Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 9
Population Sizes and Trends
In comparison with vertebrates, terrestrial gastropod faunas of British Columbia forests are relatively poorly known, and undocumented native species continue to be discovered. Vancouver Island is better surveyed than most other areas of the province, beginning with efforts of early malacologists, such as G.W. Taylor and A.W. Hanham, in the late 1800s (see Drake 1963 and Forsyth 2004 for an overview of the history of malacology in British Columbia). Surveys were sporadic over much of the 20th century, and many areas even on Vancouver Island have received scant attention.
Rollo and Wellington (1975) carried out surveys in the vicinity of Vancouver in the early 1970s. A decade later Cameron (1986) surveyed numerous sites on Vancouver Island and the Lower Fraser Valley. More recently (1990 – 2003) Forsyth surveyed hundreds of localities in these two areas (R. Forsyth, pers. comm.). Since 1999, Biolinx Environmental Research Ltd. has conducted numerous surveys for terrestrial gastropods in southwestern and coastal British Columbia, including attempts to locate species deemed rare and potentially at risk such as P. coeruleum. As of April 2005, the number of sites surveyed by Biolinx Environmental Research Ltd. as part of various projects was 152 localities in Haida Gwaii (Queen Charlotte Islands), 234 on Vancouver Island, and 98 on the Lower Mainland of British Columbia. These surveys included the following: Pacific Rim, Gulf Islands, and Gwaii Haanas National Park Reserves for Parks Canada; Department of National Defence (DND) properties on southern Vancouver Island, Lower Mainland, and Graham Island for DND/CFS Natural Resources Management Program; selected forestry lands managed by Weyerhaueser Company Limited in 1999 – 2004 on Vancouver Island, Lower Mainland, and Graham Island; numerous localities on Vancouver Island and Lower Mainland for the BC Ministry of Water, Land and Air Protection; numerous localities on Vancouver Island, Lower Mainland, and Haida Gwaii as part of projects supported by the Endangered Species Recovery Fund and Wildlife Habitat Canada.
In the autumn of 2004, the authors surveyed remnant forest habitats within Capital Regional District Parks as part of field verification associated with the preparation of this status report to better delineate the distribution of this species. Of the 21 localities surveyed intensively and repeatedly using artificial cover-objects, P. coeruleum was detected at one locality (Devonian Regional Park). They also found the species at both previously known sites on the Rocky Point Peninsula using cover-objects, and at an additional site (Sooke) during searches of natural cover on the forest floor.
To date hundreds of localities have been searched for terrestrial gastropods on Vancouver Island and the coastal mainland of British Columbia (Figures 4 and 5). Numerous localities have been surveyed throughout much of Vancouver Island, Lower Fraser Valley, and Haida Gwaii in different habitat types. Prophysaon coeruleum has been located only in the extreme southern portion of Vancouver Island in low-elevation mixed-wood forests within the Coastal Douglas-fir Biogeoclimatic Zone. However, some areas that are difficult to access remain virtually unsurveyed. These areas include the northwest coast of Vancouver Island (from Tofino to Brooks Peninsula) and coastal forests north of Powell River on the mainland of British Columbia.
Population sizes and densities for P. coeruleum are unknown both in British Columbia and the United States. Pilsbry (1948) noted that the species often occurs solitarily on the forest floor. However, at Vancouver Island sites with multiple captures, the individuals were found within a few metres from each other, suggesting either affinity to specific microhabitat features at these sites or social reasons, such as mating.
At the Vancouver Island sites, the number of individuals found ranged from one to five per locality during a survey period, and only limited information on patterns of abundance can be gleaned from these observations, summarized below. At the Rocky Point site, Ovaska and Sopuck (2002) searched a total of 35 transects (1 m wide, 100 m long) in the spring and autumn and located P. coeruleum along two transects (5.6% frequency of occurrence; a total of 6 individuals found, 5 along one transect and 1 along another transect 1 km away). Minimum densities along the two transects were 0.05 and 0.01 slugs per square metre. In contrast, Ariolimax columbianus (Pacific Banana Slug), a common species in forests of coastal British Columbia, was found along all but three transects (91.4% frequency of occurrence; in total, 117 individuals found; minimum densities up to 0.12/m2). Twenty artificial cover-objects set in each of the two localities where P. coeruleum was found along about 100 m stretches of forest floor resulted in the finding of a total of three individuals during two inspections in the autumn of 2003.
At the Colwood site, Ovaska and Sopuck (2004a) spent 21.4 h searching for gastropods along 10 meandering survey routes. They found two individuals of P. coeruleum along one of the routes. At Heals Rifle Range in Saanich, Hawkes (2004) found one P. coeruleum within an area of about 230 ha that was searched for a total of about 24 h in the autumn. In Devonian Regional Park, Ovaska and Sopuck (2004b) inspected 20 artificial cover-objects three times (60 cover-object flips) and found one individual P. coeruleum. At the Sooke site, Ovaska and Sopuck (2004b) spent 54 person-minutes searching the forest floor and found one individual P. coeruleum.
Nothing is known of population fluctuations and trends of P. coeruleum in British Columbia. In the United States, the species appears to be relatively common in southwestern Oregon but rare and declining in other parts of Oregon and in Washington State (Cordeiro 2002a, b, c). Habitat at most of the historical sites in Washington State has been heavily modified for urban developments. The species appears to be extirpated from these sites, including the type locality in Olympia (Wilke and Duncan 2004).
Immigration of slugs from Washington State to the Lower Mainland of British Columbia is conceivable but unlikely. No populations of P. coeruleum are known from mainland British Columbia, and forest habitats in the vicinity of the border and throughout the Lower Fraser Valley are fragmented amidst agricultural lands and urban developments. Vancouver Island is isolated from mainland British Columbia by the Strait of Georgia and from the Olympic Peninsula, Washington State, by the Strait of Juan de Fuca, precluding rescue effects through natural dispersal.
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