Ecology and Ecosystems: Question 5
Syllabus 19.4
A student set up a sealed culture bottle containing a fixed amount of banana-based fruit-fly medium and introduced a small starting population of fruit flies. The bottle was kept at a constant, suitable temperature, and the number of adult fruit flies present was counted every two days:
| Day | Number of fruit flies |
|---|---|
| 0 | 10 |
| 2 | 12 |
| 4 | 15 |
| 6 | 60 |
| 8 | 230 |
| 10 | 300 |
| 12 | 305 |
| 14 | 300 |
| 16 | 250 |
During which day interval was the fruit fly population growing at its fastest rate?
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Worked solution
Step 1: Recall the phases of the sigmoid growth curve
A population growing with limited resources typically shows four phases: a slow lag phase, a fast exponential (log) phase, a levelling-off stationary phase, and finally a death phase, where the population falls.
Step 2: Work out the change in count over each interval
- Day 0 to Day 2: 10 → 12, an increase of 2 (lag phase. Slow start)
- Day 2 to Day 4: 12 → 15, an increase of 3
- Day 4 to Day 6: 15 → 60, an increase of 45
- Day 6 to Day 8: 60 → 230, an increase of 170
- Day 8 to Day 10: 230 → 300, an increase of 70
- Day 10 to Day 12: 300 → 305, an increase of 5 (stationary phase. Nearly flat)
- Day 12 to Day 14: 305 → 300, a decrease of 5 (death phase begins)
- Day 14 to Day 16: 300 → 250, a decrease of 50 (death phase. Population falling)
Step 3: Identify the fastest-growing interval
The largest increase in the population count occurs between Day 6 and Day 8, where the count rises by 170 flies. This is the steepest part of the exponential phase, where food and space are still plentiful and the population is growing at its fastest rate.
Why the other options are wrong
- A (Day 0 to Day 2): this is the lag phase, where growth has barely started.
- C (Day 10 to Day 12): the count has almost stopped changing, showing the stationary phase, not fast growth.
- D (Day 14 to Day 16): the count is falling, showing the death phase, not growth at all.
Final answer
The population was growing fastest between Day 6 and Day 8, option B.