Feeding Preferences and Potential Competition between Ariolimax columbianus and Arion rufus

Within the last 100 years, the arrival in the Pacific Northwest of the invasive European slug Arion rufus has introduced a potential competitor for the iconic banana slug Ariolimax columbianus, the second largest terrestrial slug in the world. We conducted transect studies in three habitats (cedar forest, Doug fir forest, and grasslands) on San Juan Island, WA, to examine the relative demographics of these slugs and note possible food sources. Utilizing those data, we conducted feeding preference studies across leaves from 16 local plant species, incorporating slugs from the two species from different source habitats, different size classes, and both the black and brown morphs of Arion rufus, which tend to specialize in different habitats. We used ImageJ to measure the area eaten from each leaf and calculated feeding rates. The feeding preferences of Ariolimax columbianus were consistent across size classes and source populations, and the two color morphs and various source populations of Arion rufus were also largely consistent within this species. While both species are generalists, they demonstrated feeding preferences that frequently overlapped, indicating that Ariolimax columbianus may compete with Arion rufus for food resources