The relative importance of dispersal and the local environment for species richness in two aquatic plant growth forms

Local species richness can be affected by both the dispersal process and by environmental conditions (species sorting process). The evaluation of the relative roles of these two processes contributes not only to further understanding of the mechanisms determining species richness but also to biodiversity conservation. We studied the relative importance of hydrological dispersal and water chemistry for species richness of submerged and floating-leaved macrophytes using 31 sets of interconnected ponds with different numbers of component ponds (defined as connection class). Connection class was slightly more important than, or equally important to, water chemistry in determining species richness of floating-leaved macrophytes. In contrast, submerged macrophyte richness was much more influenced by water chemistry than by connection class, although increasing connection class had some positive effect. Similarly, the occurrence of a particular species of submerged macrophyte was better explained by pond water chemistry than by the occurrence of the same species in the pond immediately upstream. The reverse was true for floating-leaved macrophytes; the presence of a given species was better explained by its presence in the pond immediately upstream than by water chemistry. These results indicated that the relative importance of the two processes that shape the species richness of aquatic plants is a consequence of the growth form of the plants. However, both the dispersal process via hydrologic connection and species sorting by water chemistry play some role in determining the species richness of both floating-leaved and submerged macrophytes.