The Native Banana Slug
from buttemer 1997
Student # __
Ariolimax columbianus is the largest indigenous (native), terrestrial slug in North America. It lives in the coastal forests from California to the Alaskan Panhandle. Banana slugs in Western Washington are active from April to November. They inhabit soils in regions deficient in calcium and where snails cannot grow, and thereby compete with slugs. The streamlined body and absence of a rigid shell enable the slugs to escape from predators into crevices and hole up during periods of too dry/too cold weather.
Life Span: Banana slugs live an average of 3-5 years, although 7 years olds have been recorded. They have elaborate courting rituals that can last for 36 ours, and their eggs are laid one to two weeks later. Eggs are laid once a year, from late August to November. They can hatch before winter, or wait till spring. Banana slugs begin hibernation in mid November.
Foraging: The Banana Slug is a woodland forest species. It isn't a serious garden pest. It prefers fungi and decaying fruit to leaves. Slugs crawl about 5/10 meters a day, with speeds of up to 0.007 mph. They do not feed at temperatures below 10ūC. Slugs feed the most at temps of 13-16ūC on calm, cloudy days with high humidity. They never feed for more than 3 hours per day. So about 50-70% of their time is spent searching for food. When they find food they eat whatever it is, rather than looking for something else more tasty or with more variety. In spring and autumn they feed from 8am to 7pm. In the summer they feed from 6-10am and again from 9pm-1am. Thus they avoid the worst of the warm, dry, and sunny weather.
Foods: Banana slugs will eat Poison oak and Foxglove (both toxic), although Devil's club and Bracken fern are considered favorites. Many kinds of herbs are also eaten. They also prefer fallen leaves (esp Alder and Maple), mosses, and lichen. Lichen are an important food for Banana slugs in spring and autumn when other food is hard to find. They will also eat flowers, seeds, and decaying fruit when the toxin levels have decreased. Yum! Banana slugs need 1 calorie of food energy/day, compared to the 2-4,000 calories/day humans need.
Ecological Role. Slugs are first level consumers, decomposers, and some are carnivores. They disseminate (spread) micro flora, micro fauna, and fungal spores over the forest floor. (Many fungi are symbiotic with forest trees.) Banana slugs enhance forest litter decompostion, which is helpful as earthworms are absent from wet forest soils. They also play a role in forest succesion by selective grazing of seedlings.
Security: Banana slugs often rest above the forest floor, hiding under leaves for protection against rain. During spring and autumn they climb extensively at night to avoid the wet forest floor, cold temperatures, and frost pockets at low levels. They don't feed on the vegetation in the trees, but descend to the forest floor during the day to feed. During summer they also hide in decaying stumps and moist logs. Several slugs may lay together to lessen water loss.
Predation: As mentioned before, slugs have many predators. However young Banana slugs form a significant part of the diet of 5 species of shrews in this area. Usually slugs greater that 1 gram are not eaten. This could be because the mucus is too hard to clean off! It also could be because the bigger Banana Slugs learn to climb up off the forest floor, at night, thus avoiding the shrew's prime feeding time.
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