Niches and Tolerance Ranges

Essential Question

What decides where a species can survive?

A polar bear cannot live in a rainforest, and a palm tree cannot grow on a glacier. In this lesson, you’ll learn why. You’ll see how a species’ niche and its tolerance ranges decide which places it can live, and why some species can live in many places while others can live in only a few.

Habitat and Niche

• Habitat: the place where an organism lives
• Niche: the role an organism plays there
• A niche includes what it eats, what eats it, when it is active, and where it breeds
• Each species has its own niche

An organism’s habitat is the place where it lives, such as the edge of a pond, a desert, or a forest floor. Its niche is its role in the community: what it needs and everything it does. A niche includes what the organism eats, what eats it, when it is active, where it breeds, and how it changes its surroundings. Think of a habitat as an organism’s address and its niche as its job. Many species can share one pond, but each species has its own niche.

Abiotic and Biotic Factors

• Abiotic factors are the nonliving parts of the environment
• Examples: sunlight, temperature, water, oxygen, pH, soil
• Biotic factors are the living parts: other organisms and their effects
• Examples: predators, prey, competitors, parasites, decomposers

Organisms are affected by two kinds of factors. Abiotic factors are the nonliving parts of an environment, such as sunlight, temperature, water, dissolved oxygen, pH, and soil. Biotic factors are the living parts of an environment, meaning other organisms and the ways they affect it, such as predators, prey, competitors, parasites, and decomposers. In the pond, the water temperature and dissolved oxygen are abiotic, while the heron, fish, and algae are biotic. A change in either kind of factor can change which organisms can live in a place.

Tolerance Ranges

• Tolerance range: the span of a factor an organism can survive in
• Optimum range: best growth, survival, and reproduction
• Zones of stress: survival is possible, but growth and reproduction fall
• Beyond the limits of tolerance, individuals cannot survive

Each species can survive only within a certain span of each abiotic factor, such as temperature, pH, or salinity. This span is its tolerance range. The graph shows a pattern that is often hill-shaped. In the middle, in the optimum range, individuals grow, survive, and reproduce best, so there are the most of them. Toward the edges, in the zones of stress, individuals can survive, but growth and reproduction fall, so there are fewer of them. Beyond the lower and upper limits of tolerance, in the zones of intolerance, individuals cannot survive. A species has a separate tolerance range for each factor, and the shape of the curve is different for each species.

Where a Species Can Live

• The abiotic factors must be inside the species’ tolerance ranges
• One factor outside its range can keep a species out
• Two limits together can make a band, as on a mountain slope
• If a factor changes, the places a species can live change too

A species can live in a place only if the abiotic factors there fall inside its tolerance ranges. If even one factor is outside its range, the species cannot survive there, even when the other factors are fine. The figure shows an example (made-up values, not measurements) of a mountain tree species. Above 2,600 m it is too cold, and below 1,800 m it is too dry, so the species lives only in the band between, where temperature and moisture are both in range. Biotic factors, such as predators and competitors, also affect where a species lives, but tolerance ranges are the first test. When conditions change, the places that fall inside a species’ ranges change too.

Generalists and Specialists

• Generalist: wide tolerance range or many resources used
• Specialist: narrow tolerance range or few resources used
• Raccoons eat fruit, nuts, insects, and crayfish
• Koalas feed almost entirely on eucalyptus leaves

Species differ in how wide their niches are. A generalist tolerates a wide range of conditions or uses many kinds of resources. Raccoons are an example. They eat fruit, nuts, insects, and crayfish, and they live in forests, wetlands, and cities. A specialist has a narrow tolerance range or depends on only a few resources. Koalas feed almost entirely on eucalyptus leaves. The graph compares two sample species. Because a specialist’s range is narrow, a small change in a factor can push conditions outside it, so specialists are often more at risk when their environment changes. Many species fall somewhere between the two extremes.

↑ Back to top

---

Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Trout and Stream Temperature

The graph shows how many individuals of a cold-water trout species were counted per 100 m of stream at different summer water temperatures (sample data). Describe the optimum range and the upper limit of tolerance. Then predict what happens if removing shade trees raises a stream’s summer temperature from 14 °C to 22 °C. Use data as evidence.

