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.

Methods
During August 2018 and August 2019, ten samples were taken from each of fifteen beaches using 50 x 50cm (0.25 m2) quadrats, with five samples taken north of the beach midpoint and five taken to the south. Sites were spaced along the coastline to ensure they were independent samples and therefore representative of an independent species pool. Sample locations within each site were chosen randomly, ensuring they were ≥10 m apart and between high and low tide marks. Sampling was carried out during clear days not following a storm to avoid artefacts of changes in depositional mode. The top 5 cm of sediment within each quadrat was processed through 16, 8, and 4 mm sieves. Valves were retained and counted if they were more than 50% intact and able to be identified to morpho-species level – although this will bias our collection somewhat towards species which are naturally better preserved it will also ensure our samples are reflective of a more recent deposition. Although the spacing of the sites and sampling set up will eliminate some degree of temporal and spatial mixing, and give the collection an intertidal focus, these sites will still represent a larger mixing of habitats and time periods.