Unit 2 · Topic 2.4 Beta

Ecological Tolerance

Every species has a range of tolerance for each abiotic factor.

Practice 2: Visual RepresentationsPractice 5: Data Analysis

Part 1 · Hook

Why this matters

Brook trout crowd into a cold mountain stream in spring. In a hot, dry August the same stream warms by several degrees, and the trout gather in the few shaded pools fed by cold springs. A little warmer still and they stop feeding and growing; warmer again and they die. Every species has a band of conditions it can live in, and that band decides where it can be found.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which of these is an abiotic factor in a stream?

  1. Water temperature
  2. Algae on the rocks
  3. Trout
  4. Mayfly larvae
Show the answer

Abiotic factors are nonliving: temperature, light, water, salinity. Algae, trout and mayflies are biotic.

  • Correct: Water temperature:
  • Algae on the rocks:
  • Trout:
  • Mayfly larvae:

2. Which change usually lowers the amount of dissolved oxygen that water can hold?

  1. Warmer water
  2. Colder water
  3. Faster-flowing water
  4. More plants making oxygen
Show the answer

Warm water holds less dissolved oxygen than cold water.

  • Correct: Warmer water:
  • Colder water:
  • Faster-flowing water:
  • More plants making oxygen:

Part 4 · See it

See it first

Range of tolerance for an invented fish species against water temperature. The curve of fish numbers is highest in the optimal range, 16 to 26 °C, low in the zones of physiological stress, 8 to 16 °C and 26 to 33 °C, and zero in the zones of intolerance below 8 °C and above 33 °C.
Range of tolerance for water temperature in an invented fish species: an optimal range, zones of physiological stress, and zones of intolerance where none survive. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. An abiotic factor, such as temperature, moves out of a species’ optimal rangeindividuals enter a zone of physiological stress: they grow, feed or reproduce less
  2. Stress lasts or the factor moves further outfewer individuals survive and reproduce, so the population shrinks
  3. The factor passes the limit of toleranceindividuals die (the zone of intolerance), and the population disappears from that place or moves

Part 6 · Key ideas

Key ideas

  • Each species has a range of tolerance for each abiotic factor (temperature, salinity, light, water): the conditions it can survive in.
  • Inside the range, the optimal range is where individuals grow and reproduce best, and the population is largest.
  • Toward the edges are zones of physiological stress: individuals survive but feed, grow or reproduce poorly, so the population is small. Beyond the limits lie zones of intolerance, where none survive.
  • Tolerance applies to individuals and to whole populations. Individuals in a population vary, so some tolerate a little more than others.

Part 7 · Misconception

A common mistake

The wrong idea: If a fish survives in water that is too warm, it is doing fine.

What actually happens: Surviving is not thriving. In the zone of physiological stress fish live but eat, grow and reproduce less, so the population shrinks even before any fish die.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model or diagram

A fish species’ range of tolerance

Range of tolerance for an invented fish species against water temperature. The curve of fish numbers is highest in the optimal range, 16 to 26 °C, low in the zones of physiological stress, 8 to 16 °C and 26 to 33 °C, and zero in the zones of intolerance below 8 °C and above 33 °C.
LevlPrep original diagram.

1. According to the figure, what is the optimal range of water temperature for this fish?

  1. 16 to 26 °C
  2. 8 to 33 °C
  3. 0 to 8 °C
  4. 26 to 33 °C
Show the answer

The optimal range is the middle band where the population is largest.

  • Correct: 16 to 26 °C: Right: the curve is highest between 16 and 26 °C, labeled optimal.
  • 8 to 33 °C: That is the whole range of tolerance, including the zones of stress.
  • 0 to 8 °C: No fish survive below 8 °C; that is a zone of intolerance.
  • 26 to 33 °C: That is a zone of physiological stress, where few fish live.

2. The water in a lake warms from 22 °C to 30 °C over one summer. Which zone does the fish population move into?

  1. The optimal range
  2. A stress zone
  3. A zone of intolerance
  4. A zone that lies off the edge of the graph
Show the answer

Read the temperature on the x-axis, then the label of the band it falls in.

