Environmental <scp>DNA</scp> for invasive rat surveillance and biosecurity on islands

Abstract Managing invasive species is a key element of ecological restoration yet assessing invasion status and determining outcomes of management operations remain logistical and financial challenges, particularly on remote islands. Environmental DNA (eDNA) metabarcoding offers a promising, emerging opportunity for reliable, rapid, and cost‐efficient invasive species monitoring. Here, we tested eDNA as a tool for invasive species monitoring on islands. We collected aquatic eDNA samples from beach and lake sites on three atoll islands with different invasion statuses before and after a rat eradication operation. Two dozen eDNA samples can confirm with ≥95% probability the presence of invasive rats prior to any management intervention. Furthermore, our novel eDNA metabarcoding approach distinguished the presence of two closely related rat species ( Rattus rattus and R. exulans ) between neighboring islands. However, following the eradication operation, neither targeted eDNA nor trail camera monitoring could detect the presence of known surviving animals. Aquatic eDNA monitoring using beach and/or lake sampling offers a competitive alternative in invasive species monitoring to conventional survey tools. Particularly on remote, uninhabited, or difficult to access islands, the possibility to detect terrestrial invasive rats from coastal waters offers a substantial advantage.