Optimizing the design of flower areas to enhance their effectiveness in supporting ground-active predators: A meta-analysis across Europe

To counteract the negative effects of agricultural intensification, several European countries support the use of flower strips or fields as part of agri-environment-climate measures. These habitats provide refuge and food resources for many taxa, including ecosystem service providers. Ground-active predators are important pest control agents. However, comprehensive evidence on how to design these flower areas to maximize their benefits for ground-active predators remains limited. We conducted a meta-analysis of 36 studies spanning the Mediterranean region, Central and Northern Europe to quantify the impact of flower areas on the abundance and species richness of ground-active predators. We also examined whether the overall effects varied in terms of different reference habitats (i.e. semi-natural habitats and crop fields) and regions. Moreover, we identified which characteristics (shape, age and sown plant species richness) contribute to the effectiveness of the flower areas. Flower areas had a moderate positive effect on species richness (Hedges' g = 0.65, 95% CI = 0.22–1.09) and a small positive effect on the abundance of ground-active predators (Hedges' g = 0.21, 95% CI = −0.05–0.48). The effects varied in terms of reference habitats and across regions. Flower areas supported substantially higher species richness compared to crop fields, with an effect size 7.9 times higher than when compared to semi-natural habitats. The positive effect of flower areas on the abundance of ground-active predators in the Mediterranean region was the highest (Hedges' g = 1.03, 95% CI = 0.60–1.45). Among flower area characteristics, linear strips were more effective than large fields in enhancing the species richness of ground-active predators, with an effect size approximately 4.3 times higher. While flower area age and sown plant species richness did not significantly influence either predator abundance or species richness. Synthesis and applications. Our meta-analysis highlights the value of flower areas in supporting ground-active predators, particularly in intensively managed landscapes dominated by crop fields. The strong positive effects observed in the Mediterranean warrant further investigation into underlying mechanisms. We recommend the implementation of flower areas as linear strips, especially in conservation programmes targeting enhancing ground-active predator diversity and associated pest control services.