Sowing vegetation islets enhance plant diversity and habitat heterogeneity in Mediterranean long-established road verges.

Halting biodiversity loss requires innovative conservation approaches, and road verges offer opportunities for biodiversity-friendly measures. Although native seed mixes are increasingly used in post-disturbance revegetation, long-established road verges outside urban areas are often overlooked for proactive conservation. To promote species-rich and heterogeneous vegetation, we established vegetation islets by sowing a specifically designed seed mix of native herbaceous species along long-established road verges in a semi-arid Mediterranean agricultural landscape across two experimental blocks with different sowing years. Over four years, we conducted an integrated assessment of seed mix performance and plant diversity, composition, and structure, comparing vegetation islets and control plots and exploring the influence of local edaphoclimatic conditions. The establishment of vegetation islets had the strongest observed effect on plant composition and diversity, locally adding sown and spontaneous species absent from controls, and increasing species richness and compositional heterogeneity. Although seed mix persistence and relative cover declined over time, composition remained distinct from controls, shifting from annual to perennial dominance. Vegetation islets supported denser, shorter vegetation, enhancing local structural heterogeneity without compromising road safety. Higher rainfall, lower temperatures, and lower soil nutrient levels were associated with higher seed mix persistence and species richness. Experimental block affected only species composition and seed mix relative cover. An appropriate bespoke native seed mix, applied to stable long-established plant communities, helped support reliable conservation outcomes under semi-arid Mediterranean conditions. Vegetation islets may offer a resource-efficient approach to promote plant diversity and habitat heterogeneity in long-established road verges, highlighting their potential for proactive conservation.