Shannon-Wiener biodiversity index
Biodiversity indices, for which the Shannon-Wiener test is often employed, can be useful in any ecological assessment of a particular environment as they take in to account species richness (no. of species) and species equitability (relative abundance of species).
Whilst the Shannon-Wiener index is frequently used in ecology (there are others) two immediate problems associated with its use spring to mind. Firstly, the ecological importance of a species in the structuring of a community is not taken into consideration. For example, a large predatory organism will often be the least abundant organism in the community but its removal or absence may cause the community structure to change dramatically. Secondly, the comparison of index values from different sources (i.e., different laboratories) is potentially dangerous. The reason for this is that different scientists may exert different levels of taxonomic, or species identification, error. Furthermore, comparisons between results from different sources become meaningless when different log bases are used to obtain proportions of species.
The analysis tool bases its calculations on the use of log10 but other bases (i.e. log2, etc.) could be used. In this respect no problem should occur if the same log base is used. Many scientists do not actually state which log base they have used!
Note that you can also conduct a t-test to detect any significant difference in biological diversity at two sampling sites.
Script operation
This tool operates in much the same way as most of the others with no specific departures from the usual methods needed.
Click here for information about general script usage.
Raw sample data is entered in one or two columns as shown in the printed example below. Note that this tool will not accept sample data with unequal sample sizes (i.e., where species abundance data is available for two sites). If two sets of sample data are analysed then the script will also conduct the t-test.
Note that summary statistics are computed for the two samples.
Raw data: Spreadsheet output: Site 1 Site 2 Shannon-Wiener Index of Bio-diversity 47 48 Using Log10 as Base 35 23 7 11 Site_1 Site_2 5 13 Total Count: 99 105 3 8 No. of Classes: 6 6 2 2 Index (H'): 0.5403 0.6328 Variance: 0.00138 0.00097 Std. dev.: 0.0484 t: -1.909 d.f.(v): 195.9277 P(T<=t) one-tail: 0.028863 T-Critical (95%): 1.6527 T-Critical (99%): 2.3456 P(T<=t) two-tail: 0.057726 T-Critical (95%): 1.9722 T-Critical (99%): 2.6013
Interpretation
The biodiversity index figures calculated do not have any units of measurement. In this respect they are only really useful when used to compare sites or habitats (i.e., two different freshwater streams, etc.).
In a two-tailed t-test we are interested in whether there is any statistically significant difference in biodiversity at each site; either site could exhibit higher or lower diversity than the next. In this example, the t-critical value for a two-tailed test at the 0.05 level of significance is not exceeded by the t-statistic computed. In other words, there is no statistically significant difference in the biodiversity at both sites.