### BIDI11 — Danger Island
**CRD page:** [Danger Island Coral Surveys (Mar 2015)](https://coralreefdiagnostics.com/danger-island-201503)

**12 Mar 2015**
**GPS:** 6.48727°S, 71.29902°E
**Setting:** intermediate-exposure submerged fore reef, outside the lagoon, 23 m
**Coral cover:** 40.0% (**1 formal transect, D4 only** — see Flags; **algal cover 21.0%**) · **published image bleaching:** the `Mean(%bleaching)` cell is blank for this site — no value exists, and none may be inferred
**Live hard coral cover across methods:** formal transects 40.0% · CoralNet points — none · Coralscapes masks — none. **No imagery exists for BIDI11, so neither computer-vision product was ever generated and no multi-method comparison is possible here.** Where those products do exist elsewhere in Chagos they carry different sampling footprints and denominators from the transects and should not be averaged with them; at this site the transect figure stands alone — and it stands on one hundred points.
**Temperature:** 29.4 °C · **Salinity:** 34.1
**Photo review:** **none — no photographs exist for this site anywhere.** **Anderson did not dive here**; the CRD Danger Island page records "did not dive", and the supplement's `formal vs. informal comparison` sheet independently flags the site **"formal surveys only"**. No folder resolves to site number 11 under any prefix or casing; the site returns zero photo directories and zero frames. (**Note the contradiction in Flag 4: the published site table's `submitted to CoralNet?` column reads "yes" for BIDI11, which cannot be true if no imagery exists.**) **Everything below therefore rests on a single transect's point counts, and reef condition at BIDI11 cannot be assessed** — bleaching, tissue loss, disease, predation scarring, COTS, fish and macrofauna are all simply *unavailable*, and must never be read as zero or inferred from neighbouring Danger Island sites.

**BIDI11 is the least genus-diverse site in the 109-site dataset, and not narrowly: 4 nonzero genera, 109th of 109.** The transect records four coral genera and nothing else. All values below are **percent of total benthos, not percent of coral**, and the four genus columns sum to exactly 40.00%, matching the published cover figure: ***Porites* 30.0% of the benthos — 75% of all live coral, and the single highest *Porites* value of any Chagos site** (computed from the published site table; the runner-up is BIPB114 at 24.28%) — then ***Platygyra* 6.0% (15% of coral)**, ***Favites* 2.0% (5%)** and ***Stylophora* 2.0% (5%)**. H′ = 0.80, Simpson = 0.41, evenness J′ = 0.58, effective diversity 2.23 — the most lopsided assemblage of the seven photo-less sites. **The pairing is what makes the site interesting: 40.0% coral cover, 52nd of 109 and above the archipelago mean of 37.9%, carried almost entirely by one massive-*Porites* stand.**

The algal community is equally lopsided, and favourably so. **Algal cover of 21.0% ranks 107th of 109 — the third-lowest in the archipelago.** Of that 21.0%, **crustose coralline algae is 12.0% (66th of 109, mid-pack) — 57% of all algae — with erect algae 8.0% and cyanobacteria 1.0%. Plain turf, macroalgae and sediment-laden turf all read exactly zero**, and the site's plain-turf value of 0.0% ties for last of 109 in the central ranks. A CCA-dominated floor with no turf and no macroalgal component is a well-grazed, low-sediment signature — though on one hundred points, several of those zeros are as likely to be sampling as ecology (Flag 6). Substrate: pavement 51.0%, bare substrate 9.0%, sand 5.0%, rubble 0%, dead coral 4.0%, recently dead 0%. **Non-coral invertebrates read 30.0% — third-highest of the 109 sites** (computed from the published site table, behind BITB43 at 32.21% and BISA102 at 31.28%) — and are unresolved to taxon anywhere in the site sheet.

**No depth profile exists for BIDI11, and this is the defining limitation of the site.** The survey rests on **one transect in one depth stratum (D4), 100 points** — one of only two single-transect sites in the entire dataset (the other is BISA98, also in this pass). **That one transect is 100% of the site's value.** The single D4 row *is* the site row: coral 40.0%, algae 21.0%, bare substrate 9.0%, non-coral invertebrates 30.0%, CCA 12.0%, and the same four genera in the same order. There is therefore no standard deviation, no range, no confidence interval, no shallow-to-deep handover and no test of within-site heterogeneity — none of the depth analysis that carries the finding at the multi-stratum sites in this group. Every figure quoted above is one 100-point read, which is why they are all whole integers: the 40.0% headline is literally 40 points out of 100, and the four-genus result is four genera that happened to fall under those hundred points. **A four-genus result from 100 points is not evidence of a four-genus reef.** The site is marked **`Training`** in the published table (Validation = 0).

