## Learning Outcomes

- Define **ecological tolerance** and the **range of tolerance**
- Identify the parts of a tolerance curve, including the **optimal range**, **zone of physiological stress**, and **zone of intolerance**
- Explain how tolerance limits affect **species distribution** and **abundance**
- Distinguish **Shelford's Law of Tolerance** from **Liebig's Law of the Minimum**
- Apply tolerance ideas to abiotic factors such as temperature, pH, salinity, moisture, and dissolved oxygen
- Use tolerance concepts in AP-style graph interpretation and free-response answers

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand ecological tolerance range, with a focus on the following syllabus points:

- Know that each species has a **range of tolerance** for specific abiotic conditions
- Identify the **minimum tolerance limit**, **maximum tolerance limit**, and **optimal range**
- Explain the difference between the **zone of physiological stress** and the **zone of intolerance**
- Understand how ecological tolerance helps explain **species presence, absence, abundance, and distribution**
- Explain that both too little and too much of a factor can limit a species under the **Law of Tolerance**
- Distinguish **ecological tolerance** from **Liebig's Law of the Minimum**
- Apply tolerance language to common APES factors such as temperature, pH, salinity, moisture, light, and dissolved oxygen

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is meant by a species’ **range of tolerance**?
2. What is the difference between an **optimal range** and a **zone of physiological stress**?
3. Why might a species survive in one part of a habitat but be most abundant in another?
4. How does **Shelford's Law of Tolerance** differ from **Liebig's Law of the Minimum**?
5. Name two abiotic factors that can set tolerance limits for aquatic organisms.

## Introduction

Ecological tolerance explains a very common AP Environmental Science idea: species are not found everywhere because environmental conditions are not suitable everywhere. Even if food is present and predators are absent, a population will still fail if temperature, moisture, pH, salinity, or another abiotic factor moves outside the species' survivable limits.

This topic matters because it links biodiversity to real environmental change. If an exam question shows warming water, falling dissolved oxygen, rising soil salinity, or changing rainfall, the tolerance framework helps you predict which organisms will thrive, which will decline, and why their distribution shifts.

> **Key Term: ecological tolerance**
> The ability of an organism or species to survive under a range of environmental conditions.

## Introduction to tolerance range and the Law of Tolerance

### What a tolerance range actually means

A **range of tolerance** is the full span of values of one environmental factor within which a species can survive. That factor might be temperature, pH, salinity, moisture, light intensity, or dissolved oxygen. A species does not have one single tolerance range for “the environment” as a whole. It has separate tolerance ranges for different factors.

For example, a fish species may tolerate a certain range of water temperatures, a certain range of dissolved oxygen levels, and a certain pH range. If any one of those shifts too far, the fish population may decline even if the other conditions remain favorable.

> **Key Term: range of tolerance**
> The span of an environmental condition between the minimum and maximum values within which a species can survive.

This is why tolerance is more useful than memorizing isolated facts about species. It gives you a rule for prediction. If the abiotic factor moves toward the middle of the range, performance improves. If it moves toward either extreme, stress increases. If it passes a limit, survival is no longer possible.

### Shelford's Law of Tolerance

The APES idea often called the **Law of Tolerance** is commonly linked to **Shelford's Law of Tolerance**. The core idea is simple: the existence, abundance, and distribution of species depend on whether environmental conditions stay within the species' tolerance limits.

That means the law is not just about whether a species can be present. It is also about how many individuals can live there. A species may occur in two habitats, but if one habitat lies close to the optimum and the other lies near a stress limit, abundance will be much higher in the first habitat.

> **Key Term: Shelford's Law of Tolerance**
> The principle that a species’ distribution and success depend on whether environmental conditions remain within its minimum and maximum tolerance limits.

A common student error is to treat tolerance as a yes-or-no concept. In fact, tolerance is a gradient. As conditions worsen, abundance usually falls before the species disappears entirely.

