International A Level Biology / Biodiversity and conservation
The index of diversity revision notes
Edexcel International A Level Biology (YBI11), chapter 12: Biodiversity and conservation, specification point 4.18. Free lesson.
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The index of diversity, D, measures biodiversity using both the number of species and how many organisms belong to each species. A higher D means higher biodiversity, so it can distinguish habitats that have the same species richness.
Why species richness is not enough
Species richness is the number of different species in a habitat. It does not show whether organisms are spread fairly evenly between them or whether one species dominates. The index of diversity also takes abundance into account, meaning the number of organisms of each species. Both woods have 100 trees and a species richness of 4. Wood A has 30, 25, 25 and 20 trees of its four species. Its D is 4.04. Wood B has 94 trees of one species and 2 of each of the other three. Its D is 1.13. Wood A has higher biodiversity because its organisms are distributed more evenly.
What the formula means
The index of diversity is calculated using the total number of organisms and the count for each species: D = N(N − 1) / Σn(n − 1) N is the total number of organisms of all species together. n is the number of organisms of one species. The symbol Σ means to add the value for every species.
How to calculate D from a table
For each species, calculate n(n − 1), then add these values to get the denominator. Add all the species counts to find N, then calculate N(N − 1) for the numerator. Divide the numerator by the denominator. Rocky shore counts: limpets 20, barnacles 35, periwinkles 12, dog whelks 5 and sea anemones 8. Their n(n − 1) values are 380, 1190, 132, 20 and 56.
How to compare habitats
Compare the D values directly. The habitat with the higher value has higher biodiversity, so state which value is higher and explain what that means. To track a change, repeat the survey later and compare the results. The first shore has D = 3.55 and the second has D = 2.10. The first shore has higher biodiversity because 3.55 is greater than 2.10.
Avoid confusing N with the number of species
N counts all the organisms, not the number of species. On the rocky shore there are five species, but 80 organisms, so use N = 80. A value of D below 1 signals that there has probably been a calculation error. Do not use 5 × 4 for N(N − 1). Use 80 × 79 = 6320.