Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 8

Habitat

Habitat Requirements

Information used to describe Oregon Forestsnail habitat in BC includes the collective efforts of occurrence records in the BC Conservation Data Centre (2013) and the provincial recovery plan for Oregon Forestsnail (Oregon Forestsnail Recovery Team 2012), which is the source of information for much of what follows, unless otherwise indicated.

Oregon Forestsnail habitat is low elevation (30 - 360 m asl), deciduous and mixed-wood broadleaf forests with multi-structured vegetative microhabitat and sustained high moisture and relative humidity. High-quality habitat includes forests with high site index (forest growth productivity) including riparian areas, ravines, gullies and depressions containing both permanent and ephemeral watercourses; the wooded edges of streams, marshes, seasonally flooded and wet lowland areas; forest interfaces, and edge habitats where moisture is retained (Waldock 2002).

Forest overstory composition includes deciduous and mixed tree species, 20 to greater than 80 years old, and dominant overstory composition greater than 40%. Overstory composition includes large Bigleaf Maple (Acer macrophyllum), Black Cottonwood (Populus trichocarpa) and scattered Western Redcedar (Thuja plicata). Additional trees present include Paper Birch (Betula papyrifera), Trembling Aspen (Populus tremuloides), Red Alder (Alnus rubra) and Grand Fir (Abies grandis).

Dominant shrub species composition is typically dense shrub vegetation that functions to minimize moisture and evaporative loss from this vegetative layer. Native shrub species composition includes a suite of the following: Devil’s Club (Oplopanax horridus), Elderberry (Sambucus racemosa), False Azalea (Menziesia ferruginea), Beaked Hazelnut (Corylus cornuta), Indian Plum (Oemleria cerasiformis), Ocean Spray (Holodiscus discolor), Red-osier Dogwood (Cornus stolonifera), rose (Rosa sp.), Salmonberry (Rubus spectabilis), Salal (Gaultheria shallon), Saskatoon (Amelanchier alnifolia), Common Snowberry (Symphoricarpos albus), Thimbleberry (Rubus parviflorus) and Vine Maple (Acer circinatum).

Herbaceous plant composition consists of live and senescent vegetation, which provides food and cover during all life stages. Snails are often found at the base of large vegetation clumps or plants (e.g., leaf litter at the base of trees, shrubs and ferns). Herbaceous composition includes: bedstraw (Galium sp.), Bleeding Heart (Dicentra formosa), buttercup (Ranunculus sp.), Cow Parsnip (Heracleum maxiumum), Enchanter's Nightshade (Circaea alpina), False Lily-of-valley (Maianthemum dilatatum), Foam Flower (Tiarella trifoliata), Fringecup (Tellima grandiflora), Cooley’s Hedge Nettle (Stachys chamissonis var. cooleyae), horsetail (Equisetum sp.), miner’s lettuce (Claytonia sp.), Pathfinder (Adenocaulon bicolor), Skunk Cabbage (Lysichiton americanum), starflower (Trientalis spp.), Stinging Nettle, thistle (Cirsium sp.), Tiger Lily (Lilium columbianum), Western Trillium (Trillium ovatum var. ovatum), Clasping-leaved Twisted Stalk (Streptopus spp.), Vanilla Leaf (Achlys triphylla)and waterleaf (Hydrophyllum sp.) and Creeping Buttercup (Ranunculus repens). Ferns commonly recorded within Oregon Forestsnail habitat include Bracken Fern (Pteridium aquilinum), Ladyfern (Athyrium filix-femina), Northern Maidenhair Fern (Adiantum aleuticum) and Western Sword Fern (Polystichum munitum).

