Temperature, leaf cover density and solar radiation influence the abundance of an oligophagous insect herbivore at the southern edge of its range

Abstract

Altitudinal gradients strongly affect species distribution through the direct and indirect effects of temperature. While numerous studies have been done in the central parts or at the northern edges of species’ distributions, the patterns found at the southern edges of species’ distributions in the southern part of the temperate zone have received much less attention, especially in forest floor insect herbivores. In a case study of the altitudinal influence on insect–plant interactions in forest ecosystems, we examined the insect herbivore Cheilosia fasciata (Diptera: Syrphidae) and its hostplant ramsons Allium ursinum. We studied the influence of altitude, leaf cover density of the host plant, host plant patch size, solar radiation and forest type on C. fasciata. We investigated 0.5 × 0.5 m quadrats in patches of ramsons over several altitudinal gradients across Slovenia. The abundance of C. fasciata increased with altitude, while this pattern was not observed in ramsons. Temperature negatively affected abundance. The leaf cover density of the host plant influenced insect abundance positively only when leaf densities were low. Solar radiation had a positive effect on abundance. Forest type and host plant patch size were less important. The results are discussed in the context of mechanisms affecting altitudinal distribution and climate change.