Effet de la biodiversité et des perturbations/stress sur le fonctionnement des communautés intertidales (Directeur : Mathieu Cusson)

Habitat-forming species (HFS), such as macrophytes and mussel beds, often ameliorate harsh or extreme conditions for other organisms and are usually vital components of rocky shores providing essential ecosystem functions. Ecosystems are structured by multiple HFS and their interplay with abiotic variables. However, most of the existing knowledge on how HFS can affect associated communities in the marine ecosystem comes from studies focused on single dominants HFS or under single environmental stressors or gradients of disturbance. There are many gaps in our knowledge of the role of co-occurring HSF. There is still a need for experimental and observational studies in rocky intertidal systems that consider the strength and direction of the interaction between HFS (facilitation cascade) along multiple stress gradients. Additionally, manipulative experiments that provide insights into mechanistic approaches of the facilitating functions of HFS and their likelihood to maximize ecosystem functions are needed. My Ph.D. project focus on the understanding of how co-occurring HSF may enhance diversity and functionality, the role they play on associated communities along environmental stress gradients and the underlying mechanisms of facilitation. Observational and experimental fieldwork in the intertidal habitat of the St. Lawrence marine estuary will be carried out at different levels of wave exposure and tidal elevation, considering different treatments with combinations of macroalgae and mussel. Taxonomic indicators (Species richness, abundance, and evenness), functional diversity and taxonomic distinctness will be estimated to compare responses to environmental gradients. Additionally, historical data will be analyzed to understand how functional diversity changes across different environmental gradients of stress. Among expected results, we think that the canopy effect will be higher at intermediate levels of stress due to stress amelioration by the HFS. We also expect that the canopy effect will be lower on wave-exposed sites compared to sheltered ones, and the net effect of algae canopy will be neutral or negative at low levels in the intertidal due to algae whiplash effects. More interestingly, we expect that there will be a positive interaction of the two HSF increasing species richness, abundance and functional diversity across the elevation gradient when compared to the effect of single HFS as proposed by Angelini and Silliman (2014) in the foundation Species–Biodiversity (FSB) model. A more informed understanding of the role of co-occurring HFS on the structure and functioning of coastal communities and the underlying mechanisms will be valuable for the management and preservation of the integrity of natural resources and ecosystem services under a changing climate.