Distinguishing the Effects of Area and Habitat Type on Island Plant Species Richness by Separating Floristic Elements and Substrate Types and Controlling for Island Isolation

Abstract

An archipelago of sixty-one small islands on bare hypersaline mudflats in tropical Australia can be classified according to substrate composition into twenty-three shell, nineteen silt and nineteen mixed shell-silt islands. Their overall flora can be divided into three floristic elements characteristic of different source areas with different substrate types. Thirty-four of the islands are equidistant from source areas, providing a control on the effects of island isolation. The relative effects of area and habitat type on species richness were distinguished by examining species-area relations for different substrate sets and floristic elements independently as well as jointly. For this particular archipelago, island area is the primary determinant of total species richness. As expected, island isolation has negligible effect. Island elevation was found to be a better predictor of plant species richness than substrate composition, probably through its effect on the soil salinity profile. The relation of species richness to island area and elevation differs substantially between the different floristic elements. Overall, the approach appears valuable and worth further testing.