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Invasive alien species pose a serious threat to native biodiversity and the ecosystem structure through predation or competition, and this threat is especially severe in China. Red-eared sliders (Trachemys scripta elegans) were introduced to mainland China to meet demands for food, traditional Chinese medicine, and the pet trade. However, there has been a lack of field surveys to assess the population, behavior and ecological impacts of introduced red-eared sliders in urban aquatic ecosystems in northern China. We conducted such research in 33 urban parks in Beijing from April to November 2016. The alien sliders were recorded on 514 occasions at 19 parks, and 45 individuals were captured from 4 parks to evaluate their diet composition and reproductive status. The observations showed that red-eared sliders accounted for 95% of the recorded turtles in the parks. The male to female ratio was 1.0:3.5, and the ratio of adults to juveniles was 1.0:3.1. According to stomach content analysis, the sliders fed mainly on animals: the dominant prey were Gastropoda, Osteichthyes, Crustacea, Hexapoda, Bivalvia, Reptilia and Aves in descending order. The high frequency (83%) of follicles and oviduct development in adult females suggested a strong reproductive capacity, which was further supported by one case of early female maturity. Bacteriological examinations demonstrated that the carrying rate of Salmonella bacteria was up to 17.6%. This study demonstrated that red-eared sliders have successfully colonized the waters of urban parks in Beijing and potentially threaten native aquatic biodiversity. [ABSTRACT FROM AUTHOR]