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Global changes may direct species expansion away from their current range. When such an expansion occurs, and the species colonizes a new region, it is important to monitor the habitat used by the species and utilize the information to updated management strategies. Water deer (Hydropotes inermis) is listed as Vulnerable species in IUCN Red List and is restricted to east central China and the Korean Peninsula. Since 2017, water deer has expanded its range towards northeast China and the Russian Far East. The objective of our study is to provide support for a better understanding of habitat use and provide suggestions for developing conservation strategy. We collected occurrence data in northeast China and the Russian Far East during 2017-2021. We used MaxEnt to predict habitat suitability for water deer and applied Circuitscape to determine possible dispersal routes for the species. We used seven environmental variables, viz., altitude, slope, aspect, distance to built-up area, distance to water source, distance to cropland and distance to roads for habitat suitability prediction. We chose the MaxEnt model (AICc = 2572.86) suitable for our data with the AUC value result of 0.935±0.014. There is good quality habitat for water deer in the boundary area of the Yalu and Tumen River estuaries between China, North Korea, and the Russian Far East, as well as the east and west regions of the Korean Peninsula. We identified three main suitable habitat patches, two of them located in east (NK2) and west (NK3) North Korea, and one in the newly colonized area downstream of the Tumen River along the border of China, Russia, and North Korea (TM1). Elevation, distance to cropland and water sources, and presence of wetlands were the variables that positively contributed to modelling the suitable habitats. Two possible dispersal routes were determined using the circuit theory, one was across the area from North Korea to the downstream Tumen transboundary region (Route B), and the other was across North Korea to the boundary region in China and along the tiger national park in northern China (Route A). A series of protected areas in North Korea, China, and Russia may support the dispersal of water deer. From the study on water deer dispersal, we can understand the existing ecological network in northeast Asia, which will benefit the whole landscape and biodiversity conservation. However, there are many threats present, and there is need for continued monitoring inside and outside the protected areas. Information sharing with stakeholders and carrying out local communities awareness activities are important. The establishment of a Northeast Asia landscape conservation network would help establish monitoring and conservation planning at a broad scale, and this study provides an example of the need for such a network., Competing Interests: The authors have declared that no competing interests exist.

Simple Summary: The water deer Hydropotes inermis, one of the most primitive members of Cervidae, is classified as vulnerable by the IUCN Red List in their native range, having declined drastically in recent years. In this study, we illustrated a northern extension of water deer using the current evidence, and assessed the status, phylogeny, and genetic ancestry of the newly recorded population. Our results showed that water deer had expanded to the Northeast China and the Russian Far East, where there had previously been no record of this species; thus, this could be a genuine range expansion rather than simply an expansion of the known range. A genetic investigation 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. 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. [ABSTRACT FROM AUTHOR]