Quantifying the Sensitivity of Targeted eDNA Surveys to Improve Detection of Invasive Cane Toads (Rhinella marina)

Environmental DNA (eDNA) surveys are increasingly used to monitor biodiversity because they are often more sensitive (have higher detection probability) than conventional monitoring methods. Sensitivity is a key consideration in designing monitoring programs because it determines the survey effort (e.g., number of samples per site) required to achieve a given likelihood of detecting a species. However, assessing the sensitivity of eDNA surveys and examining the factors influencing this in the field remain understudied. Here, we quantify the importance of key factors likely to influence eDNA sensitivity and compare the results of eDNA surveys to conventional visual surveys for detecting invasive cane toads (Rhinella marina) in northern Australia. We sampled waterbodies across the invasion front and showed that both eDNA and visual surveys had similar performance in detecting cane toads. Environmental DNA sensitivity varied predictably across waterbodies as a function of several factors. Sensitivity was higher: (1) when a greater volume of water was sampled at a water body; (2) at waterbodies with higher toad densities; (3) at smaller waterbodies; and (4) when cane toad tadpoles were present. We show how these findings can be used to tailor survey effort to ensure a specified level of detection probability at individual waterbodies, for example, by scaling the number of samples taken to water body size and tadpole presence/absence. Our study highlights the value of quantitatively assessing the sensitivity of eDNA surveys in the field and understanding the factors influencing sensitivity to achieve monitoring objectives.