. This examines one dimension of a multidimensional niche.

Intertidal competition range projection

Intertidal competition range projection. In the barnacle comparison, competition restricts Chthamalus to the upper intertidal. Removing its competitor permits occupancy farther down the shore while the upper desiccation limit remains. This examines one dimension of a multidimensional niche.In the barnacle comparison, competition restricts Chthamalus to the upper intertidal. Removing its competitor permits occupancy farther down the shore while the upper desiccation limit remains. This examines one dimension of a multidimensional niche.

Stenotolerant vs. Eurytolerant

Environmental Variable Abundance Stenotolerant (narrow range) Eurytolerant (wide range)
Stenotolerant
Narrow tolerance range. Sensitive to change. Example: trout need cold, oxygen-rich water.
Eurytolerant
Wide tolerance range. Adaptable generalists. Example: rats thrive across many climates.

The Prefix System

Scientists use Greek prefixes steno- (narrow) and eury- (wide) combined with the environmental factor to describe tolerance:

Factor Narrow Tolerance Wide Tolerance
Temperature Stenothermal Eurythermal
Salinity Stenohaline Euryhaline
Moisture Stenohydric Euryhydric
Food/Diet Stenophagic Euryphagic

Shelford's Law of Tolerance

Mathematically, if we define performance \( P \) as a function of environmental variable \( x \):

A stenotolerant species has a small \( \sigma \), while a eurytolerant species has a large \( \sigma \).

Acclimation vs. Adaptation

Acclimation
Short-term, reversible adjustment to changing conditions within an individual's lifetime. The tolerance curve shifts but doesn't fundamentally change shape.
Adaptation
Long-term, genetic change across generations via natural selection. The tolerance curve itself evolves: new optimum, wider or narrower range.

How to Analyze Tolerance

1
Identify the factor. Which environmental variable is being measured (temperature, pH, salinity, etc.)?
2
Locate the optimum. Where on the curve is the peak performance or maximum abundance?
3
Find the limits. Identify the lower and upper tolerance boundaries where performance drops to zero.
4
Classify the organism. Narrow range = stenotolerant; wide range = eurytolerant for that factor.
5
Determine the limiting factor. Among all factors, which one has the narrowest tolerance range? That is the limiting factor.

Example: Brook Trout Temperature Tolerance

Temperature (\(^{\circ}\text{C}\))
 Population 0 5 10 15 20 25
\(4^{\circ}\text{C}\)
\(24^{\circ}\text{C}\)
\(14^{\circ}\text{C}\) Optimum

Example: Salmon and Salinity

Salmon are euryhaline: they tolerate a wide range of salinity. They hatch in freshwater (\(0 \text{ ppt}\)), migrate to the ocean (\(35 \text{ ppt}\)), and return to freshwater to spawn.

Salinity (ppt) Survival 0 10 20 30 36 Wide Tolerance Range (Euryhaline)

Example: Dissolved Oxygen as a Limiting Factor

A lake ecosystem contains trout, bass, and carp. Each species has a different tolerance for dissolved oxygen (DO):

Species Minimum DO (mg/L) Optimum DO (mg/L) Classification
Trout \(6.0\) \(9.0\) Stenotolerant
Bass \(4.0\) \(7.5\) Moderate
Carp \(1.5\) \(6.0\) Eurytolerant

Indicator Species and Tolerance

Because stenotolerant species can only survive within a narrow range, their presence or absence tells us about environmental conditions. These are called indicator species.

Trout
Indicate cold, clean, well-oxygenated water
Lichens
Indicate clean air with low sulfur dioxide levels
Mayfly Larvae
Indicate high dissolved oxygen in streams

Common Mistakes

✕ Confusing Acclimation with Adaptation
Acclimation is a reversible individual response; adaptation is a heritable population-level change over generations.
✕ Ignoring Interactions
Tolerance to one factor can be affected by another (e.g., high temperature lowers dissolved oxygen tolerance).
✕ Mixing Up Steno- and Eury-
Steno = narrow (think "stenography": compact writing). Eury = wide (think "Europe": wide continent).

Summary Comparison

Concept Stenotolerant Eurytolerant
Tolerance Range Narrow Wide
Curve Shape Tall, narrow bell Short, wide bell
\( \sigma \) Value Small Large
Sensitivity High (vulnerable to change) Low (resilient to change)
Indicator Use Good bioindicators Poor bioindicators
Examples Trout, coral, orchids Rats, cockroaches, carp

Question 1 of 5

An organism that can tolerate a wide range of temperatures would be classified as:

Question 2 of 5

A pond experiences eutrophication, causing dissolved oxygen to drop from \(8 \text{ mg/L}\) to \(3 \text{ mg/L}\). Which organism is most likely to disappear first?

Question 3 of 5

According to Shelford's Law of Tolerance, an organism's distribution is most limited by:

Question 4 of 5

Lichens disappear from trees near a factory. This most likely indicates that lichens are:

Question 5 of 5

A fish species gradually adjusts its gill function over two weeks when moved from freshwater to brackish water. This is an example of: