Q17 - Community Similarity: Sorensen-Dice and Jaccard

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

In ecology, the similarity of two communities can be measured by calculating the Sorensen-Dice coefficient (DSC) and Jaccard index (JI).

DSC = 2C / (A + B)

JI = C / (A + B - C)

Where A and B are the number of species in the two communities and C is the number of species common to both communities.

In the figure below, you can see two samples of invertebrates, collected from two small rivers, i and ii respectively. Similar looking animals belong to the same species. Calculate the Sorensen-Dice coefficient and Jaccard index for the rivers i and ii.

Two sample containers labeled i and ii. Container i contains various invertebrate species including worms, isopods, amphipods, mayfly larvae, stonefly larvae, and a spider. Container ii contains a subset of similar species with some overlap and some unique taxa like a snail. Samples of invertebrates from rivers i and ii. Similar looking animals belong to the same species.

Q17.1. Calculate the Sorensen-Dice coefficient (DSC) for the rivers i and ii.

Q17.2. Calculate the Jaccard index (JI) for the rivers i and ii.

Q17.3.1. For the same communities, DSC is always equal or higher than JI.

Q17.3.2. Rivers iii and i have DSC = 0.8 and JI = 0.67; faunal similarity between rivers i and iii is thus higher than between rivers i and ii.

Q17.3.3. The discriminative capacity of DSC is higher than that of JI for highly similar communities.

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF