COSEWIC Assessment and Status Report on the Threaded Vertigo Nearctula sp. in Canada – 2010

Table of Contents

Document Information

List of Figures

List of Tables

List of Appendices


Document Information

Threaded Vertigo

Photos of a Threaded Vertigo Nearctula species shell showing the arrangement of apertural denticles.

Special Concern – 2010

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

COSEWIC. 2010. COSEWIC assessment and status report on the Threaded Vertigo Nearctula sp. in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. x + 36 pp.

Production note:
COSEWIC acknowledges Kristiina Ovaska and Lennart Sopuck for writing the provisional status report on the Threaded Vertigo, Nearctula sp., prepared under contract with Environment Canada. The contractors’ involvement with the writing of the status report ended with the acceptance of the provisional report. Any modifications to the status report during the subsequent preparation of the 6–month interim and 2–month interim status reports were overseen by Robert Forsyth, Molluscs Specialist Subcommittee Co–chair.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: 819–953–3215
Fax: 819–994–3684
E–mail
Website

Également disponible en français sous le titre Évaluation et Rapport de situation du COSEPACsur le vertigo à crêtes fines (Nearctula sp.) au Canada.

Cover illustration/photo:
Threaded Vertigo -- Photo by Robert Forsyth.

© Her Majesty the Queen in Right of Canada, 2010.
Catalogue CW69–14/607–2010E–PDF
ISBN978–1–100–16023–8

Top of Page


COSEWIC Assessment Summary

Assessment Summary – April 2010

Common name
Threaded Vertigo

Scientific name
Nearctula sp.

Status
Special Concern

Reason for designation
This minute terrestrial snail species is at the northern extent of its range, and found in lowland areas around the Strait of Georgia and on southern Vancouver Island. Most individuals live on the bark of Bigleaf Maple trees and appear to have poor capacity for dispersal between trees and sites. Removal of trees and habitat degradation due to urban expansion, roads and associated infrastructure, forestry, and agriculture are the main threats.

Occurrence
British Columbia

Status history
Designated Special Concern in April 2010.

Top of Page

COSEWIC Executive Summary

Threaded Vertigo Nearctula sp.

Species information

The Threaded Vertigo is a minute land snail with a high, rather cylindrical shell. The shell is about 2.4–3.3 mm high, dull, dark brown, and somewhat coarsely marked with fine, parallel growth lines. Within the opening of the adult shell, there are four white denticles (tooth–like protuberances); the immature shell lacks denticles. The Threaded Vertigo is currently without a formal scientific name due to its convoluted nomenclatural history, but the validity of the species is not in question.

Distribution

The global range of the species extends from southwestern British Columbia through western Washington State and Oregon to west–central California. In Canada, the species occurs on southern Vancouver Island, Saturna Island, and the Sunshine Coast on mainland British Columbia. There are 24 recent (since 1984) distribution records. On Vancouver Island, clusters of sites occur within the Capital Regional District (Victoria area), but the species is known from only scattered sites along the east coast, northward to just south of Courtenay. In the Strait of Georgia, the species is known from one site on Saturna Island. On the Sunshine Coast (B.C. mainland), it occurs at several places between Gibsons and Egmont.

Habitat

In British Columbia, the snails occur in moist deciduous and mixed–wood forests at low elevations, usually below 200 m. They are often associated with Bigleaf Maples and an understory of ferns and shrubs characteristic of moist, rich sites. Older riparian forests containing groves of large maples appear to be particularly suitable. The snails are largely arboreal and encountered most frequently on trunks of maples, where they occur within crevices of grooved bark or moss mats. They are occasionally found on other deciduous trees, on fern fronds, or on the ground within the leaf litter. The snails have a patchy distribution both within and among forest stands, and aggregations occur on some trees while others are devoid of snails.

Biology

Little is known about the biology of the Threaded Vertigo, and hence information has to be extrapolated from similar, related species. Like most land snails, the Threaded Vertigo is hermaphroditic (possesses both male and female reproductive organs), but the extent of cross–fertilization is not known. In similar species, eggs are laid singly. The lifespan is probably short, two years or less. The snails hibernate in winter and probably aestivate during dry periods in summer. Movements and active dispersal are limited, but passive transport on falling leaves during windstorms is plausible. The snails have been found in small groves of trees near roadsides and busy recreational trails, suggesting that they can tolerate some habitat disturbance, provided suitable moist microhabitats remain available.

Population sizes and trends

Few data are available on population sizes or trends. On occupied trees, 1–12 snails were found during searches of the lower 2 m of the trunk during field verification surveys on Vancouver Island and the Sunshine Coast. The snails appear to occur at much lower densities within the litter layer. Surveys in 2008 showed that the species has persisted at many sites over several years, but the size and viability of local populations are unknown.

Limiting factors and threats

The distribution of the species coincides with densely populated and highly modified parts of British Columbia. Much of the land conversion is historical in these lowland coastal areas, but human developments continue to encroach on remaining natural areas concomitant with an expanding population. Housing developments, road building and other associated infrastructure, agriculture, and forestry are shrinking and fragmenting habitats. Most records for this species are from parks or federal lands protected from land conversion, but potential habitats on private lands throughout most of the species’ range continue to diminish. Populations in protected areas are not necessarily secure due to habitat degradation from intensive recreational or other uses, and invasion by introduced plants and animals.

Special significance of the species

The species is of no known socio–economic or cultural significance. It contributes to the biodiversity of the rich, extraordinary arboreal communities supported by Bigleaf Maples. At suitable moist sites, epiphytic mosses, liverworts, lichens, ferns, and other vascular plants drape large old maples, in turn providing habitat for a variety of fungi, invertebrates, and other organisms. The ecological roles and services of the different components of these arboreal communities, including the Threaded Vertigo, are yet to be elucidated.

Existing protection

The Threaded Vertigo was designated Special Concern by COSEWIC in April 2010.

Technical Summary

Nearctula sp.

