Summary

Road networks are expanding globally, altering ecosystems and facilitating the spread of invasive species. We investigated how road proximity and the invasive Argentine ant (Linepithema humile) influence native ant occurrence in a Mediterranean oak forest (Montado) ecosystem, a High Nature Value farmland. Ants were sampled by pitfall trapping across three distances from roads (0, 50, and 100 m) alongside the collection of local environmental measurements. We collected 3,287 ants (32 species), with L. humile exhibiting an incidence of 68.6%. A two-step analytical framework evaluated these species patterns. First, using univariate models, we found that the managed roadside verge (0 m) significantly altered local abiotic conditions, causing a localized spike in soil pH, an increase in understory vegetation height and plant richness, and a severe deficit in topsoil leaf litter accumulation compared to the interior forest (50 and 100 m distance from road). Second, species-specific piecewise structural equation models (pSEMs) demonstrated that these road-mediated micro-habitat modifications and the presence of L. humile may act as simultaneous, overlapping filters structuring some native ant species occurrence. The invader operated as a biotic filter, influencing the occurrence patterns of five of the eight analysed native focal species (Aphaenogaster gibbosa, Crematogaster scutellaris, Plagiolepis schmitzii, Tapinoma nigerrimum, and Tetramorium semilaeve). Our findings show that roads not only modify habitat conditions but may also mediate the impact of invasive species on native biodiversity.