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 than 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, 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.
Additionally, the spatial location of eDNA sampling within the study
environments might also influence the results. Marine eDNA studies
generally collected water samples from both near-shore and offshore
sites, whereas some lentic freshwater studies relied solely on water
sampling from shore sites (e.g., Kamoroff & Goldberg 2018; Kakuda et
al., 2019). Even if eDNA detection rates were higher at shore sites
(Hänfling et al., 2016; Jo et al., 2021), the lowered diffusion in
lentic environments and highly heterogeneous distribution (Hänfling et
al., 2016; Li et al., 2019) may not accurately reflect the overall
species abundance in studied sites, especially when large lakes and
reservoirs were targeted. Further studies are required to clarify how
the heterogeneity of eDNA distribution influences the detection
sensitivity, yield, and relationship to species abundance.