Estimation accuracy and environments
Higher R2 values were reported for laboratory conditions than natural environments, which is consistent with previous findings (Yates et al., 2019) and supports the methodological validity of our statistical analyses. As discussed in Yates et al. (2019), myriad and complex dynamics of eDNA could hamper the prediction of species abundance based on eDNA concentration in natural relative to controlled environments. However, we observed little difference in estimation accuracy among natural environments. Relative to lentic freshwater environments, such as ponds and lakes, eDNA diffusion and degradation would be substantial in riverine, coastal, and marine environments due to flow and tidal effects. Such factors can transport eDNA very long distances (Deiner & Altermatt, 2014; Andruszkiewicz et al., 2019) while also resulting in rapid dilution (Baker et al., 2018). In contrast, although eDNA may diffuse less in lentic environments (Fremier et al., 2019; Curtis et al., 2020), residual eDNA may cause false-positive inferences of species presence and inaccurate estimation of species abundance. It is noted that our results must be cautiously interpreted given the bias in the number of studies among natural environment types, but eDNA-based abundance estimation accuracy might not necessarily be worsened for lotic freshwater and marine environments relative to lentic environments.