Data-source complementarity in documenting non-native terrestrial molluscs in a seasonally dry tropical region

Abstract

Abstract Reliable, georeferenced occurrence data are essential for documenting non-native species, yet scientific collections may under-represent taxa associated with human-modified environments. This limitation is relevant for terrestrial molluscs in the Caatinga Dominion, a seasonally dry tropical region of north-eastern Brazil with sparse and spatially biased records. We evaluated how citizen-science records complement scientific collections in the taxonomic, numerical and spatial documentation of non-native terrestrial molluscs, and whether patterns were driven by the invasive snail Achatina fulica . Using 1190 occurrence records, we compared taxonomic richness, tested source-status association and mapped non-native records on a 10-km grid. Scientific collections contained 942 records and 154 species, whereas citizen science contained 248 records and 26 species. All citizen-science species were already represented in collections, so source integration did not increase recorded richness. However, non-native species accounted for 69.0% of citizen-science records, but only 2.5% of collection records, and citizen-science records had 84.94 times higher odds of representing a non-native species (95% CI 53.18–141.10). Integration expanded documented non-native coverage from nine to 51 grid cells, with no spatial overlap. After excluding A. fulica, the association persisted (odds ratio 13.50, 95% CI 7.31–25.30), and integrated coverage remained 2.22-fold greater than collection coverage alone. These findings show that the sources are complementary, but not interchangeable. Collections provide taxonomic verification and historical baselines, whereas citizen science broadens the numerical and spatial documentation of known non-native species. These records can guide expert validation, targeted surveys, and voucher deposition, but they document occurrence, not abundance, occupancy, or invasion intensity.