Species composition shifts across quartz island boundaries in the summer rainfall Nama-Karoo

• Nama-Karoo quartz islands show gradual, broad transition zones towards zonal habitats. • High species turnover indicates environmental filtering in quartz island communities. • Organic carbon, soil texture and quartz cover drive changes in species composition. • Plant traits resemble Succulent Karoo quartz islands, suggesting repeated evolution. Quartz islands of southern Africa represent an ecologically unique habitat. In the winter-rainfall Succulent Karoo biome, quartz islands exhibit sharp boundaries with the surrounding zonal habitats, which contribute to an abrupt species turnover. However, whether quartz islands in the summer-rainfall Nama-Karoo biome show different patterns has not yet been investigated. Therefore, we ask how species composition and environmental gradients change across the boundary from the surrounding habitat to the quartz islands of the Nama-Karoo. We sampled 10 transects placed perpendicularly across the habitat boundaries between quartz islands and zonal habitats. The transects were subdivided into 15 plots of 5 m x 1 m in size. We analysed species richness and community weighted means of plant traits by using generalized additive modelling and beta diversity by using beta regression. Environmental drivers were identified using generalized additive modelling and canonical correspondence analysis. Species richness slightly increased over the boundary between the two habitats. Vegetation cover decreased, but cover of succulents, quartz specialists, and nano-chamaephytes increased constantly from zonal habitats to quartz islands, revealing a gradual transition and no abrupt change across the boundary. Species turnover increased with increasing plot distance along the transect. Species change over the transect was mainly driven by gradients in soil texture, quartz cover and soil organic carbon. Our study showed for the first time that in the Nama-Karoo the habitat boundaries around quartz islands are not as abrupt as in the Succulent Karoo. However, we were able to observe significant changes in environmental gradients and plant traits within 15 m transects, reflecting the ecological distinctiveness of the quartz islands. The potential role of rainfall seasonality on soil patterns are discussed.