1. Literature search and data extraction
We searched for literature relating to eDNA persistence and degradation,
published during 2008 to 2020 (final date for the literature search was
20 Jun 2020), using Google Scholar
(https://scholar.google.co.jp/).
The terms “eDNA” or “environmental DNA”, included in the title
and/or text, were used for the literature search. We then filtered and
selected papers that (i) targeted eDNA from macro-organisms (i.e. not
from microbes, fungi, plankton, virus, and bacteria), (ii) were written
in English, (iii) were peer‐reviewed (i.e. not preprints), and (iv)
described aqueous eDNA decay rate constants using a first-order
exponential decay model (\(C_{t}=C_{0}e^{-kt}\), where \(C_{t}\) is
the eDNA concentration at time \(t\), \(C_{0}\) is the initial eDNA
concentration, and \(k\) is the first-order decay rate constant). The
eDNA decay rate constants estimated using multi-phasic exponential decay
models (e.g. biphasic or Weibull models) (Eichmiller et al., 2016;
Bylemans et al., 2018; Wei et al., 2018) were not included in our
meta-analyses, because of the limited number of such studies and
difficulty in directly comparing the constants between first-order and
multi-phasic models.
From the filtered eDNA studies, we then extracted data on the eDNA decay
rate constant (per hour), filter pore size used for water filtration
(µm), target DNA fragment size (base pair; bp), and target gene
(mitochondrial or nuclear). The decay rate constant was converted to
“per hour” if it was originally described as “per day”. Different
eDNA decay rate constants based on different experimental conditions
within the same study (e.g. species, temperature, pH, and biomass
density) were treated separately. The filter pore size in studies
involving aqueous eDNA collection via ethanol precipitation or
centrifugation was regarded as 0 µm. In addition, we extracted
information on the water temperature (°C), water source used for
experiments, and target species and taxa. Although other biotic and
abiotic factors are known to affect eDNA degradation, we extracted only
temperature and water source data, because of their consistent and
informative descriptions in all selected papers (i.e. other water
physicochemical parameters such as pH, conductivity, and dissolved
oxygen were sometimes not specified in the paper). If necessary, we used
the mean temperature obtained by averaging the maximum and minimum
temperatures during the experimental period. Water source was classified
as ‘artificial’, including tap water and distilled water (DW);
‘freshwater’, including wells, ponds, lakes, and river water; and
‘seawater’, including harbour, inshore, and offshore seawaters. Because
Moushomi et al. (2019) had estimated decay rates of Daphnia magnaeDNA at each size fraction, we calculated total eDNA concentrations
collected by a 0.2 µm pore size filter and ethanol precipitation, and
re-estimated the eDNA decay rates (Appendix S1).