2. Evidence for effects on biodiversity
Birds: A review of data and expert knowledge from across the East Asian-Australasian Flyway showed that use of artificial (i.e. human-created) wetlands is high throughout the region. The study documented records of 83 shorebird species, including all regularly occurring coastal migratory shorebirds, found at 176 artificial sites with eight different land uses (Jackson et al. , 2020). Across five important non-breeding regions of Australia, >50% of the average proportion of the regional population of 39 of 75 species-region combinations used artificial habitats at high tide (Jackson et al. , 2021). Other studies have documented birds’ use of specific types of artificial habitats. For example, in the Inner Gulf of Thailand drained aquaculture ponds are used by roosting and foraging birds, although to a lesser extent than salt ponds (Green et al. , 2015). Recently drained fishponds can be heavily visited by piscivorous birds, like herons (Ardeidae) and Black-faced Spoonbill Platalea minor . In both Hong Kong and Taiwan the majority of foraging by Black- faced Spoonbill Platalea minor is in drained down fish ponds (David Melville, pers. comm.) Another study in the Yellow Sea in China found that the banks of aquaculture ponds were used as roosting sites, with shorebirds preferring long banks with little vegetation cover (He et al. , 2016). Salt ponds are used extensively by shorebirds for both roosting and foraging in the Gulf of Thailand (Sripanomyom et al. , 2011), Australia (Jackson et al. , 2020) and China (Lei et al., 2018). The high usage of salt ponds by birds has been linked to high densities of invertebrates (Masero et al. , 2000; Rocha et al. , 2017). Rocha et al. (2017) found that salt ponds in Portugal that were drained for artisanal fishing led to a rapid increase in the number of foraging birds, suggested to be due to the high densities of polychaete worms. Some shorebirds use salt ponds more than others. A few studies have shown that small birds and short-legged birds use saltpans for roosting and foraging more so than larger birds (Masero et al. , 2000; Green et al. , 2015; Lei et al. , 2021).
3. Factors that can affect outcomes
Distance to feeding area: Shorebirds are more likely to roost in areas close to their feeding habitat (Zharikov & Milton, 2009), as this expends less energy travelling. One study of Nordmann’s Greenshank in man -made ponds in the Gulf of Thailand found roosts to be around 1 km away from foraging sites (Yu et al. , 2019). A literature review of the importance of artificial roosts for shorebirds globally showed that across 12 studies shorebirds were documented roosting from <1 km to >20 km away depending on the species (with smaller species typically moving smaller distances; Jackson, 2017). However, distances of 2 – 9 km were more typical in studies where the mean distance across multiple individuals was presented (Jackson, 2017, see Table 1). Shorebirds also appear to travel longer distances to roost sites at nighttime, presumably because of perceived increased predation risk at night (Rogers, 2003). Water depth: Some evidence suggests that water depth has the strongest influence on whether shorebirds are present (Bancroft et al. , 2002; Bolduc & Afton, 2004; Jackson et al. , 2019). A study of 94 sites containing man-made ponds found that roosts were, on average, 6 cm deep (Yu et al. , 2019).
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