AP Environmental Science

## Learning Outcomes

- Understand what **ecological tolerance** means and why it helps explain where organisms live
- Distinguish between **individual-level tolerance** and **species-level tolerance**
- Identify **tolerance limits**, the **optimum range**, and why survival is not the same as successful reproduction
- Explain how **limiting factors** control abundance and distribution in terrestrial and aquatic systems
- Compare a **fundamental niche** with a **realized niche**
- Apply tolerance ideas to examples involving temperature, salinity, pH, moisture, light, and dissolved oxygen

## AP Environmental Science Syllabus

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

- Know that organisms can survive, grow, and reproduce only within a certain range of abiotic conditions
- Identify the difference between an individual organism’s tolerance limits and a species-wide tolerance range
- Explain how **minimum**, **maximum**, and **optimum** conditions affect survival and population size
- Understand that one unfavorable abiotic condition can become a **limiting factor**
- Explain how tolerance helps determine **species distribution**
- Distinguish between **fundamental niche** and **realized niche**
- Recognise that broad tolerance is often linked to wider distribution, while narrow tolerance is often linked to greater vulnerability
- Apply tolerance concepts to terrestrial factors such as soil nutrients, water, light, and temperature, and aquatic factors such as pH, dissolved oxygen, salinity, and light

## 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 ecological tolerance?
2. How is the tolerance range of a species different from the tolerance range of one individual?
3. What is a limiting factor, and how does it affect species distribution?
4. What is the difference between a fundamental niche and a realized niche?
5. Why might a species survive in an area but still fail to maintain a stable population there?

## Introduction

Ecological tolerance is one of the key ideas that links organism biology to ecological distribution patterns. If you know what conditions a species can tolerate, you can often predict where it will be common, where it will be rare, and where it will be absent. That makes this topic highly useful on AP Environmental Science multiple-choice questions, data questions, and free-response questions.

This topic also helps connect Unit 2 to later units. Tolerance helps explain why some species are generalists, why others are specialists, why populations decline when conditions shift, and why climate change, acidification, salinity changes, or drought can alter biodiversity so quickly.

> **Key Term: ecological tolerance**
> The range of abiotic conditions within which an organism can survive, grow, and reproduce.

### The Law of Tolerance and what it really means

The main principle behind this topic is often called the **Law of Tolerance** or **Shelford’s Law of Tolerance**. The basic idea is simple: each organism has upper and lower limits for environmental conditions such as temperature, pH, salinity, moisture, light, or dissolved oxygen. If conditions move too far outside those limits, the organism cannot function properly.

> **Key Term: Law of Tolerance**
> The principle that the existence, abundance, and distribution of a species depend on whether environmental conditions remain within its tolerance limits.

What students often miss is that tolerance is not just about staying alive. AP questions often expect you to include **survival, growth, and reproduction**. That matters because a population can appear present in a habitat even when conditions are poor. A fish may survive in warm water for a short time, for example, but if that temperature reduces egg development or juvenile survival, the population will decline over time.

A useful way to picture tolerance is a curve across one abiotic factor. In the middle is the **optimum range**, where organisms perform best and population size is usually highest. On each side are **zones of physiological stress**, where individuals may survive but grow less well and reproduce less successfully. Beyond those lie the **zones of intolerance**, where survival is not possible.

> **Key Term: tolerance limits**
> The minimum and maximum levels of an abiotic factor beyond which an organism cannot survive, grow, or reproduce successfully.

This explains why abundance often changes across an environmental gradient. Along a stream, for example, dissolved oxygen may be high upstream and lower downstream. A sensitive aquatic insect might be abundant where oxygen is high, sparse where oxygen is lower, and absent where oxygen falls beyond its tolerance limit.

> **Test Tip:** If an exam question asks why a species is “less abundant” rather than “absent,” think about stress near the edge of the tolerance range, not complete intolerance.

### Individual tolerance and species tolerance

A subtle but important distinction is the difference between an individual organism’s tolerance and the tolerance range of the species as a whole.

An individual has its own physiological limits. One trout in one stream can tolerate only a certain range of temperature, oxygen, and pH. If conditions cross that line, that individual becomes stressed or dies.