> **Test Tip:** If an APES question asks where a species will be _most abundant_, do not just identify where it can survive. Choose the condition closest to the **optimal range**, not merely inside the tolerance limits.

## The structure of a tolerance curve

### Minimum limit, optimum, and maximum limit

A tolerance curve is usually shown from low values of an abiotic factor on the left to high values on the right. Moving across the curve, the standard order is:

- zone of intolerance
- zone of physiological stress
- optimal range
- zone of physiological stress
- zone of intolerance

At the center sits the **optimal range**, where survival, growth, and reproduction are highest. Near the edges are the **zones of physiological stress**, where the organism can survive but does not function well. Beyond those are the **zones of intolerance**, where the species cannot survive.

> **Key Term: minimum tolerance limit**
> The lowest value of an environmental condition at which a species can survive.
>
> **Key Term: maximum tolerance limit**
> The highest value of an environmental condition at which a species can survive.
>
> **Key Term: optimal range**
> The part of the tolerance range where survival, growth, and reproduction are greatest.

Imagine a plant species with a moisture tolerance range from very dry to very wet soil. It may survive in somewhat dry soil and somewhat wet soil, but grow fastest and produce the most seeds in moderately moist soil. That middle zone is where its population will usually be densest.

### Stress zones and intolerance zones

The **zone of physiological stress** is where the species is still alive, but under strain. Growth may slow. Reproduction may decline. Individuals may be smaller, weaker, or less numerous. This matters because a population can appear present in an area but still be struggling.

> **Key Term: zone of physiological stress**
> Conditions outside the optimum but still within tolerance limits, where survival is possible but growth, reproduction, or abundance are reduced.

The **zone of intolerance** lies beyond the minimum or maximum tolerance limit. Here the species is absent because individuals cannot survive.

> **Key Term: zone of intolerance**
> Conditions beyond the tolerance limits where a species cannot survive.

This distinction appears often in APES questions. A stream may still contain some fish if oxygen is low, but not all fish species. Sensitive species may disappear first, while more tolerant species remain. Presence alone does not mean conditions are good.

A useful example is dissolved oxygen in freshwater systems. Organisms such as mayfly nymphs usually need high dissolved oxygen and are often treated as indicators of clean water. If dissolved oxygen drops, they may move from optimum to stress and then disappear. More tolerant organisms may still survive under the same conditions.

> **Test Tip:** In free-response answers, use precise verbs. Say a species is **most abundant** in the optimal range, **reduced** in the stress zone, and **absent** in the zone of intolerance.

## How tolerance range explains species distribution

### Why species live where they do

Tolerance range helps explain **species distribution**, meaning where a species is found geographically or within a habitat. If a condition stays within tolerance limits, the species can occur there. If conditions approach the optimum, abundance increases. If the condition goes beyond a limit, the species disappears.

> **Key Term: species distribution**
> The pattern of where a species is found, shaped in part by whether abiotic conditions remain within its tolerance range.

This is why the same species can be common in one section of a river and rare in another. Upstream water might be cooler and better oxygenated, while downstream water is warmer and lower in dissolved oxygen. If the species has a narrow temperature or oxygen tolerance, its distribution will reflect that.

The idea also works across larger scales. As climate changes, temperature and precipitation patterns shift. Species whose tolerance ranges are narrow may be forced to move poleward, upslope, or into deeper water. Species that cannot move or adapt quickly enough may decline.

### Real APES-style abiotic examples

Here are common abiotic factors used in APES questions:

- **Temperature:** Trout are associated with cooler, oxygen-rich water. If stream temperature rises, their abundance may fall and eventually reach zero if water moves beyond their tolerance range.
- **pH:** Many aquatic organisms do best near neutral conditions. Acidification can move sensitive species from optimum to stress to intolerance.
- **Salinity:** Freshwater fish usually have low tolerance for saltwater. Estuarine organisms often have broader salinity tolerance because salinity changes regularly where river water meets seawater.
- **Moisture:** Desert plants survive with low water availability because they are adapted to that range, but even they still have lower and upper moisture limits.
- **Dissolved oxygen:** Many aquatic species need a minimum oxygen level. Low DO can sharply reduce abundance.

