Environmental heterogeneity as a driver of species and phylogenetic diversity in coastal salt marshes

• Estuaries with higher environmental heterogeneity tend to have higher species richness. • Species diversity was positively correlated with sediment moisture organic content. • Species and phylogenetic diversity were positively correlated with variation in elevation. • Restoration efforts should focus on enhancing heterogeneity to increase species diversity. Studying the relationship between species diversity, phylogenetic diversity and environmental heterogeneity in coastal salt marshes will provide a more holistic understanding of the structure and functioning of these important habitats. This study aimed to determine how environmental heterogeneity drives species and phylogenetic diversity in coastal salt marshes. In eight estuaries along the South African coast, species composition, sediment and groundwater characteristics were measured. To determine phylogenetic diversity indices of transects and estuaries phylogenetic analysis was conducted. Environmental heterogeneity (EH) for each estuary was determined by Principal Component Analysis (PCA). To determine the influence of EH on macrophyte species richness, species diversity and phylogenetic diversity a generalized linear model (GLM) was applied to identify significant variables. Results indicated a significant positive relationship between EH and species richness ( t = 2.192; p = 0.028). Species richness significantly correlated with mean sediment moisture content ( t = 3.09; r² = 0.2; p = 0.00). Simpson diversity showed a significant negative relationship with sediment electrical conductivity ( t = -2.86; r² = 0.16; p = 0.01). There was a negative correlation between phylogenetic diversity and elevation (MPD: t = -3.73; r² = 0.3; p = 0.00) and sediment redox potential (MNTD: t = -2.05 r² = 0.1; p = 0.05). This suggests that elevation, and the variation of the physicochemical variables, are important predictors of salt marsh biodiversity. However, the greatest species diversity may arise not solely from larger salt marshes but from increased variation in elevation. It is expected that salt marshes along the west coast will experience the greatest loss of species richness and diversity under anticipated climate change because of lower rainfall and drier conditions. Restoration of salt marshes should be focused on the creation of habitat mosaics, where multiple habitat types are created along an elevation gradient.