QUESTION 45 (5 marks)
Two teams independently sample equal areas of a grassland using the same quadrat size. Team A selects the five greenest patches. Team B uses 25 random locations. Their SDI estimates are 0.82 and 0.61 respectively.
a) Identify one limitation of Team A’s estimate and explain its effect on representativeness. [2 marks]
b) Explain why Team B’s greater sample number can improve reliability. [2 marks]
c) Explain why the different index values do not, by themselves, demonstrate that biodiversity changed between surveys. [1 marks]
Practice marking scheme
Answer
a) Preferentially selecting green patches biases habitat representation. b) More independent samples reduce the influence of unusual quadrats. c) The sampling procedures differ.
Working
Green patches may differ systematically from the rest of the grassland in moisture or species composition. Their preferential selection gives some microhabitats little or no chance of inclusion. More randomly distributed quadrats generally reduce the influence of chance local variation on the estimate. Because methods differ, the observed SDI difference could be methodological rather than temporal change.
Marking criteria
- Identifies preferential location selection. [1 mark]
- Explains how this can misrepresent the whole grassland. [1 mark]
- States that more independent samples reduce random sampling variation. [1 mark]
- Links this to a more repeatable estimate. [1 mark]
- Recognises differing methods as an alternative explanation for different SDI values. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.