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).