Invasive species frequently affect urban ecosystems, but few studies investigate primary and secondary mechanisms to determine overall ecological effects. Previous work demonstrates slugs can negatively affect seedling recruitment and vegetation biomass, and positively affect litter decomposition, nutrient cycling, and litter invertebrate populations. This study aimed to assess components of how invasive European black slugs (Arion rufus) affect native Cascade Oregon grape (Mahonia nervosa), in Pacific Northwest forests. We hypothesized that slug herbivory would decrease leaf area, but excretion of leaf and litter matter would enhance soil and litter conditions. Field experiments aimed to alter slug densities to measure effects on leaf area, litter biomass, soil nutrients, and non-slug litter invertebrates. Leaf area was lower in areas with high slug densities along with slight increases in non-slug invertebrate abundance; whereas leaf area, number of leaves, and branches per plant were higher in areas with low slug densities. However, slugs did not affect soil nitrate or litter decomposition significantly. Laboratory experiments found slugs consumed fresh leaves faster than litter but there were no differences in assimilation. By testing potential primary and secondary ecological mechanisms, our results suggest the overall effect of invasive slugs on a common forest understory plant is negative, as expected, but secondary positive effects on the litter invertebrate community, which could mitigate cumulative impact, are subtle and detection may require longer-term study.