Degree Name

Doctor of Philosophy


School of Biological Sciences


Invasive species can exert significant deleterious effects on native individuals, populations and communities. Freshwater ecosystems are especially vulnerable to invasion, due to their close association with anthropogenic activity and limited biogeographic connectivity. Understanding the behavioural mechanisms which underlie predator-prey and competitive interactions between invasive and native species is critical when predicting the impacts of invasive species on freshwater habitats. However, due to the predominance of indirect experimental methods, these mechanisms are seldom identified. Furthermore, behavioural mechanisms, and subsequently, invasive-native species interactions may be mediated by environmental and individual-level context. Here I used both direct and indirect methods to determine whether and how predator-prey and competitive interactions between the invasive eastern mosquitofish, Gambusia holbrooki and native Australian freshwater fauna are mediated by environmental variables. Throughout, I also assessed how individual-level morphological variables, specifically size and sex, modulate behavioural interactions depending on environmental context. In the first section of this thesis, I investigated whether the consumptive and non-consumptive effects of predator-prey interactions between G. holbrooki and the glass shrimp, Paratya australiensis, are mediated by the invader’s density and the diel cycle. In the next section, I determined how the density of G. holbrooki influenced its competitive interactions with juvenile Australian bass, Macquaria novemaculeata, in the laboratory and field. Finally, I tested whether the intensity of interference competition between G. holbrooki and M. novemaculeata was dependent on a combination of temperature and salinity stress. While I found a positive relationship between G. holbrooki density and the frequency of its direct interactions with P. australiensis and M. novemaculeata, this did not translate into reduced survivorship or growth of the native species. Temperature and salinity interacted in a non-additive manner on inter-specific aggression, highlighting the importance of considering multiple stressor effects. The responses of both P. australiensis and M. novemaculeata to predation and interference competition were modulated by interactions between body size and environmental context. In contrast, sex-specific aggression in G. holbrooki was shown to be dependent on its density. These results demonstrate the importance of considering the complex interplay between environmental and individual-level variables when predicting the strength and outcome of interactions between G. holbrooki, native prey and competitors.