A student investigates the effect of water acidity (pH level) on the survival of freshwater micro-crustaceans (Daphnia magnaDaphnia\;magnaDaphniamagna).
After 48 hours, she counts how many of the DaphniaDaphniaDaphnia are still alive.
The table below shows her results:
| pH Level | Number of the 30 DaphniaDaphniaDaphnia alive after 48 hours |
|---|---|
| 7.0 | 28 |
| 6.0 | 26 |
| 5.0 | 22 |
| 4.0 | 11 |
| 3.0 | 2 |
Write down one other factor that the student should have kept the same in this investigation.
Describe what this investigation shows about the effect of pH level on DaphniaDaphniaDaphnia survival.
One way to compare the vitality of the population is to use a Population Health Index (PHI\text{PHI}PHI).
Use the formula:
PHI=mean swimming speed×number of Daphnia alivenumber of hours \text{PHI} = \frac{\text{mean swimming speed} \times \text{number of Daphnia alive}}{\text{number of hours}} PHI=number of hoursmean swimming speed×number of Daphnia aliveFor the DaphniaDaphniaDaphnia at a pH of 5.05.05.0, the mean swimming speed was 2.4 mm/s2.4\text{ mm/s}2.4 mm/s. Calculate the PHI\text{PHI}PHI for these DaphniaDaphniaDaphnia.
Look at the equation for PHI\text{PHI}PHI. Explain why this is a better way of measuring the effect of acidity on the population than just counting the number of surviving DaphniaDaphniaDaphnia.
44 exam-style questions on OCR GCSE Biology Practical skills. Each one has a worked solution and a mark scheme showing where the marks go.