Improved detection of an alien invasive species through environmental DNA barcoding: the example of the American bullfrog Lithobates catesbeianus

1. Alien invasive species (AIS) are one of the major causes of biodiversity loss and global homogenization. Once an AIS becomes established, costs of control can be extremely high and complete eradication is not always achieved. The ability to detect a species at a low density greatly improves the success of eradication and decreases both the costs of control and the impact on ecosystems. 2. In this study, we compare the sensitivity of traditional field methods, based on auditory and visual encounter surveys, with an environmental DNA (eDNA) survey for the detection of the American bullfrog Rana catesbeiana = Lithobates catesbeianus, which is invasive in south-western France. 3. We demonstrate that the eDNA method is valuable for species detection and surpasses traditional amphibian survey methods in terms of sensitivity and sampling effort. The bullfrog was detected in 38 sites using the molecular method, compared with seven sites using the diurnal and nocturnal surveys, suggesting that traditional field surveys have strongly underestimated the distribution of the American bullfrog. 4. Synthesis and applications. The environmental DNA approach permits the early detection of alien invasive species (AIS), at very low densities and at any life stage, which is particularly important for the detection of rare and/or secretive aquatic species. This method can also be used to confirm the sensitivity of control operations and to better identify the distributions of vulnerable species, making this a very relevant tool for species inventory and management.