Different interaction effects between two invasive harvestmen and a native relative in the laboratory

Interspecific exploitation and/or interference competition may be operating when a successful invasion of an animal species leads to decrease or disappearance of native relatives, in particular if inter-assemblage predation and other interactions are unlikely. The harvestman Opilio canestrinii arrived in Denmark around 1980 and quickly became a dominant species in urban habitats. This was followed by a nearly complete disappearance of the formerly co-dominant species, 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 effects 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 ). Food resources offered were insufficient for both individuals. Opilio canestrinii gained significantly more weight than the other species. Leiobunum gracile exhibited fewer startle behaviours 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.