Understanding the distribution of species in different regions of the planet has been a central question in biology. In recent decades, ecological niche theories have driven the use of correlative models to describe the ecological spaces occupied by populations by analyzing relationships between known locations and environmental data. However, such correlative models often assume ecological equivalence among populations, which limits their ability to take into account diverse sources of variation inherent in population dynamics. The main objective of this dissertation was to evaluate niche equivalence among populations of Galaxias maculatus, an amphidromous fish with wide distribution and trophic resource use variation. We investigated the sources of niche variation at individual, population and species level, considering different niche definitions (existing fundamental, realized and occupied). Furthermore, we assessed the impact of loss of equivalence in existing fundamental, realized and occupied niche ecological spaces. In terms of the existing fundamental niche, the biogeographic assessment provided evidence of synergy between vicariance and marine dispersal as explanatory mechanisms for the disjunct distribution of G. maculatus in America and Oceania. This suggest the conservation of the existing fundamental niche and consequently its population equivalence since approximately 30 million years ago. The evaluation of the trophic niche revealed first source of niche variation among populations. Specifically, the variation of isotopic niches (food resource uses) among populations was significantly associated with the characteristics of riverine habitats they occupied within each river system and the selection of specific life-history strategies. We detected loss of equivalence at realized niche level among populations with different life-history strategies. The selectivity of specific habitats that offer freshwater recruitment conditions, trophic specialisation and the loss of amphidromy due to local adaptation processes may have driven to the complex patterns of ecological differentiation detected in realized niche of G.s maculatus. This study highlights the need to evaluate niche variation at different hierarchical levels and to consider the sources of variation. Furthermore, it points out that loss of population equivalence may affect niche estimates, especially in species with complex life history strategies.