Non‐native ants that coexist in abundance with many native ant species are the most widespread on an oceanic island

Non‐native ant establishment is one of the greatest threats to native biodiversity on oceanic islands. Interactions with native ant species may provide beneficial ecological resistance that prevents non‐native ants from spreading across island landscapes. We linked the fine‐scale co‐occurrence of non‐native and native ants with the island‐scale establishment of non‐native ant species across the Caribbean island of Montserrat. We hypothesized that non‐native ants that co‐occurred in high abundance with many native ant species at fine scales would occur at a greater number of sites across the island. Fifty‐seven ant species were collected using the Winkler extraction method at seven forest‐edge transects across human‐accessible parts of Montserrat, representing independent habitat gradients across which co‐occurrence was quantified. Of the collected species, 14 species were identified as non‐native and included the globally important invasive ants Pheidole megacephala (Fabricius 1793), Solenopsis invicta (Buren 1972) and Wasmannia auropunctata (Roger, 1863). Among non‐native species, there was no significant correlation between the number of transects occupied and co‐occurrence at fine scales within transects. The same was true among different native species. However, there was a strong positive correlation between fine‐scale co‐occurrence with native species and the number of transects occupied by non‐native species. There was also a strong negative correlation between co‐occurrence with non‐native species and the number of transects occupied by native species. While based solely on spatial associations in occurrence, our findings suggest that non‐native ant establishment may scale up from interspecific interactions within habitats. Non‐native ant species that can co‐occur at a fine scale with many native species may infiltrate native ant communities, spread further and potentially reduce island‐scale occurrence of native ants.