Threaded Vertigo – vertigo à crêtes fines
Range of Occurrence in Canada: British Columbia

Demographic Information

Generation time (usually average age of parents in the
population; indicate if another method of estimating
generation time indicated in the IUCN guidelines(2008) is being used)
yr
Is there an [observed, inferred, or projected] continuing
decline in number of mature individuals?
Unknown
Estimated percent of continuing decline in total number of
mature individuals within [5 years or 2 generations]
Unknown
[Observed, estimated, inferred, or suspected] percent
[reduction or increase] in total number of mature individuals
over the last [10 years, or 3 generations].
Unknown
[Projected or suspected] percent [reduction or increase]
in total number of mature individuals over the next [10 years, or 3 generations].
Unknown
[Observed, estimated, inferred, or suspected] percent
[reduction or increase] in total number of mature individuals
over any [10 years, or 3 generations] period, over a time period including both the past and the future.
Unknown
Are the causes of the decline clearly reversible and understood and ceased?N/A
Are there extreme fluctuations in number of mature individuals?Unknown

Extent and Occupancy Information

Estimated extent of occurrence
Areas of ocean (unsuitable habitat) in the Strait of Georgia were included.
9,530 km²
Index of area of occupancy (IAO)
Based on a 2 km × 2 km grid. The actual area of occupancy at each site is unknown and the total biologically based AO is much smaller.
72 km²
Is the total population severely fragmented? Unknown but not likely
Number of locations*
There are > 10 locations; 16 recent locations (since 1984) are identified (Table 1), and additional locations likely exist.
16
Is there an [observed, inferred, or projected] continuing |
decline in extent of occurrence?
Unknown; likely not
Is there an [observed, inferred, or projected] continuing
decline in index of area of occupancy?
Unknown; likely not
Is there an [observed, inferred, or projected] continuing
decline in number of populations?
Unknown
Is there an [observed, inferred, or projected] continuing
decline in number of locations?
The number of distribution records has increased in the past
10 years with increased search effort, and new locations
continue to be found. The actual trend in number of locations through extirpations or colonization is unknown.
Unknown
Is there an observed, inferred, and projected continuing
decline in area, extent and quality of habitat?
The distribution of the species coincides with lowland coastal areas with dense and expanding human population; deciduous
and mixed wood stands inhabited by the snails are shrinking
and being degraded as a result of human activities.
Declining in area, extent and quality
Are there extreme fluctuations in number of populations? Unknown; probably not
Are there extreme fluctuations in number of locations*? Unknown; probably not
Are there extreme fluctuations in extent of occurrence? No, as far as known
Are there extreme fluctuations in index of area of occupancy? Unknown

* See definition of location.

Number of mature individuals in each population

Population N Mature Individuals
Vancouver Island Unknown
Saturna Island Unknown
Sunshine Coast, mainland B.C.Unknown
TotalUnknown
Number of populations Minimum 3 populations – probably more

Quantitative Analysis

Probability of extinction in the wild is at least [20% within 20 years or 5 generations, or 10% within 100 years]. N/A

Threats (actual or imminent threats to populations or habitats)

Main threats:

  • Habitat loss and degradation primarily due to urban expansion, roads, and associated infrastructure, forestry, and agriculture

Potential threats:

  • Habitat degradation through edge effects, recreational activities and other human uses, and by invasion of introduced plants and animals

Rescue Effect (immigration from outside Canada)

Status of outside population(s)? USA: Apparently secure (N3N5); unranked or with no special status in California, Oregon, and Washington
Is immigration known or possible? No immigration is known; immigration is very unlikely
Would immigrants be adapted to survive in Canada? Yes
Is there sufficient habitat for immigrants in Canada? Yes
Is rescue from outside populations likely? No

Current Status

COSEWIC:Designated Special Concern in April 2010.
British Columbia: S2(imperilled); red–listed
Additional Sources of Information: None

Status and Reasons for Designation

Status: Special Concern
Alpha–numeric code:None
Reasons for designation:
This minute terrestrial snail species is at the northern extent of its range, and found in lowland areas around the Strait of Georgia and on southern Vancouver Island. Most individuals live on the bark of Bigleaf Maple trees and appear to have poor capacity for dispersal between trees and sites. Removal of trees and habitat degradation due to urban expansion, roads, and associated infrastructure, forestry, and agriculture are the main threats.

Applicability of Criteria

Criterion A (Decline in Total Number of Mature Individuals):
Not applicable.  The total number of mature individuals is unknown.

Criterion B (Small Distribution Range and Decline or Fluctuation):
Not applicable.  Although EO (9,530 km²) is <20,000 km², IAO (72 km²) is <500 km², and a continuing decline in area and quality of habitat is inferred, the species is not severely fragmented, is found at >10 locations (likely more), and extreme fluctuations (unlikely) in EO, IAO, number of locations or populations, or number of mature individuals are unknown.

Criterion C (Small and Declining Number of Mature Individuals):
Not applicable.  The total number of mature individuals is unknown.

Criterion D (Very Small Population or Restricted Distribution):
Not applicable.  The number of mature individuals is unknown and likely much >1000; IAO >20 km² and number of locations >5.

Criterion E (Quantitative Analysis):
No data available.

Top of Page

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal–Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non–government science members and the co–chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2010)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)***
A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Top of Page


COSEWIC Status Report on the Threaded Vertigo Nearctula sp. in Canada – 2010

Species Information

Name and classification

The Threaded Vertigo, Nearctula species, is a member of the family Vertiginidae, a group of small land snails with worldwide distribution. Some authorities include this group as a subfamily within the family Pupillidae (Pilsbry 1948; Turgeon et al. 1998), while others regard it as sufficiently distinct to warrant a separate family designation (Schileyko 1998; Roth and Sadeghian 2003, 2006; Bouchet and Rocroi 2005). The classification of the Threaded Vertigo, following Forsyth (2004), is as follows:

  • Phylum Mollusca
  • Class Gastropoda
  • Subclass Orthogastropoda
  • Order Pulmonata
  • Superfamily Pupilloidea
  • Family Vertiginidae
  • Subfamily Vertigininae
  • Genus Nearctula
  • Nearctula species

The genus Nearctula Sterki, 1892, corresponds to the Vertigo californica group of Pilsbry (1948). Smith et al. (1990), following Coney (1988), separated Nearctula from Vertigo. This generic designation is used in the American Fisheries Society list of names of molluscs (Turgeon et al. 1998). The genus Nearctula, as currently known, contains N. rowellii with several subspecies (California and Baja California), N. species, currently without a formal name(British Columbia to central California) (Turgeon et al. 1998), and N. dalliana (Oregon and California) (Roth and Sadeghian 2006). Another undescribed and possibly new species, from the Olympic Peninsula and colloquially known as the Hoko Vertigo, may belong to Nearctula (Burke et al. 1999). The nomenclatural history of the Threaded Vertigo is complicated due to misapplications of names and taxonomic revision, currently leaving this species without a formal name. The validity of the species is not in question (Turgeon et al. 1998); rather, the issue is with nomenclature.

