Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 10

Limiting Factors and Threats

This species exists at the northern extremity of its geographic range in southwestern British Columbia. It is unknown whether areas farther north are outside physiological tolerances of the species, or whether the present distribution in British Columbia reflects Pleistocene extinctions and glacial history of the area and subsequent barriers to dispersal.

Main threats to populations include habitat loss, fragmentation and degradation. Additional threats include competition with exotic gastropods and predation by native and introduced species.

Habitat loss – Forest habitats outside protected areas are shrinking at an alarming rate on southern and eastern Vancouver Island due to urban developments and associated infrastructure (see Habitat trends). Habitats at localities within Capital Regional District regional parks are protected from land conversions but are often degraded by heavy recreational use and introduced plants and animals. Habitats at known sites within Department of National Defence properties also receive some degree of protection.

Habitat fragmentation – Fragmentation of habitats concomitant with habitat loss is a major threat for this species with presumably poor dispersal capabilities. Isolation of populations in remnant forest patches increases the probability of local extinctions, as recruitment and recolonization through immigration is curtailed. Fragmentation of forest habitats within the Capital Regional District is extensive.

Habitat degradation – All human activities that result in reduced moisture and cover on the forest floor have the potential to degrade habitats of this species. Logging, thinning, grazing, and firewood cutting decrease canopy closure and potentially result in drier forest floor conditions, reducing habitat suitability. Off-road recreational activities, such as all-terrain-vehicles or mountain biking, compact the substrate. Trampling of the forest floor through other intensive recreational activities is similarly detrimental. Forest harvesting practices that prevent the accumulation of coarse woody debris, including large logs in all stages of decay, deplete essential cover for this species on the forest floor. Intense fires that modify the forest floor are thought to be detrimental for P. coeruleum (Burke et al. 2000). Invasive introduced plants (such as Scotch broom, gorse, and laurel-leaved daphne) are extensively modifying natural ecosystems within the range of P. coeruleum. Habitat degradation is prevalent in small forest stands that are subject to invasion by introduced species and changes in microclimate, including drying of the forest floor. 

Competition with exotic gastropods – Competition from exotic gastropods for shelter or food is considered a threat to populations of P. coeruleum in the United States (Burke et al. 2000). There are at least 25 exotic gastropods, most of them of European origin, in British Columbia (Forsyth 2004). Exotic gastropods are prevalent within populated areas of Vancouver Island, and several are present at sites where P. coeruleum occurs (see Interspecific interactions). Some invasive exotic species, including the giant gardenslug, are aggressive competitors (Rollo and Wellington 1979), whereas the wormslug includes eggs of other slugs in its diet (Forsyth 2004). Both species have been found at localities where P. coeruleum occurs (Ovaska and Sopuck, unpublished data).

Predation by native and exotic animals– As habitats become more fragmented, the risk from predation is likely to increase. Predation may be a problem for P. coeruleum in small habitat patches that have a relatively large amount of edge and are subject to invasion by predators that use disturbed habitats, such as crows, some ground beetles, and exotic gastropods. Autonomy of the tail in the genus Prophysaon is believed to be an evolutionary adaptation to predation from beetles, among other organisms. In a related species, P. foliolatum, there is strong evidence that tail autonomy is an effective escape response from carabid beetles (Deyrup-Olsen et al. 1986). Native predatory beetles are unlikely to be a threat to P. coeruleum unless the size of the population declines to such an extent that it can no longer cope with natural predation. The carnivorous snail Oxychilus draparnaudi has been identified as a potential threat to native gastropods in areas where it has been introduced (Frest and Rhodes 1982). The diet of the exotic slug Boettgerilla pallens includes eggs of other gastropods (Reise et al. 2000). The availability of suitable shelter, including large decaying logs, is expected to be particularly important in small habitat patches to provide refuges from predation.

Global climate change – Prolonged summer droughts and increased frequency of extreme events are predicted to occur under global climate change scenarios (Gates 1993, Environment Canada 2002). Drier conditions on the forest floor during the activity period of the slugs from spring to autumn are likely to adversely affect populations. The effects of prolonged summer droughts can be expected to be particularly severe in marginal, degraded habitat patches that may lack microhabitats suitable for refuges.

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