Most habitats of Oregon Forestsnail contain patches of Stinging Nettle (Figures 7, 8, 9, 10). Stinging Nettle appears to have high importance to Oregon Forestsnail populations especially for mating and egg-laying (Waldock 2002; Steensma et al. 2009). The consumption of Stinging Nettle is likely needed for healthy shell growth, as the plant contains high levels of calcium and other essential minerals needed to maintain shell durability. Stinging Nettle is also important to other land snails (Iglesias and Castillejo 1998). Waldock (2002) examined the association of Oregon Forestsnail with Stinging Nettle in detail at TWU-ESA (Trinity Western University Ecological Study Area)in Langley, and found a positive correlation between the abundance of the snails and Stinging Nettle. The presence of Stinging Nettle indicates moist, rich soils with high amounts of nitrogen and phosphorus (Pojar and MacKinnon 1994).


Figure 7. Open and forested habitat at Campbell Valley Regional Park, Langley.

Photo showing open and forested habitat at Campbell Valley Regional Park, Langley (see long description below).

Mixed-wood forest with Bigleaf Maple and patches of Stinging Nettle represent potential Oregon Forestsnail habitat, June 1, 2010. Photo Laura Parkinson.

Description of Figure 7

Photo showing open and forested habitat at Campbell Valley Regional Park, Langley. This mixed-wood forest with Bigleaf Maple and patches of Stinging Nettle represents potential Oregon Forestsnail habitat.

 


Figure 8. Oregon Forestsnail (Allogona townsendiana) habitat

Photo of Oregon Forestsnail habitat at Aldergrove Lake Regional Park, Abbotsford (see long description below).

Aldergrove Lake Regional Park, Abbotsford. A patch of Stinging Nettle where Oregon Forestsnail were observed, May 19, 2010. Photo Laura Parkinson.

Description of Figure 8

Photo of Oregon Forestsnail habitat at Aldergrove Lake Regional Park, Abbotsford. The foreground shows a patch of Stinging Nettle where Oregon Forestsnail were observed in May 2010.

 


Figure 9. Oregon Forestsnail (Allogona townsendiana) habitat at Bridal Veil Falls Provincial Park, July 9, 2011.

Photo of Oregon Forestsnail habitat at Bridal Veil Falls Provincial Park (see long description below).

Note dense patch of Stinging Nettle (Urtica dioica) adjacent to a road right-of-way and parking area for recreational vehicles (partially shown). Photo by Jennifer Heron.

Description of Figure 9

Photo of Oregon Forestsnail habitat at Bridal Veil Falls Provincial Park. A dense patch of Stinging Nettle can be seen on the left of the image. To the right of this patch is a grassy area and farther right still a parking area for recreational vehicles. The background is wooded.

 


Figure 10. Oregon Forestsnail (Allogona townsendiana) habitat at Colony Farm Regional Park, June 11, 2010.

Photo of Oregon Forestsnail habitat at Colony Farm Regional Park (see long description below).

The trail edges and habitat beyond have dense Stinging Nettle (Urtica dioica). Photo by Jennifer Heron.

Description of Figure 10

Photo of Oregon Forestsnail habitat at Colony Farm Regional Park. The habitat occurs beside a trail and contains dense patches of Stinging Nettle.


Soil composition at Oregon Forestsnail sites includes rich, mesic and soft, productive, moist, well-developed mull-type[2] litter layer soils and is an important habitat requirement at all life stages (Cameron 1986; Steensma et al. 2009). Litter depth (leaves and needles) is typically 5 - 10 cm (Durand 2006) and often greater than 15 cm. This deep litter layer provides shelter, hibernation and aestivation sites (Steensma et al. 2009; BC Conservation Data Centre 2013). Soil pH was 6.4 - 6.9 from three sites in Langley (Steensma et al. 2009) while soil temperature was 9.9 - 13°C.

