Diverging responses of taxonomic, functional, and phylogenetic plant diversity to biogeographic parameters in arid habitat islands of southern Africa

Habitat islands conceptually bridge true islands and habitat patches,
adding new perspectives to island biogeographic principles that shape
community assembly processes. Despite growing interest, empirical studies
are scarce, and the drivers of biodiversity in these systems, such as
different aspects of island shape, isolation, and habitat diversity,
remain unclear. Our aim was to assess how biogeographic parameters
influence taxonomic, functional, and phylogenetic plant diversity on
habitat islands using quartz islands in South Africa as our model system.
We conducted floristic surveys on 20 quartz islands in the harsh and arid
summer-rainfall region around Pofadder (Nama–Karoo, South Africa) to
quantify species richness, functional dispersion, and phylogenetic mean
pairwise distance. Via remote sensing, we derived island metrics and
evaluated their effects on the diversity metrics, using multivariate
regression analysis. We fitted negative binomial generalized linear models
for species richness and linear models for functional and phylogenetic
diversity. Island area emerged as the strongest predictor of species
richness, consistent with island biogeographic theory. Connectivity was a
significant predictor for functional dispersion and explained most of the
variation. The mean pairwise distance was significantly
influenced solely by the area of the nearest neighbour island, although
its explanatory power was only moderate. Our findings highlight
that different facets of diversity respond distinctively to biogeographic
parameters. Integrating taxonomic, functional, and phylogenetic
perspectives is therefore essential for a comprehensive understanding of
biodiversity patterns on habitat islands and for providing valuable
insights for conservation.