Practice question

Similarity ordering compares the overlap in species composition for every pair of communities. The ranking A–B > B–C > A–C means A and B share the greatest fraction of their species, B and C share an intermediate fraction, and A and C are least alike. With presence–absence data, this may be tested using Jaccard similarity, C/(A + B − C), or Sørensen similarity, 2C/(A + B), where C denotes shared species. Both coefficients rise with overlap, although their numerical scales differ. Abundance data may instead use Bray–Curtis similarity, which also considers how evenly shared species are represented. The ecological implication is that community B is compositionally intermediate or connected to both others, whereas A and C show the strongest species replacement. This pattern corresponds to increasing beta diversity in the reverse order: turnover is greatest between A and C and least between A and B. Geographic separation, habitat filtering, or an environmental gradient can generate such a sequence. The species table or clustering diagram needed to compute the coefficients is absent from the row, so the stated ordering cannot be independently derived, but its interpretation follows directly from pairwise overlap.