flat ecosystem, especially impacting its role as shorebird feeding ground (Erftemeijer & Lewis, 2000; Choi et al. , 2022; Beeston et al. , 2023).
Control: Options for management intervention include the controlled removal of accreting seedlings and saplings from key shorebird feeding grounds (see: Mai Po marshes case study below). Other more intrusive control methodologies are sometimes deployed, such as the use of herbicides in combination with mowing to control invasive Spartina (Jackson et al. , 2021). Periodic flooding with seawater, mowing or herbicides are some of the ways used to control colonisation and expansion of unwanted vegetation in ponds and other artificial habitats used by shorebirds in Australia (Erftemeijer, 2019). Management should be timed to avoid or minimise impacts on non-target species, for example avoiding bird migratory or breeding seasons. Other important considerations include selecting access points to removal areas that minimise or avoid trampling adjacent habitat types (Lundquist et al. , 2017). In New Zealand, many methods of removing mangroves have been used with varying success (see review by Lundquist et al. , 2017). These include manually pulling small seedlings, removal using chainsaws and axes at above ground level, and mechanical removal. Manual removal of seedlings is cheapest and can be effective in containing local spread, whilst causing the least adverse environmental impacts, but must be conducted regularly to avoid re- establishment. Mechanical operations using tractors and diggers to remove vegetation and some below-ground root material from tidal flats are more expensive and rarely result in a return of tidal flats, whilst often having detrimental effects on the local ecosystem and amenity (sight and smell). Removal by hand or light equipment (pulling, shovels, chainsaws) may be preferable to use of heavy machinery in areas of archaeological interest (Rauzon & Drigot 2002). Herbicide application by drone (as used in China for Spartina control) is a method that could also potentially be trialled to control mangrove expansion on mudflats if an appropriate herbicide can be identified (David Melville, pers. comm.). Some herbicides have been implicated in devastating mangrove dieback (see Duke et al. , 2005), so caution is warranted. Other vegetation (e.g. low-growing herbaceous plants, shrubs or algae) can be controlled, at least temporarily, by covering it with sediment. This will reset succession. Deposition of shell and gravel debris has been tried as a means to help control thick vegetation at important shorebird sites in the USA (Plauny, 2000). Temporary reduction of vegetation cover can also be achieved by ploughing. In Tokyo Port Wild Bird Park (Tokyo-ko Yachoen), the tidal flats are ploughed before and after the shorebird season (SSS 2023). In Hawaii, Amphibious Assault Vehicles have been used to control invasive pickleweed Batis maritima growing on mudflats (Rauzon & Drigot, 2002). ‘Checkerboard patterns’ or ‘donuts’ of ploughed muddy sediment with islands of vegetation were created, which the authors suggested were attractive to Hawaiian stilts Himantopus mexicanus knudseni . Also note that vegetation removal can affect physical processes, which in turn can affect local biodiversity. For example, mangrove removal can exacerbate estuarine infilling through landscape-scale bio-morphodynamic feedbacks, enhancing estuary-scale sediment trapping due to altered sedimentation patterns (Xie et al. , 2023). In the Waikareao Estuary, New Zealand, a gradual seaward erosion of salt marsh was observed following mechanical removal of the mangrove buffer (Lundquist et al. , 2017).
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