Specific island biogeographic and landscape features shape plant diversity and habitat specialism on edaphic quartz islands in an arid ocean

Edaphic habitat islands offer unique environmental conditions for plants and often harbour specialized floras, thus having high nature conservation value. Besides edaphic uniqueness, distinct spatial features and landscape filters characterize habitat islands. However, their role as drivers of biodiversity on habitat islands remains unclear. We aim to quantify the extent to which spatial parameters (island shape characteristics, habitat diversity) and landscape filters (insularity parameters, surrounding matrix effects) explain plant diversity on natural edaphic islands in an arid biodiversity hotspot. The quartz islands in our study area (Knersvlakte, South Africa), resemble natural edaphic islands within a semi-arid vegetation matrix, hosting unique, predominantly dwarf and locally endemic succulent plants. In a comprehensive field survey, we sampled plant diversity and richness of 47 quartz islands and analysed their spatial characteristics derived from satellite imagery. Island area and habitat diversity were the most reliable predictors of plant diversity. Including measures of landscape features, (e.g. matrix contrast) improved the predictive power of several models. However, distance-based isolation measures had little explanatory value in relation to the observed variance. These results suggest that the diversity of quartz islands can be effectively described using specific island biogeographic parameters, such as island area or habitat diversity. However, measuring isolation in edaphic island systems may require alternative quantification methods such as incorporating matrix properties. Based on the results of this study, we recommend combining different parameters in a habitat island biogeography to quantify the biodiversity of such natural terrestrial edaphic islands.