European agroforestry is no universal remedy for biodiversity: a time-cumulative meta-analysis

Background Agroforestry is a production system combining trees with crops or livestock. It has the potential to increase biodiversity in relation to single-use systems, such as pastures or conventional agriculture, by providing a higher habitat heterogeneity. In a literature review and subsequent meta-analysis, we investigated the relationship between biodiversity and agroforestry and critically appraise the underlying evidence of the results. Results Biodiversity in agroforestry was higher than in conventional agriculture, but could not outcompete pastures, forests and abandoned agroforestry systems. There was no overall biodiversity benefit in agroforestry systems. Data were available for plants, birds, bats and arthropods. Arthropods and birds were the two taxonomic groups profiting from agroforestry systems. A time-cumulative meta-analysis shows that there was no general benefit of biodiversity at any point in the past besides in early 2015. Time-cumulative meta-analysis can unravel missing robustness of meta-analytical results if conclusions alternate between significant to non-significant summary effect sizes over time. Conclusion Agroforestry increases biodiversity only in silvoarable systems compared with conventional agriculture. But even this result is based on a small magnitude and single-study effect sizes were heterogeneous with sometimes opposing conclusions. The latter suggests the importance of other usually unmeasured variables, such as landscape parameters or land-use history, influencing biodiversity in agroforestry systems.