Plant diversity on inselbergs along the aridity gradient of the central Namib Desert

Abstract

Inselbergs are isolated, rocky outcrops that rise above the surrounding plain landscape. They are globally recognised as important biodiversity hotspots, and support a variety of plant communities because of their ability to create microclimatic conditions and stable microhabitats, often with higher moisture availability than the surrounding matrix in arid or semi-arid environments. Studies on plant diversity and the ecological factors that influence diversity on inselbergs have largely focused on tropical regions, with comparatively few studies conducted in arid environments, where inselberg refugia may contribute considerably to overall diversity. In particular, limited knowledge exists regarding the influence of large-scale aridity gradients on plant diversity patterns of inselbergs in the central Namib Desert. This study aimed to document and compare plant diversity patterns on inselbergs of varying sizes and different geological substrates (marble, schist, and granite) along an east-west transect that represented an aridity gradient in the central Namib Desert. Transect-based field surveys were conducted at 25 sites to assess plant diversity patterns. Vascular plant species including trees, shrubs, and forbs, were recorded on inselbergs and the surrounding matrix. Data were analysed using univariate (diversity indices, one-way ANOVA and Kruskal-Wallis ANOVA) and multivariate techniques to evaluate compositional differences, and Similarity Percentage (SIMPER) and Indicator Species Analysis (IndVal%) to identify species contributing most to group dissimilarities and habitat associations. Species richness and composition differed significantly between inselbergs and the surrounding matrix. Inselbergs supported a significantly higher species richness per plot compared to the surrounding matrix. Topography influenced species richness and composition only between inselbergs and the matrix; however, no significant differences were detected among topographical positions on the inselbergs themselves. Geology influenced species richness between the inselbergs and matrix, but no significant differences were observerd among the geological types (marble, schist and granite), although species composition was significantly affected by geology. Euphorbia virosa and Pogonospermum cleomoides were strongly associated with schist inselbergs, whereas Ceropegia pedicellata, Euphorbia giessii and Arthraerua leubnitziae were associated with marble inselbergs. Slope aspect had no significant effect on species richness or composition. Contrary to island biogeography predictions, species richness showed no relationship with inselberg size, precipitation (aridity gradient proxy) or spatial isolation, but species composition was significantly influenced by precipitation and isolation, while inselberg size had little effect on species composition. An important finding was that species richness remained invariant along the aridity gradient, whereas species composition shifted significantly with precipitation. Environmental variables explained 7.7% of the total variation in species composition (total inertia = 12.22%), suggesting that other variables not explored in this study also shape the species assemblages on inselbergs in the central Namib Desert and require further investigation. These findings demonstrate that, in hyper-arid environments, inselbergs function as compositional refugia, rather than following classical island biogeography predictions, with species richness decoupled from spatial variables and community assembly primarily structured by environmental filtering. The results highlight their importance as ecological refugia and conservation value as biodiversity hotspots in the central Namib Desert.