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Stage 4: Data analysis

Stage 4: Data presentation | Statistics

Data presentation

Plot a profile graph for the dunes. You will need to exaggerate the vertical scale. Use bar charts to display the abiotic data you collected at each point.

The kite diagram below shows the results of an interrupted belt transect from the strand line inland by 300 metres.

Statistics

Simpson's Diversity Index

You can use the Spearman's Rank Correlation Coefficeient to investigate changes with distance inland from the seashore, and the Mann-Whitney U test and Chi-squared tests to compare two or more different areas of the dunes. Another useful statistic is the Simpson's Diversity Index, which is a measure both of species richness (i.e. the number of different species present) and species evenness (i.e. how evenly distributed each species is).

D = N(N-1) / Σ n(n-1)

where

D = Simpson's Diversity Index
n = the number of individuals of each species
N = the total number of individuals

Worked example

The table shows mean % cover for two contrasting areas of a single dune system.

TOTAL

	Mobile dune		Fixed dune
Species	n	n(n-1)	n	n(n-1)
Marram	10	90	4	12
Sea holly	3	6	0	0
Sand fescue	1	2	11	110
Saltwort	2	2	0	0
Dandelion	0	0	8	56
TOTAL	17	100	23	178
	D = 17(16) / 100		D = 23(22) / 178
	D = 2.72		D = 2.84

The larger the value of D, the higher the species diversity. A low value of D could be due to low overall species richness (like at the strand line) or to the dominance of one species (as in dune scrub).

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