Abstract

Predicting species responses to climate change is an increasingly important objective in ecological research and natural resource management. However, heterogeneity in demography, life history, and habitat characteristics across multiple spatial scales can generate substantial diversity in population responses, complicating species-level assessments. Therefore, for widely distributed species consisting of many fragmented populations, understanding the mechanisms that underlie population variation can improve predictions of climate impacts across the species range. Using salmonid fishes as a model system, my thesis investigates variation in population responses to climate change at global, regional, and local scales. First, through a global meta-analysis of 156 studies of 23 species, I demonstrated that population responses to temperature and precipitation exhibit significant spatial, temporal, and biological patterns that broadly align with predictions based on salmonid thermal limits. Importantly, I showed that salmonid populations at low latitudes and elevations tend to be most negatively impacted by rising temperatures. Subsequently, I analyzed mark-recapture and stream temperature data collected during field surveys in Cape Race (Newfoundland, Canada) since 2010 to characterize local-scale variation in demography, climate impacts, and thermal regimes among eleven populations of brook trout (Salvelinus fontinalis) separated by <5 km. I showed that variation in recruitment, growth, and demographic relationships combined to generate diverse population dynamics that stabilized brook trout abundance across Cape Race, and that thermal regimes driven by groundwater inputs contributed to population diversity. Finally, using population-specific demographic and life history data, I built eco-genetic models that simulated responses to future climate warming across Cape Race brook trout populations, which emphasized the role of life history evolution and thermal habitat variation in determining population persistence. Together, my thesis shows that large-scale gradients in latitude and elevation structure salmonid responses to climate change, but substantial fine-scale variation is embedded within these trends due to heterogeneity in habitat characteristics, human impacts, and eco-evolutionary dynamics experienced by populations. Similar research frameworks that employ diverse methodologies and integrate data across scales will be crucial for understanding the complex impacts of climate change on salmonids and other freshwater fish populations, and should inform the conservation of a wide range of species.