Biogeography and Diversity of the Inselberg (Laja) Vegetation of Southern Venezuela

Granitic inselbergs represent xeric islands with a patchy distribution world-wide. Comparative studies on the floristic inventory of these ecosystems give an illuminating glimpse into mechanisms of diversity. The Venezuelan study area comprises the northwestern margin of the Guayana Shield. Here the granitic outcrops, locally known as 'lajas', are grouped along a macro-climatic gradient. In total, 614 species of vascular plants are recorded. Characteristic families and endemic elements are assessed, with the result that eighty-six species are obviously restricted in their occurrence to laja sites of the Guayana region. Evaluation of the distributional ranges of these species reveals that a northern and a southern phytogeographical subunit of the inselberg flora can be distinguished. In the area where the two subunits overlap, a centre of endemism (Atures centre of endemism) can be located. The effects of climate and isolation on floristic diversity are discussed, giving emphasis to the importance of intermediate isolation for florisitic diversification.