The Ecology of Slugs
from buttemer 1997
The slug and its environment
Student # __
Evolution - Think of a slug as a snail that lost its shell. That's what they did, although some slugs actually carry a tiny, vestigial shell - momentos of their snail like ancestors. Slugs no longer have shells because there is very little calcium in the soil. It takes calcium to build a shell. (and bones too, by the way) The famous rains of Western Washington have leeched (washed slowly) calcium out of the soil. So, the slug became shell-less, open to droughts, floods, and dangerous predators. On balance, it lives in the perfect environment to thrive, with a cool, moist environment, and plentiful food supplies.
Who eats whom? A Slug's niche in the environment depends on what food is available. They are first level consumers chowing down on native herbs, lichens, and ripe fruits. They are decomposers, eating fungi, like mushrooms, and decaying vegetation. Some are carnivores, eating the flesh of other critters.
Slugs are eaten by a variety of predators. Birds, like geese and ducks, small mammals, carabid beetles, snakes, lizards, centipedes all partake of a tasty slug snack at times. Slugs have an awesome protection from most animals and external parasites in their mucus. However they are often infested with internal parasitic worms, like nematodes, cestodes, and planarians.
Competition between Slugs - Slugging it out! 100 years ago, the only large native slug in the Pacific Northwest was the Banana Slug (Ariolimax columbianus) . Since then many species of foreign slugs have been introduced on imported European garden plants. These foreign invaders are extremely fierce!! They attack weaker individuals of their own and other species, driving them out of shelters and away from food. Combat begins when the aggressor slug touches its victim with its tentacles. Then it lifts the forepart of its body off the ground and slashes down at its victim with its toothy radula. Argh - death by rasping!!! A quick acting victim will side-swipe the aggressor with its tail and move away. Sometimes the victim exudes a large puddle of mucus to distract its aggressor before it flees. At other times the aggressor successfully continues the attack, striking and slashing until either the victim succumbs or escapes.
Foreign invaders are much more prolific than the gentler Banana Slug. Foreign slug lay eggs twice a year, Bannana Slug's lay once. They are more adaptable to cultivated areas and are voracious feeders. It's these European invaders that have given slugs a bad name amongst Northwest gardeners. Every time we cut down trees, lay down lawn, make gardens, and clear forested areas we create a situation where the foreign imports can compete better. Thus the Banana Slug is rarely found in cultivated areas and developments unless the area backs onto a forested greenbelt. There are some urban (city) reservations, however. It is estimated that there are 5-10,000 Banana Slugs per acre in Carkeek Park, in the heart of Seattle. Imagine the influence that this many slugs have on the natural balance of the forest habitat.
Presently, introduced slugs only live in larger urban areas. They are being spread into the Cascades on the boots of unwitting hikers. There they attack and outcompete Banana Slugs for territory. It is unclear what effect this change will have to our natural community and the Pacific Northwest forests. But think about it. The Banana Slug is destined to eventually join so many other creatures on the endangered species list. Will this make a difference to us? We may never know its true effect until it's too late!
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