The Threaded Vertigo was formerly and erroneously known as Vertigo rowellii prior to 1998 (Turgeon et al. 1998), but Nearctula rowellii (Ribbed Vertigo) is the valid name for another (Californian) species (Smith et al. 1990; Roth and Sadeghian 2003, 2006). The Californian species now bearing the name N. rowellii was once called Vertigo californica (Pilsbry 1948), but these two names are synonyms, with Pupa rowellii Newcomb, 1860, the senior synonym (Turgeon et al. 1998; Roth pers. comm. 2008). A lectotype designation by Clarke (1960) confirms this change. Hence, the Californian species rightfully claims the name N. rowellii, which leaves the Threaded Vertigo without a formal name. Roth and Sadeghian (2003, 2006) referred to this species as Nearctula species “p”. Turgeon et al. (1998) retained listing for this temporarily unnamed species, for which they had applied the common name Threaded Vertigo in the first edition of the list in 1988. The formal naming of the Threaded Vertigo is in progress (Turgeon et al. 1998; Roth and Sadeghian 2006; Roth pers. comm. 2008).

Top of Page

Morphological description

The Threaded Vertigo is a minute snail with a high, subcylindrical shell. The average shell height of 47 adult specimens from British Columbia measured 2.7 mm (SD = 0.2 mm, range = 2.4–3.3 mm; K. Ovaska, unpublished data). The shell is slightly tapered with a blunt apex. The crest (a ridge behind lip of the aperture on the last whorl), sinulus (an indentation of the lip of the aperture), and palatal callus (the thickened region inside the apertural back from the lip), which are present in some vertiginids, are absent in the Threaded Vertigo. The periostracum (outer layer that covers the shell) is dull and dark brown; somewhat irregularly spaced, raised, minute axial threads give the shell a coarse surface texture (Figure 1). The body of the animal is dark grey dorsally, grading into lighter grey on the sides and sole of the foot.

Figure 1. Threaded Vertigo in groove of furrowed bark of a large Bigleaf Maple, near Sooke River, Vancouver Island (shells 2.5–3.0 mm high; October 2008; photo in situ by K. Ovaska).

Photo of Threaded Vertigo in the furrowed bark of a Bigleaf Maple near Sooke River, Vancouver Island.

Top of Page

The lip of the aperture of the adult shell flares outwards and is only slightly thickened. There are four white denticles within the aperture: parietal, columellar, lower palatal, and upper palatal (Figure 2). The broad, slightly twisted parietal denticle is the most prominent. The upper palatal is usually the smallest and lacking in some specimens.

Figure 2. Shell of the Threaded Vertigo, showing the arrangement of apertural denticles. Photo by Robert Forsyth; Vancouver Island (Metchosin), British Columbia.

Two line drawings and two photos of a Threaded Vertigo shell, showing the arrangement of apertural denticles.

Top of Page

The Threaded Vertigo can be distinguished from sympatric vertiginids by features of the shell. It can be distinguished from Vertigo species by its larger size and arrangement of denticles. The shells of Columella species are broader, have a simple lip (not thickened or flared), and lack apertural denticles. Undescribed species of Nearctula might occur within the range of the Threaded Vertigo, and it can be difficult to discern whether individuals with unusual features are aberrant or within normal range of variation for this species, or belong to a new species. In these cases, examination of a number of specimens from the same site is required to determine the frequency of individuals with unusual features and the consistency of these features.

Top of Page

Population spatial structure and variability

The population spatial structure of the Threaded Vertigo has not been studied, and no genetic information is available. The geographic range of the species in Canada is disjunct, and stretches of ocean form barriers to movements of snails among Vancouver Island, Saturna Island, and the Sunshine Coast on mainland British Columbia (see Distribution). These areas are also isolated from populations in the United States by ocean or, in the case of the Sunshine Coast, by Metro Vancouver and surrounding developed areas.

Top of Page

Designatable units

There is one designatable unit. More than one designatable unit is not applicable because no subspecies are recognized and all sites are found in similar habitat within one ecozone. While natural disjunction appears to exist among populations on islands and the mainland, there are no data available on the genetic distinctiveness or evolutionary significance (such as local adaptations) of these populations.

Top of Page

Distribution

Global range

The geographic range of the Threaded Vertigo extends from southwestern British Columbia southward through western Washington State and Oregon to west–central California (Frest and Johannes 2000; Roth and Sadeghian 2006; Duncan pers. comm. 2008; Burke pers. comm. 2008; Figure 3). The species’ distribution appears to be relatively continuous from west–central Oregon to California, where records exist from all coastal and several sub–coastal counties from Lane County, Oregon, to Monterey County, California at the southernmost limits of the species’ range (Frest and Johannes 2000; Roth and Sadeghian 2006). In northern Oregon and Washington, it is known from scattered records only, including several sites away from the coast in the northern Oregon Cascades (Duncan pers. comm. 2008). The eastern limits of the distribution throughout the United States are poorly known.

Figure 3. Global distribution of the Threaded Vertigo in western North America.  Distribution in USA compiled from information from Roth and Sadeghian (2006), Frest and Johannes (2000), Burke pers. comm. 2009, Duncan pers. comm. 2008, Roth 2009 pers. comm. Eastern limits of the distribution are approximate. Map prepared by L. Sopuck.

Map showing the global distribution of the Threaded Vertigo in western North America.

Top of Page

Canadian range

In Canada, the Threaded Vertigo is known from Vancouver Island and from the Sunshine Coast on the southwestern mainland of British Columbia (Appendix 1; Figure 4). It was recently (March 2009) found on Saturna Island in the Strait of Georgia, and it is likely that it occurs on other Gulf Islands as well. On Vancouver Island, records exist from near Sooke in the southwest to Union Bay on the east–central coast and from several places in between along the south and east coast of the island. There are clusters of records from the Metchosin–Sooke area and from the Saanich Peninsula within the Capital Regional District around Victoria, and scattered records from coastal forests farther north along the east coast of Vancouver Island. On the Sunshine Coast, records exist from a 100 km–long stretch of coast from Roberts Creek to near Egmont on the Sechelt Peninsula. The northern extent of the species’ distribution on Vancouver Island and along the mainland coast north of Sunshine Coast remains unknown, but it is unlikely that the species occurs much farther north from where it is presently known (see Habitat requirements).