A species-level tolerance range is broader because it includes variation among many individuals and populations. Different populations may be adapted to slightly different local conditions. That means not every individual has identical limits.

> **Key Term: species-level tolerance**
> The full range of abiotic conditions tolerated by a species as a whole, reflecting variation among individuals and populations.

This distinction matters because students sometimes overstate tolerance by treating all members of a species as identical. They are not. A species found across a wide geographic area may include northern populations adapted to colder conditions and southern populations adapted to warmer ones. No single organism may tolerate the full range seen across the species.

That helps explain why species can occupy many habitats even though one individual cannot. The species persists because its populations differ.

This also links to adaptation and natural selection. If some individuals survive better under changed conditions, they are more likely to reproduce. Over generations, the population may shift toward different tolerance limits. That is one reason ecological tolerance is closely tied to evolution.

A broad species tolerance often means a broader distribution. Generalist species usually tolerate wider conditions. A narrow tolerance often means a narrower range and greater risk from environmental change. Specialist species, such as those tied to one food source or one habitat type, are often more vulnerable when conditions shift.

The giant panda is a useful case. It depends heavily on bamboo and a limited habitat type. Even if some local conditions remain acceptable, warming that changes bamboo distribution can still threaten the species because its overall tolerance as a specialist is narrow in practical terms.

### Limiting factors and species distribution

Tolerance matters most when one condition becomes the factor that restricts a population. That factor is called a **limiting factor**.

> **Key Term: limiting factor**
> A biotic or abiotic factor that restricts the growth, abundance, reproduction, or distribution of a population or species.

A limiting factor does not have to be the only important condition. It is simply the one that is closest to or beyond the organism’s tolerance limit at that place and time. A habitat may have enough food, suitable shelter, and few predators, but if the water is too acidic or the temperature is too high, the species may still be absent.

The textbook examples are high yield for APES:

- In **terrestrial habitatss**, limiting factors often include water availability, soil nutrients, light, and temperature.
- In **aquatic habitatss**, important limiting factors include pH, dissolved oxygen, salinity, and light.

Why these matter:

- **Temperature** affects enzyme activity, metabolism, and development.
- **Salinity** affects osmotic balance. Organisms adapted to freshwater often cannot maintain water balance in salty water.
- **pH** affects internal chemistry and can increase physiological stress.
- **Dissolved oxygen** controls whether aquatic animals can respire effectively.
- **Light** controls photosynthesis for producers, which then affects the whole food web.
- **Moisture** affects plant water balance and the survival of eggs, larvae, and amphibians.

Take salinity as an example. An estuary changes in salinity with tides and freshwater inflow. Species with broad salinity tolerance can use more of the estuary. Species with narrow salinity tolerance are restricted to smaller zones. So the abiotic gradient creates a pattern in distribution.

Take temperature in streams. Cooler upstream water may support trout or other cold-water fish. If downstream temperatures rise, dissolved oxygen usually drops as well. Now two factors shift together: higher temperature and lower oxygen. A fish population may first decline in abundance, then disappear if conditions cross its tolerance range.

This is a common APES reasoning chain:

1. Abiotic factor changes
2. Organism experiences stress
3. Growth or reproduction declines
4. Population size falls
5. Distribution becomes narrower
   > **Test Tip:** In FRQs, do not stop at “the species dies.” A stronger answer traces the effect on reproduction, abundance, and distribution.

The factor that limits a species in one place may be different elsewhere. In a desert, water is often the main limit. In a cold biome, temperature may be the stronger limit. In a polluted lake, pH or oxygen may become the main constraint.

### Tolerance curves: optimum, stress, and intolerance

Tolerance questions often become easier if you mentally picture the classic tolerance curve. Even if the test does not show a graph, it may describe one.

At the center is the **optimum range**. Here the organism is most successful. Growth rates, survival, and reproduction are highest. Population size tends to peak in this zone.

On either side are **zones of physiological stress**. The organism can still exist, but not under ideal conditions. Energy must be spent coping with the environment. That leaves less energy for growth or reproduction.

Beyond these lie **zones of intolerance**. Conditions are so extreme that the organism cannot persist there.

