Benthic functional diversity as a valuable tool to identify vulnerable mangrove forests

• Small mangrove patches can support high functional diversity, regardless of area. • Sites with low functional redundancy and high vulnerability are especially threatened. • Functional indices are effective for current assessment but limited for future predictions. • Conservation should prioritise both vulnerable and functionally rich mangrove sites. Mangrove forests are biodiverse systems that harbour a plethora of highly specialised species. The size of a mangrove forest can influence its biodiversity, but the relationship between these two characteristics is not always linear. In the present study, carried out in 36 medium- to small-size mangrove patches across Hong Kong, we determined the size of each mangrove stand, its taxonomic richness, and the functional diversity, redundancy and vulnerability of crab assemblages, to identify those that were more at risk of functional loss. Eight sites displayed a combination of critically low levels of functional richness, functional redundancy and high functional vulnerability. These sites have either lost some of its ecosystem functions or are at risk of losing them. We also highlighted five sites in good condition that could potentially act as reservoirs or supply sites for nearby threatened locations, and that should be the focus of conservation efforts. Our data show that species composition, diversity and functional role of the associated fauna plays a more important role than size in the functioning of such ecosystems. We then simulated the local extinction of eight crab species in light of future climate change and recalculated the previous functional analyses. Our analyses show that the adopted functional indices are effective in comparing the functionality of mangroves at local scales and determining their degradation. Such indices, however, were less reliable for predicting future trends in functionality of the studied mangrove forests, due to lack of long-term datasets and shifted baselines.