Figure 4. Canadian distribution of the Threaded Vertigo in southwestern British Columbia. Map prepared by Alain Filion from data in Appendix 1(place names added to original map).

Map showing the global distribution of the Threaded Vertigo in western North America.

Top of Page

As of March 2009, there are 24 recent (since 1984) and three historic (from the mid–1890s to early 1920s) records of the species from British Columbia (Appendix 1). It is likely that additional sites exist, as suggested by seven new distribution records documented during field verification surveys targeting this species and its microhabitats in 2008 (Appendix 1).

Based on the premise that open expanses of water form barriers for dispersal (see Dispersal/migration), there is a minimum of three populations (Saturna Island, the Sunshine Coast, and Vancouver Island), but more than three populations are probable. In particular, the distribution records for the species on the east coast of Vancouver Island are scattered and may represent several populations. The ability of this species to disperse over long distances is unknown and survey effort is incomplete, making delineation of these possible populations unfeasible at this time.

The extent of occurrence (EO) is about 9,530 km², using the minimum polygon method and including areas of ocean in the Strait of Georgia. Ocean, which is unsuitable habitat, covers about a third of this area. The index of area of occupancy (IAO) is 72 km², calculated by superimposing a grid with 2 km × 2 km cells on the range and counting the number of cells that encompass records of the species. The actual area of occupancy is unknown. The EO represents about 7% of the global range of the species, but this value is uncertain because all areas with potential habitat have not been adequately surveyed for the species. There is no information on the historic range of the species in Canada, but it was probably larger in the past because riparian zones and other suitable moist forest habitats have shrunk due to human activities over the past century.

Top of Page

Habitat

Habitat requirements

In British Columbia, the Threaded Vertigo occurs in moist low–elevation deciduous and mixed–wood forests (Forsyth 2004). The species occurs in restricted habitats within the Coastal Douglas–fir and Coastal Western Hemlock biogeoclimatic zones (Meidinger and Pojar 1991). All records from the province are from below 200 m above sea level (Appendix 1). Farther south, however, the species has been found at elevations up to about 1650 m in the Oregon Cascades (Duncan pers. comm. 2008), but it appears to prefer low– to mid–elevation areas (Frest and Johannes 2000). In British Columbia, the snails most often occur in forest stands containing Bigleaf Maple (Acer macrophyllum) and with an understory of Sword Fern (Polystichum munitum), Salmonberry (Rubus spectabilis), and other shrubs and herbs characteristic of moist, rich sites and riparian areas (Figure 5). Bigleaf Maples are restricted to middle and low elevations (usually below 300 m) in the southwestern part of the province, including Vancouver Island and the adjacent mainland coast, and occur mainly at latitudes below 51°N (Peterson et al. 1999; B.C.Forests and Range, undated). Maples decrease in occurrence with increasing elevation and latitude and inland from the coast. Stands with the highest proportion of Bigleaf Maples occur at scattered places on southern and southeastern Vancouver Island and along the extreme southwestern portion of the mainland. Older riparian forests containing groves of large Bigleaf Maples appear to be particularly important for the snails, as they provide a high level of humidity, an abundance of moss and epiphytes, and a deeply furrowed bark for shelter.

Figure 5. Habitat of the Threaded Vertigo in mainly deciduous riparian forest with Bigleaf Maple and Sword Fern (Rocky Point, Vancouver Island; photo by K. Ovaska).

Photo showing Threaded Vertigo habitat in mainly deciduous riparian forest with Bigleaf Maple and Sword Fern.

Top of Page

The Threaded Vertigo is largely arboreal, similar to some other vertiginid snails, such as Vertigo bollesiana, which in Nova Scotia was found exclusively on trunks of deciduous trees (Davis 1995). Other species of vertiginids that occur in British Columbia, however, are usually found within the leaf litter or climbing on ferns or low bushes (Forsyth 2004). During field verification for this status report in 2008, the Threaded Vertigo was most often found on trunks of large Bigleaf Maples, where they occupied microhabitats within crevices of grooved bark or within or under moss growing on the bark (Figures 1 and 6). They were found infrequently on the bark of other deciduous trees (Cottonwood, Populusbalsamifera, Red Alder, Alnus rubra) and on Sword Fern fronds. Other records from the province indicate that the snails are also found within the leaf litter, but whether they regularly occur in the litter layer is unknown. These records might represent snails that have dropped from trees or ferns. In Oregon, the snails occur primarily on perennial bushes and small trees with smooth bark, including California Hazel (Corylus cornuta californica), Red Alder, Cascara (Rhamnus purshiana), and Vine Maple (Acer glabrum), where they are often found on the underside of small branches (Frest and Johannes 2000). In British Columbia, however, searches of smaller trees and bushes were unproductive during the field verification surveys. It is possible that the snails use these microhabitats seasonally; it is also possible that habitat use at the northern limits of the species’ distribution here is different from that farther south in the United States.

Figure 6. Large, moss–covered Bigleaf Maple, which provided arboreal microhabitats for the Threaded Vertigo (Rosewall Creek, Vancouver Island; photo by K. Ovaska).

Photo of part of a moss-covered Bigleaf Maple that provided microhabitats for the Threaded Vertigo.

Top of Page

Some degree of habitat connectivity both within and among stands is likely required, but no data are available. Fragmented forest stands and stands with only scattered maples or other suitable deciduous trees may not support viable populations, as subpopulations of snails on individual trees or small groups of trees may become isolated and prone to extirpation.

Top of Page

Habitat trends

The distribution of the Threaded Vertigo coincides with the highly modified landscape and dense human population in southwestern British Columbia, where the species is associated with forest stands containing Bigleaf Maples. Although Bigleaf Maples are not rare in this region, forest stands containing old (> 121 years), large trees comprise only a small proportion of maples on both Vancouver Island (2.4% of age classes) and the southwestern mainland (1.5% of age classes; values based on the volume of maples in areas that allow harvesting; Peterson et al. 1999). Furthermore, stands that include more than scattered maples (at least 5%) are restricted to narrow, low–elevation coastal strips in the extreme southwest of the province (Figure 7, top). Unfortunately, these areas are also where people prefer to live (Figure 7, bottom), and hence have been fragmented.

Figure 7. Distribution of Bigleaf Maple (top) in relation to human population density (bottom) in southwestern British Columbia.

Map shows the distribution of Bigleaf Maple

Source: Generated from digital map provided by Dr. Andreas Hamann, Department of Renewable resources, University of Alberta, Edmonton (Hamann pers. comm. 2009)

Map shows human population density.