Oregon Forestsnail is recorded from habitats with abundant coarse woody debris at various stages of decay. Size ranges from large-diameter pieces to a forest floor composed of thin, compact leaf litter. Coarse woody debris is an important habitat attribute for Oregon Forestsnail activity: mating, nesting, aestivation, hibernation and egg laying (Steensma et al. 2009) and offers protection against daily or seasonal variations in temperature and water availability (as summarized in Prior 1985; Steensma et al. 2009). Decaying logs retain moisture and allow for the growth of a thick and healthy moss layer, both of which provide essential shelter during warm and dry weather conditions. It is important for Oregon Forestsnail to have a suitable resting site where moisture can be absorbed through the foot; contact re-hydration is crucial for survival of gastropods (Prior 1985). Large diameter, damp rotten logs provide sites for aggregating and mating (Steensma et al. 2009; BC Conservation Data Centre 2013). Oregon Forestsnail has been occasionally observed ovipositing within well-decayed wood (Steensma et al. 2009; BC Conservation Data Centre 2013) but also build nests in soil and moss (see Life Cycle and Reproduction). Soils soft enough for digging but firm enough for the nesting chamber to hold its shape is an important habitat requirement.

One study at TWU-ESA recorded Oregon Forestsnail mating pairs requiring humidity greater than 76% with optimum humidity 81 - 100%. These results suggested this environmental factor may have more of an influence over mating activity than air temperature (ranged from 7.1 - 17.0°C) (Steensma et al. 2009). Soil moisture measured at three of seven Oregon Forestsnail mating sites at TWU-ESA was 30 - 37% (Steensma et al. 2009).

Habitat Trends

The Canadian range of Oregon Forestsnail coincides with the most densely populated and highly fragmented region of BC. The Lower Fraser Valley and southern Vancouver Island regions have had extensive habitat loss, fragmentation, and modification over the past 100 years. Such modifications include extensive logging of forested hillsides, valley bottoms and riparian areas before the 1950s; large-scale changes to watercourses and reduction of wetlands such as the draining of a large lake (Sumas Lake in the Fraser Valley) in the 1920s to reclaim land for agriculture and flood control; diversion and channelling of rivers and creeks; and extensive urban and rural settlement (Sleigh 1999).

Habitat trends include the cumulative and widespread loss of habitat due to land conversion; the cumulative spread of invasive species and resultant changes to ecosystem composition, structure and function, including soil structure and biotic species composition; as well as the gradual impacts from climate change. Each of these factors is discussed further in Threats and Limiting Factors.

Lower Fraser Valley habitat trends

Oregon Forestsnail inhabits riparian lowland forests and areas next to wetlands, some of which may be seasonally flooded (see Habitat Requirements). Development throughout the Lower Fraser Valley from the 1860s to present has resulted in a reduction of wetland cover from approximately 10% to less than 1.5% (as of 1996) (Boyle et al. 1997 as cited in BC Ministry of Water, Land and Air Protection 2002). These wetland areas include lowland swampy and marshy land that would have been seasonally flooded (Boyle et al. 1997 as cited in BC Ministry of Water, Land and Air Protection 2002). If the loss of wetlands is used as a surrogate for loss of natural Oregon Forestsnail habitat, it is assumed there has been an 85% decline of wetland habitat and riparian areas since European settlement began. Most recently, between 1999 and 2009, 306 ha of wetland were lost in the lowlands of the Fraser Valley (Buffet et al. 2011).

Oregon Forestsnail habitat also includes open riparian habitats adjacent to streams and more permanent wetlands (see Habitat Requirements). Since European settlement, there has been extensive alteration to stream flow in the Lower Fraser Valley. As a result, approximately 15% of streams that once existed in the Lower Fraser Valley no longer exist and 71% are considered threatened or endangered (Fraser River Action Plan 1998 as cited in BC Ministry of Water, Land and Air Protection 2002). If the loss of streams is used as a surrogate for loss of natural Oregon Forestsnail habitat, it is assumed there has been a 15% decline in streamside potential habitat since European settlement began.

Over the past ten years there has been exponential urban and agricultural land development within the Lower Fraser Valley. The human population within the Lower Fraser Valley has increased 10.4% between 2001 and 2007 (last census date) to approximately 2.5 million people. This increase is one of the highest human population growth rates on the continent (Ipp 2007). At a minimum, there have been 17 urban housing developments (Greater Vancouver Real Estate 2011) within the municipalities of Mission, Abbotsford, and Chilliwack that may have impacted Oregon Forestsnail habitat or populations (as observed through satellite imagery). These urban developments include large-scale new communities that include new infrastructure, such as schools and roads. Most of this development has been within privately owned natural land within the Sumas Mountain, Vedder Mountain and Whatcom areas of the Lower Fraser Valley.