Students often miss that tolerance applies to one factor at a time. A crab might have broad salinity tolerance but narrow temperature tolerance. A plant might tolerate acidic soil but not drought. So the factor mentioned in the question matters.

### Generalists, specialists, and tolerance width

Species with broad tolerance ranges are often more **generalist** in their environmental needs. Species with narrow tolerance ranges are often more **specialist**. This does not mean every generalist has a broad tolerance for every factor, but it is a useful pattern.

A broad tolerance range gives a species more flexibility when conditions vary. A narrow range makes a species more vulnerable to environmental change. That is one reason specialist species are often at greater risk when habitats are disturbed.

If a question asks why one species persists after environmental change while another declines, tolerance width is often part of the answer. The surviving species may tolerate a wider range of the altered abiotic condition.

## Tolerance range compared with limiting factors

### Shelford vs Liebig: the distinction students must know

Students often confuse the **Law of Tolerance** with **Liebig's Law of the Minimum**. Both help explain limits on growth and distribution, but they are not the same.

**Shelford's Law of Tolerance** focuses on whether an environmental factor is too low or too high. It is about a survivable range bounded by minimum and maximum tolerance limits.

**Liebig's Law of the Minimum** focuses on the essential resource in shortest supply relative to need. It is about scarcity.

> **Key Term: limiting factor**
> An environmental factor that restricts the growth, abundance, or distribution of a population.
>
> **Key Term: Liebig's Law of the Minimum**
> The principle that growth is limited by the essential resource in shortest supply relative to need.

A worked comparison makes the difference clear:

- A plant has enough sunlight and adequate temperature, but almost no available nitrogen in the soil. Growth is limited by nitrogen. That is **Liebig**.
- A fish species can live only between cool and moderately warm water temperatures. If the water becomes too warm, it declines because temperature exceeds its tolerance range. That is **Shelford**.

Sometimes both apply at once. A species may need enough nitrogen, phosphorus, and light, while also needing temperature and moisture within its tolerance range. On APES questions, identify which principle the wording points to.

If the question says “shortest supply,” think Liebig. If it says “too low or too high,” think tolerance.

### How to answer AP-style questions clearly

Strong answers name the abiotic factor and then trace the biological consequence. For example:

- “As water temperature rises above the trout's tolerance range, the fish experience physiological stress, reproduction falls, and the population declines.”
- “Because soil nitrogen is the scarcest essential nutrient, plant growth is limited by Liebig's Law of the Minimum.”

That level of precision earns more credit than vague phrases such as “the species will struggle.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ecological tolerance describes a species' ability to survive across environmental conditions
- A **range of tolerance** extends from a minimum tolerance limit to a maximum tolerance limit
- The center of a tolerance curve is the **optimal range**, where survival, growth, and reproduction are highest
- The **zone of physiological stress** allows survival but reduces growth, reproduction, and abundance
- The **zone of intolerance** lies beyond tolerance limits, where the species cannot survive
- **Shelford's Law of Tolerance** explains species distribution using lower and upper environmental limits
- Ecological tolerance helps explain species presence, absence, and abundance in habitats
- Common APES abiotic factors include temperature, pH, salinity, moisture, light, and dissolved oxygen
- A species may survive in a habitat without thriving there
- Broad tolerance ranges are often linked to greater flexibility under environmental change
- **Liebig's Law of the Minimum** is about the scarcest essential resource, not upper and lower limits
- In APES answers, always identify the abiotic factor and explain whether it causes optimum conditions, stress, or intolerance

## Key Terms and Concepts

- ecological tolerance
- range of tolerance
- Shelford's Law of Tolerance
- minimum tolerance limit
- maximum tolerance limit
- optimal range
- zone of physiological stress
- zone of intolerance
- species distribution
- limiting factor
- Liebig's Law of the Minimum


Ecological Tolerance - Ecological tolerance range