Human population density in 2001 (1 dot = 50 people)
Source: BC. Ministry of Environment. State of the Environment
Reproting - Environement Trends 2007.

Top of Page

On Vancouver Island, most of the human population is concentrated in low elevation areas on the south and east coast. Much of the conversion of the habitat into residential areas, agriculture, and other human uses is historical, but human developments continue to encroach on remaining natural areas. For example, the human population within the Capital Regional District (CDR, encompassing 13 municipalities on southern Vancouver Island), where concentrations of records of the Threaded Vertigo occur, grew by an average of 6% annually within the five–year period from 2001 to 2006, to a total of 345,164 people in 2006 (CDR 2007). Within CDR, West Shore communities exhibited the fastest growth (11%) during this period. They include Metchosin and Sooke areas, where much of remaining forests are located and where the snail has been found. Population growth within CDR is projected to continue over the next decades, increasing from 151.09 people/km² in 2006 to 176.94 people/km² by 2031 (B.C. Ministry of Environment 2007). Smaller but steady increases are also projected for the east–central coast with records of the Threaded Vertigo (Comox–Strathcona Regional District: from 5.36 people/km² in 2006 to 7.13 people/km² by 2031).

The portion of the Sunshine Coast where the Threaded Vertigo occurs is subject to similar trends of habitat loss and fragmentation. The Sunshine Coast Regional District covers an area of 3,778 km² and is relatively sparsely populated (27,759 persons in 2006; SCORE 2006). However, most of the human population is concentrated in the relatively narrow coastal strip of lowlands that support mixed–wood forest with abundant Bigleaf Maples and provide habitat for the Threaded Vertigo, rather than in the mainly coniferous, mountainous areas away from the coast. This area is within commuting distance from metropolitan Vancouver, the fastest growing area in the province, and is experiencing population expansion. The total population of the district had grown by 8.4% in the five–year period from 2001 to 2006 (SCORE2006). Population growth was more rapid in some of the coastal communities, including Roberts Creek (16.2% increase), where the Threaded Vertigo has been found. Population growth is projected to continue into the future, increasing from 7.75 people/km² in 2006 to 11.18 people/km² by 2031 (B.C. Ministry of Environment 2007).

Top of Page

Habitat protection/ownership

Lands over most of the geographic range of the species are privately owned, both on Vancouver Island, the Gulf Islands, and the Sunshine Coast, and are primarily used for residential and forestry purposes. Moist mixed–wood and deciduous stands containing suitable habitat for the Threaded Vertigo are scattered in riparian areas and other low elevation sites amid rural, agricultural, and forestry lands, and it is impossible to estimate the extent of suitable habitat that is on private versus public lands. Most known sites of the Threaded Vertigo are on public lands, including regional and provincial parks, largely reflecting the distribution of survey effort that has focused on these relatively undisturbed areas. However, many additional sites probably exist on private lands. The species and its habitats have no protection on private lands.

On Vancouver Island, the Threaded Vertigo is known from two federal properties used by the Department of National Defence for military training activities (Rocky Point Ammunition Depot and Heals Rifle Range). At present, the habitat at both sites is relatively intact and under no immediate pressure from development; however, their long–term future is not secure. The species is also known from two regional parks and from a narrow right–of–way of a regional trail within the Capital Regional District Parks and Trails System. Records exist from three provincial parks on Vancouver Island (John Dean, Horne Lake, and Rosewall Creek Provincial Parks) and two on the Sunshine Coast (Skookumchuck Narrows and Roberts Creek Provincial Parks). On Saturna Island, the species occurs within the Gulf Islands National Park. Habitats within parks are protected from development but are not necessarily safe from habitat degradation, resulting from invasive species, intensive recreational activities, or other disturbances.

Top of Page

Biology

Virtually no information is available on the life history and other aspects of the biology of the Threaded Vertigo in British Columbia or within its range in the United States. Therefore, information provided below is extrapolated from relevant aspects of the biology of other land snails, especially vertiginids or other species of Nearctula, when available.

Top of Page

Life cycle and reproduction

Like most pulmonate gastropods, the Threaded Vertigo is hermaphroditic (possesses both male and female reproductive organs) but probably exchanges gametes with other individuals through copulation. Cross–fertilization appears to be the norm in most terrestrial gastropods (Tompa 1984), but self–fertilization and aphally (loss of male genitalia) is widespread in European species of Vertiginidae (Pokryszko 1987). It is unknown whether self–fertilization occurs in the Threaded Vertigo. In European vertiginids, eggs are laid singly (Pokryszko 1987), but it is unknown if the Threaded Vertigo does the same.

Little is known of the life cycle and patterns of survival of the Threaded Vertigo. In British Columbia, most records are from the autumn, but adult snails have also been found in March and June, suggesting that the lifespan is more than one year. Applegarth and Duncan (2005) speculated that the related Hoko Vertigo is probably short–lived with a lifespan of two years or less. Timing of reproduction is unknown. Some pupilloids in temperate areas overwinter while gravid, giving birth in early spring (Tompa 1984). Generation time for the Threaded Vertigo might be about one year, but no definite information is available. The snails probably hibernate in the winter and aestivate during dry periods in the summer. Threaded Vertigo individuals appeared to be inactive and likely aestivating during a dry period in June 2008 (Forsyth pers. comm. 2009).

Top of Page

Herbivory/predation

The Threaded Vertigo probably gleans microorganisms from the bark of trees and shrubs, as suggested for Nearctulaspecies in the United States based on their arboreal habits (Applegarth and Duncan 2005). The snails may also feed on decaying vegetable matter and fungi while within the leaf litter, but no information is available.

Predators likely include songbirds that forage on trees and shrubs, such as chickadees, bushtits, and wrens. The Winter Wren (Troglodytes troglodytes) has been observed feeding in bushes with aggregations of Nearctula sp. (cited as Pupa rowellii), apparently feeding on these snails (Andrews 1897 cited in Pilsbry 1948). Various arthropod predators may prey on the Threaded Vertigo in both arboreal and terrestrial habitats. For example, carabid beetles, such as Scaphinotus species, are known snail–predators (Digweed 1993) and commonly climb tree trunks at night (Ovaska and Sopuck, unpublished data).

