Invasive Mammals Outweigh Soil Condition in Limiting Quercus ilex Recruitment: Implications for Forest Restoration in Mediterranean Insular Context

Ecosystem restoration on Mediterranean islands is often hindered by the residual effects of past land use and invasive species. Decades of holm oak forest exploitation, the establishment of secondary pine plantations, and the introduction of invasive mammals have altered habitat configurations. Consequently, converting these conifer stands to promote the recovery of native Quercus ilex L. communities has become a conservation priority. This study investigates the regeneration constraints of Q. ilex in Mediterranean insular environments, focusing on the inhibitory role of conifer-derived litter and seed predation by invasive rodents and lagomorphs. We integrated an ex situ experiment (384 acorns) testing germination under varying local pine-forest soil and commercial substrate conditions, with an in situ field experiment (300 acorns) across five areas, comparing three treatments: closed cages (exclusion of all mammals), open cages (exclusion of lagomorphs), and unfenced controls. Results indicate that, while ex situ, local pine-forest soil significantly favoured germination over the commercial mixture, predation represents the main obstacle in situ, outweighing any soil-mediated effects. Seedling emergence was substantially reduced by early predation and, to a lesser extent, by litter presence. These findings highlight the necessity of integrated management strategies in insular ecosystems.