In many zoos, the area separating the enclosures of zoo animals harbours highly diverse communities of free-living animals and plants. These organisms have received little attention so far. Using an all-taxa-biodiversity-inventory approach, a team of 46 zoologists and botanists carried out a 3 year study to assess the free-living organisms (plants, fungi, animals) occurring in the areas between the enclosures of zoo animals at Basel Zoo. A total of 3110 free-living species could be documented in this relatively small city zoo. However, not all taxonomical groups could be considered, mainly owing to the lack of experts. It was estimated that the actual richness of free-living species in Basel Zoo may exceed 5500. Thus, the number of free-living species is approximately eight to ten times higher than the 646 species of zoo animals maintained at Basel. The findings are important for preserving both the valuable remnants of natural and semi-natural habitats and threatened free-living species at the Zoo. The project combines research and outreach, which can improve the understanding of native biodiversity while simultaneously raising the awareness of the threats to it. The success of sustainable garden management in protecting native biodiversity may motivate the public to aspire to their own ‘wildlife-friendly’ gardening activities.