Scaevola taccada as a keystone nesting resource for sympatric cavity-nesting bees and its implications for coastal biodiversity conservation

Coastal ecosystems present critical nesting habitats for stem-nesting bees, yet the role of littoral shrubs as nesting substrates remains poorly documented. We investigated the nesting biology of bee communities utilizing dead twigs of the widespread coastal shrub Scaevola taccada (Goodeniaceae) on Hainan, China. From 172 dissected nests, we identified five bee species: Braunsapis hewitti , Ceratina dentipes , Ceratina smaragdula , Ceratina taiwanensis , and Heriades sauteri . All species constructed linear nests in the pithy stems, but exhibited significant interspecific differences in nest architecture. Nest entrance diameter varied significantly among species, with C. dentipes using the narrowest twigs (2.73 ± 0.31 mm) and C. taiwanensis the widest (4.13 ± 0.52 mm). For the four cell-building species, the number of brood cells per nest also differed significantly, with H. sauteri (5.29 ± 2.63) and C. taiwanensis (5.39 ± 3.53) producing more cells than C. smaragdula (2.54 ± 1.68) and C. dentipes (2.25 ± 1.89). Uniquely, B. hewitti did not construct cell partitions, rearing brood communally in a single linear chamber with progressive provisioning. Nest materials varied from chewed pith partitions in Ceratina spp. to resin partitions in H. sauteri . Microscopic analysis of pollen slides prepared from pollen-nectar provisions and larval feces confirmed that intact and identifiable pollen grains are well-preserved, demonstrating a direct and effective approach for elucidating the dietary breadth of these bees in pollinating costal plants. Our findings, particularly the differential use of S. taccada twigs by diameter, suggest resource partitioning that may facilitate species coexistence, and highlight the critical importance of preserving coastal shrublands as essential nesting resources for maintaining diverse pollinator communities in tropical and subtropical coastal ecosystems.