Estimation accuracy and eDNA production source
We found that eDNA-based estimation accuracy of species abundance was
significantly lower for crustaceans 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), while 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. Consequently, crustaceans
infrequently and irregularly shed eDNA, which may impede sufficient eDNA
detection in the field and prevent accurate abundance estimationvia eDNA analysis (Dougherty et al., 2016; Mächler et al., 2016;
Johnsen et al., 2020). In contrast, estimation accuracy of species
abundance was marginally higher for coral and seastars than fish. These
species are sessile or move relatively slower than other marine animals.
Accordingly, their eDNA after released into environments might reflect
species presence abundance more precisely. In particular, under the
recent climate change and anthropogenic disturbances, rapid and
sensitive monitoring of tropical coral reefs via eDNA would be
helpful for the effective conservation of biodiversity and ecosystem
functioning on reefs (Nichols & Marko, 2019).
The mean R2 value between herptile eDNA quantity 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 herptiles
relative to fish. This point could simply be explained by biases derived
from the smaller number of corresponding studies or different
experimental conditions. Most of the relationships between herptile eDNA
quantity and abundance were studied in natural environments and depended
on visual counts for abundance estimation (e.g., Thomsen et al., 2012;
Kakuda et al., 2019), which might make the relationships less certain
than other capture-based survey. On the other hand, Everts et al. (2021)
assessed the relationships between eDNA concentration and abundance of
American bullfrog (Lithobates catesbeianus ) tadpoles and
juveniles using mesocosm experiments, and reported relatively high
R2 values (0.64 to 0.99). 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 improving eDNA-based
abundance estimation.