Given current anthropogenic pressures and climate change, wildlife range expansion offers a second chance for species conservation. The water deer Hydropotes inermis is a native to China and the Korean peninsula, but populations in North Korea and mainland China have declined drastically in recent years. However, the range of this species appears to be rapidly expanding northward. In this study, we employed camera traps and molecular technology to assess the status, phylogeny, and genetic ancestry of the newly recorded population. Our results showed an ongoing northward expansion of water deer, reaching at least 500 km from their historical distribution limit. We provided updated information on this species’ geographical distribution in Northeast China and the Russian Far East. Based on historical survey data before the 1990s, there had previously been no record of this species in these two regions; thus, this could be a genuine range expansion rather than simply an expansion of the known range. A genetic investigation based on mitochondrial DNA indicated that the expanding population had a close phylogenetic affinity with Korean water deer. The likely migration route and causes of the species’ distribution range expansion are discussed. We recommend revising the range of water deer in the IUCN Red List to facilitate the effective conservation and management of this threatened species, especially in new locations.

Non-invasive genetic approaches, particularly using fecal samples, are commonly used to study endangered and elusive species, as they are easy to collect with minimal permission and cause little disturbance to the subject population. However, such studies face limitations due to poor DNA yield, which affects the overall utilization of collected samples and increases data errors. Here, we evaluated the impact of sample age and collection season on the performance of DNA extracted from feces (spraints) of the Eurasian otter (Lutra lutra), a semi-aquatic apex predator in South Korean freshwater ecosystems. We found that PCR amplification success rates decreased more rapidly in summer (79.3–58.2%) compared to winter (99.2–84.8%) with extended environmental exposure. Genotyping error rates were higher in samples collected during summer, with the rate of error increase over time being significantly greater in summer than in winter. The hot and humid South Korean summer fosters microbial growth and fecal degradation, which negatively impacts DNA yield, reducing PCR amplification success and increasing genotyping errors. We recommend collecting otter feces during winter for better DNA quality. If sampling in summer is unavoidable, it is crucial to collect fresh samples, which can be facilitated by conducting frequent surveys of latrine sites.