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Community structure and functioning is shaped by resource partitioning between cooccurring species. Niche differentiation among sympatric species can be reached through trophic, spatial or temporal segregation to avoid competitive exclusion. Intraspecific segregation in the use of habitats and resources might determine, in turn, a population's niche width and interspecific segregation. The Alboran Sea is the only area in the Mediterranean where common and striped dolphins coexist abundantly. Therefore, these putative competing species provided the opportunity to investigate niche partitioning through spatial modelling and trophic analysis. Density surface modelling and nitrogen and carbon stable isotopes (δ 15 N and δ 13 C) were used to investigate spatial and trophic niche partitioning at inter-and intraspecific levels. The 2 species showed high isotopic overlap. However, we could not rule out the possibility of interspecific trophic segregation, as isotopic similarity does not necessarily mean true ecological or dietary similarity. Among conspecifics, variations in δ 15 N and δ 13 C values with dolphin length pointed to ontogenetic dietary changes in striped dolphins, while sex played only a minor role in δ 13 C values. Spatially, these species tended to segregate their core areas of distribution, with common dolphins being more coastal than striped dolphins, which occupied adjacent, deeper waters. Overall, the main enabler for the coexistence of common and striped dolphins in the Alboran Sea was spatial segregation.

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