Conceptual and applied approaches to marine invasions

The accelerating rise in global trade and travel means that our world is more interconnected than ever before. This trend could severely impact species and ecosystems globally, as it increases opportunities for species to invade regions beyond their natural range. In this thesis, I combine ecological theory and data synthesis with empirical field-studies to tackle the questions of what makes some communities more easily invaded than others, and how can both natural and anthropogenic control interventions affect the persistence and impacts of invasive species. I first evaluate the relationship between native species diversity and invasibility, or the vulnerability of a community to invasion. Using a meta-analytic approach, I show that the conflicting patterns between diversity and invasibility that are often observed in the literature are likely due to not only differences in spatial scales between studies but also to differences in the metrics researchers use to measure invader success. I then use the invasion of Caribbean coral reefs by the predatory Indo-Pacific lionfish as a model system to test natural and anthropogenic means of controlling the invader. Using a combination of fisheries-derived sampling of native grouper predators and a field experiment conducted across a gradient of grouper abundance, I examine the ability of native grouper predators to mitigate the negative effects of lionfish predation in the Bahamas. I reveal little evidence for direct predation by groupers on lionfish, but show that fear of native groupers alone by lionfish is sufficient to evoke behavioural changes in lionfish that could potentially reduce their impact on native prey. Finally, I use a long-term field experiment to investigate the ecological effectiveness of infrequent culling (i.e., the physical removal of lionfish from reefs by divers). I demonstrate that infrequent culling can reduce lionfish abundance, but is insufficient to halt the decline in native prey fish biomass. Moreover, I show that large-scale natural disturbances, like hurricanes, and density-dependent movement by lionfish from neighbouring reefs can undermine culling efforts. Overall, my thesis reveals that the development of standardized metrics is key to generate a holistic understanding of invasion dynamics, and that both natural and anthropogenic control over invaders is unlikely to stymy biological invasions at the scale currently observed for Indo-Pacific lionfish in the Caribbean.