Estimating the carrying capacity of tidal flats for migratory shorebirds: A comparison between the Changjiang and Western Scheldt estuaries from 1990 to 2020.

Estuarine tidal flats are vital stopover and wintering sites for migratory shorebirds. The tidal flat carrying capacity (CCA) of birds depends mainly on the energy supply of the habitats and the energy demand of the birds. Comparing CCA across different sites allows for the optimization of natural resource management and biodiversity conservation, particularly in estuaries undergoing transitions due to global change. We developed a pixel-based model to estimate the CCA of the Changjiang (China) and Western Scheldt (the Netherlands) estuarine tidal flats, using Landsat 5 and 8 satellite data from 1990 to 2020 to derive the available food density and feeding time. We then combined a sensitivity analysis to identify potential drivers of the spatial and temporal CCA variability between the estuaries. The results revealed that the average CCA in the Western Scheldt estuary was double that of the Changjiang estuary from 1990 to 2020; however, the Changjiang estuary exhibited greater temporal fluctuations over the past 30 years. The model-estimated average CCA aligned well with locally observed shorebird data, showing stronger correlations in inter-estuarine comparisons than within individual estuaries. The dominant drivers included both human activities and natural succession, with their influence varying over time and space. We emphasize the need for large-scale assessments that integrate human impact analysis to develop effective shorebird protection strategies amidst future environmental risk.