  • The optimal range: 30 °C is above the optimal range, which ends at 26 °C.
  • Correct: A stress zone: Right: 30 °C lies between 26 and 33 °C, a stress zone.
  • A zone of intolerance: Intolerance starts above 33 °C, and 30 °C is below that.
  • A zone that lies off the edge of the graph: The graph runs from 0 to 40 °C, so 30 °C is on it.

3. Which statement best explains why the curve is low but above zero between 26 and 33 °C?

  1. No fish can survive at those temperatures
  2. Fish grow fastest at those temperatures
  3. Fish survive but feed and breed poorly, so few remain
  4. Those temperatures are rare in lakes and streams in nature
Show the answer

Low but not zero means survival with stress.

  • No fish can survive at those temperatures: That describes the zone of intolerance, where the curve is zero.
  • Fish grow fastest at those temperatures: Growth is best in the optimal range, where the curve is highest.
  • Correct: Fish survive but feed and breed poorly, so few remain: Right: the stress zone has living fish, but conditions keep the population small.
  • Those temperatures are rare in lakes and streams in nature: The graph is about how fish respond, not how often a temperature occurs.

4. A factory would release heated water that keeps one bay at 35 °C all summer. What does the figure predict for this fish species in that bay?

  1. It would grow faster, because warm water holds more food
  2. It would not change, because fish can swim to cooler water
  3. It would shift into its optimal range for temperature
  4. It would vanish, as 35 °C is in a zone of intolerance
Show the answer

Past the upper limit of tolerance, a species cannot live in a place.

  • It would grow faster, because warm water holds more food: Above 33 °C the figure shows no fish at all.
  • It would not change, because fish can swim to cooler water: Some could leave, but the bay itself would no longer hold the species; the figure shows zero at 35 °C.
  • It would shift into its optimal range for temperature: The optimal range is 16 to 26 °C, far below 35 °C.
  • Correct: It would vanish, as 35 °C is in a zone of intolerance: Right: above 33 °C none survive, so the population would die or leave.

Data

Snail survival at different salinities

Percent of 200 marsh snails alive after 10 days at each salinity, in parts per thousand (ppt) (invented data for practice)
Salinity (ppt)Snails alive after 10 days (%)Snails that laid eggs (%)
000
5405
108530
159570
209672
259035
30455
3500

5. Based on the table, what is the optimal range of salinity for these snails?

  1. About 15 to 20 ppt
  2. About 0 to 35 ppt
  3. About 5 to 30 ppt
  4. About 25 to 35 ppt
Show the answer

The optimal range is where individuals both survive and reproduce best.

  • Correct: About 15 to 20 ppt: Right: survival is highest and egg laying peaks at 70% and 72% there.
  • About 0 to 35 ppt: At 0 and 35 ppt no snails survive; those are the limits.
  • About 5 to 30 ppt: That is the range where some survive, including the stress zones where few lay eggs.
  • About 25 to 35 ppt: Survival and egg laying fall at 25 ppt and above.

6. In which part of a range of tolerance do individuals grow and reproduce best?

  1. A zone of physiological stress
  2. A zone of intolerance
  3. The upper limit of tolerance
  4. The optimal range
Show the answer

Optimal range: thrive. Stress zone: survive poorly. Intolerance: die.

  • A zone of physiological stress: Individuals survive but grow and reproduce poorly there.
  • A zone of intolerance: Individuals die there.
  • The upper limit of tolerance: At the limit, survival ends.
  • Correct: The optimal range: Right: the population is largest there.

Part 9 · Summary

Summary

Every species has a range of tolerance for each abiotic factor. In the optimal range individuals thrive and populations are largest; in the zones of physiological stress they survive but grow and reproduce poorly; beyond the limits, in the zones of intolerance, they die. Where conditions move out of a species’ range, its population shrinks, moves or disappears.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Share to Google Classroom (opens in a new tab)

Question set for this topic