Habitat effects on slug assemblages and introduced species

Habitat disturbance alters environmental conditions and can affect biotic exchange. While the process of biotic exchange is difficult to quantify in the field, it affects community assembly and thus species abundance distributions, diversity, faunal homogeneity and biogeographic patterns. Here, we provide the first comprehensive assessment of habitat-specific assemblage structure and turnover in slow, active dispersers, namely slugs (Mollusca: Gastropoda). We compare species richness, densities, assemblage homogeneity and spatial turnover from nine differently disturbed habitat types (total: 729 sites) within an area spanning the border between two major European biogeographic regions, the Atlantic and the Continental Regions. Gardens, mesic open habitats and successions tended to harbour many introduced species. The nonmetric multidimensional scaling plot revealed a gap, rather than a transition, between disturbed habitats (including woody successions) and mature forest stands. This gap indicates a tipping point for slug assemblages related to food sources and microclimate. Anthropogenic disturbance blurred the effect expected from the border between the biogeographic regions. When compared with broadleaved forests as the natural vegetation cover, human disturbance doubled the range before spatial faunal dissimilarity occurred in physically highly fragmented gardens, and more than tripled it in continuously-disturbed habitat types such as mesic open habitats. Our results support the idea that continuously-disturbed habitats are associated with altered passive dispersal and colonization dynamics, distorted biogeographic boundaries and faunal homogenization.