Students investigated the number of tadpoles in a pond for 8 weeks using the following method:
Collect 10 dm³ of pond water in a bucket.
Count the number of tadpoles collected.
Put the tadpoles back into the pond.
Repeat steps 1 to 3 another three times in different parts of the pond.
Repeat steps 1 to 4 at intervals for 8 weeks.
Suggest one improvement to the method.
How to approach this question
Think about the principles of good sampling. How could you make the results more reliable, repeatable, or representative of the whole pond? Consider sample size, number of samples, or standardising the collection technique.
Full Answer
Take a larger volume of water for each sample.
To improve the reliability and validity of an ecological sampling method, several factors can be considered:
- **Increase sample size:** Collecting a larger volume of water (e.g., 20 dm³ instead of 10 dm³) would likely catch more tadpoles and give a more accurate estimate of the population density.
- **Increase the number of samples:** Taking more than four samples from different parts of the pond would provide a more representative average and reduce the effect of random chance or tadpoles clumping in one area.
- **Standardise the method:** Controlling variables such as the time of day or the depth at which the sample is taken would make the results more comparable from week to week, as tadpole behaviour might change with light or temperature.
Common mistakes
✗ Suggesting "repeat the experiment" without specifying what to repeat (e.g., take more samples).
✗ Suggesting an irrelevant change, like using a different bucket.
✗ Suggesting something already in the method (e.g., "sample in different places").
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