Estimation accuracy and eDNA production source
Since the first study targeting common carp (Cyprinus carpio )
(Takahara et al., 2012), most research investigating the correlation
between eDNA concentration and species abundance has targeted fish
species (e.g., Klymus et al., 2015; Jo et al., 2017; Capo et al., 2021;
Table 1). According to these studies, fish eDNA concentration predicts
relative species abundance with relatively high accuracy regardless of
environmental conditions, which was also confirmed in our meta-analysis.
However, the meta-analysis showed that such tendencies might not
necessarily be similar among target taxa, and the abundance of some taxa
may not be accurately estimated via current eDNA applications.
We found that eDNA-based estimation accuracy of species abundance
(R2) was significantly lower for crustaceans and
mussels than fish. A previous study hypothesized that species morphology
and/or behavior might affect eDNA production, reporting lower eDNA
shedding rates in grass shrimp than in fish and jellyfish
(Andruszkiewicz et al., 2021). Fish and jellyfish are likely to
constantly produce eDNA as epidermis and/or muco-substances (Merkes et
al., 2014; Sassoubre et al., 2016). In contrast, crustaceans are
characterized by their hard exoskeletons and segmented bodies plans
(Hadley, 1986) and are thus unlikely to shed large amounts of eDNA from
their body surfaces unless they are molting. Similarly, mussel soft
tissue is covered with a hard, calcified shell that is less likely to
shed eDNA (Sansom & Sassoubre, 2017). Consequently, crustaceans and
mussels infrequently and irregularly shed eDNA, which may impede
sufficient eDNA detection in the field and prevent accurate abundance
estimation via eDNA analysis (Dougherty et al., 2016; Mächler et
al., 2016; Johnsen et al., 2020).
The mean correlation between amphibian eDNA concentration and abundance
was similar to that of fish, which is reasonable given amphibians likely
shed eDNA constantly from their epidermis and/or mucus. However, the
variation (95 % CI) was much larger for amphibians relative to fish.
The discrepancy could simply be explained by biases derived from the
smaller number of corresponding studies or different experimental
conditions. Among the studies collected for our meta-analysis, most of
the correlations between amphibian eDNA concentration and abundance were
studied in natural environments (e.g., Thomsen et al., 2012; Pilliod et
al., 2013). This is likely because of the difficulty in conducting
controlled laboratory experiments using amphibians due to its rarity,
unless some invasive species such as American bullfrog (Lithobates
catesbeianus ). In addition, possibly for the same reason, many of the
collected studies depended on visual counts for abundance estimation,
which could ambiguate correlations between amphibian eDNA concentration
and abundance. On the other hand, Everts et al. (2021) assessed the
correlation between eDNA concentration and abundance of American
bullfrog tadpoles and juveniles using mesocosm experiments, and reported
relatively high R2 values (0.64 to 0.99). Thus,
amphibians are potentially suitable for accurate estimation of species
abundance via eDNA analysis. Similarly, given that the study
targeting mussels was not conducted under laboratory conditions (Currier
et al., 2018), the relatively low R2 values for
mussels may partly include such biases. Accumulating studies targeting
various taxa in laboratory conditions and natural environments could
help us understand the effects of ecological characteristics
(morphology, physiology, and ethology) on the process of eDNA
production, and may provide us with keys for improved approach of
eDNA-based abundance estimation.