Humid grassland fallows promote spider diversity in a traditionally managed landscape.

• In the Westerwald mountain range, mid-western Germany, humid long-term fallows rather than traditional hay meadows harboured the highest spider richness and diversity. • Spider assemblages were structured by on-site management, and additionally by eutrophication, fallow-related parameters, and the proportion of forests in the surrounding landscape. • In traditional landscapes of rather low land-use intensity, enhancing structural heterogeneity, e.g. through fallows, may be of key importance to promote spider diversity. • Long-term grassland fallows should be subsidized under agri-environmental schemes. Agricultural intensification is an important driver of biodiversity decline. Regarding grassland ecosystems, traditionally managed hay meadows, as opposed to highly productive silage grassland, are often of high conservation value. Here, we compare spider assemblages among five grassland management regimes along an intensification gradient, ranging from long-term fallows (not farmed for ≥ 30 years) via traditionally managed hay meadows to high-intensity silage grassland. Humid long-term fallows harboured highest species richness and diversity, and their assemblages differed strongly from other management regimes. There was surprisingly little variation though among different types of hay meadows and silage grassland. We also included environmental parameters and land-cover types in the surroundings of study sites, indicating that spider assemblages were most strongly shaped by fallow-related parameters and the proportion of forests. The high spider diversity on fallows may result from their undisturbed vegetation structure, while management, regardless of frequency, may have detrimental effects. Our results demonstrate the high importance of unmanaged areas even in regions with an overall low land-use intensity, such as grassland ecosystems. We thus recommend the inclusion of temporal grassland fallows (unmanaged for 3-5 years) in agri-environmental schemes to increase landscape heterogeneity and preserve spider diversity. [ABSTRACT FROM AUTHOR]