Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 7

Habitat

Habitat Requirements

Because only a few sites for P. coeruleum are known from British Columbia, a review of the species’ habitat requirements based on studies in the United States is useful. However, inferences to British Columbia populations should be made with caution, as populations at the northern limits of the species’ range in Canada may differ in their ecology and habitat requirements from populations farther south.

Records of P. coeruleum exist from near sea level (in British Columbia) to 1650 m above sea level (in Oregon) (Wilke and Duncan 2004). All records from British Columbia are from low elevations (< 100 m). In the United States, P. coeruleum occurs in a wide range of coniferous and mixed-wood forests, where it is usually associated with moist plant communities such as those containing bigleaf maple (Acer macrophyllum) and sword fern (Polystichum munitum) (Kelley et al. 1999, Burke et al. 2000). The species is frequently encountered in mature and old growth forests but also occurs in younger stands, especially where attributes of older forest are present (Miller et al. 1999, Burke et al. 2000). Burke et al. (2000) stated that in Washington the species appears to be largely confined to old-growth forests. In one study in Oregon, a negative association was found with bracken fern (Pteridium aquilinum), an indicator of disturbed habitats, suggesting that the slugs might prefer more natural habitats to disturbed sites (Miller et al. 1999). Required microhabitat features include abundant coarse woody debris or other cover, deep forest litter layer, and shaded, moist forest floor conditions (Burke et al. 2000). The availability of mycorrhizal fungi for food and the plants that support them is also thought to be important (see Diet and predation).

In British Columbia, most locality records are from the Coastal Douglas-fir biogeoclimatic zone, a narrow strip of land that encompasses the southeast coast of Vancouver Island, Gulf Islands in the Strait of Georgia, and a tiny fringe of the southern mainland coast (Meidinger and Pojar 1991). One record (from Sooke) is from a transitional forest between the Coastal Douglas-fir and Coastal Western Hemlock biogeoclimatic zones. All records are from second-growth, mixed-wood stands, but the details of habitat features at these sites are variable (Table 1). For example, on the Rocky Point Peninsula, one locality is in a fringe of Bigleaf Maple in an otherwise largely coniferous forest, whereas the other locality is along the edge of a small wetland in a small stand of trembling aspen (Populus tremuloides) (Ovaska and Sopuck 2002, Ovaska et al. 2004). All sites are moist and productive, as indicated by varied and abundant understory vegetation, which often includes sword fern. Abundant cover (coarse woody debris, talus, and/or leaf litter) is present at these sites.


Habitat Trends

The Coastal Douglas-fir (CDF) biogeoclimatic zone encompasses about 200,000 ha of land, most of which is below 150 m in elevation (Meidinger and Pojar 1991). It is one of the most disturbed ecosystems of British Columbia, and little of the original forest remains (MacKinnon and Eng 1995, MSRM 2004). According to the Sensitive Ecosystems Inventory, older forests (with average age of trees 100 years or more) comprise only 2.6% of eastern Vancouver Island, from Campbell River in the north to Sooke in the south, and on the Gulf Islands (MSRM 2004). This value includes old-growth and mature second-growth forest within the CDF and the Coastal Western Hemlock (CWH) biogeoclimatic zones. The CWH zone occurs at higher elevations and/or moister sites, particularly in the northern and western portions of the Sensitive Ecosystems Inventory area. Maturing second-growth forest (with average age of trees 60 – 100 years) are relatively common on forested lands on southeastern Vancouver Island and consist of coniferous or mixed-wood stands, often with a deciduous component of red alder (Alnus rubra) or bigleaf maple, or more rarely trembling aspen. Due to continued pressures on the land-base, however, it is unlikely that much of the regenerating forest outside protected areas will be allowed to reach maturity. Efforts are under way to protect the larger of these stands in key areas (MSRM 2004).

