Multidimensional isotopic niches inform coexistence mechanisms in an Alpine ungulate community

<jats:p>Understanding how species coexist is essential for explaining how biodiversity is maintained and how ecological communities respond to environmental change. In Alpine ecosystems, where strong environmental gradients intersect with increasing anthropogenic pressure, mechanisms of coexistence among sympatric large herbivores remain poorly resolved. Here, we investigated niche segregation among three mountain herbivores — Alpine chamois (&lt;i&gt;Rupicapra rupicapra&lt;/i&gt;), red deer (&lt;i&gt;Cervus elaphus&lt;/i&gt;), and roe deer (&lt;i&gt;Capreolus capreolus&lt;/i&gt;) — in the south-eastern Alps. Using five stable isotope ratios (&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;13&lt;/sup&gt;C, &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;15&lt;/sup&gt;N, &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;34&lt;/sup&gt;S, &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O, &lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt;H) in summer-grown hair, we quantified multidimensional niche hypervolume structure within a Bayesian framework and evaluated patterns of niche overlap and differentiation. Despite substantial spatial overlap, and broad dietary overlap typically observed among these ungulates, the three species exhibited clear isotopic niche segregation, with mean pairwise overlap below 40%. Niche differentiation was primarily structured along gradients of water sourcing (&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;18&lt;/sup&gt;O), diet quality (&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;15&lt;/sup&gt;N), and habitat openness (&lt;i&gt;δ&lt;/i&gt;&lt;sup&gt;13&lt;/sup&gt;C). Specifically, Alpine chamois appeared to derive more water from plants in their diet rather than from drinking, and to consume a higher-quality diet compared to Cervids, whereas red deer relied more heavily on forested habitats for foraging compared to roe deer and chamois. Additional isotopic differences between red deer and roe deer may reflect abiotic variation such as microclimate and topography. Together, these patterns indicate functional differentiation across multiple ecological axes, allowing coexistence despite potential for resource overlap. Our results highlight how multidimensional niche differentiation can structure large herbivore communities in mountain ecosystems and provide a mechanistic framework for anticipating how such assemblages may reorganise under accelerating environmental change.</jats:p>