Patterns and drivers of forb diversity in South African grasslands

Abstract

Biodiversity forms the basis of ecosystem functioning, which forms the foundation of ecosystem services. Quantifying diversity patterns and its drivers is needed to effectively conserve and maintain ecosystems. Diversity is scale-dependent and can be measured as richness at a local scale (alpha-diversity), compositional dissimilarity among sites (beta-diversity) and the accumulation of these two components as diversity across a landscape (gamma-diversity). The effect of the spatial scale of sampling on alpha- and beta-diversity patterns in the same highly diverse ecosystem has rarely been assessed. South African grasslands are both diverse and understudied, providing an ideal location to assess diversity patterns, its drivers, and how spatial scale impacts these. Despite being a grassland, the diversity of this biome is ascribed to herbaceous forb species, even though the majority of the biomass is comprised of grasses. The first aim of this dissertation was to assess patterns of forb alpha- and beta-diversity at species-, genus-, and family-level and the drivers thereof. The second aim was to assess the effect of spatial scale on the species-level diversity patterns and drivers. Forty-two grassland sites were sampled and the species, genus, and family alpha- and beta-diversity, calculated. A total of 487 species, belonging to 146 genera and 49 families were recorded. Beta-diversity was also examined at each taxonomic level via the use of the Jaccard distance index with the dissimilarity at species, genus and family level being 89.52 ± 5.93 %, 79.14 ± 8.82 % and 65.54 ± 9.85% respectively. Spatial grain had a pronounced effect on the non-parametric interpolation of gamma-diversity. Large grain alpha-diversity significantly increased with slope and certain soil and geology types and decreased with an increase in the degree of northness. Intermediate grain alpha-diversity was also significantly higher on certain soil types and textures. Small grain alpha-diversity significantly increased with longer fire return intervals and decreased with higher grazing pressure. The mean beta-diversity at large, intermediate and small grains were 89.52 ± 5.93 %, 94.42 ± 5.12 % and 97.34 ± 5.49 % respectively. Turnover was the most important component of beta-diversity at all spatial grains. Geographic distance and the difference in the degree of northness between sites significantly increased beta-diversity across all spatial grains. In addition to this, the presence of the vertisol soil type and the difference in the days since the last fire increased beta-diversity at large, and intermediate grains respectively. Little variation in beta-diversity was explained at a small grain. The high levels of diversity of the South African grassland biome across all taxonomic levels was demonstrated by this study. Grassland sites were not only very species rich but demonstrated high compositional turnover, which indicated that unique suites of species can be found in different regions. The pronounced effect of spatial scale on the patterns and drivers of diversity became apparent. Therefore, taking spatial scale and environmental gradients into account when making decisions with regards to systematic conservation planning or environmental impact assessments is critical, especially in highly diverse systems, such as the grassland biome. This will ensure the effective conservation of these systems and the associated ecosystem services.