Practice question

Sørensen’s coefficient compares the composition of two communities using shared species. For presence–absence data it is commonly calculated as Cₛ = 2c/(a + b), where a and b are the numbers of species in the two communities and c is the number occurring in both. The coefficient ranges from 0, indicating no shared species, to 1, indicating identical species lists. Because shared species are weighted twice, the numerator corresponds to their contribution to both lists. It is useful for comparing sites, seasons, successional stages, or treatment plots and is closely related to beta diversity: lower similarity generally implies greater species turnover. Shannon and Simpson indices, by contrast, summarize diversity within a community from relative abundances; they do not directly state how many species are shared between two sites. Alpha diversity likewise names the within-site scale rather than a pairwise comparison statistic. Sørensen’s measure can be sensitive to sampling completeness because undetected rare species appear as absences, so comparable effort is important. Abundance-based extensions exist, but the classic coefficient specifically answers the compositional similarity question.