This structure helps you interpret tricky questions. If a species is present but uncommon, conditions may be in a stress zone. If the species is absent entirely, conditions may be in a zone of intolerance. If a species dominates the habitat, conditions are more likely close to the optimum.

A major misconception is assuming that the entire tolerance range is equally good. It is not. The optimum is only the best part of the whole range.

Another common mistake is assuming tolerance to one factor guarantees success. In reality, multiple factors act together. A plant may tolerate the temperature of a habitat, but if the soil phosphorus is too low, or the moisture is too scarce, that one unfavorable factor can still control distribution.

### Fundamental niche and realized niche

Tolerance range is closely related to niche, but it is not identical. Tolerance focuses mainly on abiotic conditions. A niche includes more: resources used, interactions with other species, and ecological role.

> **Key Term: fundamental niche**
> The full range of abiotic conditions and resources a species could use in the absence of limiting biotic interactions.
>
> **Key Term: realized niche**
> The actual range of conditions and resources a species occupies in nature after competition, predation, parasitism, and other biotic interactions are taken into account.

The **fundamental niche** is the bigger possibility. It is where a species could live based mainly on abiotic tolerance.

The **realized niche** is usually smaller. It reflects what happens when real natural communitiess add competition, predation, disease, and mutualist availability.

For example, a bird species may be able to tolerate the climate across an entire forest type. That defines much of its fundamental niche. But if a stronger competitor occupies part of that habitat, or if predators are concentrated there, the bird may use only part of that range. That smaller actual range is the realized niche.

This connects strongly to **competitive exclusion**. If two species use exactly the same limited resource in the same way, one will usually outcompete the other in the long run. The weaker competitor may shift habitat, shift food use, or decline. As a result, its realized niche becomes smaller than its fundamental niche.

Resource partitioning reduces that pressure. If two species use slightly different parts of the habitat or feed at different times, both may persist. Their realized niches remain narrower than the maximum possible, but coexistence becomes possible.

An AP-style way to think about it:

- **Abiotic factors** often set the broad outer boundaries of where a species could live.
- **Biotic factors** then reduce or modify where it actually lives.

This is why a species may be absent from a seemingly suitable area. The temperature, moisture, and light may all fit within its tolerance range, but competition or predation may still prevent establishment.

### Applying ecological tolerance to real environmental change

Tolerance becomes especially important when environments change. Climate change, habitat modification, water pollution, and acidification all shift abiotic conditions. Species nearest their tolerance limits are the first to show decline.

In mountain or polar systems, organisms often have little room to shift further. If temperatures rise, they may not be able to migrate to cooler areas. That is one reason specialists in tundra or alpine systems are often highly vulnerable.

In wetlands or estuaries, rising salinity can exclude freshwater species that have narrow salt tolerance. In lakes or streams, eutrophication can increase biological oxygen demand and reduce dissolved oxygen, pushing fish or macroinvertebrates beyond tolerance limits. In acidified waters, pH-sensitive species may disappear first, changing the whole food web.

This also explains why biodiversity often supports resilience. Communities with more species are more likely to include organisms with different tolerances. If one pathway is disrupted by environmental stress, another species may still function. A species-poor system has fewer backups.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ecological tolerance is the range of abiotic conditions within which organisms can survive, grow, and reproduce
- The Law of Tolerance states that species distribution and abundance depend on whether conditions remain within tolerance limits
- Each factor has minimum, maximum, and often optimum conditions
- The optimum range is where organisms perform best, while stress zones allow survival but reduce growth and reproduction
- Conditions outside tolerance limits create zones of intolerance
- Individual organisms have narrower limits than the species-wide tolerance range
- Species-level tolerance is broader because individuals and populations vary
- Limiting factors are the conditions that most restrict growth, reproduction, abundance, or distribution
- Common terrestrial limiting factors include water, soil nutrients, light, and temperature
- Common aquatic limiting factors include pH, dissolved oxygen, salinity, and light
- A fundamental niche is the full potential range based mainly on abiotic tolerance
- A realized niche is the narrower actual range after biotic interactions such as competition and predation

## Key Terms and Concepts

- ecological tolerance
- Law of Tolerance
- tolerance limits
- species-level tolerance
- limiting factor
- fundamental niche
- realized niche


Ecological Tolerance - Tolerance of individuals and species