Combining Multispectral and Radar Imagery with Machine Learning Techniques to Map Intertidal Habitats for Migratory Shorebirds

Abstract

Migratory shorebirds are notable consumers of benthic invertebrates on intertidal sediments. The distribution and abundance of shorebirds will strongly depend on their prey and on landscape and sediment features such as mud and surface water content, topography, and the presence of ecosystem engineers. An understanding of shorebird distribution and ecology thus requires knowledge of the various habitat types which may be distinguished in intertidal areas. Here, we combine Sentinel-1 and Sentinel-2 imagery and a digital elevation model (DEM), using machine learning techniques to map intertidal habitat types of importance to migratory shorebirds and their benthic prey. We do this on the third most important non-breeding area for migratory shorebirds in the East Atlantic Flyway, in the Bijagós Archipelago in West Africa. Using pixel-level random forests, we successfully mapped rocks, shell beds, and macroalgae and distinguished between areas of bare sediment and areas occupied by fiddler crabs, an ecosystem engineer that promotes significant bioturbation on intertidal flats. We also classified two sediment types (sandy and mixed) within the bare sediment and fiddler crab areas, according to their mud content. The overall classification accuracy was 82%, and the Kappa Coefficient was 73%. The most important predictors were elevation, the Sentinel-2-derived water and moisture indexes, and Sentinel-1 VH band. The association of Sentinel-2 with Sentinel-1 and a DEM produced the best results compared to the models without these variables. This map provides an overall picture of the composition of the intertidal habitats in a site of international importance for migratory shorebirds. Most of the intertidal flats of the Bijagós Archipelago are covered by bare sandy sediments (59%), and ca. 22% is occupied by fiddler crabs. This likely has significant implications for the spatial arrangement of the shorebird and benthic invertebrate communities due to the ecosystem engineering by the fiddler crabs, which promotes two vastly different intertidal species assemblages. This large-scale mapping provides an important product for the future monitoring of this high biodiversity area, particularly for ecological research related to the distribution and feeding ecology of the shorebirds and their prey. Such information is key from a conservation and management perspective. By delivering a successful and comprehensive mapping workflow, we contribute to the filling of the current knowledge gap on the application of remote sensing and machine learning techniques within intertidal areas, which are among the most challenging environments to map using remote sensing techniques.

Multi-Sensor Remote Sensing of Intertidal Flat Habitats for Migratory Shorebird Conservation

“…Though not concerned with bird habitat per se, Zhang et al [32] employed multi-temporal feature extraction, machine learning classification using GEE, and morphological post-processing to map intertidal vs. supratidal barren vs. vegetated flats in China's eastern coastal zone. Henriques et al [57] combined Sentinel-1 SAR and Sentinel-2 multispectral imagery and a digital elevation model (DEM), along with random forest machine learning techniques, to map intertidal habitat types of importance to migratory shorebirds and their benthic prey at a stopover location along the east Atlantic flyway in West Africa. Mapped habitat types included rocks, shell beds, and macroalgae and distinguished between areas of sandy and mixed bare sediment and areas occupied by fiddler crabs.…”

Spatial and seasonal variation in macrozoobenthic density, biomass and community composition in a major tropical intertidal area, the Bijagós Archipelago, West-Africa

“…The Bijagós Archipelago is a group of 88 islands and islets off the coast of Guinea-Bissau, in West Africa, encompassing ca. 450 km 2 of intertidal flats [ 48 , 49 ]. Its topographical characteristics promotes large tidal amplitudes reaching up to 4.5 m in spring tides [ 50 ].…”

“…699 km 2 ) [ 36 , 51 ] and the intertidal flats are mainly composed by bare sediments, with some areas covered by large macroalgal mats (ca. 12% of the intertidal flats) and few rocky reefs (4% of the intertidal flats) [ 35 , 36 , 49 , 52 ]. The vast adjacent mangrove forests, the continental runoff during the rainy season, and the upwelling from the northern coast [ 36 ], contribute to biological productivity of this system in ways that remain to be discovered (but see [ 44 , 45 ]).…”

Trophic ecology of a migratory shorebird community at a globally important non-breeding site: combining DNA metabarcoding and conventional techniques

“…The high consumption of fiddler crabs recorded in our study is mostly likely explained by their high availability in the Bijagós Archipelago. Indeed, approximately 25% of the intertidal areas of the archipelago (corresponding approximately to 113 km 2 ) is occupied by dense patches of fiddler crabs (Henriques et al 2022). The distribution and availability of fiddler crabs is expected to be even larger since small individuals also occur outside typical patches (Paulino et al 2021).…”