Results
We compiled 204 R2 values in total from the collected
56 literature (Tables 1 and S1). Most studies reported the relationships
between eDNA quantity and species abundance by the species-specific
assay (47 studies) and targeting fish species (40 studies). Contrary,
such relationships were reported less frequently for metabarcoding assay
(9 studies) and other taxa (7, 4, 3, and 3 studies for herptiles,
crustaceans, mollusks, and coral and seastars). The filter pore size and
amplicon size ranged from 0 to 10 μm and 52 to 719 bp, respectively.
Almost all the studies used filters with less than 3 µm pore size and
amplified less than 400 bp target DNA fragments. The number of studies
conducted in laboratory, lentic freshwater, lotic freshwater, and marine
environments were 18 17, 18, and 6, respectively.
A GLMM with MCMC algorithm showed the significant effects of target
taxa, filter pore size, and study environments (Table 2). Relative to
fish, estimation accuracy (Fisher’s z-transformed R2values) was significantly lower for crustaceans (posterior mean = -0.544
[95% CI: -1.074, -0.030]) and marginally higher for coral and
seastars (0.500 [-0.116, 1.063]). Among the target taxa, the
relationship between crustacean eDNA quantity and abundance was the
weakest and the estimated R2 values for herptiles and
crustaceans were more variable than other taxa (Figure 2a). Laboratory
experiments produced higher estimation accuracy than natural
environments, whereas there seemed to be no substantial differences in
the estimation accuracy among natural environment types (Figure 2b).
Filter pore size had a significantly positive effect on the estimation
accuracy (0.094 [0,011, 0.180]; Figure 3). In contrast, we did not
confirm any statistically significant effects of assay strategy,
amplicon size, and abundance metrics (Figures S1 and S2). These
statistical trends were not different even if the outliers were excluded
(Table S2). We confirmed the convergences of MCMC algorithm for all the
run and no substantial multi-collinearity among the factors (adjusted
GVIFs: 1.23 to 1.68).
The heterogeneity across studies/datasets was relatively high
(I2 = 77.4 % for the overall dataset; Table 3).
Although the values were re-calculated by dividing the dataset into each
categorical factor (assay strategy, target taxa, study environment, and
abundance metrics), the heterogeneities were generally high
(> 70 %) except for some categories. In addition, Egger’s
regression and funnel plot showed the significant publication bias in
our dataset (P < 0.01; Figure 4).