Sika deer Cervus nippon out of the blue: a cryptic invasion in Italy

Hybridization between native and introduced species is one of the most important reasons underlying the impact on biodiversity exerted by biological invasions and also one of the hardest to be detected. The East-Asian sika deer Cervus nippon has been introduced to several European countries, where it is known to generate fertile offspring with the congeneric native red deer C. elaphus, threatening its local populations with genetic pollution. Most studies on hybrid populations have been carried out in northern and eastern Europe, whereas introductions of this deer and their effects are still poorly known in Southern countries. In this study, we updated the distribution of sika deer in Italy, where the species has been present since at least 25 years ago, but where no formal assessment has ever been conducted. We confirm the occurrence of sika deer genes in formerly pure red-deer populations in central Italy by means of genetic analyses. Further range expansion of invasive sika deer and red × sika deer hybrids is likely to lead to further hybridisations and introgression, with implications for the genetic integrity of native red deer populations.