Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 3
Oregon Forestsnail Allogona townsendiana
Wildlife Species Description and Significance
Oregon Forestsnail (Escargot-forestier de Townsend) is a member of a large and diverse family (Polygyridae) of North American land snails. The shell of mature individuals is pale brown or straw-yellow, round and flattened, and ranges from 28-35 mm in diameter. The apertural lip of the shell is pale and broadly expanded. Unlike some related species, the aperture (shell opening) lacks tooth-like structures (denticles). The shell is smooth and without hair-like projections as in some related snails.
Relatively few native, large land snails inhabit coastal forests in British Columbia (BC). The presence of the Oregon Forestsnail in Canada is of both scientific and conservation interest, as populations at the northern limits of their geographical range might possess unique adaptations.
Distribution
Oregon Forestsnail occurs in the western Cascade Range, Puget Trough, and eastern lowlands of the Olympic Peninsula in the United States north into extreme southwestern BC. Oregon Forestsnail are found from Tsawwassen through the Lower Mainland towards Langley, with most records occurring within the Lower Fraser Valley in the Mission, Abbotsford, Chilliwack area, and as far east as Hope. There is one outlying site on southern Vancouver Island near Crofton.
Habitat
Oregon Forestsnail occupies mixed-wood and deciduous forest habitat, typically dominated by Bigleaf Maple, Balsam Poplar and scattered Western Redcedar. Many records are from riparian habitats and forest edges, where dense cover of low herbaceous native vegetation is typically present. The presence of Oregon Forestsnail is correlated with the presence of Stinging Nettle, although the specific connection between these two species has not been studied. Stinging Nettle is known to have high levels of calcium, which is necessary for healthy shell growth in snails. All known Canadian Oregon Forestsnail populations are from habitats less than 360 m above sea level.
Biology
Oregon Forestsnail is hermaphroditic. The surface activity of the snails appears to peak in spring and early summer, coinciding with mating and oviposition. They appear to be slow maturing and long-lived, reaching maturity at two years and living from five to eight years. Their dispersal ability is likely poor, based on the scattered distribution of the species throughout its geographic range.
Population Sizes and Trends
Increased search effort since the initial species’ assessment by COSEWIC in 2002 has resulted in numerous additional records of Oregon Forestsnail. There is no information available on population size from previously known sites to allow the assessment of trends. The Canadian population for Oregon Forestsnail is estimated to be in the 100,000s.
Fifty-two of the 66 known sites (78.8%) were assessed as fragmented and isolated. At least 50 sites are less than 5 ha in size and/or are subject to urban development within the next 10 years. These sites are isolated by distances of more than 1 km of unsuitable habitat as a result of past land development (since 2002) and most likely will not be able to maintain a viable snail population in the future. While only 108 ha of the 328 ha (33%) of the biological area of occupancy of known mapped sites is fragmented and isolated, nearly 80% of the total number of known sites and 67% of the 75 occupied 2 km x 2 km grid cells are considered too small and isolated to maintain viable populations. These remnant fragments are indicative of larger pieces of habitat that once existed.
Threats and Limiting Factors
The Canadian range of Oregon Forestsnail coincides with the most densely populated and highly fragmented region of BC. The most serious threat to Oregon Forestsnail is continuing residential and commercial development, resulting in further fragmentation and isolation of populations. Snails have limited dispersal capabilities and cannot move between habitat patches or colonize suitable habitat if there are barriers to dispersal. Additional threats include recreational activities and invasive non-native/alien species.
Protection, Status, and Ranks
Oregon Forestsnail is listed as Endangered under the federal Species at Risk Act and a BC Recovery Plan is completed. The species is recommended for listing as Identified Wildlife under the BC Forest and Range Practices Act, the BC Wildlife Act and Wildlife Amendment Act. Oregon Forestsnail habitat is protected within parks and protected areas by the BC Park Act and Ecological Reserves Act. Where Oregon Forestsnail occurs in parks and properties owned by local governments, land managers are aware of the snail and its habitat needs although efficacy of protection varies. The provincial Water Act and Riparian Areas Regulation under the provincial Fisheries Act have the potential to indirectly protect Oregon Forestsnail habitat.
Oregon Forestsnail is critically imperilled-imperilled (S1S2) in BC and is nationally ranked in Canada as critically-imperilled-imperilled (N1N2). The global conservation status rank is vulnerable-apparently secure (G3G4). In Washington State the species has a conservation status rank of vulnerable-apparently secure (S3S4) but it is not ranked (SNR) in Oregon State.
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