Office for Environmental Programs - Theses
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ItemNiche similarity assessment for understanding sea rockets replacement in the Australian coast( 2012)Sea Rockets (Cakile: Brasssicaseae) are plant species specialized for inhabiting coastal fringes and are well known for being successful invaders worldwide. A characteristic patter of invasion in Western North America and New Zealand, has been observed in two species of this genus. Cakile edentula (Bigel.) Hook, native from eastern North America invades and colonizes a coastal area, and is then replaced by Cakile maritima Scop., native from the Atlantic coast of Europe and coast of the Mediterranean Sea. This pattern of invasion and replacement has been repeated in coastal areas of West, South, and East Australian, coast. Currently, few sites in Tasmania are the only places in which these two species coexist. A greater breeding performance of Cakile maritima is one explanation for this replacement pattern In this research, the niche of each species is determined, through ecological niche modelling with MaxEnt, and contrasted to assess the importance of environmental conditions in Cakile edentula's replacement Species' niches were determined through comparison of habitat suitability of each species in the Australian coast. Ecological niche models were developed using data and environmental variables of Cakiles' native range and extrapolated onto Australian records of both species. To compare Cakile edentula's and Cakile maritima's realized niche differences it was necessary to test the ability of each species to predict the distribution of the other. Modelling results showed there are niche differences between both species. However, contrary to what was expected from replacement dynamics, models predicted greater habitat suitability for Cakile edentula than Cakile maritima in the South and East Australian coast. This result, in addition to low model performance and unexpected environmental factors driving C. maritima's distribution, could indicate that model assumptions of equilibrium and unbiased distribution of species records are not being fulfilled. In addition, there are other factors such as the choice of environmental variables, niche similarity methods and biological knowledge of Cakile that may affect niche comparison. Therefore, modelling limitations together with potential solutions are discussed, reinforcing the importance of taking into account the assumptions of species equilibrium and unbiased records, as well as the use of relevant environmental variables.
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ItemModelling wind and seed dispersal patterns to identify high threat willow population in Bogong High Plains( 2013)The invasion of Salix cinerea (grey sallow willow) is one of the major threats to Sphagnum bogs on the BHP. Sphagnum bogs is a threatened ecological community protected under the EPNC Act. The genus Salix is a Weed of National Significance (WONS) and S. cinerea being one of the few seed producing willows in Australia is highly invasive. Identifying the high threat locations of these willows is hence a top priority of the managers. The Salix cinerea seeds are dispersed mainly by wind but the inability to estimate this dispersal distance of these seeds makes it very hard to decide which areas of the established Salix cinerea population around the BHP pose high threat to the wetland ecosystems. Recent research has revealed that high threat seed source populations can be identified by improving our knowledge of the dispersal capability of the propogule. The literature suggests that the long distance dispersal of the seeds is mainly governed by the weather and topographical parameters of the region such as wind uplift, elevation and horizontal winds near ground. Thus, it was essential to identify the driving factors behind the dispersal of Salix cinerea on the BHP. In this project we have characterised the wind and seed dispersal patterns over the BHP region using a meso scale model (TAPM) by simulating the weather conditions from the synoptic data. As topographical variation is significant in the region of BHP, TAPM was used to model the weather patterns in the region at a smallest grid resolution of 500 m. The weather data was further analysed to study the effect of topography on the wind patterns. These weather simulations were further used to simulate various seed dispersal events all over the region with a certain set of predetermined parameters. We combined field measurements of seed production and information from literature to parameterise a seed dispersal regime which was used to run the above said seed simulations in the model to predict the seed dispersal patterns. The simulations were run for the month of November as the Salix cinerea have been known to seed during this month. Being a mechanistic model, TAPM's simulations were analysed for individual seed sources with respect to individual parameters to study a comprehensive pattern of seed dispersal in the region. This helped in identifying the key driving factors of the dispersal mechanism. It was found that weather patterns do affect the seed dispersal mechanism to a great extent and the dispersal kernel is almost always anisotropic as the prevailing winds drive the seeds along a specific direction. The dispersal pattern was found to be highly skewed with a long tail suggesting that most of the seeds were being deposited very close to the source and very few seeds were being carried to long distances on rare occasions.