The garden biodiversity index: A self-assessment tool for evaluating biodiversity in private gardens

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.