Urbanization is presenting a growing problem for biodiversity conservation, notably by increasingly isolating over half of the world's population from the experience of nature. This separation of people from nature is an important environmental issue, as it could fundamentally influence the way people value nature and their willingness to conserve it. Here we provide the first experimental study that jointly explores how urban biodiversity can be enhanced and how these changes may influence some aspects of people-biodiversity interactions. We significantly increased the diversity of flowers, birds and pollinators in small public gardens (Paris, France) by providing additional resources (i.e., planting flower-meadows and placing nesting-boxes). Semi-structured interviews were conducted in situ with 1116 regular garden users before and after the manipulation. Close-ended questionnaires were completed exploring the respondents' biodiversity perception and their sensitivity to the changes in biodiversity. Our results highlight a people-biodiversity paradox between people's perceptions and biodiversity awareness. Respondents expressed a strong preference for a rich diversity of species (excluding insects) and related this diversity to their well-being in the gardens. However, they did not notice the diversity of species. Respondents underestimated species richness and only noticed the changes in native flower richness in those gardens where advertisement and public involvement were organized. More experimental interdisciplinary studies are needed to further explore the people-biodiversity interactions. This would help expose the role that urban biodiversity plays in people's daily life and the importance of this interaction for raising public support for general conservation policies.