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Snails and slugs are commonly found in a wide variety of habitats but many species are very small and easily overlooked because they seek moisture and typically make their home away from direct sunlight. Seasonal differences in weather and moisture affect the presence and numbers of species. Some land snails and slugs are large and easy to find but small and medium sized species require a much more careful search. Look under logs, rocks, roots, fallen branches and fallen leaves or under rubbish or anything which offers some protection from exposure. At times of heavy rainfall, many species crawl up vegetation, finding shelter under the loose top layer of newly fallen leaf litter or on the undersides of fern fronds or other vegetation. Very small species are best found during such times; a handful of loose leaf litter may contain a number of minute snails. More is known about the coastal faunas than those inland, but the generalizations probably hold true for the fauna of the Kootenay region. Myers (1972) determined that the amount of organic matter in the soil and soil texture, as related to water retention, corresponds directly to the diversity of terrestrial gastropods in alder, cedar and fir habitats in western Washington. These studies and Cameron (1986) found that the greatest diversity and abundance of terrestrial gastropods were under cedar, and least in Douglas-fir habitats. Karlin (1961) had earlier observed a strong association between terrestrial gastropods and deciduous trees in many localities from northern Montana south to Colorado. Cameron (1986) investigated the coastal faunas on Vancouver Island and the Fraser and Chilliwack Valleys and found that the type of litter, soil and associated vegetation influence diversity and abundance of snails. He characterised litter as either mull - a granular forest humus that forms a layer of mixed organic matter that merges gradually into the mineral soil beneath; or mor - a forest humus forming a layer of largely organic matter abruptly distinct from the mineral soil beneath. High diversity and abundance of terrestrial molluscs is associated with mull litter, which is richer in nutrients and damper than mor litter. Soil, litter and vegetation types are usually correlated. Localities with mull litter are dominated by cedar and/or various deciduous trees, such as bigleaf maple. Localities with mor litter are dominated by Douglas-fir, spruce, or pine. Cameron found that faunas on mor litter and litter intermediate between mor and mull are impoverished. This is congruent with observations made by others; Boag & Wishart (1982) and Karlin (1961) found that coniferous forests (characteristically having a mor litter) have about the same number of species as deciduous and mixed forests but with significantly lower densities. Cameron also determined that disturbed sites, usually dominated by deciduous forests and therefore having corresponding mull litter, are often rich in species, and may also contain additional non-indigenous species. He also noted that burning and logging does not significantly reduce diversity. The distributions of species are dependent upon several factors, including climate, geography, topography and their ancestral migration. Distributions of terrestrial molluscs do not correspond to the geopolitical boundaries of British Columbia but rather follow natural limitations. Along the coast, the ocean moderates the climate. In the east, the Rocky Mountains form a natural barrier for migration. Towards the north, climatic conditions become progressively more severe. As a rule, British Columbia has considerably
fewer species of terrestrial gastropods than southern Washington,
Oregon, Idaho and other areas south. British Columbia has no known
endemics (species confined to a particular geographic area). In
British Columbia several species of snails and slugs occur only
in the Kootenay region, their main distribution being south into
the United States. After the retreat of the last glaciers, the Columbia
Basin was repopulated by terrestrial gastropods from the south.
The genera Oreohelix, Anguispira and Allogona have
their origins south of B.C. The greatest species diversity is consequently
in the southern part of the Columbia Basin region. The northern
portion of the Columbia Basin lacks the distinctive elements of
the southern fauna, and has fewer species, but these are more widespread.
Approximately 25% of the terrestrial mollusc species known to occur in British Columbia are exotic (introduced from another geographic area). All are ultimately European in origin, except perhaps Vallonia pulchella and Vallonia excentrica which are native to Eastern North America as well as Europe. For the vast majority of these exotic species, the time, place and method of introduction is not known. However, the spread of exotic species subsequent to their initial introduction appears to be with plants and related material (i.e. compost or soil) (Forsyth in press), and earlier Getz & Chichester (1971) noted that for all European slugs introduced into North America transport was with plant material. In the Columbia Basin region, there have been six species of non-native terrestrial gastropods identified to date. They are Arion rufus, Deroceras reticulatum, Limax maximus, Vallonia pulchella, Vallonia excentrica, and Cepaea nemoralis. Another exotic species yet to be identified to species, but known to belong to the Arion fasciatus complex of species, has also been recorded. Other species could also be present in and near towns, cities and agricultural areas, but our collecting did not focus on such areas. Important papers on exotic terrestrial gastropods of British Columbia are Hanna (1966), Rollo & Wellington (1975), Forsyth (in press) and Reise et al. (in press). |
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