The same per-station species lists gathered for the count (Module 5) can be pushed one step further — from how many organisms to how the community is structured: whether the individuals are spread across many species or dominated by a few (its diversity), and how much the species mix shifts between visits (its turnover). A reef with a hundred fish of one species and a reef with a hundred across twenty have the same density and a very different structure, and the same data already in hand can show it at no extra field cost.
This is a genuine second dimension of reef demography — but it is an optional, advanced topic, and it is not assessed. The core census is complete without it: the diver runs the ring, computes the density and its noise floor, and states the limitations, with no diversity index required. Divers who want to measure and track community structure will find the full treatment — the standard diversity indices, a worked example, and the turnover measures — in Appendix A.9, developed under the same conspicuousness caveat that governs the count: richness is the most bias-sensitive measure, so the abundance-weighted indices lead, and every value is relative, read at the same place over time.