Regional databases demonstrate macroecological patterns less clearly than systematically collected field data

The analysis of macroecological patterns has necessitated the use of
large, composite datasets recording local-scale species occurrences
distributed across the globe. These datasets, however, have various
spatial and temporal biases, including taxonomic under sampling, range
gaps for many species, and geographic uncertainty. They have rarely been
compared to data collected in the field across large spatial gradients. In
this paper we use two datasets built from online repositories plus
standardised field collections of death assemblages to reconstruct
macroecological patterns for marine bivalves along the eastern coastline
of Australia – spanning over 20° of latitude and the transition between
tropical and temperate regions. We test the strength of the latitudinal
diversity gradient using four diversity measures and identify a
biogeographical boundary. The field collection demonstrates a strong
latitudinal gradient, but results from the composite datasets were varied.
Adding observation-based records to the composite dataset obscured the
latitudinal gradient. The location of the biogeographic boundary was the
same in all datasets, and the location mirrored two previously published
bioregionalisations. Although broad patterns seen in the field can be
uncovered from composite macroecological datasets, care both in dataset
construction and choice of methods is needed to ensure robust results.