Using quantitative eDNA analyses to accurately estimate American bullfrog abundance and to evaluate management efficacy

Abstract

AbstractBiological invasions contribute now more than ever to the global homogenization of fauna and flora. Large‐scale monitoring programs are, therefore, needed to detect incipient invasions and to evaluate management interventions. As conventional monitoring methods are constrained by large costs, environmental DNA (eDNA)‐based methods are increasingly recognized as valuable monitoring tools. However, accurately estimating species abundance from eDNA concentrations in natural systems remains challenging and consequently hinders their integration in management applications. Here, we used droplet digital PCR (ddPCR) in eDNA surveys to estimate the abundance of invasive American bullfrogs (Lithobates catesbeianus). We first introduced bullfrog tadpoles in natural ponds to assess the relationship between abundances and eDNA concentrations under field conditions. Next, we combined eDNA sampling with fyke netting in naturally colonized ponds to investigate whether bullfrog eDNA concentrations can estimate bullfrog capture success and conventional abundance measures obtained via depletion sampling. Finally, we evaluated eradication measures by comparing bullfrog eDNA concentrations before and after fyke netting. We found a strong linear relationship between the numbers of introduced tadpoles and eDNA concentrations (r2 = 0.988). Bullfrog eDNA concentrations were not only linearly related to the catch‐per‐unit‐effort (r2 = 0.739), but also to conventional abundance estimates (r2 = 0.716), particularly when eDNA concentrations were standardized for pond area (r2 = 0.834) and volume (r2 = 0.888). Bullfrog tadpoles were only captured when eDNA concentrations exceeded 1.5 copies µl−1, indicating that quantitative eDNA analyses enable the localization of breeding ponds. We found a significant reduction in eDNA concentrations after fyke netting proportional to the number of captured bullfrogs. These results demonstrate that eDNA quantification is a reliable tool that accurately estimates bullfrog abundance in natural lentic systems. We show that quantitative eDNA analyses can complement the toolbox of natural resource managers and facilitate the coordination of eradication campaigns targeting alien invasive species.