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.