A biogeographical analysis of the freshwater plants of Australasia

Abstract

Patterns in the distribution of Australasian species of freshwater aquatic plants were sought, to determine whether vicariance, distance dispersal, local speciation, or a mixture of these could best explain the distribution. The distribution was recorded from 10 regions of Australasia that include sizeable areas of wetland: Papua New Guinea, Cape York Peninsula, south-east Queensland, eastern New South Wales, Victoria, Tasmania, New Zealand, northern Northern Territory, the Kimberley, and south-west Western Australia. Matrices of 553 species by 10 regions, 139 genera by 10 regions, and 56 families by 10 regions were analysed using hierarchical fusion, nearest neighbour and ordination techniques. The results indicate that there are two distinct elements in the aquatic flora, tropical and temperate. The diffuse boundary between these two climatic zones could be interpreted as a barrier in the sense used in definitions of vicariance. There is little effective spread between tropical and temperate areas but, within each of these climatic zones, the species are mobile and many spread reasonably readily between regions, provided suitable habitats and dispersal opportunities are available. Where geographic barriers to distance dispersal have been great then these may become as important as the climatic barrier. This is demonstrated, at least in part, by the differences between some of the generic and species dendrograms. Local speciation (not shown by our PATN analyses because of the endemic species being ignored in them) has been important where some primarily aquatic genera have proliferated when conditions have been suitable. Local speciation has occurred in cosmopolitan aquatic genera that have presumably arrived in regions via long distance dispersal. The significance of bird migration and dispersal patterns are discussed. The aquatic flora of the monsoon tropics has evolved mainly from long distance dispersal but with significant local speciation in some genera such as Nymphoides, Utricularia, Nymphaea and Vallisneria. The pattern of distribution was compared with those recorded from other ecologically defined groups such as the Australian arid and alpine floras.