Plant species' range type determines local responses to biotic interactions and land use

Geographic plant distribution is often assumed to be predominantly limited directly by the climatic tolerances of species. However, the role of climate is now known to be mainly an indirect one mostly mediating dispersal and establishment, species interactions, or habitat characteristics, which all are often modified by human land use. In these complex systems, negative biotic interactions are predicted to increase in relative importance toward benign climatic conditions. We tested this hypothesis experimentally by exposing plant species with different geographic distribution ranges to different climates, biotic interactions, and land use. Thereby, species predominantly distributed in regions with benign climatic conditions were expected to be better able to cope with negative biotic interactions than species from regions with environmentally stressful climatic conditions. We present results of a fully crossed two-year transplantation field experiment replicated in 45 plots in three study regions along a precipitation gradient across Germany. We manipulated biotic interactions (presence/absence of competition and mollusk herbivory) in grasslands of different management regimes (meadows, mown pastures, pastures). The transplanted phytometers consisted of six congeneric species pairs, each representing one oceanic and one distinctly more continental range type. The oceanic range type is predominantly distributed in benign climatic conditions in Western Europe, while the more continental type is distributed in regions with more stressful climatic conditions in Eastern Europe. This experimental setting allowed us to study the impact of negative biotic interactions along an abiotic stress gradient under realistic land-use conditions. Under competition and mollusk herbivory, growth performance was more strongly reduced in continental compared to oceanic species. Range types also differed in their responses to grassland management. Differences in survival between the congeneric species were found to be region-specific and largely unaffected by biotic interactions and land use. In consequence, our results suggest that local responses to biotic interactions and land-use practices of otherwise very similar plant species can differ strongly depending on species' large-scale geographical distribution. Regionally differing responses to biotic interactions also show that local conditions can drastically change responses expected from macroecological theory.