Literature search and data extraction
Using Google Scholar (https://scholar.google.co.jp/), we conducted a
literature search relating to eDNA production and species abundance
published during 2008 to 2021. the literature search was 1 June, 2021).
The terms “eDNA” or “environmental DNA”, included in the title
and/or text, were targeted for the literature search. We then filtered
and selected papers that (i) targeted eDNA from macro-organisms (not
from microbes such as bacteria and fungi, or viruses), (ii) were
published in international journals, (iii) were peer-reviewed (not
preprints), and (iv) described the relationships between eDNA
concentration, quantified by real-time PCR or digital PCR, and species
abundance (biomass and/or density) by fitting linear or linear-mixed
models. For criterion (iv), most studies reported positive relationships
with statistical significances, while some of results were not
significant.
We then extracted R2 values from linear fitting,
indices of species abundance estimation accuracy based on eDNA
concentration, target taxa, filter pore sizes used for water filtration
(μm), and PCR amplicon sizes (base pair; bp) from the selected studies.
Taxa were classified as fish, amphibians, other vertebrates,
crustaceans, mussels, and other invertebrates. In studies involving
aqueous eDNA collection via precipitation or centrifugation, the
filter pore size was regarded as 0 μm. We did not include Pearson’s
correlation coefficients in our meta-analyses because the index was not
available in all collected literatures. If the manuscripts only reported
Pearson’s correlation coefficients, we squared the coefficients and
substituted them for R2 values. Different
R2 values based on different experimental conditions
within the same study (e.g., species, filter type, and amplicon size)
were treated separately. Abundance metrics (biomass/density) were pooled
here because of its mere effect on correlation between eDNA
concentration and species abundance (Yates et al., 2019). In addition,
we extracted information on the study environments and classified them
as laboratory, lentic freshwater, lotic freshwater, and marine
environments. Moreover, we calculated the sample size (the number of
water samples or sampling sites) required for fitting each linear model
based on figures and/or text in the corresponding literature.