Avian nest-boxes enhance ant assemblage diversity across altitudinal and habitat gradients in Mediterranean woodlands

ABSTRACT 1. Tree cavities create unique microclimatic conditions and, when occupied by animals, can accumulate organic materials that attract ants. However, their role in structuring ant assemblages across ecological contexts and their potential function as buffers against the effects of global change, remains poorly understood. 2. Here, we used nest-boxes as surrogates for natural cavities to test whether nest-box presence and bird use influence ant assemblages in Mediterranean woodlands. Using a paired design, we compared ant assemblage structure between trees with and without nest-boxes along a ∼400 m gradient of altitude and habitat transformation measured as a decrease in natural vegetation cover. We also assessed whether birds affect nest-box microclimate and its altitudinal buffering. 3. Nest-boxes partially buffered external environmental variation relative to tree trunks, with those containing bird nestlings being warmer and drier. Compared to trunks, nest-boxes maintained more stable temperatures throughout the day and were generally more humid, particularly during early hours, although these differences diminished at higher altitudes and during warmer periods. Trees with nest-boxes supported ant assemblages that were taxonomically and functionally richer, and arboreal ant species occurred more frequently in trees with nest-boxes than in control trees. Phylogenetic diversity showed metric-dependent responses: neither Faith’s PD nor mean pairwise distance (MPD) varied with nest-box presence; however, MPD increased with altitude in more transformed landscapes of lower vegetation cover and decreased in less transformed ones. However, we found no significant interaction between altitude, habitat transformation, and nest-box presence for any assemblage feature, indicating that nest-boxes did not function as ants’ microrefugia. Neither nest-box occupancy by birds nor nest material type influenced ant assemblages. 4. Our findings suggest that nest-boxes primarily act as additional nesting sites that increase ant diversity in Mediterranean woodlands, rather than microrefugia shaping assemblages through microclimatic buffering. Nest-box supplementation may therefore influence tree-associated arthropod communities by modifying habitat availability.