Decadal changes in intertidal biodiversity of the Benguela upwelling region

The southern Benguela upwelling region, one of the most productive coastal ecosystems globally, supports diverse intertidal communities and is strongly affected by strong oceanographic and climatic forces. However, this region has been facing various anthropogenic pressures, primarily caused by human activities in the past few decades, including climate change, commercial and subsistence harvesting, coastal development, and mining. These pressures have resulted in noticeable ecological changes, particularly in intertidal communities and changes in land use. Replication of historical surveys has allowed scientists to track and document these shifts, which helps them better understand the trends and shifts in biological communities. This thesis examines changes in coastal ecosystems over the past three decades through two complementary approaches, (1) through a fine- scale field resurvey of rocky intertidal shore communities and (2) through landscape-scale remote sensing data and spatial analysis of land cover changes along the west coast of South Africa.Intertidal resurveys conducted at 13 sites originally studied in 2001–2003 revealed significant community shifts by 2020–2021. Multivariate analyses indicated strong site-level differences, yet consistent temporal changes. Alien functional groups played a key role: Alien species played a key role: <i>Balanus glandula</i> showed a positive association with the periwinkle <i>Afrolittorina knysnaensis</i>, while <i>Mytilus galloprovincialis</i> declined at wave-exposed sites but increased in sheltered habitats. The arrival of the alien mussels <i>Semimytilus patagonicus</i> marked one of the most striking changes, alongside declines in native limpets and mussels, reduced grazing intensity increased algal cover, pointing to possible trophic restructuring.Land use land cover analyses (1990-2020) highlighted substantial habitat alteration along the west coast, particularly linked to agricultural expansions, urban growth, mining and extractive industries. Three case studies, Verlorenvlei, the Alexcor mining area, and the Tormin Mine were selected a priori to represent distinct and dominant forms of anthropogenic landscape change in the region, including wetland contraction and both historical and recent large-scale coastal mining. The case studies showed a concerning water loss at Verlorenvlei (a Ramsar-listed wetland) and major landscape transformation around the Alexcor and Tormin mines, including vegetation loss, wetland degradation and altered sediment flows. These findings demonstrate the extent and spatial pattern of anthropogenic landscape transformation along the west coast over the past three decades.The combination of these two chapters offers a wide-ranging, multidisciplinary assessment of decadal changes in anthropogenic pressures and their impacts on the ecology of southern Benguela coastal ecosystems. By linking biodiversity patterns in rocky intertidal zones with spatial land cover dynamics onshore, the thesis demonstrates how localised disturbances, such as the harvesting of limpets altering grazer population structure, or mining-driven land cover change reshaping coastal landscapes, contribute to broader socio-ecological change across the land-sea interface. The study’s findings highlight the importance of sustained long-term ecological monitoring and the protection of critical habitats, such as estuarine coastal wetlands. There is an urgent need to implement integrated coastal zone management in South Africa, given the vulnerability of coastal ecosystems to both local and regional disturbances. The research highlights the necessity for repeat surveys to accurately track ecological change, especially under the ongoing anthropogenic impacts and climate change pressure.