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).