Shallow coverage in shallow waters: the incompleteness of intertidal species inventories in biodiversity database records

The availability of online biodiversity data has increased in recent decades, aiding our

understanding of diversity patterns and species richness–environment relationships

across temporal and spatial scales. However, even the most exhaustive databases are

prone to sampling biases, which create knowledge gaps in species distributions and

increase uncertainty in model predictions. Regarding marine environments, intertidal

zones are globally distributed and considered early warning systems for climate change

impacts and species’ range shifts. Owing to their relative accessibility, intertidal records

should – supposedly – be less incomplete and biased compared to open-ocean and

deep-sea areas. Yet, the extent and coverage of intertidal records available in global bio-

diversity databases remains unknown. In this study, we used a high-resolution world-

wide tidal flat map to identify intertidal records of 11 563 benthic species from the

OBIS (Ocean Biodiversity Information System) portal. Following a thorough data-

cleaning process, we evaluated geographic patterns in observed species richness, site

accessibility, sampling effort, and inventory completeness across latitudes. We demon-

strate that observed species richness has mid-latitudinal peaks while the tropics accu-

mulate species with missing records, similar to patterns described for the entire marine

realm. These patterns correlate with disproportionate mid-latitude sampling efforts

and poor tropical sampling coverage. Sixty-five percent of the mapped intertidal sites

are located within 3 hours of a city, but sampling records remain almost absent along

African Atlantic, South American Pacific, and Indo-Pacific coasts. Thus, even for the

accessible and well-studied intertidal shorelines, database records are not free from

geographical biases and their associated implications for biodiversity estimates. Our

results highlight the need for a better data-sharing culture, and we hope to encourage 

initiatives promoting more and better-distributed research efforts on intertidal biodiversity, which could improve global scale

detection and prediction of climate change impacts at regional and global scales.