Abstract
Understanding the distribution of invasive species and their reproductive area is crucial for their managements after invasion. While catch and observation surveys are still embraced, environmental DNA (eDNA) has been increasingly utilized as an efficient tool for identifying these species in the wild. In this study, we developed a Bufo-specific eDNA assay for detecting an invasive, toxic, and terrestrial toad species Bufo japonicus formosus in Hokkaido, Japan, and applied it to their reproductive area at watershed scale. The eDNA assay was field-validated in ponds where B. japonicus were observed, as well as in rivers downstream of the reproductive ponds. Thus, the assay provided us an opportunity to screen watersheds that include their reproductive area by collecting downstream water samples. Applying it to the Ishikari river basin, the largest river basin in Hokkaido (c.a., 14,330 km2), we detected toad eDNA at 32 out of 73 sampling sites. They are composed of eleven sites with species observation records nearby (all the sites with observation records within a 500 m radius) and 21 sites without such records. And those eDNA detections were from twelve out of 31 river systems in the entire river basin. A Bayesian, multiscale occupancy model supported high eDNA detectability among those sites. These results suggest that the eDNA assay can efficiently estimate the presence of reproductive area of the terrestrial toad even from a distant downstream of the watershed, and that it provides a powerful means of detecting new reproductive area and monitoring further spread of invasive species.
Highlights
Difficulties in monitoring of current distribution of invasive species, especially at the front-line of their invasions, are one of the biggest issues in biodiversity management
A Bayesian, multiscale occupancy model supported high Environmental DNA (eDNA) detectability among those sites. These results suggest that the eDNA assay can efficiently estimate the presence of reproductive area of the terrestrial toad even from a distant downstream of the watershed, and that it provides a powerful means of detecting new reproductive area and monitoring further spread of invasive species
EDNA techniques based on river water samples are useful for screening the presence of focal species at large spatial scales because eDNA can be carried from their actual habitat to downstream by the river current (Deiner et al 2016)
Summary
Difficulties in monitoring of current distribution of invasive species, especially at the front-line of their invasions, are one of the biggest issues in biodiversity management To address this issue, detections of the presence of the invasive species through visual and acoustic assessments have been employed for a variety of taxa including amphibians (e.g., Boback et al 2020; Diamond and Ross 2020). Detections of the presence of the invasive species through visual and acoustic assessments have been employed for a variety of taxa including amphibians (e.g., Boback et al 2020; Diamond and Ross 2020) They are often ineffective to grasp distribution and reproductive area of the invasive species because of many restrictions (large potential distributions, high labour cost, difficulty in species identification, especially at their early developmental stages, etc.) (Lepczyk 2005; Ficetola et al 2008). EDNA from pond-dwelling species can be presumably detected by collecting river water at distant downstream, empirical evidence to support this idea is scarce at best (Deiner et al 2016; Bista et al 2017)
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