Habitat island biogeography of natural edaphic islands

The term habitat island refers to various island systems that are potentially isolated by a contrasting surrounding. To predict biodiversity patterns on these habitat islands, island biogeographic parameters which were developed on true islands are commonly used, e.g. island area and geographic isolation. However, besides these important parameters, other factors arise in habitat island archipelagos that can have a major impact on biodiversity but are less frequently considered. Therefore, we selected quartz islands in western South Africa as an edaphic island model system to test the effect of different biogeographic and landscape ecological parameters on the diversity of the local, unique flora. We investigated the effects of different island-biogeographical (island area & isolation, habitat diversity), landscape ecological (matrix contrast, edge effects) and edaphic factors (soil abiotic parameters) on the taxonomic diversity, habitat specificity as well as on endemism of the local, very unique flora. Results revealed scale-dependent effects of the different parameters: edge effects across the boundaries between the edaphic islands and the matrix were driven by small-scale changes in the soil abiotic variables; island area and habitat diversity best-predicted species richness and the number of habitat-specialized plants, while classical measures of spatial isolation proved to be weak predictors in all models. The matrix contrast index we developed specifically for the island system increased the explained variance of some models but was a weak predictor on its own. We conclude that edaphic islands like the studied quartz islands represent a kind of intermediate island system between true oceanic islands and fragmented habitats. As a result, studies of such island systems would therefore benefit from a separate habitat island biogeography that critically discusses the use of classical metrics and also incorporates new factors developed specifically for habitat islands.

Quartz islands in the Knersvlakte