Department of Botany and Zoology

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Browsing Department of Botany and Zoology by Subject "Acacia -- Biological invasions"

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    The invasion ecology of Acacia elata (A. Cunn. Ex Benth.) with implications for the management of ornamental wattles
    (Stellenbosch : Stellenbosch University, 2013-12) Donaldson, Jason Ernest; Richardson, David M.; Wilson, John R.; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: This thesis explores how human dictated methods of introduction and species-specific traits interact to define spatial patterns in invasive plant populations using Acacia elata as a model species. I initially asked whether the relatively small invasive extent (when compared to congeners introduced for forestry or dune stabilization) of a species used widely for ornamental purposes (A. elata) is due to low rates of reproduction in South Africa. Results indicate that A. elata has similar traits to other invasive Australia Acacia species: annual seed input into the leaf litter was high (up to 5000 seeds m-2); large seedbanks develop (>20 000 seeds m-2) in established stands; seed germinability is very high (>90%); seeds accumulate mostly in the top soil layers but can infiltrate to depths of 40cm; and seed germination appears to be stimulated by fire. I argue that the current limited distribution of invasive A. elata populations is not attributable to species-specific reproductive traits. Following on from this I addressed the relative importance of species traits, the recipient environment, and introduction dynamics using species distribution models and spread models defined using a range of parameters representative of invasive Australian acacia species. Results indicate that in the early stages of simulated invasions, the number of trees introduced was the most important parameter influencing abundance and extent, while the placement of introduction foci in urban areas associated with ornamental introductions limited the extent and abundance of invasive populations. I suggest that conditions relating to human-mediated introduction events initially mask the influence of intrinsic species traits and help to explain the failure and success of species associated with specific human-shaped pathways. Third, I used scale-area curves to determine how these human-dictated introduction conditions influenced the spatial structure of populations. The fractal dimension (Dij) of A. elata populations indicates plot scale (2.5-25m) spread with high densities that are increasing. Artefacts of introduction history were observed at the regional scale (2.5-25km) - populations are contiguous around introduction foci such as towns and roads. Moreover, fragmentation of A. elata populations at local to landscape scales (0.25-2.5km) is due to a combination of the haphazard regional placement and limited secondary dispersal vectors, both of which can be linked to their introduction history. In summary, this study used a combination of field-based data, modelling techniques and broad-scale sampling methods to assess how intrinsic species traits and introduction conditions mediate invasive spread. I conclude that introduction history can have a long-lasting (100+ years) influence on the spatial structure and distribution of invasive plants, which can mask the influence of individual species traits.