Relations between forest management and slug assemblages (Gastropoda) of deciduous regrowth forests

The degradation–deforestation–afforestation-enrichment cycle many west European forests have undergone during the last centuries equals a large-scale disturbance experiment. The aim of this study is to analyze effects of past disturbances and recent management on forest-floor slugs, i.e. a group with comparatively low mobility and presumably low recolonizing ability. To account for spatial variability within forests, slugs were collected from moist leaf litter from sites close to and distant from coarse woody debris (CWD). In total, 68 different forests were sampled, and 1736 slugs from 15 species were found. The species response curves illustrated that most slugs have defined habitat preferences and thus may be suitable indicators. A canonical correspondence analysis with 10 environmental factors revealed that the slug assemblages were predominantly structured by anthropogenic disturbance. Rarefaction diversity was higher in disturbed forests, but the proportion of stenoecious forest species (Arion silvaticus, A. subfuscus, Limax cinereoniger, Lehmannia marginata, Malacolimax tenellus) within the assemblages was higher in comparatively undisturbed forests than in forests with high anthropogenic disturbance. The proportion of these species increased with forest age and the amount of woody debris on the forest floor. Soil acidity did not influence densities, but species richness was highest on soils with pH values between 4 and 5. The proportion of stenoecious forest species within the assemblages increased with decreasing pH. Slug densities and species richness were higher close to CWD than distant from CWD. The proportion of Limacoidea (Limacidae, Agriolimacidae, and Boettgerillidae) increased close to CWD whereas that of the Helicoidea (Arionidae) decreased. The results show that the slug assemblages are relatively resilient. However, stenoecious forest slugs profit from nature-oriented forestry, whereas exotic species such as Arion “lusitanicus” profit from high anthropogenic disturbance.