Among its neighbours, BIDI11 is **second of nine Danger Island sites by coral cover** — behind BIDI08 (59.01%), ahead of BIDI09 (37.71%) — against an island mean of 26.4%, so on abundance it is one of the better reefs on the atoll. But both of those neighbours carry 28 and 31 genera on far heavier sampling. **BIDI11 is an outlier in composition, not in abundance:** a *Porites*-dominated, CCA-floored fore reef whose apparent monodominance is amplified by a very thin survey. The *Porites* result itself is probably real — 75% of coral cover is hard to reach by undersampling alone, and the absolute figure of 30.0% of the benthos is the highest recorded anywhere in Chagos — but the *absence* of everything else is not established.

<b>Assemblage composition.</b> A single 100-point transect at D4 gives 40.0% coral against 21.0% algae on four nonzero genera — *Porites* 30.0% of benthos (75% of coral), *Platygyra* 6.0%, *Favites* 2.0%, *Stylophora* 2.0% — over a floor that is 57% crustose coralline algae with no turf, no macroalgae and no sediment-laden turf, and 30% unresolved non-coral invertebrates. Coral cover is 52nd of 109 and genus count is last of 109; the same hundred points produce both, and the two numbers should not be given equal weight. **There is no photographic record whatsoever, no depth profile, and no replication, so nothing here can be corroborated and no statement about the reef's condition — bleaching, mortality, disease, predation or fauna — is possible at BIDI11.**

**Flags:** (1) **SINGLE TRANSECT, SINGLE DEPTH STRATUM, 100 POINTS — the weakest replication in the dataset alongside BISA98.** **That one transect (`BIDI11_d4_T1`) carries 100% of the site's headline value.** No SD, no range, no depth profile, no confidence interval, no heterogeneity estimate. Treat every figure at this site as one 100-point sample and say so wherever the 40.0% is quoted. (2) **THE FOUR-GENUS RICHNESS IS PARTLY A SAMPLING ARTEFACT.** Recorded richness scales with survey effort across the archipelago, and BIDI11 has the least effort of any of the 109 sites against a dataset median in the low thousands of points. The central ranks file places it 109th of 109 for genus count, which is a true rank of a number that is itself effort-limited. **The *Porites* monodominance is likely real — 75% of coral cover, and the highest absolute *Porites* value in Chagos — but do not cite "4 genera" as a community result** without the caveat. **Anderson's judgement needed** on whether to report richness here at all. (3) **THE PUBLISHED `Mean(%bleaching)` CELL IS BLANK.** BIDI11 appears in `formal vs. informal comparison` only as a placeholder row (keyed under `site code 2` as `BiDI11`, with `site code` blank) carrying no cover, bleaching or image count. **% bleached is unavailable and must not be computed from anything else.** Corpus-wide the published bleaching figure is a **computer-vision product** rather than an in-situ observation, with per-image human-vs-model agreement of r = 0.40 for bleached against r = 0.72 for live hard coral (`CHAGOS_MULTIMETHOD_README.md`) — but here there is no figure at all to caveat. (4) **⚠️ SOURCE CONTRADICTION WORTH RESOLVING: the site table's `submitted to CoralNet?` column reads "yes" for BIDI11**, while the CRD page says "did not dive", the comparison sheet says "formal surveys only", no photo folder resolves to site 11, and the site carries no row in `chagos_multimethod_cover_by_site.csv`. **BIDI11 is the only one of the seven photo-less sites whose `submitted to CoralNet?` cell is not "no photos".** Either imagery exists somewhere and has been lost from the local tree, or the cell is wrong. **Recommend checking the CoralNet source before this cell is used to drive anything.** (5) **The two documented reasons for the absence differ in wording** — CRD page "did not dive", supplement "formal surveys only". They are compatible (LOF ran the transect, Anderson was not in the water) but the discrepancy is recorded, not resolved. (6) **PLAIN TURF, MACROALGAE, SEDIMENT-LADEN TURF AND RUBBLE ALL READING EXACTLY ZERO** on a single 100-point transect is a plausible sampling outcome, not necessarily a real absence. The site's last-of-109 rank for plain turf rests on that one zero. Do not cite any of these as confirmed absences. (7) **NON-CORAL INVERTEBRATES AT 30.0% ARE UNEXPLAINED** — third-highest of the 109 sites, a third of the benthos, unresolved to taxon, and on one 100-point transect that is 30 individual points. **Worth checking against the raw LOF datasheet.** (8) **Integrity checks pass:** the four genus columns sum to 40.00% against a published 40.0%; the three nonzero algal categories sum to 21.00% against a published 21.0%. (9) Per-site workbook `BiDi11-benthic data.xlsx` is present. Site depth is published as 23 m, but **the transect is recorded only as stratum D4, with no metre value**, so the 23 m cannot be attached to the survey depth with confidence.

**Benthic data:** [Download BIDI11 benthic survey data (CSV)](../benthic_data/BIDI11_benthic.csv) — scleractinian cover by genus with full names, plus substrate and algal categories (bare substrate, CCA, turf, sediment-laden turf, macroalgae, erect algae, non-coral invertebrates), from the LOF BIOT-1 survey (Mayfield & Dempsey 2026, *Oceans*, supplemental file S1).