A BIOGEOGRAPHICAL ANALYSIS OF THE FRESH-WATER PLANTS OF AUSTRALASIA

Patterns in the distribution of Australasian species of freshwater aqu atic plants were sought, to determine whether vicariance, distance dis persal, local speciation, or a mixture of these could best explain the distribution. The distribution was recorded from 10 regions of Austra lasia that include sizeable areas of wetland: Papua New Guinea, Cape Y ork Peninsula, south-east Queensland, eastern New South Wales, Victori a, Tasmania, New Zealand, northern Northern Territory, the Kimberley, and south-west Western Australia. Matrices of 553 species by 10 region s, 139 genera by 10 regions, and 56 families by 10 regions were analys ed using hierarchical fusion, nearest neighbour and ordination techniq ues. The results indicate that there are two distinct elements in the aquatic flora, tropical and temperate. The diffuse boundary between th ese two climatic zones could be interpreted as a barrier in the sense used in definitions of vicariance. There is little effective spread be tween tropical and temperate areas but, within each of these climatic zones, the species are mobile and many spread reasonably readily betwe en 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 o ur PATN analyses because of the endemic species being ignored in them) has been important where some primarily aquatic genera have prolifera ted 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 d ispersal patterns are discussed. The aquatic flora of the monsoon trop ics has evolved mainly from long distance dispersal but with significa nt local speciation in some genera such as Nymphoides, Utricularia, Ny mphaea and Vallisneria. The pattern of distribution was compared with those recorded from other ecologically defined groups such as the Aust ralian arid and alpine floras.