When to mow and how: Short-term effects of river dike grassland management on arthropod abundance, species richness, and community composition

Species-rich grasslands and their associated arthropods have become rare across Europe. Linear green infrastructure, such as river dikes, can serve as secondary habitats and improve the connectivity among isolated grasslands. Although applied extensively on dike grasslands, mowing impairs local arthropod populations, depending on the date and the techniques applied, while leaving areas uncut over winter can support arthropods. To investigate the short-term effects of different mowing dates, hay handling techniques, and overwintering strips on arthropod abundance, species richness, and community composition, we conducted mowing experiments at eleven sites on dikes of River Inn (South Germany). We sampled carabids, spiders, true hoppers, and true bugs, and investigated combinations of mowing dates (June - 'early'; September - 'late') and hay handling techniques (raking or suction), and the effects of overwintering strips located in the middle or upper part of the dikes. Carabids were favored by late mowing and hay suction. Spider species richness profited from overwintering strips in July and showed an overall seasonal decline. True hoppers benefited from early mowing, and true bugs were insensitive to the treatments. Abundance and/or richness of all groups were favored by overwintering strips. The arthropod communities changed across the season, and the mowing dates significantly drove their assemblages. An indicator analysis revealed a non-random distribution of species among treatments. Our results indicate that there is no optimal mowing date for all studied taxa. We conclude that a spatio-temporal mosaic of mowing treatments helps to meet the needs of different arthropods in dike grasslands, and that rotational management promotes arthropod diversity.