Results
We selected and analyzed 44 published papers in total, from which we
extracted 100 R2 values ranging from 0.02 to 0.99
(Tables 1 and S1). All the collected studies targeted mitochondrial DNA
fragments, and no study reported the positive correlation between
nuclear eDNA concentration and species abundance. Most studies reported
the correlation between eDNA concentration and species abundance,
targeting fish species (30 of 44 studies). The filter pore size and
amplicon size ranged from 0 to 10 μm and 66 to 719 bp, respectively. The
majority of studies used filters with less than 3 µm pore size and
amplified less than 200 bp target DNA fragments. The number of studies
conducted in laboratory, lentic freshwater, lotic freshwater, and marine
environments were 15, 14, 14, and 4, respectively.
The effect sizes of R2 values and their variances were
estimated using Fisher’s z‐transformation and forest plots (Figure S1).
Relative to fish (R2 = 0.68 [95 % CI: 0.61,
0.74]), estimated R2 values were significantly lower
for crustaceans (0.22 [−0.04, 0.46]) and mussels (0.50 [0.44,
0.55]); among the target taxa, the correlation between crustacean eDNA
concentration and abundance was the weakest (Figure 1). Laboratory
experiments (0.78 [0.69, 0.84]) produced higher estimated
R2 values than natural environments, and there was no
substantial difference in the estimation accuracy among natural
environment types (Figure 2). In addition, the LMM showed a significant
positive effect of filter pore size on R2 values
(P < 0.05; Table 2 & Figure 3). R2values tended to be higher as filter pore size increased, but some
datasets also reported high R2 values with smaller
filter pore sizes. PCR amplicon size did not significantly influence
R2 values (P > 0.1; Table 2 &
Figure S2).