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q-bio.PE · Populations and Evolutionstat.AP · Applications

In current OBIS records, the equatorial dip in marine animal species richness did not deepen from 1955–1974 to 1995–2015; with equal effort in shared 1° cells it became shallower

Chaudhary et al. (2021, PNAS) used OBIS records and reported that the dip in marine species richness near the equator became deeper from 1955–1974 to 1995–2015, especially for pelagic species. We tested this with all 7,129 OBIS open-data datasets (snapshot of 2026-10-23; 125.6 million animal records of 109,162 species for 1955–2025; eDNA datasets removed). Richness was calculated per 5° latitude band and period. The dip index (DI) is the mean log richness at 10°S–10°N minus the mean of the log richness peaks at 20–50°N and 20–50°S. With the original estimator (iNEXT q=0 asymptotic richness, all records), DI did not become progressively more negative. The change from 1955–1974 to 1995–2015 was +0.08 (95% dataset-bootstrap interval −0.22 to +0.49) for all animals, +0.03 for benthic species and −0.08 (−0.41 to +0.21) for pelagic species. Pelagic DI deepened from 1955–1974 to 1975–1994 but became shallower from 1975–1994 to 1995–2015. When we used only 1° cells sampled in every period and the same number of records per cell, the dip became shallower for all animals (+0.27, +0.13 to +0.42) and for benthic species (+0.23, +0.11 to +0.41). For pelagic species, the change was small and its sign depended on the cell set. Equal records per band gave no significant deepening. Coverage-standardised richness was unstable. The 2016–2025 data did not continue a deepening. The northern richness peak of pelagic species moved from 37.5°N to 42.5–47.5°N under equal effort. This agrees in direction with the reported poleward shift. Limitations: we used current OBIS, not the 2015 extract, and an approximate habitat classification. The dataset bootstrap is approximate for asymptotic estimators. Equal effort does not equalise gear or depth.

latitudinal diversity gradient · equatorial dip · OBIS · marine biodiversity · sampling bias · rarefaction · species richness · climate change · pelagic · benthic · replication

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References

5
  1. challengesdoi:10.1073/pnas.2015094118

    We test its main claim, a deepening equatorial dip in OBIS-based marine richness from 1955-1974 to 1995-2015, on current OBIS data with effort controls.

  2. doi:10.1016/j.tree.2016.06.001

    Earlier OBIS-based proposal that marine richness is bimodal with latitude.

  3. doi:10.1038/s41467-018-07217-7

    Shows that gaps in OBIS species distributions peak near the equator, which motivates the effort controls.

  4. usesdoi:10.1890/11-1952.1

    Coverage-based rarefaction and extrapolation of richness.

  5. usesdoi:10.1111/2041-210X.12613

    iNEXT asymptotic richness estimator replicated here.

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