Determining the status of ecosystem degradation trends and their implications for ecological integrity in the southern African grassland biome

The value of the southern African grassland biome as a biodiversity hotspot will be amplified by its potential to sequester carbon and contribute towards climate change mitigation. Despite this, several studies have reported alarming degradation and transformation of this biome, threatening its ecosystem functioning and services. Using a whole-biome assessment, we aimed: (1) to ascertain the dynamics in the vegetation cover over 34 years (1986-2020), assess the extent and trends of anthropogenic activities; (2) to assess the intensity of alien plant species (APS) invasions in different parts of the grassland biome; and from this (3) anticipate the conservation implications for this biome and identify and prioritise management needs for vulnerable parts of the biome (e.g., critical biodiversity and water sources). Our results showed that sparse vegetation (i. e., equivalent to indigenous grassland) lost 31.5 % of its cover between 1986 and 2020, while the "moderate vegetation cover" and "dense vegetation cover" (comparable to transformed grassland and associated with bush encroachment) increased by 16.8 % and 24.7 % respectively, indicating a substantial loss of biodiversity and ecosystem services. Overall, 39.2 % (365,629 km2) of the grassland biome has been permanently modified, while 60.8 % of the basic ecosystems remain unchanged. We recorded eleven prominent land cover use classes dominated by agricultural activities accounting for 31.9 % (365,629 km2) of which 27.4 % was cultivated area, and 4.5 % was forest plantation, and human settlement covered 4.2 %. Of the 1332 alien plant species recorded, 17.8 %-36.2 % were invasive. The hotspots for APS covered 19.7 % (374,002 km2) of the biome, were concentrated in one province, and were highly dominated by herbs and woody species (46 % and 38 %, respectively). The abundance of APS increased by 13.64 % since the 1900s. Alien woody species increased from 2.2 % occupancy in 1980-4.0 % in 2020, covering 0.4 % (17,217 Km2) of protected critical biodiversity areas and 9.6 % (102,123 Km2) of important water source areas. Our study suggests that management efforts for woody alien invasions could usefully prioritise water source areas and sensitive biodiversity areas to achieve multiple objectives simultaneously, including carbon sequestration. We conclude that the functioning, biodiversity and services supplied by the southern African grassland biome is at risk that could be addressed by urgent context-specific management action, policy and selective habitat restoration/ rehabilitation interventions. In particular, we recommend that the management efforts for woody alien invasions prioritise water sources and critical biodiversity areas.