Motorway proximity affects spatial dynamics of orthopteran assemblages in a grassland ecosystem

Major roads have become significant components of environmental change but their impacts on invertebrate assemblages are insufficiently understood, and therefore rarely considered in road-planning and management. In the current study, we test (i) whether and how orthopteran assemblages change with distance from a motorway, and (ii) how road-induced changes in noise level, vegetation height and microclimate affect assemblage metrics and spatial distribution of orthopteran species. In 2018, we sampled orthopterans at five distances from a motorway: 10, 25, 50, 100 and 500 m, in eight locations within homogeneous portions of a grassland habitat in Lika region, Croatia, using two sampling methods: pitfall trapping and sweep-netting. Orthopteran abundance, species richness, true diversity and conservation value decreased at the sites closest to the motorway in the pitfall dataset, primarily due to negative responses of species with low-frequency acoustic signals. Road-influenced vegetation height had a stronger overall impact on orthopteran assemblages than traffic noise and/or microclimate; increased diversity and conservation value at 25 m from the motorway suggest an effect similar to early stages of vegetation succession. This study shows, for the first time, that a major road can induce changes in adjacent orthopteran assemblages, but negative impacts are confined to a narrow zone. Our results indicate that road-induced changes in orthopteran assemblages could be more efficiently assessed using pitfall trapping than sweep-netting. Increasing heterogeneity of roadside habitats by appropriate vegetation management could help mitigating negative road impacts on Orthoptera in the study area, while contributing to higher diversity of their assemblages.