Show the solution

1. Optimum range: the highest counts, 28 and 30 trout per 100 m, are at 10 °C and 14 °C. The optimum range is about 10 to 14 °C.
2. Upper limit: the count falls to 14 at 18 °C and 2 at 22 °C, and there are no trout at 26 °C. The upper limit of tolerance is between 22 °C and 26 °C.
3. Calculate the change: (30 − 2) trout per 100 m ÷ 30 trout per 100 m × 100 = 93% decrease
4. Claim: the trout population would decline sharply.
5. Evidence: streams at 14 °C had 30 trout per 100 m, but streams at 22 °C had only 2, which is a decrease of about 93%.
6. Reasoning: temperature is an abiotic factor with a tolerance range. 22 °C is in the upper zone of stress, close to the upper limit, so the trout that survive grow and reproduce less. Warmer water also holds less dissolved oxygen.
7. Answer: The trout would decline by about 93%, from 30 to 2 per 100 m.

Example 2: A Fern and Two Factors

A woodland fern survives at average temperatures of 10 to 28 °C and soil moisture of 30 to 70% (sample values). The graph and table show five sites. (a) At which sites can the fern survive? For each site where it cannot, name the factor that is outside the range. (b) Suppose the climate warms every site by 4 °C. Which sites still work?

Site	Temperature (°C)	Soil moisture (%)
A	20	50
B	32	55
C	15	20
D	8	25
E	26	75

Show the solution

1. Test each factor: a site works only if the temperature is from 10 to 28 °C and the moisture is from 30 to 70%.
2. (a) Site A: 20 °C and 50% are both in range. Site B: 32 °C is too hot. Site C: 20% is too dry. Site D: 8 °C is too cold and 25% is too dry. Site E: 75% is too wet.
3. (b) Add 4 °C: A is 24 °C, still in range. B is 36 °C, too hot. C is 19 °C, but the soil is still too dry. D is 12 °C, now in range, but the soil is still too dry. E is 30 °C, too hot, and still too wet.
4. Reasoning: the fern needs both factors inside their ranges at the same time, which is the box on the graph. Fixing one factor does not help when another is still outside its range.
5. Answer: (a) Only site A. (b) Still only site A.

Example 3: Comparing Two Insects

Stream insect A tolerates 4 to 24 °C. Stream insect B tolerates 8 to 14 °C. The graph places 10 streams by their temperatures (sample data). Find the width of each tolerance range and the number and percent of the 10 streams where each insect could survive. Which insect is the generalist for temperature?

Show the solution

1. Width of each range: insect A: 24 °C − 4 °C = 20 °C. Insect B: 14 °C − 8 °C = 6 °C. Insect A’s range is 20 ÷ 6 ≈ 3.3 times as wide.
2. Insect A: streams 1 to 9 are between 4 and 24 °C, and stream 10 (26 °C) is too warm. 9 streams ÷ 10 streams × 100 = 90%.
3. Insect B: only streams 2, 3, 4, and 5 (9 to 13 °C) are between 8 and 14 °C. 4 streams ÷ 10 streams × 100 = 40%.
4. Claim and reasoning: insect A is the generalist for temperature. Its wider range means more streams meet its needs. Other factors, such as oxygen and food, could still limit where each insect lives.
5. Answer: A: 20 °C wide, 9 of 10 streams (90%). B: 6 °C wide, 4 of 10 streams (40%). Insect A is the generalist.

↑ Back to top

---

Practice

Sort abiotic and biotic factors, separate habitat from niche, label a tolerance range, and use data to compare how species respond to change. Try it on your own first, then check your work with the answer key.

• Niches and Tolerance Ranges Practice Worksheet (PDF)
• Niches and Tolerance Ranges Practice Answer Key (PDF)

↑ Back to top

---

Lesson Slides

• 2.1 Niches and Tolerance Ranges Slides (PDF)

↑ Back to top

---

Big Ideas

• A habitat is where an organism lives; its niche is its role
• Abiotic factors are nonliving; biotic factors are living
• A tolerance range has an optimum, zones of stress, and limits
• A species lives only where its abiotic factors are in its tolerance ranges
• Specialists have narrow ranges and are often more at risk

Back to the essential question: what decides where a species can survive is the match between a place and the species’ tolerance ranges. Abiotic factors such as temperature and pH must fall within the range the species can tolerate, and biotic factors such as predators, prey, and competitors also shape its niche. A generalist has a wide range and can live in many places, while a specialist’s narrow range limits it to fewer. Next, you’ll see how scientists estimate how many individuals of a species live in a place.

↑ Back to top

---

↑ Back to top