PhD Oral Exam - Alix Brusseau, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Freshwater ecosystems are particularly vulnerable to anthropogenic change, as they are increasingly exposed to co-occurring environmental stressors. These stressors, such as warming, turbidity, and acidification, may disrupt the availability of chemical and/or visual information through which prey detect risk and predators locate prey. While much research has focused on the effects of single stressors on prey behaviour, fewer studies have examined how combinations of stressors, especially those impacting different sensory modalities, affect both prey and predator behaviours. My thesis investigates how multiple environmental stressors influence predator-prey interactions in freshwater fish, focusing on the behavioural responses of Trinidadian guppies (Poecilia reticulata) and Jack Dempsey cichlids (Rocio octofasciata). Using a combination of laboratory and field experiments, I test the hypothesis that combined stressors reduce an individual’s ability to assess risk or locate prey, thus affecting both predators and prey. I first examine the effects of paired stressors on prey and predator behaviour individually, then on both simultaneously, and finally assess the impacts of anthropogenic disturbances in wild streams. I show that paired stressors reduce antipredator behaviours while increasing predatory behaviours, potentially putting prey more at risk than predators in a changing environment. Specifically, the pairing of warming and acidification has more severe behavioural consequences for prey than the pairing of warming and turbidity, potentially due to sensory compensation. Finally, I show that anthropogenic uses of rivers increase uncertainty among prey, modifying their antipredator behaviours. Overall, my research highlights how multiple environmental stressors can asymmetrically affect predators and prey, with potentially cascading consequences for ecological communities. My results also demonstrate that the impacts of stressors are modulated by prey sex and the presence of a moving predator, underscoring the importance of context-dependent responses. These findings contribute to our understanding of how information flow is disrupted in stressed environments. It can also help build a framework for future studies on multiple stressors, by highlighting the importance of using ecologically relevant experimental designs that incorporate both sexes, both predator and prey species, and both laboratory and field approaches when studying multiple stressor effects.