Distribution of a Non‐Native Amphibian in Britain Explained by a Complex History of Introductions and Translocations

ABSTRACT Aim Success in a biological invasion often depends on whether the non‐native species can disperse from its introduction site and spread across the invaded landscape. The alpine newt, Ichthyosaura alpestris , has achieved a widespread distribution in Britain in the last century, with new records continuously reported. The objective of this study was to utilise population genomic analyses to disentangle the invasion history of I. alpestris in Britain, focusing on whether distribution was attained through natural dispersal or human‐aided translocations. Location England, Wales, and Scotland. Methods We sampled 95 individuals from 11 geographical locations across Britain and used ddRAD‐sequencing to genotype over 20,000 single‐nucleotide polymorphisms. We investigated the invasion history of I. alpestris using population genomic analyses, migration estimates and by identifying historical admixture and population splits. Results Our results demonstrate strong population structure among sites and distinct genetic clusters. This is consistent with multiple, independent introductions or a single introduction followed by translocation and population sub‐structuring due to isolation and drift. By building a maximum likelihood tree and inferring gene flow, we find evidence for the multiple introduction scenario. Population structure analyses combined with genetic diversity analysis also provide evidence for human‐assisted movement of newts across the British landscape, resulting in geographically distant populations being assigned to the same genetic cluster. Based on contemporary gene flow and migration analyses, we demonstrate that introduction followed by long‐distance natural dispersal is unlikely to be responsible for the spread of this invasive species in the UK. Main Conclusions Our findings imply that multiple introductions and human‐mediated translocation play a central role in the spread of alpine newts across the British landscape, allowing them to overcome barriers to natural dispersal and achieve widespread distribution. This lack of natural dispersal and low risk of unaided long‐distance spread suggests that management should focus on localised eradication if needed. Our study also illustrates how genomics can be used in conjunction with historical data to better understand the invasion history of non‐native organisms.