Fine-scale habitat quality drives isotopic niche variation in a didelphid opossum in fragmented semideciduous dry forests of central Brazil

Studies on the effects of habitat fragmentation on animal trophic ecology have traditionally emphasised patch size or habitat loss, while the role of habitat quality has been comparatively overlooked. We investigated how habitat quality – assessed through as fine-scale variation of forest structure and food resource availability within patches – alongside patch attributes and landscape configuration, shapes the isotopic niche of the opossum Gracilinanus agilis in fragmented semideciduous dry forests of the central Brazilian Cerrado. Using a multiscale approach, we investigated the trophic responses of the species to environmental variation across 36 forest patches during two field campaigns conducted in 2018. Small mammals were live-trapped, habitat quality descriptors were quantified, and patch- and landscape-scale metrics (1-km buffer) were measured. Isotopic niche metrics were estimated from δ13C and δ15N values obtained from hair samples. Regression analyses showed that variation in δ¹⁵N range (NR) increased with patch core area, indicating a broader range of trophic levels in larger and better-preserved patches. At the habitat-quality scale, the evenness of niche packing (SDNND) decreased with increasing vine abundance, suggesting that preferential understory use in more degraded forests may increase microhabitat overlap among individuals and leads to a more homogeneous trophic structure in isotopic space, with individuals more evenly packed within a narrower isotopic niche. SDNND also declined with higher termite abundance, indicating increased isotopic similarity among individuals as this resource became more intensively exploited. At the patch scale, SDNND increased with patch core area, suggesting greater trophic differentiation among individuals in larger patches with more pristine vegetation. Overall, trophic responses varied across spatial scales, with patch characteristics and habitat quality exerting stronger effects than landscape-level structure. Our findings highlight the importance of fine-scale environmental conditions in shaping the trophic niche structure and reinforce the need to incorporate habitat quality into fragmentation research.