An investigation of long-distance dispersal based on species native to both Tasmania and New Zealand

Some 200 species of plants are currently recognised as being native to both Tasmania and New Zealand. It is argued that dispersal across the 1500-2000 km Tasman Sea has occurred in all of these species. Almost all (187) are herbs, and constitute over 20% of the herbaceous flora of Tasmania. Common species, non-dioecious species, species with very small seeds, species from aquatic, coastal or wet habitats and possibly species with hooked fruit are all over-represented among the disjunct species of herbs. The incidence of disjunct species also varies significantly among families. In contrast, fleshy-fruited species, or species with plumes or very hairy disseminules are not over-represented among the herbaceous disjunct species. This data is used to model the probability that a species (past or present) with given traits would show a within-species trans-Tasman disjunction, and it is inferred that this can be used to give a crude approximation of the rates of long-distance dispersal for different types of species. The model can be tested using molecular clock methods, and could be made more robust by incorporating equivalent data from other disjunct regions.