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.