University of Tasmania
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Biogeographical and ecological predictors of disjunction in the Tasmanian and New Zealand flora

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posted on 2023-05-26, 02:36 authored by Hsu, E
Long distance dispersal, the migration and establishment of propagules across large biogeographical barriers, has played a critical role in shaping the historical and current biogeography of the Southern Hemisphere flora. Vicariance, the physical separation and subsequent isolation and speciation of populations, alone cannot account for the geographic distribution of the plant lineages. The extant vascular flora of Tasmania and New Zealand provides insight into the process of long distance dispersal and its consequences. This thesis investigates the relative abundance, directional dispersal, and long distance dispersal-mediated traits of these species by considering the overall flora of these island masses, and in particular, the 293 species that occur in both landmasses (referred to herein as disjunct species). These disjunct species are likely to be all recent migrants, and therefore they can be used to infer the characteristics and processes involved in dispersal. Analysis of within-species disjunctions showed a significant directional bias for immigration from Tasmania to New Zealand, rather than vice versa. Disjunct species that are common in both areas, defined here as ones that have widespread distributions of populations, were over-represented compared to species that were rare. This observation supports a direct association between relative abundance and frequency of dispersal. Furthermore, disjunct species were more common in their source area than in the sink area, highlighting the effect of source size on the degree of dispersal. However, the observed ecological range of disjunct species in the source area was not greater or less than the observed ecological range of these species in the sink area, although disjunct species were likely to have wider ranges than nondisjunct species. No evidence was found to indicate that ecological release was an influence on establishment due to the under-representation of disjunct species common in the sink area. Herbaceous species were more likely to be disjunct than woody species in New Zealand and Tasmania. High proportions of disjunct species inhabited freshwater aquatic, salt marsh and estuaries, coastal areas, and bogs and wet peats while low proportions were in forests (including open), grasslands, and heaths. Disjunct species were more likely to have specialist features for wind dispersal and external transport on vertebrates, but less likely to have fleshy fruits. Despite these features, small disseminules were over-represented. Dioecy was significantly under-represented in disjunct species. Groups with high frequencies of disjunct species include ferns and the angiosperm families Caryophyllaceae, Chenopodiaceae, Cyperaceae, and Juncaceae. The under-representation of disjunct species that are more common in the sink area than the source implies that there is no evidence that ecological release was an influence on establishment. The within-species disjunctions of Tasmania and New Zealand reinforce the shifting acceptance of long distance dispersal in the Southern Hemisphere and the relevance of relative abundance in evaluating the frequency of dispersal and ecological patterns. The results argue for a comparative evaluation of other island disjunctions.


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Copyright Copyright 2011 the author

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