The Effect of Mucus and Feces from the Invasive Slug Arion rufus on the Growth of Annual Ryegrass Lolium multiflorum

On San Juan Island, WA, the invasive slug Arion rufus lives in two habitats, the Douglas-fir/red cedar forests and the grasslands. The native slug of comparable size, Ariolimax columbianus, the banana slug, only lives in the forests, so Arion represents an inherently new type of herbivore/decomposer in the local grassland ecosystems. At some sites, Arion is present at extremely high densities, making it likely that this new species may be altering the overall rate of nutrient cycling in its new habitat. We examined the effect of live Arion slugs and Arion feces on the growth of the annual ryegrass, Lolium multiforum, over a month in July 2015. We found no significant difference among our treatments (live slug, slug feces only, no slug impact) with any measure of biomass production. This finding seems contrary to what we expected based on prior research by other researchers which typically occurred in forested areas. Our results prompt the larger questions of how Arion impacts grassland ecosystems, whether the impact of Arion slugs on nutrient cycling in forests is different than its impact in grasslands, and, if this difference does occur, why it does so