Effects of landscape composition on hoverflies (Diptera: Syrphidae) in mass-flowering crop fields within forest-dominated landscapes

Agricultural intensification has led to structurally simplified landscapes with reduced and fragmented resources for farmland insects. However, studies on the effects of landscape composition on farmland insects have mainly been performed in areas dominated by open arable land and semi-natural grasslands, while studies from forest-dominated landscapes are scarce. This research examined the effects of landscape composition on hoverfly species richness and abundance in arable land in boreal forest-dominated landscapes. Hoverflies were sampled in 22 mass-flowering caraway (Carum carvi) fields in Central Finland using pan traps. The effects of landscape composition on species richness and abundance were examined for all hoverflies, and for species groups with different adult habitat preferences. Landscape composition was measured as proportions of land cover classes within two different radii. Species richness and abundances of all hoverflies, forest species and open-habitat species increased with decreasing arable land cover and/or increasing forest cover within a 500 m radius (the two land cover classes strongly negatively correlated). Wetland species were most abundant in landscapes with an intermediate cover of arable land and forest, and most species-rich in landscapes with intermediate (10%) water cover. The species richness and abundance of mixed-habitat species increased with increasing cover of transitional woodland. Implications for insect conservation Our results show that most hoverfies in arable land benefit from increasing surrounding forest cover even in relatively heterogeneous, forest-dominated landscapes. Preserving or increasing the area of forests and other non-arable habitats is needed to safeguard a diversity of resources for hoverflies, and associated ecosystem services in farmland. Agricultural intensification has led to structurally simplified landscapes with reduced and fragmented resources for farmland insects. However, studies on the effects of landscape composition on farmland insects have mainly been performed in areas dominated by open arable land and semi-natural grasslands, while studies from forest-dominated landscapes are scarce. This research examined the effects of landscape composition on hoverfly species richness and abundance in arable land in boreal forest-dominated landscapes. Hoverflies were sampled in 22 mass-flowering caraway (Carum carvi) fields in Central Finland using pan traps. The effects of landscape composition on species richness and abundance were examined for all hoverflies, and for species groups with different adult habitat preferences. Landscape composition was measured as proportions of land cover classes within two different radii. Species richness and abundances of all hoverflies, forest species and open-habitat species increased with decreasing arable land cover and/or increasing forest cover within a 500 m radius (the two land cover classes strongly negatively correlated). Wetland species were most abundant in landscapes with an intermediate cover of arable land and forest, and most species-rich in landscapes with intermediate (10%) water cover. The species richness and abundance of mixed-habitat species increased with increasing cover of transitional woodland. Implications for insect conservation Our results show that most hoverfies in arable land benefit from increasing surrounding forest cover even in relatively heterogeneous, forest-dominated landscapes. Preserving or increasing the area of forests and other non-arable habitats is needed to safeguard a diversity of resources for hoverflies, and associated ecosystem services in farmland.