What evidence exists on wild bee trends in Germany? A systematic map

Background Wild bees have attracted growing attention from both the scientific community and civil society, alongside increasing evidence of biodiversity losses. Declining wild bee populations threaten both the quality and quantity of pollination, which also affect crop production and are therefore critically important for human wellbeing. Landscape homogenisation, land use changes, land use intensity, and climate change are driving the decline. Despite concerns about the wild bee decline, knowledge of wild bee population patterns and long-term trends across Germany remains limited. Here, we present a systematic map, including a newly developed comprehensive database that compiles available data on temporal trends in wild bee communities across Germany. Our goal is to provide an overview of the frequency of wild bee trend studies over time and the land use types and geographical areas they have covered. Methods Our search for data on wild bee trends was conducted in November 2020 and included peer-reviewed literature (from Web of Science databases and Scopus) and grey literature in English and in German. After screening the literature by title and abstract, relevant data were extracted from eligible studies. All eligible studies included data on wild bee taxa from at least two years at the same site within Germany and using the same sampling method. The database consists of data sheets on studies (bibliographic context), on covariates (methodological and spatio-temporal context) and on data (bee species sampled at a specific time on a specific location – exemplarily for two German regions). Review findings The database contains 382 studies out of 24,486 initial records. Nearly 75% of the full texts screened did not include field data on wild bees from at least two different years and were therefore excluded. Studies date back to the 1880s, with a consistently high number of studies since the 1990s. Most studies were published in German-language journals of entomological societies in Germany. Data originate from different types of land use throughout the country, mostly from southern and north-western Germany and from urban areas. Conclusions The systematic map shows that there is a lack of long-term monitoring studies. Moreover, there are research gaps in land use categories and federal states, which hinder more regional or land use-specific analyses. We encourage researchers and practitioners to use the database for further analyses on bee trends and their drivers, and the potential success of mitigation measures.