AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/2/2-4)
Unit 2 · Topic 2.4
2.4 Ecological Tolerance
Every species can survive only within a certain range of conditions, like temperature, salinity or pH. In the middle of that range it thrives, near the edges it's stressed, and beyond the limits it dies, which explains where species live and why changing conditions can harm them.
Key terms
- range of tolerance
- optimal range
- zone of physiological stress
- zone of intolerance
- limiting factor
The range of tolerance
Ecological tolerance is the range of conditions an organism can survive. Each species has a range for each abiotic factor, such as temperature, salinity, pH, sunlight, water and dissolved oxygen. A trout, for example, needs cold water with lots of dissolved oxygen, while a carp can live in warmer, less oxygenated water.
Tolerance applies at two levels. An individual has a range it can survive. A species also has a range, which sets where it can live across the landscape.
The zones
On a graph, this usually looks like a bell-shaped curve. The horizontal axis is the condition (like temperature) and the vertical axis is population size or how well the organism does. The peak is the optimal range, the slopes are the stress zones, and the flat ends at zero are the zones of intolerance.
Physiological stress means the body is working hard just to cope. A fish in water that's too warm uses extra energy to breathe and may stop reproducing even though it survives.
| Zone | Where it is | What happens |
|---|---|---|
| Optimal range | Middle of the range | Organisms thrive, grow and reproduce; population is largest |
| Zones of physiological stress | On both sides of the optimal range | Organisms survive but struggle; growth and reproduction drop; population is smaller |
| Zones of intolerance | Beyond the upper and lower limits | Organisms can't survive; no population |
Limiting factors
A limiting factor is whatever condition is furthest from the optimum and so holds a population back. A desert plant may have plenty of sunlight and warmth but be limited by water. A lake's algae may be limited by phosphorus. Improving any other factor won't help much until the limiting factor improves.
Organisms may also be able to adjust to conditions somewhat over time, a process called acclimation. A person who moves to a high mountain gradually makes more red blood cells. But acclimation only stretches the range a little; it doesn't remove the limits.
Why tolerance matters for the environment
When people change conditions, species can be pushed out of their optimal ranges. Thermal pollution from power plants warms rivers beyond what trout can tolerate. Warming oceans push corals past their upper temperature limit, causing coral bleaching, when corals expel the algae that feed them. Road salt raises the salinity of streams. Climate change also shifts species' ranges as they move to stay within their tolerance limits.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading a tolerance range
A fish species has an optimal range of 12–18°C, zones of physiological stress from 6–12°C and 18–24°C, and cannot survive below 6°C or above 24°C. Describe how the fish would do in water at (a) 15°C, (b) 21°C and (c) 26°C.
Show the solutionHide the solution
- Step 1: Locate each temperature in the zones.
- Step 2: (a) 15°C is inside 12–18°C, the optimal range: the fish thrive and reproduce well.
- Step 3: (b) 21°C is inside 18–24°C, a zone of physiological stress: the fish survive but grow and reproduce less.
- Step 4: (c) 26°C is above 24°C, in the zone of intolerance: the fish die.
Answer: (a) Optimal, thriving; (b) stressed, surviving with reduced growth and reproduction; (c) intolerance, death.
- Example 2
Predicting the effect of warming
A power plant releases warm water into a river, raising its temperature from 14°C to 22°C near the outlet. Using the fish from the previous example, predict the effect on the fish population near the outlet and explain.
Show the solutionHide the solution
- Step 1: Before: 14°C is in the optimal range, so the population is large.
- Step 2: After: 22°C is in the upper stress zone.
- Step 3: Stressed fish spend energy coping with heat (warm water also holds less dissolved oxygen), so fewer reproduce.
- Step 4: The population near the outlet will likely shrink, and some fish may move to cooler water.
Answer: The population near the outlet will decline, because 22°C is in the stress zone, where fish survive but reproduce less.
Common mistakes
- Thinking organisms die as soon as they leave the optimal range. In the stress zones they survive but do poorly.
- Forgetting there are two stress zones and two zones of intolerance, one at each end.
- Ignoring the limiting factor. Improving a condition that's already optimal doesn't help a population held back by a different factor.
On the exam
- Questions often describe a tolerance curve and ask what happens at a given value. Name the zone and describe survival and reproduction.
- Link tolerance to human impacts like thermal pollution, coral bleaching and climate change when asked to explain a decline.
Connected topics
Videos
Check yourself
4 questions on 2.4 Ecological Tolerance. Pick an answer to see if you got it, and why.
| Water temperature (°C) | Survival after 30 days (%) | Average growth (g per week) |
|---|---|---|
| 2 | 0 | no survivors |
| 6 | 70 | 1.0 |
| 10 | 100 | 3.0 |
| 14 | 100 | 3.2 |
| 18 | 85 | 1.4 |
| 22 | 30 | 0.3 |
| 26 | 0 | no survivors |
Hypothetical laboratory data for juvenile trout
Based on the data, which temperature range is closest to the trout's optimal range?
At 22 °C, the trout are best described as being in which zone?
A power plant begins releasing cooling water that raises a stream's temperature from 12 °C to 20 °C. Based on the data, which prediction is best supported?
Many reef-building corals live only 1–2 °C below the highest temperature they can tolerate. A marine heat wave raises the water temperature about 2 °C above normal for several weeks. Which outcome is most likely?
0 of 4 answered