The challenges of the 21st century, namely, climate change and loss of biodiversity, especially present in heavily populated areas, should be addressed. Nature-based Solutions (NBS) seem to offer a suitable answer to these challenges. However, this new concept is not always easy to implement in a sustainable manner. In an effort to identify the barriers and levers for the implementation in cities of these sustainable NBS, several professionals working on them in France were interviewed. The first analysis with the most quoted words shows that the constraints would be mainly related to technique, management, biodiversity and people. The levers would be related to projects, roofs, people, legislation and services. Further analysis shows that most of the interviewees feel that the main barriers are the lack of knowledge, political will, financial resources and regulations. There are also cultural limitations, such as the use of exotic horticultural plants rather than wild local species. According to them, the technical problems should be easy to solve. To address these issues, the interviewees propose the development of transdisciplinary research disciplines, as well as on-field collaboration between all NBS actors in cities. They also recommend specific funds for NBS and their implication in related regulations. Demonstrative examples of urban NBS highlighting their multiple benefits are also needed to encourage their replication or upscaling. Education and communication are essential to shift the traditional points of view on nature in the city.

City-dwellers are often confronted with the presence of many spontaneous animal species which they either like or dislike. Using a questionnaire, we assessed the appreciation and perception of the pigeon (Columba livia), the rat (Rattus norvegicus), and the hedgehog (Erinaceus europaeus) by people in parks, train stations, tourist sites, community gardens, and cemeteries in Paris, France. Two hundred individuals were interviewed between May 2017 and March 2018. While factors such as age, gender, level of education or place or location of the survey did not appear to be decisive in analyzing the differential appreciation of these species by individuals, there was a clear difference in appreciation based on the species and the perceived usefulness of the animal, which is often poorly understood. The rat was disliked (with an average appreciation score of 2.2/10), and the hedgehog was liked (with an average appreciation score of 7.7/10). The case of the pigeon is more complex, with some people loving or hating it, and many others being indifferent (with an average appreciation score of 4.7/10). The most common descriptor associated with pigeons and rats is the word "dirty", while for for hedgehogs it is "cute". Like Driscoll (1995) who found a positive correlation between perceived usefulness an appreciation given to the species, we assume that providing information to the citizens about the ecosystem services provided by unappreciated urban species could improve their perception of these animals, thus enabling better coexistence.