page_title: Black slug
section_title: Invasive species
hierarchical_section_title: Black slug / Invasive species
caption_attribution_description: English: Black slug, Arion ater, munching fungi in Snoqualmie valley. Washington state
context_page_description: The black slug Arion ater L. is a large terrestrial gastropod mollusk in the family Arionidae, the round back slugs. Land slugs lack shells like other terrestrial mollusks. Without such shells, slugs produce unappetizing mucus, that may also contain toxins—to deter predators. Terrestrial slugs produce two other forms of mucus that facilitate locomotion and prevent death from drying. Such mollusks are hermaphroditic. Slugs most often function as decomposers but are also often omnivores. Arion ater is one such slug, decomposing organic matter, preying on other organisms, and consuming vegetative matter including agricultural crops. Native to Europe, the black slug is an invasive species in Australia, Canada, and the United States.
context_section_description: Arion ater has been introduced to southeastern Australia and to North America, where it occurs in Newfoundland, southern British Columbia, the Pacific Northwest of the United States and some parts of Alaska. In the past two decades, its mating with the non-native (at northern latitudes) pest species Arion vulgaris (or Spanish slug) has resulted in a more resilient hybrid exhibiting increased tolerance to cold.
  These slugs might endanger sensitive ecosystems, especially as an invasive species, and it is yet unclear how drastically these slugs might alter plant community compositions. The black slug is a voracious seedling predator. Terrestrial slugs are considered to be especially dangerous because they alter plant species abundance, adult plant fecundity, and the production of plant defensive compounds. Black slugs are of special concern in fragmented ecosystems and areas with high shrub and tree cover. In Alaska, the black slug threatens seedling populations of lilies and orchids after already having diminished sensitive populations of deltoid balsamroot and yellow montane violet in BC Canada.
  There is much debate concerning black slug effect upon plant species diversity. Slug impacts change over successional stages, and Alaska conservationists observed the black slug’s impact on species diversity depends upon community composition. If a system is composed of sensitive species, the black slug will likely have a negative impact by pressuring said species. If a system presents more evenness with less sensitive species, the black slug may promote species diversity and encourage healthy succession rates.
  Around the 1970s, the black slug came to Alaska via nursery plants, potting soil, or among shipments containing wood pallets. The Alaska National Heritage Project published an invasive ranking for the black slug, providing a recent case study of the species as an invasive. They ranked the species based upon distribution, biological characteristics, ecological impacts, and feasibility of control. These categories were assigned points, and Arion ater received a 62/100, awarding it a qualitative score of “moderately invasive.” This report stated the black slug can often be found in disturbed soils or areas with well-irrigated soils such as lawns. Frequent sightings along roads suggest the black slug might move more easily along human traffic corridors; however, the black slug is believed to migrate little during its one-year life span. Arion ater’s biology awarded it its highest risk factor. Again, the black slug is a prolific, omnivorous consumer, capable of inhabiting a wide range of conditions with high fecundity. Its effect upon local ecology received a moderate score; while the black slug puts pressure on native systems, it does not appear to catalyze entire regime shifts within the habitats where it has invaded. Greatest concern appears to be its possible pressure on early successional plants and native slugs. The black slug often feeds upon seedlings, diminishing plants’ reproductive success, but the black slug does not appear to be feeding selectively, endangering one plant species over another. And the black slug’s effect on native-Alaska slugs is unknown, but the black slug might be displacing the banana slug in BC, Canada; however, such displacement might not have implications beyond those for the banana slugs. Slugs facilitate seed and spore dispersal through fecal matter. The black slug facilitates nutrient cycling by decomposing decaying and fecal matter; additionally, slug mucus left behind in the soil helps cycle nutrients, and Arion ater may merely step into this niche in place of native slugs. Arion ater has spread in Alaska in the following places: Anchorage, Cordova, Yakutat, Gustavus, Juneau, Sitka, Tenakee Springs, Ketchikan, and Kodiak Island, and in some of these areas, the black slug threatens lilies and orchids. It is unlikely complete removal can be achieved given high fecundity, but fencing and educating the public to hand pick black slugs might mitigate associated invasion risks.
  Conservationists w
caption: Arion ater eating fungi
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