Assessing Plant Species Turnover in Grasslands of South Africa

Beta diversity represents the degree of variation in species composition between plant communities, and is thus an important indicator of the spatial distribution of biodiversity within regions. Patterns of beta diversity are shaped by deterministic processes relating to environmental conditions and species interactions, or by stochastic processes that include speciation, extinction, and dispersal limitation. Knowledge of the mechanisms that generate and maintain beta diversity is important and can inform management strategies for the conservation of biodiversity. The study aimed to assess the influence of environmental gradients on beta diversity in the grassland Biome of South Africa by comparing plant species composition between selected protected areas within the biome. Similarity in species composition between the protected areas was compared with the Jaccard index (βJ). In addition, constrained (CCA) and unconstrained (DCA) ordination, variation partitioning, and linear regression were used to analyse species turnover along environmental gradients. Jaccard similarity values were low, indicating high species turnover. There was an average of only 9% species composition similarity between the protected areas. Composition similarity decreased significantly with geographical distance between protected areas, but it increased significantly with mean annual temperature and assumed a hump-shaped pattern with mean annual rainfall. In general, geographic and climatic factors each explained approximately 20% of the variation in species composition. The patterns of beta diversity between the study locations suggest an interplay of both stochastic and deterministic processes in shaping community structure and composition, with environmental filtering as possibly one of the major drivers.