Environmental DNA for invasive rat surveillance and biosecurity on islands

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 greater 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 neighbouring 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.