Habitat island biogeography of natural edaphic islands

Abstract

Habitat islands – terrestrial isolates embedded in a contrasting environment (matrix) – share several features with true islands which are surrounded by water. However, there is empirical evidence that apparent differences between true and habitat islands limit the applicability of the theory of island biogeography to the latter. Edaphic islands are a sub-type of habitat islands, which are characterized and differentiated to the matrix by contrasting soil abiotic parameters. On the one hand, they are similar to true islands because they are of natural origin. On the other hand, they show some characteristics similar to fragmented ecosystems which have been created by anthropogenic influences, such as edge effects with the adjacent matrix habitats. Therefore, it is debated which theoretical model best predicts the biodiversity of such habitat islands: island biogeography theory or concepts developed specifically for habitat fragments.

As our edaphic island model system, we chose the quartz islands of the Knersvlakte nature reserve in western South Africa, which are embedded in a zonal, semi-arid matrix of the Succulent Karoo biome. We investigated the effects of different island-biogeographical (island area, spatial isolation, habitat diversity), landscape ecological (matrix contrast, edge effects, habitat amount) and edaphic factors (soil chemical and physical parameters) on the taxonomic alpha & beta 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 and beta diversity 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 also 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 habitat islands that were formed by fragmentation process. As a result, some of the classical island biogeographic parameters such as island area or habitat diversity can be well used as predictors for the biodiversity of these islands. On the other hand, especially when quantifying the isolation of such systems, other landscape ecological parameters such as the surrounding matrix or edge effects along the island boundaries must also be included. 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.