Plant size differences between quartz islands and their surroundings in a South African desert ecosystem.

Islands constitute excellent models to disentangle the effect of different biotic and abiotic factors generating plant size variation. However, plant size variation in terrestrial habitat islands remains unexplored. This is a meaningful gap as these insular systems are expected to have both similarities (e.g. isolation defined by habitat contrast and permeability against the surrounding matrix) as well as differences (e.g. large herbivores' presence). Here, we address this gap by examining size-related plant trait differences between specialist species of terrestrial habitat islands (quartz islands) and species not restricted to the islands. We collected size-related traits (plant height, leaf length, area and volume) for 67 species belonging to the three most representative families (Aizoaceae, Asteraceae, Crassulaceae) in the Richtersveld desert, South Africa. Species were categorized as quartz specialists (restricted to the islands), generalists (occurring on islands and matrix) and matrix species (found only in the matrix). We ran univariate, bivariate and multivariate analysis to examine whether quartz specialists differ in trait values and coordination compared to generalists and matrix species. Quartz specialists are consistently more variable in their plant size traits, are shorter, yet are characterized by similar leaf size (and succulence) compared to generalists and matrix species. The variation of whole-plant stature is decoupled from that of leaf traits. Our findings suggest that environmental filtering associated with quartz islands' harsh conditions (low resource availability and solar radiation) and related ecological specialization may operate as the main eco-evolutionary driver shaping size-related traits, selecting for compact (dwarf) succulent growth forms.