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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.
The importance of density in the regulation of animal populations is well established, but the mechanisms by which it operates are still equivocal. More specifically, the extent to which density influences individual fitness remains uncertain, and the variability in responses to density across gradients of environmental conditions, and between distinct populations, species, taxonomic classes, and experimental designs has received limited empirical investigation. Using a combination of field experiments with salmonids and meta-analyses at a broader scale, my thesis investigates the relationship between density, somatic growth, and survival, and the extent by which these relationships can be related to biological and methodological predictors. In chapter 1, the mechanisms of density dependence are investigated by manipulating the density (range: 0.3 - 7 fish/m^2) of young-of-the-year brook trout (Salvelinus fontinalis) in three genetically distinct populations (θST = 0.13-0.30) during three consecutive summers in sections of streams in Cape Race, Newfoundland. I found that populations exhibited population-specific patterns of density dependence that were consistent across years but different across populations, which were partially related to environment conditions. In chapter 2, the mechanisms of competition that cause density dependence in these populations were investigated. To do so, I quantified the consumption and depletion of invertebrate prey communities across brook trout densities in the same experiment as chapter 1. Our results demonstrated that strong density dependence can occur without prey depletion or reductions in consumption, suggesting that alternative mechanisms can be important. In chapter 3, a meta-analysis was conducted to quantify the relative importance of biological and methodological predictors on the patterns of density dependence in salmonids. This meta-analysis demonstrated that methodological biases (experimental design, density gradient) were better predictors of the shape and strength of density dependence across salmonids than biological predictors (food abundance, predators, habitat, species). However, salmonids differ from other animals in several key ways (e.g. territoriality, life history, habitat, etc.), and whether relationships derived from salmonids can be applied to other animals is uncertain. Therefore, in chapter 4, I conducted a similar meta-analysis at a broader scale across all animals, to quantify the prevalence of the same biological and methodological predictors on density dependence, and to quantify potential differences across taxonomic classes. Patterns of density dependence across animals varied according to both biological (taxonomic groups, food abundance, age) and methodological predictors (density gradient). However, these relationships were different than those present in salmonids, suggesting that importance variation occurs at multiple taxonomic levels. Overall, my thesis demonstrates that the patterns of density dependence can vary according to multiple factors simultaneously (environment, populations, taxonomic classes, methodology). These findings have important implications for the management of populations and our understanding of density dependence. More specifically, they demonstrate that the outcome of density dependence is highly context-dependent, and that care should be exercised both for research and the management of endangered populations.