Pollen limitation affects reproductive outcome in the bird-pollinated mangrove Bruguiera gymnorrhiza (Lam.) in a highly urbanized environment

• Pollen limitation is detected in Bruguiera gymnorrhiza populations.
• Low pollinator availability resulted in low fruit set.
• Pollinator visitation rate is likely to depend on the habitat size.
• Populations were largely outcrossed (>80% of the progeny) despite pollen limitation.

Urban mangrove systems are increasingly common in SE Asia but little is known about the potential changes in pollinator ecology in highly urbanized and fragmented landscapes. We detected pollen limitation across four Bruguiera gymnorrhiza populations in Singapore with a pollen limitation index (compromised seed production due to insufficient pollen receipt) ranging from 0.633 to 0.878. Trees in larger patches had higher pollinator visitation rates than in smaller patches, and lower pollinator visitation rates resulted in reduced fruit sets. These results suggest a cascading link between habitat patch size, pollinator visitation rate, and reproductive outcomes in B. gymnorrhiza. Mating system analysis indicated that B. gymnorrhiza may have an internal mechanism acting against inbreeding depression, leading to reduced fruit set under circumstances of reduced pollinator visitation. Our results underscore the importance of pollinator service in mangroves and highlight the need for an in-depth understanding of changing plant–pollinator relationships in urban mangroves systems.