Historical changes in anthropogenic pressures, distribution and population structure of mangrove forests at a distributional range limit

Understanding how mangroves respond to natural and anthropogenic pressures at global distributional range limits informs protection and restoration strategies. Mangroves along the South African coastline are at one of the most southerly global distribution limits. These endangered ecosystems have been studied for more than 25 years to determine their vulnerability and responses to global climate change and the impacts of natural and anthropogenic pressures. This study assessed the drivers of change in mangrove area cover, species composition and population structure between 2011 and 2021 from field surveys and manual GIS digitizing of 17 estuaries. There was a small increase in mangrove cover over this 10-year period by 3 ha to a total of 274 ha attributed to natural regeneration along tidal sand banks and into areas previously covered by salt marsh. Bruguiera gymnorrhiza was the only mangrove tree species found in all the estuaries, Avicennia marina occurred in 71% and Rhizophora mucronata in 47% of all estuaries. Anthropogenic pressures have persisted since 2011 resulting in localized mangrove degradation indicated by a decrease in the number of seedlings and saplings and increase in canopy gaps. Major anthropogenic pressures included trampling, livestock browsing, and wood harvesting that reduced mangrove cover and caused shifts in population structure. These results provide input to the National Biodiversity Assessment and are relevant to the implementation of the Global Biodiversity Framework informing site specific restoration strategies such as the exclusion of livestock browsing to ensure healthy mangrove populations. The research also informs global studies on range limit populations and their resilience. The study recommended that adaptive management and monitoring frameworks are used to track mangrove changes.