# Ecological Trade-Offs Between Mangrove Expansion and Waterbird Diversity: Guild-Specific Responses to Pond-to-Mangrove Restoration

## TL;DR

Mangrove restoration in Bamen Bay boosted bird diversity and ecological functions, but may threaten mudflat-dependent species like shorebirds.

## Contribution

This study provides empirical evidence of guild-specific waterbird responses to pond-to-mangrove restoration in a subtropical setting.

## Key findings

- Restored mangrove areas showed higher species and functional diversity compared to aquaculture ponds.
- Herons thrived in restored habitats, while shorebirds showed no improvement due to reduced open mudflats.
- Long-term monitoring is needed to assess risks to mudflat specialists as mangroves continue to expand.

## Abstract

Mangrove restoration is rapidly expanding across coastal regions, yet its benefits and risks for waterbirds remain uncertain. In this study, we examined how waterbird communities responded to pond-to-mangrove restoration in Bamen Bay, Hainan Island, by comparing restored mangrove ponds with nearby aquaculture ponds. We found that restoration quickly increased the number of bird species and supported a wider range of ecological functions. Herons adapted well to the new habitats created by tidal reconnection and young mangrove growth. Shorebirds, which depend on open mudflats, showed no clear improvement in this short-term comparison and may be more sensitive if mangroves continue to expand, a possibility that will require longer-term monitoring. Our study highlights that mangrove restoration can enhance biodiversity but also reduce the habitats needed by mudflat specialists. Successful restoration should therefore balance mangrove expansion with the conservation of open tidal flats to support diverse waterbird communities.

Coastal pond-to-mangrove restoration has become a prominent Nature-based Solution, yet its short-term ecological effects on waterbird communities remain unclear. We assessed taxonomic, functional, and compositional responses of waterbirds to large-scale restoration in Bamen Bay, Hainan Island, using BACI-style comparisons between restored and unrestored aquaculture ponds in 2021 and 2023. Restored areas exhibited higher taxonomic α diversity and functional richness (p < 0.001), coinciding with rapid habitat diversification following hydrological reconnection. Species richness (p < 0.001), Shannon diversity (p < 0.01), and functional richness (p < 0.01) were consistently higher in restored areas than in aquaculture ponds. In contrast, β diversity patterns diverged between habitats: restored areas remained relatively stable, whereas aquaculture ponds showed greater between-year compositional change (p < 0.05). Guild-specific responses revealed contrasting patterns: herons showed higher diversity in restored habitats (p < 0.05), whereas shorebirds exhibited no significant changes (p > 0.05), consistent with their dependence on open mudflats that were only partially retained. Although no significant declines were detected, functional richness tended to be lower in 2023 (p > 0.05), and ongoing mudflat loss suggests potential long-term risks for mudflat specialists, warranting extended monitoring. Taken together, our findings suggest that effective pond-to-mangrove restoration in Bamen Bay should balance mangrove expansion with the retention of tidal flats and managed shallow-water habitats to support diverse waterbird assemblages.