Top of Page

Physiology

Applegarth and Duncan (2005) noted that pupilloid snails, including Nearctula species, presumably possess some degree of freeze–tolerance, as they overwinter exposed on tree limbs in the Pacific Northwest of the United States where ambient temperatures frequently drop below freezing during winter months. However, the physiological mechanisms for doing this are unknown. Little is also known about summer dormancy or aestivation of the Threaded Vertigo or other Nearctula species. Like most land snails, the Threaded Vertigo produces an epiphragm of dried mucus that temporarily seals off the aperture, so reducing water loss, during periods of dormancy. Studies of another pupilloid snail (Lauria cylindracea) demonstrate specialized physiological mechanisms that enable the snails to withstand water loss during prolonged dry periods (Zeev et al. 1998). This species inhabits open, often rocky, areas and forest edges and is probably able to withstand drier conditions than the Threaded Vertigo. Nevertheless, the Threaded Vertigo may possess similar physiological adaptations that enable it to survive summer droughts in a dormant stage.

Top of Page

Dispersal/migration

Individual snails of this species are likely to be sedentary, perhaps confining their movements to individual trees. Active dispersal of land snails in general is extremely limited (reviewed in Cordeiro 2004), and long–distance dispersal usually occurs through passive means with wind, water, or animal vectors, or aided by human activities. Small snails such as the Threaded Vertigo could be transported by wind while adhering to falling leaves, but no data are available, and on trees the snails have been found on bark, not leaves. Extrapolating from movements of sediment particles, Kirchener et al. (1997) calculated that a small vertiginid snail (Truncatellina rothi), comparable in size to the Threaded Vertigo, could be transported up to several kilometres during severe storms, contributing to the colonization of oceanic islands and genetic interchange among populations from different islands. The Threaded Vertigo may similarly be transported from stand to stand by storms with high winds. Passive transport with human activities could also occur, for example, with firewood removal.

It is likely that the Vancouver Island, Saturna Island, and mainland B.C. populations are isolated from each other (and populations in the U.S.) due to stretches of ocean. Within each of the above geographic areas, low–elevation deciduous and mixed–wood forest stands containing Bigleaf Maple are highly fragmented by urban development and other human activities. However, uncertainties exist about the implications of this fragmentation for the snails because (a) their long–distance dispersal capabilities through passive means are unknown; (b) scattered small groves of Bigleaf Maples could potentially provide stepping–stones for dispersal; and (c) Bigleaf Maples are able to regenerate relatively rapidly, within decades, after disturbance (Peterson et al.1999), although older stands that may be more suitable continue to be lost. For the above reasons, habitats of the snails within each of the three larger areas are not considered severely fragmented, although some sites on southern Vancouver Island, in particular, are isolated amid developed areas.

Top of Page

Interspecific interactions

Surveys conducted in 2008 indicate a close association of the Threaded Vertigo with the Bigleaf Maple (see Habitat requirements). The furrowed bark apparently provides preferred habitat for these snails. The snails may obtain minerals directly from the bark, as the bark is rich in calcium (Peterson et al. 1999), which is important for shell development in snails. All forest stands where the Threaded Vertigo has been found in British Columbia contain Bigleaf Maples (Appendix 1), but the snails have occasionally been found on trunks of other deciduous trees and within leaf litter on the forest floor. Whether populations would be able to persist in mixed–wood or deciduous forest stands devoid of Bigleaf Maples is unknown and requires more extensive, comparative surveys throughout the species’ potential range in British Columbia.

Top of Page

Adaptability

Due to its minute size, resource requirements of the Threaded Vertigo are also small. It may be able to persist in small forest patches or groves of trees that contain suitable microhabitats. It can probably coexist with a variety of human activities as long as moist habitat with deciduous trees is retained; it has been found alongside busy recreational trails and near roadsides (Appendix 1). However, the snails occupy specialized arboreal microhabitats, increasing their vulnerability to changes in conditions that change the structure or microclimate of these habitats.

Top of Page

Population Sizes and Trends

Sampling effort

Terrestrial gastropod faunas of British Columbia 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). Survey effort has been sporadic and negligible over much of the 20th century.

Recent surveys in the province include those by R.A.D. Cameron, Robert Forsyth, and Biolinx Environmental Research Ltd. (Kristiina Ovaska and Lennart Sopuck). Surveys until 2008, outlined below, did not target tree trunk microhabitats preferred by the Threaded Vertigo, and the species might have been missed. Therefore, these surveys do not adequately reflect efforts expended to locate these snails in British Columbia.

Cameron (1986) surveyed 38 lowland forest sites on Vancouver Island, Lower Fraser Valley, and Chilliwack Valley in 1984. He found the Threaded Vertigo at two sites, both on Vancouver Island. Robert Forsyth has surveyed numerous places throughout British Columbia since 1990, including hundreds of sites on Vancouver Island and the southwestern mainland (Forsyth pers. comm. 2008). He found the species at five sites, on Vancouver Island and the Sechelt Peninsula. From 1999 to 2008, Biolinx Environmental Research Ltd. conducted numerous surveys for terrestrial gastropods in British Columbia, including the following areas: 152 sites in the Queen Charlotte Islands (Haida Gwaii), 108 sites in the Kootenay region, about 300 sites on Vancouver Island and the Gulf Islands, and about 200 sites on the coastal mainland. These surveys included the following areas: 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/CFSNatural Resources Management Program); selected forestry lands managed by Western Forest Products (formerly Weyerhaeuser Company) on Vancouver Island, Graham Island, and southwestern mainland near Powell River; numerous other sites on Vancouver Island, southwestern mainland, and interior of the province (for the B.C. Ministry of Environment); numerous sites on Vancouver Island, Lower Mainland, and the Queen Charlotte Islands as part of projects supported by the Endangered Species Recovery Fund and Wildlife Habitat Canada; selected regional parks on southern Vancouver Island (for Capital Regional District Parks). While some of the above surveys consisted of one–time visits, several others included intensive, repeated surveys over several years using artificial cover–objects and/or extraction of small snails from litter samples. The Threaded Vertigo was found at three sites surveyed by Biolinx Environmental Research Ltd. before the field verification surveys conducted for this status report.

Field verification associated with the preparation of this status report included surveys of 17 sites (13 on Vancouver Island and four on the Sunshine Coast) for a total of 20.3 person–hours in 2008, targeting habitats of the Threaded Vertigo in low elevation mixed–wood or deciduous forest (Appendix 1, 2). The species was found at nine sites, including all four sites on the Sunshine Coast (Appendix 1). These surveys were intended to provide presence/absence level information to confirm the presence of the species at historical sites and to fill in distribution gaps; more intensive methods, such as quadrat searches, needed to provide density estimates were not deployed. During searches of trees, only the lower portion of tree trunks below 2 m were available for examination without specialized climbing equipment; when covered by moss, moss was not stripped but the trunk was selectively sampled to avoid excessive disturbance.

