Literature search
We conducted a literature search according to The Preferred Reporting
Items for Systematic reviews and Meta-Analyses (PRISMA) statement (Page
et al., 2021) (Figure 1). First, we compiled all the studies described
in the literature list of Jo et al. (2021) (N = 728), which screened the
literature regarding macrobial eDNA published until 2020 on the
following criteria; (i) targeted eDNA from macro-organisms (not from
microbes such as bacteria and fungi, or viruses), (ii) published in
international journals, (iii) peer-reviewed (not preprints), and (iv)
not review articles, news, views, introductions, opinions, responses, or
perspectives. Additionally, using Google Scholar
(https://scholar.google.co.jp/), we searched for the latest eDNA
literature, where the search included the terms “eDNA” or
“environmental DNA” in the title and/or text (the final date of the
literature search was 15 September 2021) and screening strategy was the
same as above (N = 195). We then assessed the abstracts of the remaining
923 literature and excluded 805 literature that were out of the scope of
our meta-analysis (i.e., not describing the relationship between eDNA
quantity and species abundance).
We further assessed the full-texts of the remaining 118 literature and
excluded 62 literature that did not present R2 values
and Pearson’s correlation coefficients of the relationship between
species abundance and eDNA quantity, and the values of eDNA
quantification were not reliable (< limit of quantification).
Besides, a part of dataset in Akamatsu et al. (2020) was excluded
because the sample size was too small to estimate variance under a
Fisher’s z-transformation (see below). 56 literature were finally
included in our meta-analysis, assessing the relationships between eDNA
quantity in water samples and species abundance. Quantity of eDNA was
categorized into (a) eDNA concentration measured by real-time PCR or
digital PCR (species-specific assay) or (b) eDNA read number measured by
high-throughput sequencing (metabarcoding assay). Species abundance
metrics were categorized into (a) biomass or (b) number of individuals.
Most studies reported positive relationships with statistical
significance, while some results were not significant (Table S1).