Many of the remaining natural ravines in the Lower Fraser Valley are surrounded by urban development (existing or planned) and as such there is an increase in proposed development projects that involve infilling, diverting or channelling existing natural water courses to accommodate access among urban developments. Such developments require approvals under the BC Water Act. In the past five years, there have been at least eight development project sites (and Water Act approvals) where Oregon Forestsnail has been recorded and where at least a portion of the species’ habitat was converted to roads, or other infrastructure (Malt pers. comm. 2012; Robbins pers. comm. 2012). During at least three projects, there has been salvage of Oregon Forestsnail adults (Malt pers. comm. 2012; Robbins pers. comm. 2012). Much Oregon Forestsnail habitat within the Lower Fraser Valley under potential urban development pressure may not require Water Act approvals because there are no requested changes to a watercourse thus triggering this legislative requirement.

Industrial and business park expansion plans are published for some municipalities within the Lower Fraser Valley, such as the City in the Country Plan specific to the City of Abbotsford. This plan projects the need for "1,300 acres of employment-generating industrial and business park lands over the next 20 years" with "future residential development accommodated through hillside development…not accommodated by expansion into the Agricultural Land Reserve" (City of Abbotsford 2004).

While the overall amount of habitat loss that has occurred in the past and is projected to occur into the future over the entire range of Oregon Forestsnail is untallied, a geographic information system (GIS) mapping exercise was done (Appendix 2). The exercise consisted of overlaying known Oregon Forestsnail occurrence records available to the BC Conservation Data Centre (2013) with projected urban growth boundary layers for the Fraser Valley Regional District. Each municipality has separate bylaws that require development proposals to consider environmental values such as species at risk (see Habitat Protection and Ownership). Abbotsford has some of the most stringent environmental bylaws in the province and requires proponents to collect data on species at risk under specific Wildlife Assessment Report Guidelines (City of Abbotsford 2010). The best data on Oregon Forestsnail distribution as well as GIS habitat overlays is available for the Abbotsford region, which is in the centre of the Canadian range (Figure 4). The conclusion of the mapping exercise was that all areas within the urban growth boundary are slated for future land conversion and development. This development will eliminate most large contiguous Oregon Forestsnail habitats, and the small pieces of habitat that remain as part of municipal set-aside requirements, riparian corridors, or compensation are likely to be sinks with snail populations declining to or close to zero in the short term (< 10 years). While it is currently not possible, given the available data, to quantify the effects of this urban development on reductions in EO, IAO, number of populations, and number of mature individual Oregon Forestsnail, the outlook is not good.

Habitat trends within southeastern Vancouver Island and Gulf Islands

Habitat trends within southeastern Vancouver Island follow a similar decline to those in the Lower Fraser Valley. Agricultural and urban development, logging and infilling have impacted riparian areas, broadleaf deciduous forests, wetland habitats and seasonal flooding regimes and thus have likely led to an overall decline in the natural habitat available for Oregon Forestsnail. There is little information on overall low elevation, deciduous broadleaf forests and wetland or riparian habitat loss within southern Vancouver Island.

Sensitive ecosystem mapping inventory for east Vancouver Island and Gulf Islands completed from 1993 - 1997 showed approximately 8,800 ha (11%) of nine sensitive ecosystem types (in the study) in the early 1990s had been disturbed by 2002. Potential Oregon Forestsnail habitat within this study area lost/disturbed includes riparian (4.6%), woodland (2.6%) and wetland (2.0%) ecosystems (Kirkby and Cake 2004).

2Rich moist soil composed of a thick humus organic layer, decomposing deciduous leaf litter, some mineral soil content and the presence of invertebrate soil fauna.

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