The contribution of wetland flora to regional floristic diversity across a wide range of climatic conditions in southern Africa

The assessment of regional diversity of wetlands at a country-wide scale can be challenging for data-poor countries. However, plant species records and datasets can provide insight in the drivers and geographic extent of the possible diversity in wetland flora at a country-wide scale. The species pool concept was applied to wetland plants in South Africa and the kingdoms of Lesotho and ESwaitini, whereby the land surface was subdivided into 35 bioregions, and wetland plants were characterized either as facultative (occurring in wetlands but not restricted to them) and obligate (restricted to the wetland environment). The total number of species and the number of facultative and obligate wetland species was calculated for each bioregion. This led to the development of 35 bioregional wetland floras and the number of species in these floras could be correlated to various climatic parameters and other explanatory variables of the bioregion. The relationships between several factors, connecting wetland plant diversity to the landscape scale, were used to explore the wetland biodiversity of the region. These included the fraction of the species pool consisting of wetland species, the number of wetland vegetation types, the overall wetland surface area and Mean Annual Precipitation (MAP). These relationships were established for both absolute species numbers (using Generalized Linear Models with a Poisson link function) and fractional species numbers (using beta regression). The best model was obtained using beta regression whereby the fraction of wetland species in the species pool was explained by MAP (pseuso-R² = 0.8 for obligate species). For facultative species the correlations were weaker and for absolute values, no simplified model could be obtained, even though it was clear that surface area was not significant, whereas the correlation with MAP was stronger. It is suggested that higher amounts of precipitation lead to a higher volume of water in the landscape, which can be available in the form of groundwater, interflow or surface water. This available water leads to a larger number of wetland habitats which contains steeper environmental gradients and more niches for wetland plants, and this in turn leads to higher wetland species diversity. This shows a direct connection to wetland species diversity and the climatic regime over large areas.