1. Scattered trees are thought to be keystone structures for biodiversity in landscapes worldwide. However, such trees have been largely neglected by researchers and their importance for biodiversity remains unclear. 2. We completed a global meta-analysis to quantify relationships between scattered trees and the species richness, abundance and composition of vertebrates, arthropods and plants. First, we tested whether areas near scattered trees support higher levels of species richness and abundance than nearby open areas. Second, we compared levels of species richness and abundance in matrix areas with scattered trees and areas embedded within nearby habitat patches. We also compared the composition of biological communities inhabiting habitat patches, open areas and areas with scattered trees. 3. A total of 62 studies contained suitable data for our quantitative analyses. The local abundance of arthropods, vertebrates and woody plants was 60-430% greater and overall species richness was 50-100% higher in areas with scattered trees than in open areas. Conversely, for herbaceous plants, there was no consistent relationship between species abundance and the occurrence of scattered trees, although species richness was, on average, 43% lower. 4. The abundance and richness of all taxonomic groups was similar in matrix areas supporting scattered trees and habitat patches, although the species richness of epiphytes was, on average, 50% higher in habitat patches. Communities inhabiting habitat patches were more similar in composition to the communities inhabiting areas with scattered trees, and less similar to the communities of open areas. 5. Synthesis and applications. Areas with scattered trees support greater levels of biodiversity than open areas, as well as communities that are more similar to those inhabiting habitat patches. Scattered trees can be regarded as keystone structures for vertebrates, arthropods and terrestrial plants in landscapes worldwide. The maintenance of scattered trees may be compatible with livestock grazing in some agricultural landscapes. Greater management effort and targeted, long-term policies are needed to retain or re-establish scattered trees in many farming landscapes in both forest and non-forest biomes around the world.04-May-2017