Multiple threats and conservation opportunities in the forgotten coral rag forests of Zanzibar, Tanzania

Coral rag forests are a dry, and usually dwarfed, forest type scattered along an extensive coastline from Kenya to Mozambique, representing major hubs for biodiversity within the Coastal Forests of Eastern Africa Biodiversity Hotspot and the Swahilian Centre of species endemism. In Zanzibar, coral rag forests, although heavily degraded, are the largest remaining natural terrestrial forest type, still occupying 41% (± 18.2%) of the total land area across the archipelago. Characteristic of this biome, these forests are being rapidly lost to urbanization, tourism, infrastructure, and changes from rotational to permanent agriculture driven by accelerating development and population pressure. Several are being lost within established protected areas that have been protected for decades. Here, we delineate the distribution of coral rag forest on Zanzibar and review the geology, vegetation, fauna, and the cultural values derived from this largely unstudied ecosystem. Using remote sensing, we show that the total area of the remaining intact ‘high forest’ and ‘thicket’ sub-types currently cover 20% of the islands' land area and 44% of their historical range. These should be the focus for renewed conservation and sustainable use. The heavily degraded ‘coral rag agriculture’ has strong potential for forest restoration; it covers 20% of the islands and 46% the historical range of coral rag forest in Zanzibar. We suggest a twin strategy of enhanced conservation of remaining high forest and intermediate thicket within established protected areas, and forest landscape restoration and re-wilding efforts of degraded coral rag agriculture. We finish by setting out a series of measures to retain this threatened forest on the Zanzibar archipelago which can serve as a model of restoration along the African coastline.