Quantifying the extent and rates of change in wetland ecosystem functional groups in the Maputaland Coastal Plain of South Africa

Abstract

Despite global concerns highlighting the threats to wetlands, monitoring and quantifying changes in palustrine wetland ecosystem extent remains inadequate. The feasibility of mapping the extent and rates of change of wetland Ecosystem Functional Groups (EFGs) in the Maputaland Coastal Plain, South Africa, using Earth Observation (EO) was evaluated. Seven wetland EFGs were mapped, including two estuarine (Coastal saltmarshes, and Intertidal forests and shrublands (mangroves)) and five freshwater EFGs (Lacustrine wetlands and palustrine wetlands: Large macrophytes, Permanent marshes, Seasonal marshes, and Subtropical-temperate forested wetlands). Changes in their extent were quantified across seven epochs across a 32-year period (1990-2022), including three above-average rainfall years (2000, 2006, and 2022), and four years that corresponded with the South African National Land Cover datasets (SANLCs: 1990, 2014, 2018, and 2020). Landsat images between 1990 and 2014 and a combination of Sentinel-1 and -2 images between 2018 and 2022 were modelled with a Random Forest classifier using EFG reference spectra informed by fieldwork. The classifications achieved overall accuracies between 78% and 87%, with user accuracies of the EFGs ≥ 73% for all years. Over the last 32 years, 53% of the extent of wetland EFGs remained unchanged, whereas 35% experienced interclass transformation and 8% were converted to anthropogenic pressures (5% speckle ignored). Four of the wetland EFGs showed an annual decline of 1% to 3%. Projections indicate that, under current conditions, four EFGs could face total collapse by 2050, with Intertidal forests and shrublands at the highest risk. The findings highlight the need for enhanced EO-based monitoring and protective measures to preserve wetland biodiversity and its ecosystem services.