Invasion of alien slugs in disturbed habitats: role of behavioural phenotype, plasticity and interspecific competition.
Habitat disturbance can promote the invasion success of alien species, but the underlying mechanisms remain unclear, especially in animal species. Alien species may have trait values that facilitate invasion (ideal weed hypothesis) or greater phenotypic plasticity than native species (plasticity hypothesis), thus promoting their invasion success in disturbed habitats. Invasion success and ultimately alien species dominance in disturbed habitats can also be a direct consequence of competitive exclusion of native species. We performed experiments in controlled conditions to determine which of these mechanisms could explain the invasion success of Arion subfuscus s.l. (hereafter Arion), an alien slug species complex that lives in various disturbed ecosystems of eastern Canada such as managed forests. To do so, we compared the behavioural response of Arion and native sympatric philomycid slugs (hereafter Philomycus) to a decrease in soil moisture content, which often occurs after forest harvesting (i.e. habitat disturbance). We also investigated whether the presence of Arion alters the behaviour of Philomycus, which would indicate that Arion has a greater ability to compete by interference than these native species. Our findings show that Arion did not have higher boldness or locomotor activity (i.e. behaviours that promote invasion success) or greater plasticity than Philomycus, and that the presence of Arion did not affect the behaviour of Philomycus. However, Arion may be able to exploit resources more efficiently than native species since it mainly used the closest refuge from the food source in the present study. These findings are in line with the ideal weed hypothesis but do not support the phenotypic plasticity hypothesis or any role of interference competition in Arion invasion success in disturbed habitats such as harvested forests. Our results also suggest that the direction, but not the magnitude, of behavioural plasticity might be a key aspect in alien species invasion success in disturbed habitats.