Forecasting biological invasion risks: a screening of non-native mammals in a shifting climate

Biological invasions are a primary cause of global biodiversity loss and ecosystem homogenization, with terrestrial mammals historically having received poor attention despite their significant ecological impacts. Climate change further exacerbates this threat by altering species distributions and invasion dynamics. Italy, with its diverse landscapes and central Mediterranean location, is particularly vulnerable to the establishment and spread of non-native species, being in the middle of important trade routes. This study screened 18 non-native terrestrial species under present and future climate scenarios for Italy, to address the urgent need for preventive risk assessments. The list included 13 established (extant) species and five horizon species, i.e. not yet present but with potential for future introduction or arrival in the risk assessment area. Six native species were also screened as non-invasive controls to compute a calibrated risk threshold to distinguish between medium-risk and high-risk species. Under current climate conditions, 15 (83.3%) species were classified as high risk, increasing to 16 (88.9%) under future climate scenarios. Eastern grey squirrel Sciurus carolinensis was ranked as very high risk under both scenarios, while coypu Myocastor coypus and raccoon Procyon lotor reached this ranking under future conditions, confirming their significant invasive potential. This study highlights the amplifying effect of climate change on invasion risk, even for species initially considered marginal threats like small Indian mongoose Urva auropunctata, and underscores the critical need for proactive biosecurity measures and early detection programmes to mitigate ecological and economic impacts.