Top of Page

Abundance

Little information is available on densities or relative abundance of the Threaded Vertigo, and no attempts have been made to estimate population sizes. In general, arboreal vertiginids tend to have patchy distribution patterns within suitable forest stands, with relatively large numbers of snails found in some trees and none in other seemingly similar trees (Applegarth and Duncan 2005 and references therein). During field verification surveys for this report, an average of 2.9 snails/occupied tree were found on the portion of trunk below 2 m that was searched (range: 1–12 snails per tree, n = 27 occupied trees). The snail populations on entire trees could have been much larger.

In contrast to arboreal habitats, the snails appear to exist at low densities in the litter layer on the forest floor. Cameron (1986) found one and four snails, respectively, at two sites where the species was detected. At both sites, the snails were within a 30 m x 30 m sampling area that was intensively searched both visually and by extracting small snails from litter samples (see Cameron [1986] for details of methods). At three sites on southern Vancouver Island in the autumn of 2008, Ovaska and Sopuck (2008, 2009) found only one individual snail per site using artificial cover–objects placed on the forest floor (100–650 cover–object checks per site). Additional cover–objects in other parts of the same sites resulted in no records of this species. Cardboard cover–objects effectively sample a variety of forest floor gastropods (Boag 1990; Hawkins et al. 1998; Ovaska and Sopuck 2001), but may underestimate abundance of the Threaded Vertigo because of their mainly arboreal habits.

Top of Page

Fluctuations and trends

Few data are available on population fluctuations or trends. Loss of habitat over the past century due to human activities may have resulted in a population decline of the Threaded Vertigo, but no historic baseline data are available. The increase in the number of distribution records over the past 10 years is due to the increased search effort. Field verification surveys carried out in 2008 indicate that the species has persisted at many of the sites surveyed over several years. The species’ continued presence was confirmed near the site of a previous record on the Sunshine Coast, where it was first found near Egmont in 1996, and within the Capital Regional District on southern Vancouver Island, where it was first found at Rocky Point in 2002. In contrast, it was not found near three historic sites on the east coast of Vancouver Island north of Victoria (Union Bay in 1996; Departure Bay and Quamichan Lake, undated, from the mid–1890s to early 1920s). The latter two records are inexact, but little original habitat remains in the vicinity of these populated areas.

Top of Page

Rescue effect

Dispersal of the snails from western Washington State to the mainland of British Columbia is conceivable but unlikely. On the Canadian side, the nearest known populations of the Threaded Vertigo are on the Sunshine Coast north of the metropolitan area of Vancouver, which is likely to act as a barrier to movement if the snails were to cross the border. The distribution of the species appears to be scattered in Washington. There are several records from the San Juan Islands, from Lopez and Patos islands (Burke pers. comm. 2008); Patos Island is only 6 km from Saturna Island on the Gulf Islands in British Columbia. If long–distance transport occurs by severe windstorms, a low frequency of natural dispersal between Washington and Vancouver Island via the San Juan Islands and Gulf Islands cannot be ruled out.

Top of Page

Limiting Factors and Threats

The Threaded Vertigo reaches the northern extremity of its geographic range in southwestern British Columbia. Its expansion to the north and higher elevations in the province are probably limited by the distribution and abundance of Bigleaf Maple and associated ecosystems, and possibly by physiological tolerances of the snails to low temperatures, but these are presently unknown.

Main known threats to the Threaded Vertigo are from habitat loss, fragmentation, and degradation associated with an expanding human population in the lowland, coastal areas where the snails occur (Figure 7; see Habitat trends for details). Human activities that threaten habitats within the range of the Threaded Vertigo include housing developments, road building and other associated infrastructure, agriculture, and forestry. Although Bigleaf Maples are not usually commercially harvested, many trees are felled or damaged as a result of clear–cut logging operations, and harvesting of conifers around small maple stands or individual trees exposes them to the drying effects of wind and sun, so deteriorating habitat conditions for snails. Any human activity that alters natural patterns of hydrology can result in conditions that are either too dry or too wet for maples or the epiphytic communities they support. Bigleaf Maples are intolerant of flooding for prolonged periods (Peterson et al. 1999). Conversion of riparian hardwood stands to conifers as part of forestry activities or aquatic habitat restoration is also a potential threat to Nearctula species, as suggested for the Hoko Vertigo in Washington State (Applegarth and Duncan 2005).

Fragmentation of forest habitats is a potential threat for terrestrial gastropods with poor dispersal capabilities. On one hand, their ability to persist in small habitat patches for long periods is likely to ameliorate requirements for habitat connectivity at landscape and larger scales (Roth 1993). On the other hand, snail populations in small habitat patches are vulnerable to extirpation from catastrophic events, such as fires, or from stochastic environmental or population fluctuations. Hence, both the number of occupied habitat patches and their distribution within the landscape are an important consideration for assessing threats and probability of persistence of the species over larger geographic areas. For the Threaded Vertigo, many more occupied habitat patches than are currently known probably exist, and the species occurs at widespread places, suggesting that the total population might be somewhat resilient to habitat fragmentation. However, there is incomplete information both on the number of forest stands that are occupied and on the number that are necessary for the species to persist and be viable over the long term within a given area. There is insufficient evidence to determine whether the distribution can be considered severely fragmented, as the size of occupied areas and habitat patches and viability of subpopulations are unknown. During field verification surveys in 2008, the smallest stand where the Threaded Vertigo was found was a small isolated grove of about 10 maples in a moist depression amid unsuitable habitat (a clear–cut on one side and a highway and lake on the other) on the Sechelt Peninsula. Unless the snails are passively transported by wind or other means, small populations such as this one might not be viable over the long term. Similarly, populations of the Threaded Vertigo in small, protected areas, such as several CDR Parks and narrow trail right–of–ways surrounded by residential developments, may be isolated and vulnerable to extinction.

