Interactions between invasive and native harvestmen

Abstract The harvestman Opilio canestrinii arrived in Denmark around 1980 and quickly became a dominant species in urban habitats. This was followed by nearly complete disappearance of the formerly co-dominant Leiobunum gracile . A second invasive species, Dicranopalpus ramosus , arrived in 2007 and is still under population increase. Old forests have been a refuge for L. gracile , but both invasive species are now invading this habitat. We investigated the possible presence of exploitative and interference competition in relation to limited food resources between the three species in the laboratory using pairs of individuals, either heterospecific ( O. canestrinii/L. gracile ; D. ramosus/L. gracile ) or conspecific ( L. gracile/L. gracile ) and offered food resources that were insufficient for both individuals. O. canestrinii gained significantly more weight than other species. L. gracile exhibited less startle behaviour and a higher contact time with its partner in the conspecific than in the heterospecific treatment. Overall, O. canestrinii performed better than L. gracile , when faced with food scarcity and restricted space, whereas D. ramosus did not. Thus, competition (both exploitative and interference) is a possible mechanism for explaining the disappearance of L. gracile from urban habitats following the successful establishment of O. canestrinii in Denmark; additionally, O. canestrinii may be a potential future threat to L .gracile in forests.