Private gardens have considerable potential to support and enhance urban biodiversity. However, many garden owners have limited knowledge of biodiversity conservation in their gardens and of biodiversity-enhancing interventions. Capturing garden biodiversity is a first necessary step to utilise this potential. Here, we propose a new Garden Biodiversity Index (GBI), developed based on a detailed survey of the vegetation and structural features present in 28 private gardens in North-Western Germany. We calculated the GBI and four existing feature-based indices for 55 samples taken in these gardens in the years 2022 and 2023 and assessed their applicability for detecting temporal changes. Furthermore, we applied the GBI to 2,000 private gardens distributed across Germany using data from a nationwide survey. The distribution of GBI values across the 2,000 German gardens follows a bell-shaped curve, with most gardens clustering around the mean. This highlights the potential for many gardens to further enhance their contributions to biodiversity. Unlike existing indices, we designed the GBI as a self-assessment tool for garden owners and weight the features according to their contribution to biodiversity. We anticipate it will serve as a valuable tool for many to encourage and guide biodiversity-enhancing measures in their gardens.

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With this study we seek to address three goals: First, to design a new index that garden owners can use as a self-assessment tool, with features weighted according to their impact on plant species richness. We call this the Garden Biodiversity Index (GBI). Second, to calculate the four existing indices (WRI, EGI, HHI, HRI) and the new GBI for 55 garden samples. We aim to test for correlation with plant species richness as a proxy for biodiversity and to test to what extent the indices are comparable. Third, to apply the new GBI to 2,000 private gardens using data from a nationwide survey for a comprehensive assessment of the potential of private gardens across Germany in supporting biodiversity. This study was conducted as part of the interdisciplinary project “gARTENreich Preferences and Constraints for Biodiversity Conservation in Home Gardens” (see www.gartenreichprojekt.de and www.NABU.de/gartenreich), a collaboration between practitioners and researchers. The project aimed to assess the role of private gardens in biodiversity conservation, identify drivers and barriers to biodiversity-friendly garden management, and develop strategies to promote such practices in private gardens.