Conclusions and perspectives
To our knowledge, the present meta-analysis is the first to report variations in eDNA-based estimation accuracy of species abundance among different target taxa and filter pore sizes (reflecting eDNA particle size distribution). Some recent studies have suggested the possibility improving the accuracy of eDNA-based abundance estimation by statistically accounting for the processes of eDNA production, transport, and degradation (Carraro et al., 2018; Cerco et al., 2018; Fukaya et al., in press). In contrast, our meta-analyses shed a new light on the importance of what characteristics of eDNA should be targeted for more accurate estimation of species abundance. In particular, our findings on the effects of eDNA state imply that ‘more recently released’ eDNA, existing as larger eDNA particles and potentially longer eDNA fragments, more precisely reflect species abundance in the field. This knowledge will complement abundance estimation approaches that consider eDNA spatiotemporal dynamics; that is, understanding eDNA characteristics, including production source, particle size, and fragment length, as well as eDNA production, transport, and degradation processes, will enable us to further enhance the potential of eDNA analysis as a non-disruptive and cost-efficient tool for species abundance estimation. Therefore, accumulating knowledge of eDNA states and their interactions with the dynamics is crucial (Jo & Minamoto, 2021), which could facilitate the development of a novel eDNA marker suitable for accurate eDNA-based estimation of species abundance.
There are some potential biases and limitations in our meta-analyses. First, our collected dataset was concentrated toward studies targeting fish species, which might cause biased and over-dispersed estimation for other taxa. Second, our meta-analyses excluded some eDNA studies because these studies were regarded to be inadequate for the methodology of our analyses (see above) or did not directly estimate the indices of abundance estimation accuracy (Pearson’s correlation coefficients or R2 values; e.g., Jo et al., 2020b). Accumulating additional empirical studies for various taxa and environmental conditions are necessary to validate the findings of our meta-analyses and further elucidate the influence of eDNA characteristics on eDNA-based estimation of species abundances.
Furthermore, although not considered in the present study, the applicability of nuclear eDNA, particularly targeting multiple copies of ribosomal RNA genes, should be noted for more accurate eDNA-based species abundance estimations. Relative to mitochondrial eDNA, targeting multi-copy nuclear eDNA can improve detectability and yield (Minamoto et al., 2017; Jo et al., 2020b) and nuclear eDNA may degrade more rapidly due to potential differences in membrane and DNA structures (Bylemans et al., 2018; Jo et al., 2020b). In addition, nuclear eDNA production may also be less biased by individual growth and developmental stages, whereas mitochondrial eDNA production is expected to be suppressed with maturity and aging (Jo et al., 2020b). Understanding both the characteristics and dynamics of eDNA will fill a gap between eDNA concentration and species abundance in the field, and update current eDNA analysis as a more refined tool for biodiversity and ecosystem monitoring and stock assessment.