Table 1: Habitat Features at Localities where Prophysaon coeruleum has been Found on Vancouver Island, British Columbia
Locality
(reference)
Stand age
(years)
Context
and land
ownership
Dominant
tree
species
Dominant
understory
shrubs & ferns
Dominant
herbaceous
plants
Microhabitat
features
Disturbance
Metchosin: Rocky Point, Site 1
(Ovaska and Sopuck 2002)
70 – 80 Maple fringe of largely coniferous stand; federal land (DND property) Douglas Fir, Bigleaf maple, Grand Fir, Western Redcedar Sword Fern, Salal, Huckleberry, Ocean Spray, Indian Plum, Oregon Grape   Deep layer of leaf litter; abundant coarse woody debris Within 50 m from a paved road; introduced plants and gastropods present
Metchosin: Rocky Point, Site 2
(Ovaska and Sopuck 2002)
70 – 80 Edge of small wetland adjacent to an older coniferous stand; federal land (DNDFootnote a property) Trembling aspen Sword Fern, Salal, Nootka Rose, Trailing Blackberry Western Trumpet Honeysuckle, grass Deep layer of leaf litter; some coarse woody debris Gravel road and clearing within 50 m; introduced plants (Scotch Broom); introduced gastropods
Colwood
(Ovaska and Sopuck 2004a)
70 – 80 Edge of mostly coniferous stand at the base of a rocky outcrop with Garry Oak/ Arbutus; federal land (DND property) Douglas Fir, Western Hemlock, Grand Fir, Bigleaf Maple Sword Fern, Oregon Grape, Ocean Spray, Trailing Blackberry, Nootka Rose Fringecup, Sweet Cicily, Bedstraw, Pathfinder Abundant coarse woody debris; compact, mostly needle litter Adjacent to access road within military training area; recent firewood cutting; introduced plants and gastropods present
Saanich
(Hawkes 2004)
Young second-growth Douglas Fir – Salal plant community; base of hill with Garry Oak/ Arbutus; federal land (DND property) Douglas Fir, Arbutus, Western Redcedar Oregon Grape, Salal (well-developed understory)   Abundant coarse woody debris; rock and talus piles; closed canopy Gravel access road within 50 m; adjacent to a popular recreational area; introduced plants and gastropods present
Metchosin: Devonian Regional Park 60 Remnant patch of forest within agricultural area; CRDFootnote b regional park Douglas Fir, Arbutus, Bigleaf Maple Ocean Spray, Oregon Grape, Indian Plum, Rose, Holly, Snowberry, Bracken Fern, Sword Fern Pathfinder, Bedstraw, grass Little coarse woody debris; needle/leaf litter Forest edge by agricultural/residential area; recreational trails; many introduced plants and gastropods present
Sooke: Galloping Goose Trail 50 – 60 Moist, productive mixed-wood, second-growth forest; CRD regional park Bigleaf Maple, Red Alder, Western Hemlock, Western Redcedar, Douglas Fir, Grand Fir, Arbutus Salal, Oregon Grape, Salmonberry, Sword Fern     Adjacent to recreational trail near parking lot


All distribution records for P. coeruleum from British Columbia are from within the Capital Region, which covers the extreme southern portion of Vancouver Island and the southern Gulf Islands in the Strait of Georgia. The Capital Region encompasses 245,103 hectares of land within 13 regional municipalities. Only about 6200 ha of older forest (with trees over 100 years) remain within this area (CRD 2003). Most of the older forest is located within the Western Communities, a region immediately west of Victoria, extending from Esquimalt Harbour to Rocky Point peninsula along the shoreline of the Juan de Fuca Strait (about 4500 ha). Western Communities include two of four municipalities with records of P. coeruleum (City of Colwood and District of Metchosin). Very little older forest (< 100 ha) remains on Saanich Peninsula, which includes one locality of the species (in the District of Saanich). The remaining record is from near Sooke, in the southwestern part of Vancouver Island. The west coast of Vancouver Island, north of Sooke, is very moist and consists mainly of coniferous forest, dominated by western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and Sitka spruce (Picea sitchensis). This type of wet, coastal forest may be unsuitable for P. coeruleum.

The Capital Region is densely populated (325 754 people in 2001; CRD 2004a). The population grew at a relatively rapid rate of 8.7% from 1991 – 2001. The areas experiencing highest rates of growth during this period were the Western Communities (16.6%) and the Gulf Islands (18.1%), but most of the growth occurred within the first half of the period. By 2026 the population within the Capital Region is predicted to continue to grow by about 30% from population size in 1996. The population increase is forecast to be greatest for Western Communities (92.1% increase), where relatively large areas of natural habitats still exist. As the human population within the Capital Region continues to grow, the remaining natural areas are under increasing pressures from development.


Habitat Protection/Ownership

About 13% of the land-base on Vancouver Island is designated as parks or other protected areas. About 24% of the island is privately owned; of this proportion about 75% consists of privately owned forestry lands (Sierra Club 2003, van Kooten 1995). Most of the private lands are on the southeastern portion of the island and include extensively logged forestry lands and those converted to agriculture, industrial uses, and housing. Crown lands prevail in the northern and western portions of the island, where more old-growth forest remains. However, to date P. coeruleum has been recorded only from the extreme southeastern portion of Vancouver Island from the Coastal Douglas-fir Biogeoclimatic Zone. Only about 5% of forested land-base within this zone is within protected areas (MWLAP 2002).

