Bottom-up effects of mangrove expansion on transient and resident salt marsh fauna
In many subtropical coastal environments, warming winter temperatures are contributing to a regime shift from grass- and forb-dominated marshes to woody mangrove swamps. Our goal was to quantify the bottom-up effects of this foundation species shift on fauna using a novel combination of surveys and a large experiment. In 2012, we thinned black mangroves in ten 24 x 42 m experimental plots on Harbor Island (Port Aransas, TX) to create a gradient (0-100%) of mangrove cover; cleared areas were colonized by marsh vegetation. Within these plots, we periodically counted epifauna, deployed pit traps to sample nekton, and recorded bird abundance. We conducted similar nekton and epifauna measurements at several large survey sites with either marsh or mangrove vegetation. Finally, we used the citizen science database eBird to compare bird use between the marsh and mangrove survey sites over a two year period.
Adjacent to marsh or mangrove stands, subtidal nekton abundance and richness were generally similar, though species composition differed. Nekton density was likely related to subtidal features such as seagrass density, which in turn may be determined by processes that are linked to emergent vegetation type (e.g., nutrient cycling, carbon flux, erosion rates). Within marsh or mangrove stands, benthic epifauna and nekton densities were generally similar. These data suggested that tidal transients and resident fauna were not selective in terms of aboveground vegetation structure. However, mangroves generally increased carbon retention in the soil and caused more heterogeneous wrack accumulation patterns, suggesting that there may be differences in the trophic availability of marsh and mangrove carbon.
In experimental plots with fewer mangroves, we observed many wading birds feeding in cleared patches, though we also noted some passerine roosting and shorebird nesting in the mangroves. At a larger spatial scale, citizen science data indicated that the species richness of waders, shorebirds, and passerines was similar between marsh and mangrove areas, though there were differences in composition, especially for passerines. It is likely that birds selected habitat based on vegetation features, and a heterogeneous marsh-mangrove mosaic may facilitate a range of behaviors, including foraging and sheltering. Overall, our results indicated that the transition from marsh to mangrove may not have dramatic effects on marsh epifauna or transient nekton abundance. Bird abundance and behavior appeared to differ between localized mangrove and marsh areas, but patterns of habitat use at this scale may not affect bird densities at the landscape scale.