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A reliable description of any spatial pattern in species richness requires accurate knowledge about species geographical distribution. However, sampling bias may generate artefactual absences within species range and compromise our capacity to describe biodiversity patterns. In a study published in Nature Communications in November 2018, researchers from Federal University of Goiás explored this topic by analyzing more than 3 million occurrence records from OBIS and other public datasets to identify missing occurrences (gaps) across species latitudinal range. The records included the spatial distribution of almost 35,000 marine species varying from copepods to sharks.
The researchers found a latitudinal gradient of species absence peaking near the equator, a pattern observed in both shallow and deep waters. The tropical peak in missing species richness coincides with the dip in species diversity that characterizes the recently suggested bimodality of the marine realm. This result suggests that spatial gaps in species distribution are the main cause of the bimodal pattern of marine diversity. The tropical gap in species distribution was strongly associated with the poor inventory completeness and reduced sampling effort at low latitudes, indicating a sampling bias effect. However, the authors concluded that only the increasing sampling effort at low latitudes will reveal if the absence of species in the tropics, and the consequent dip in species richness, are indeed a sampling artefact or if it may be a natural phenomenon.
Reference: Menegotto A. and Rangel T.F. Mapping knowledge gaps in marine diversity reveals a latitudinal gradient of missing species richness. Nature Communications, 2018. https://www.nature.com/articles/s41467-018-07217-7