As of December 2004, three of five known localities for P. coeruleum are from federal lands managed by the Department of National Defence (DND). Canadian Forces Ammunition Depot Rocky Point (1,100 ha) in the District of Metchosin consists largely of coniferous and mixed-wood forest and includes relatively large, continuous stands of old-growth and mature forest. Colwood Supply/Fuel Oil Depot (98 ha) is bounded by housing and commercial developments within the City of Colwood. About 46 ha of the property are forested, mostly by mature (>80 years old) second-growth stands. The Heals Rifle Range (230 ha) in the District of Saanich includes large areas of mature coniferous and mixed-wood forests and in the west abuts Mount Work Regional Park, which in turn is contiguous with Gowland-Tod Provincial Park in the District of Highlands. Together these areas form an important ecosystem of continuous forest cover amidst residential and agricultural lands on the Saanich Peninsula.

DND properties contain relatively large tracts of older forest within otherwise highly modified landscapes on the southern tip of Vancouver Island and might form refuges for P. coeruleum and other inhabitants of the vanishing coastal Douglas-fir ecosystems. These areas are presently protected from development but are not secure, however, as DND retains the option of disposing of lands that are not used for training or other military purposes. It should be noted, however, that it would be impossible for a federal department to dispose of lands without first ensuring that the habitat needs of Species at Risk occupying those lands are met.

The remaining two records for P. coeruleum are from Capital Regional District parks (Devonian Regional Park; Galloping Goose Trail). Devonian Regional Park protects a small stand of forest (14.4 ha) amidst residential and agricultural lands. The Galloping Goose Trail is used mainly for recreational purposes and consists of a 30 m-wide right-of-way that the Capital Regional District Parks department manages as a linear park corridor. The lands adjacent to the corridor are extensively forested along portions of the trail, especially in the south in Sooke and Metchosin, but are mostly privately owned.

Protected areas within the Capital Regional District consist of a network of 27 regional parks and park reserves, and two regional trails with a total area of 10 583 hectares (CRD 2004b). The largest regional parks and regional park reserves include Sooke Hills Wilderness (4100 ha), East Sooke (1436 ha), Thetis Lake (778 ha), Sea to Sea Green Blue Belt (609 ha),Mt. Work (536 ha), and Elk/Beaver Lake (442 ha). Several regional parks are small (<10 ha), and some are highly modified and subject to intense recreational use. Of the larger parks, Thetis Lake and Elk/Beaver Lake in particular, receive heavy recreational use, and numerous introduced plants and animals are present, further modifying the ecosystems.

Provincial parks within the Capital Region include Gowland Tod (1219 ha), John Dean (174 ha), and Goldstream (388 ha). Other larger provincial parks on southern Vancouver Island are Koksilah River (210 ha), Cowichan River (873 ha), and Chemainus River (128 ha) on the southeast coast and Juan de Fuca (1277 hectares) on the southwest coast. These parks have not been systematically surveyed for terrestrial gastropods. The Gulf Islands National Park Reserve, established in 2003, covers about 33 km2, in total, on 16 islands in the Strait of Georgia (Parks Canada 2004). Most lands incorporated into the new park were already protected as provincial or regional parks, but some new properties were acquired. Surveys for terrestrial gastropods, including P. coeruleum, were carried out on two of the larger islands (Pender and Saturna) within the Gulf Islands National Park Reserve in 2004 (Ovaska and Sopuck 2005). There are no records of P. coeruleum from the Gulf Islands or from the southwest coast Vancouver Island.

Three of the known sites (Rocky Point, Colwood, and Saanich) are in the immediate vicinity of the following Indian Reserves: South Saanich 1 (Tsarlip Nation); Esquimalt (Esquimalt Nation); New Songhees 1A (Songhees Nations); Beecher Bay 1 and 2 (Beecher Bay Nation). The species’ range also overlaps other aboriginal lands that may contain suitable habitat: T’Sou-ke 1 and 2 (T’Sou-ke Nation); East Saanich 2 (Tsawout Nation); Cole Bay 3 (Pauquachin Nation); Union Bay 4 (Tseycum Nation) (CRD 2001).

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