Upper tidal flats are disproportionately important for the conservation of migratory shorebirds

The decline of migratory shorebird populations along the EAAF has been largely attributed to the loss of tidal flats in the Yellow Sea region due to coastal development. It is intriguing, however, that the rate of population decline of many coastal shorebirds is much faster than the rate of tidal flat loss. Can it be the case that the tidal flats are not of equal value to shorebirds, and it is the disproportionate loss of more valuable or more important part of the tidal flats that leads to the precipitous decline of shorebirds in EAAF?

The tidal flats are both spatially and temporally heterogeneous. At the flyway scale, different stopover sites support different sets and numbers of shorebirds, and the Saemangeum project clearly exemplified the detrimental consequences of the loss of key stopover sites. It was unclear, however, whether it is also the case at the local scale.

Shorebirds congregating on the upper tidal flats at Tiaozini in Jiangsu Province, China. Erosion of the tidal flats after the completion of seawall in year 2013 might have greatly changed the landscape and the distribution of foraging shorebirds, a major but poorly understood change that may severely affect shorebird populations using this newly inscribed World Heritage Site © Tong Mu

Focusing on the foraging distribution of 17 shorebird species at two key stopover sites in the Yellow Sea region of China, a recent study published in Proceedings of the Royal Society B shows that different parts of the tidal flats are indeed of different importance to foraging shorebirds: the upper tidal flats, the area that is usually closest to land and highest in elevation, is the most valuable part of the tidal flats for shorebirds. Unfortunately, the upper tidal flats is also the area that is usually the first and most often to be developed.

This study also shows that shorebirds can be largely grouped into three types based on their foraging distributions on the tidal flats, namely, generalists, zone specialists, and tide followers. The 17 species studied show substantial interspecific and site-specific patterns in their foraging distribution. The generalists and zone specialists spend the majority of the foraging time on the upper tidal flats, thus may be more severely affected by coastal development locally.

These findings stress the need for detailed understanding in the foraging and stopover ecology of migratory shorebirds in guiding effective conservation planning at key stopover sites along the EAAF. Not only are species using the same tidal flats in different ways, but the different parts of the same tidal flat differ in their value to the foraging shorebirds. Conservation actions need to go beyond protecting the size of the habitat and look into habitat quality and species-specific habitat requirements at local stopover sites, particularly for the species of high conservation concerns (e.g., spoon-billed sandpiper and spotted greenshanks) at key stopover sites showing signs of habitat degradation (e.g., Tiaozinin in southern Jiangsu, China).

Development of tidal flats is not limited to the Yellow Sea, and similar studies on the spatiotemporal distribution of foraging shorebirds should be conducted elsewhere along the EAAF to determine if the upper tidal flats are of disproportionate importance to these birds elsewhere. Monitoring and modeling of changes in the tidal flats after coastal development are also urgently needed to understand the long term changes in habitat size and quality that may affect shorebird populations in addition to the instantaneous loss of the upper tidal flats.

The study can be accessed at https://royalsocietypublishing.org/doi/10.1098/rspb.2020.0278. A PDF version of the study is also available by contacting the corresponding author.

Full citation:

Mu, T. and Wilcove, D.S., 2020. Upper tidal flats are disproportionately important for the conservation of migratory shorebirds. Proceedings of the Royal Society B, 287(1928), p.20200278.

Article prepared by Tong Mu from Princeton University.



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