Dr. Contreras-Quintana, Sergio

1) Although key biotic interactions can shape species distributions across large geographic areas, the specific roles and spatial scales of these interactions are not well understood. Correlational models, often used in ecological studies, assume niche equivalence among populations of a species. However, this assumption overlooks how different factors can affect defined ranges and the reliability of predictions. 2) In this study, we evaluate ecological niche equivalence among populations of the facultative amphidromous fish Galaxias maculatus, which includes both migratory amphidromous and freshwater resident populations. We identify sources of niche variation across hierarchical levels of biological organization (individuals, populations, species) and operational niche definitions (existing fundamental niche, realized niche). 3) Our findings reveal structural niche differences based on population movement ranges and the operational niche definition used. Notably, no overlap exists in realized niches between amphidromous and freshwater resident populations, with isotopic niche breadth driving this differentiation. This niche difference resulted in models that poorly predicted distributions of populations with different life-history strategies, indicating low niche equivalence between these groups when considering biotic factors. 4) Our results suggest that differing life-history strategies are key drivers of G. maculatus distribution. Habitat selectivity based on freshwater recruitment conditions and loss of diadromy through local adaptation may also contribute to niche divergence. We emphasize the importance of assessing niche breadth changes across hierarchical levels to understand species' distributional patterns better, particularly in species with complex life-history strategies.

1. Ecological processes that are behind distributions of species that inhabit isolated localities, complex disjunct distributions, remain poorly understood. Traditionally, vicariance and dispersion have been proposed as explanatory mechanisms that drive such distributions. However, to date, our understanding of the ecological processes driving evolution of ecological niches associated with disjunct distributions remains rudimentary. 2. Here, we propose a framework to deconstruct drivers of such distribution using World's most widespread freshwater fish Galaxias maculatus as a model and integrating marine and freshwater environments where its life cycle may occur. Specifically, we assessed ecological and historical factors (Gondwanan vicariance, marine dispersion) and potential dispersion (niche‐tracking) that explain its distribution in the Southern Hemisphere. 3. Estimated distribution was consistent with previously reported distribution and mainly driven by temperature and topography in freshwater environments and by primary productivity and nitrate in marine environments. Niche dynamics of G. maculatus provided evidence of synergy between vicariance and marine dispersion as explanatory mechanisms of its disjunct distribution, suggesting that its ecological niche was conserved since approximately 30 Ma ago. 4. This integrated assessment of ecological niche in marine and freshwater environments serves as a generic framework that may be applied to understand processes underpinning complex distributions of diadromous species.