Objective: remove or contain vegetation to maintain open intertidal flats
Definitions
● Intertidal zone = area between high and low tide.
1. Description
Most migratory shorebird species require open areas of tidal flat habitat where they can forage and maintain unobstructed sight-lines to allow early detection of predators (Erftemeijer, 2023). The expansion of mangrove and salt marsh vegetation into shorebird feeding habitats due to sea level rise and increased sedimentation is a problem in estuaries such as in Hong Kong, Taiwan, New Zealand and eastern Australia (Straw and Saintilian, 2006; Jackson et al ., 2021; Choi et al. , 2022). Planting of mangrove on open tidal flats (that were not mangrove before) should in general be avoided as this would be substituting one habitat for another, thereby losing the specific value provided by tidal flats (Erftemeijer & Lewis, 2000; Choi et al. , 2022; Beeston et al. , 2023). Although some algae may be naturally present on tidal flats, prolific colonisation by opportunistic algae, such as Agarophyton (Besterman et al. , 2020) , Ulva (Zhang et al. , 2019) and Lyngby (Estrella et al. , 2011) resulting from eutrophication can significantly reduce benthic diversity and attractiveness of tidal flats to some shorebird species (Estrella et al ., 2011; Besterman et al ., 2020). The same applies to colonisation by the invasive seagrass Zostera japonica (Durance, 2002). However, most native seagrasses (as well as seagrass wrack) seem to enrich benthic fauna in intertidal flats and enhance their attractiveness to feeding shorebirds (see: Unsworth and Butterworth, 2021). Removal of such vegetation may be desirable (a) where the tidal flat is a particularly important habitat for other species, such as shorebirds, and/or (b) the vegetation is not native. There are separate guidance documents on removal of invasive cordgrasses Spartina spp. (see Cutts et al., 2024a-c).
2. Evidence for effects on biodiversity
Birds: In the Danshuei River estuary, Taiwan, wintering shorebirds roosted in open mudflats created by removing mangroves. The expansion of mudflat habitat in the estuary increased the richness of wintering shorebirds (Huang et al ., 2012). In Hawaii, the Hawaiian Stilt Himantopus mexicanus knudseni numbers increased once vegetation was managed to restore bare mud patches (Rauzon & Drigot, 2002). Invasive pickleweed Batis maritima was ploughed and mangroves were physically removed. Invertebrates: Despite concerns prior to removal, adjacent shellfish beds appeared unaffected by mechanical mangrove removal in the Waikareao Estuary, New Zealand (Lundquist et al ., 2012). Studies in Mangawhai Harbour, New Zealand (Alfaro, 2010) and Siangshan Wetland, Taiwan (Chen et al ., 2018) reported increases in the abundance, richness
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