The Northwest native Banana slug (Ariolimax columbianus) is close to nonexistent in many urban forests, largely replaced by the non-native European black slug (Arion ater). Previous studies show that slugs have negative effects on plants through herbivory, while others indicate that slugs consume leaf litter and release nutrients into the soil via their feces, potentially positively affecting plant growth. To investigate these effects, treatment sites at ten locations within Seward Park in Seattle, Washington, were created to simulate the effect of high, normal, and low (nonexistent) A. ater densities on soil nitrate availability, leaf litter consumption, and Oregon grape (Mahonia nervosa) growth. Similar laboratory experiments with A. columbianus were completed for species comparisons and to investigate mechanisms and impact of their local extinction. Results/Conclusions
Results indicate that A. ater has a negative net effect on M. nervosa, although studies concerning leaf litter consumption and A. columbianus were inconclusive. Future research building on this study would include modeling as an approach to investigate the loss of A. columbianus in Pacific Northwest forests.