Comparing traditional vs. environmental DNA monitoring protocols. The case of the Eurasian otter (Lutra lutra) re-expansion in Central Italy

Once widespread in Italy, in the last century the Eurasian otter (Lutra lutra) went to the brink of extinction in this country, only surviving in a few southern river basins. Being included in the Habitats Directive 92/43/EEC, it is mandatory for State Members to monitor its conservation status over time, making cost effective and accurate monitoring protocols a relevant issue. We compared the efficiency of environmental DNA (eDNA) and traditional field survey techniques in detecting the presence of Eurasian otter at the border of the current expansion range of the Eurasian otter in Latium, Central Italy. Water sampling for eDNA detection through species-specific assay and systematic field surveys were run in six river basins. Results from the two approaches were compared at the scale of sampling site, 5 km and 10 km radius upstream. Both techniques agreed in revealing the expansion of Eurasian otter in southern Latium and in confirming otter absence in the northern part of the study area. eDNA allowed to detect otters at two more river catchments compared to field surveys, one of which is located northward in a tributary of the river Tiber, and showed a higher detection rate (0.41), both at sampling site (0.06, p = 0.01), 5 km (0.23) or 10 km radius (0.35) scale. These results suggest that eDNA is a more sensitive method in revealing Eurasian otter presence, providing a useful reference for future standardized monitoring of the species at the country level, especially in expansion areas.