AbstractRubber bush (Calotropis procera) was recognised during industry and expert weed prioritisation processes as an important economic and environmental weed in northern Australia. It establishes at high densities hindering pastoral management and reducing range productivity. It is estimated to infest 1.8-3. 7 million hectares of northern Australia, however, the drivers of its spread are not well known. This thesis documents various aspects of its reproductive biology and ecology, seed ecology, competitiveness and distribution in Australia; and how these attributes contribute to its invasiveness. A combination of greenhouse, field, laboratory, observational and computer simulation studies, covering the competitive ability, reproductive biology and ecology, seed ecology and species habitat modeling were performed at multiple locations in the Northern Territory and Queensland over three and a half years. This work addressed the question whether rubber bush invasions are a symptom of rangeland management practices or a consequence of natural biological interactions with native species. The results demonstrated that rubber bush is not a strong competitor of native grasses and its invasion is dependent on anthropogenic disturbance. The spread of rubber bush is facilitated by a self-compatible breeding system and an ability to recruit new pollinators during range expansion. Additionally, it exerts considerable propagule pressure through prolific wind-dispersed and highly germinable seeds in Australia compared to native environments. The seeds are quiescent under unfavourable temperature conditions but germinate rapidly when conditions improve. Species habitat modeling showed that it has not saturated the current suitable area, and its suitable area will expand under future climate scenarios. These abilities combine with ruderality to make rubber bush a high risk weed in northern Australia. Nevertheless, because feasibility of control is low, targeted control and protection of priority areas is recommended. Given the important role of disturbance in the spread of rubber bush, careful management of disturbance regimes on rangelands is necessary to limit the spread of rubber bush across northern Australia. For greater effectiveness, it is recommended that policy frameworks regarding the management of rubber bush be harmonised across affected Australian States and Territories.
Note: Please note that articles in this thesis have been removed due to copyright restrictions.
|Date of Award
|Sean Bellairs (Supervisor)