Global Mangrove Watch: Monthly Alerts of Mangrove Loss for Africa

Abstract

Current mangrove mapping efforts, such as the Global Mangrove Watch (GMW), have focused on providing one-off or annual maps of mangrove forests, while such maps may be most useful for reporting regional, national and sub-national extent of mangrove forests, they may be of more limited use for the day-to-day management of mangroves and for supporting the Global Mangrove Alliance (GMA) goal of halting global mangrove loss. To this end, a prototype change mangrove loss alert system has been developed to identify mangrove losses on a monthly basis. Implemented on the Microsoft Planetary Computer, the Global Mangrove Watch v3.0 mangrove baseline extent map for 2018 was refined and used to define the mangrove extent mask under which potential losses would be identified. The study period was from 2018 to 2022 due to the availability of Sentinel-2 imagery used for the study. The mangrove loss alert system is based on optimised normalised difference vegetation index (NDVI) thresholds used to identify mangrove losses and a temporal scoring system to filter false positives. The mangrove loss alert system was found to have an estimated overall accuracy of 92.1%, with the alert commission and omission estimated to be 10.4% and 20.6%, respectively. Africa was selected for the mangrove loss alert system prototype, where significant losses were identified in the study period, with 90% of the mangrove loss alerts identified in Nigeria, Guinea-Bissau, Madagascar, Mozambique and Guinea. The primary drivers of these losses ranged from economic activities that dominated West Africa and Northern East Africa (mainly agricultural conversion and infrastructure development) to climatic in Southern East Africa (primarily storm frequency and intensity). The production of the monthly mangrove loss alerts for Africa will be continued as part of the wider Global Mangrove Watch project, and the spatial coverage is expected to be expanded to other regions over the coming months and years. The mangrove loss alerts will be published on the Global Mangrove Watch online portal and updated monthly.

The Spatial and Temporal Distribution of Mangrove Forest Cover from 1973 to 2020 in Chwaka Bay and Menai Bay, Zanzibar

“…Like many other parts of the world, the loss of mangroves in Zanzibar is linked to human activities such as timber/pole cutting, firewood collection, charcoal burning, and crop cultivation [39]. This is due to rapid population growth coupled with poverty and a lack of alternative livelihoods [20].…”

“…Global mangrove forest losses exceed those of tropical rain forests and coral reefs; therefore, global mangrove distribution maps, e.g., the World Atlas of Mangroves-1 (WAM-1), the World Atlas of Mangroves-2 (WAM-2), the Global Distribution of Mangroves (GDM), and the Global Mangrove Watch (GMW), were initiated to monitor mangroves [38,39]. The global mangrove maps may be most useful for mapping the national and regional coverage of mangroves [39]. Therefore, they may be of limited use for those managing relatively small/protected areas containing mangrove forests.…”