Temporal and spatial patterns of common hippopotamus populations in the Okavango Delta, Botswana

Conservation requires reliable estimates of a species' population and their spatial distribution. Knowledge of large-scale habitat use and population trends in common hippopotamus (Hippopotamus amphibius; hereafter hippo) in wetland ecosystems is limited, with no studies of hippo populations in the Okavango Delta (the Delta), Botswana. We examined the drivers of long-term trends and spatial patterns in the Delta's hippo population, using 13 years of aerial surveys (1996-2018) informing on the potential impacts of changes in inflow, flooding patterns and rainfall on hippo populations. We estimated temporal changes in hippo populations and relationships with rainfall and inflow. We also examined how spatially explicit hippo counts related to flood and surface water extent and vegetation class at different scales. The Delta's hippo population has increased, probably due to increasing long-term rainfall and inflow, following a period of severe drought/low flooding. Hippos were positively associated with areas with high variation in flooding and negatively associated with broad-scale surface water extent, indicating hippos avoided permanent swamps and main channels, probably due to water depth, lack of nearby grazing, and thick riparian vegetation. At a fine scale, hippos relied on large lagoons for daytime refuge. The most recent population estimate indicates that Botswana has the third highest hippo population in Africa, reflecting the importance of the Delta. Reductions in inflow and rainfall from climate change and water resource development would threaten these hippo populations by reducing grazing availability, lagoon sizes, and seasonal swamps. Ongoing monitoring of hippos should continue as they represent a good indicator for the entire floodplain ecosystem.