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NESA Biology Communicating

1 sample question with marking guides and sample answers · Avg. score: 100%

Q25

2023

QCAA

Paper 1

6 marks

Q25

The effect of an invasive species on plant biodiversity was investigated by collecting this data from an ecosystem.

	Percentage cover (invasive species)
	0–20%	>20–40%	>40–60%	>60–80%	>80–100%
Species richness	7	7	7	4	2
Simpson’s diversity index	0.83	0.77	0.55	0.49	0.30

Q25a

1 mark

Contrast species richness in areas of low invasive species cover (0–20%) with areas of high invasive species cover (>80–100%).

Species richness is higher in areas with low invasive species cover (seven different species compared to two).

Marking Criteria

Descriptor	Marks
Identifies species richness is higher in areas with low invasive species cover	1

Q25b

2 marks

Draw a conclusion about the effect of the invasive species on plant biodiversity in this ecosystem. Justify your response.

The presence of the invasive species has a negative effect on biodiversity. While the same number of species were observed in the area when invasive species cover was 0–60% (7, as indicated by species richness), the SDI decreased from 0.83 to 0.3 as percentage cover increased. This indicates that overall, biodiversity decreased as percentage cover increased.

Marking Criteria

Descriptor	Marks
Identifies a relevant trend in the data	1
Draws a valid conclusion about the effect of the invasive species	1

Q25c

3 marks

Explain why having data on species richness and Simpson’s diversity index is more informative than a single measure for the purpose of this investigation. Refer to the table to support your response.

Species richness indicates the number of species that can coexist in the area; however, it doesn’t account for the abundance of each species, so one or two species may be dominating. SDI considers the number of species and their relative abundance (evenness), but the number of species is not obvious from the index.
Having both values is more informative because it allows the researcher to identify the number of species in the area (7) and infer evenness from the SDI. For example, the same number of species was present for 0–60% coverage (7), but the evenness was not the same, as the SDI decreased as % coverage got higher.

Marking Criteria

Descriptor	Marks
Recognises species richness is the total number of species present	1
Recognises SDI considers species number and abundance/evenness	1
Uses data to explain why data on species richness and SDI is more informative than a single measure	1

NESA Biology Communicating

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