Potential threats include habitat degradation from edge effects that reduce moisture, increased disturbance from recreational and other uses, and introduced plants and animals. Invasive introduced plants (such as English Ivy [Hedera helix], Scotch Broom [Cytisus scoparius], Gorse[Ulex europaeus], and Laurel–leaved Daphne [Daphne laureola]) are extensively modifying natural ecosystems within the range of the Threaded Vertigo and are common at many sites where the snails occur. English Ivy is extremely abundant at some sites both on the forest floor and in trees and directly modifies arboreal communities; its effects on snail populations are unknown but potentially detrimental. Habitat degradation is expected to be particularly prevalent in small forest stands that are subject to invasion by introduced species and changes in microclimate, including drying of the forest floor and arboreal habitats.

Introduced carabid beetles, which are known predators of land snails, may be a threat. There has been a widespread change in the carabid beetle fauna with 22 introduced species now in B.C. (C. Copley pers. comm. 2010). Just how many of these non–native carabids are in the area of the Threaded Vertigo and whether they predate the Threaded Vertigo, or what the effects might be, are unknown.

Detrimental effects of climate change on the snail populations are plausible, particularly in degraded and fragmented habitats. Prolonged summer droughts and increased frequency of extreme climatic events are predicted to occur in British Columbia under climate change scenarios (B.C. Ministry of Environment 2007). Drier conditions during the activity period of the snails from spring to autumn could adversely affect snail populations, especially here at the northern extremity of the species’ range where the growing season is already relatively short. Increased frequency and severity of flooding and fires would also be detrimental to snails and their habitats.

Based on an analysis of main threats at each occupied site, there are 16 recent locations for the Threaded Vertigo, applying the COSEWIC definition based on threats to define location (Table 1). Historical distribution records were excluded from the analysis. The number of locations is somewhat arbitrary, because landownership, zoning, habitat fragmentation, and a myriad of other factors affect the geographic scope and immediacy of threats to this species including development, land clearing, edge effects, and fire. The following locations were identified: one location on Saturna Island; four locations on the Sunshine Coast; and11 locations on Vancouver Island within the Capital Regional District and on the east coast north of this area. The records for the species are mostly widely spaced point records from areas that are usually under different land ownership and different immediacy of threats. Supported by the patchy distribution and apparently low dispersal capability of the species, most of these points are considered a separate location. A notable exception is the cluster of records from southwest of Victoria, which was divided into three locations, based on land ownership, habitat fragmentation, habitat type, and potential threats.

Top of Page

Special Significance of the Species

The Threaded Vertigo is of no known socio–economic or cultural significance, and no Aboriginal traditional knowledge is available. In fact, few people apart from malacologists and dedicated naturalists are even aware of the existence of these minute snails. These snails contribute to the biodiversity of the rich, extraordinary arboreal communities supported by large Bigleaf Maples. These communities include abundant epiphytic mosses, liverworts, lichens, ferns, and other vascular plants that grow on the branches and boles of old maples (Peterson et al. 1999). These communities are particularly well developed in moist riparian areas, inhabited by the snails. Some epiphytic bryophyte communities associated with Bigleaf Maples are unique and contribute significantly to British Columbia’s biodiversity (Peterson et al. 1999). Epiphytes, in turn, enhance conditions for other organisms, ranging from fungi and minute invertebrates to birds and mammals. These communities are poorly understood, and the ecological roles and services of the different components, including the Threaded Vertigo, are yet to be elucidated.

Table 1. Threats for sites with records of the Threaded Vertigo and the assignment of the sites to COSEWIC locations.
Geographic areaSite1 ID #
(land status)
Threat (in order of priority)Location #
Vancouver Island: east–central coast1 (private)
  • Logging
  • Development
  • Intensive wildfire
1
Vancouver Island: east–central coast1 (private)
  • Logging
  • Development
  • Intensive wildfire
1
Vancouver Island: east–central coast2 (provincial park)
  • Recreational activities
  • Conversion to conifers through succession
2
Vancouver Island: east–central coast3 (provincial park)
  • Recreational activities
  • Conversion to conifers through succession
3
Vancouver Island: southeast coast1 (private)
  • Logging
  • Development
  • Recreational activities
4
Vancouver Island: CDR2
(North Saanich)
1 (provincial park)
  • Conversion to conifers through succession
  • Recreational activities
  • Intensive wildfire
5
Vancouver Island: CDR (Saanich)2 (federal)
  • Military training activities
  • Conversion to conifers through succession
6
Vancouver Island: CDR (Highlands)3 (regional park)
  • Recreational activities
  • Conversion to conifers through succession
  • Intensive wildfire
7
Vancouver Island: CDR (Metchosin)4 (regional park)
  • Recreational activities
  • Invasive species
  • Severe weather or stochastic events (small, isolated population)
  • Intensive wildfire
8
Vancouver Island: CDR (Metchosin)5 (federal)
  • Military training activities
  • Conversion to conifers through succession
9
Vancouver Island: CDR (Metchosin)6 (right–of–way of regional trail surrounded by private lands)
  • Recreational activities
  • Development
  • Invasive species
  • Edge effects
  • Intensive wildfire
10
Vancouver Island: CDR
(Sooke)
7 right–of–way of regional trail surrounded by private lands)
  • Development
  • Recreational activities
  • Invasive species
  • Intensive wildfire
11
Saturna Island1 (national park)
  • Recreational activities
  • Conversion to conifers through succession
  • Intensive wildfire
12
Sunshine Coast: SE of Sechelt1 (provincial Park)
  • Recreational activities
  • Conversion to conifers through succession
  • Invasive species
  • Intensive wildfire
13
Sunshine Coast: Sechelt Peninsula2 (private)
  • Development
  • Logging
  • Recreational activities
  • Conversion to conifers through succession
  • Invasive species
  • Edge effects
  • Severe weather or stochastic events (small, isolated population)
  • Intensive wildfire
14
Sunshine Coast: Sechelt Peninsula3 (private)
  • Development
  • Intensive wildfire
15
Sunshine Coast: Sechelt Peninsula4 (private at edge of B.C. crown land)
  • Recreational activities
  • Development
  • Edge effects
  • Intensive wildfire
16

1 Site – single distribution record or cluster of records

2 CDR – Capital Regional District (Victoria).

Top of Page

Existing Protection or Other Status Designations

The Threaded Vertigo was designated Special Concern by COSEWIC in April 2010. The species is on the provincial red list of species at risk. NatureServe (2009) lists the species as “apparently secure” globally (G3G5; rounded status G4) with national status in the United States also “apparently secure” (N3N5) but “imperilled” in Canada (N2) and British Columbia (S2). No other sub–national rankings for the United States are available from NatureServe, which last assessed the species on 30 January 2006.

Top of Page

Acknowledgements and Authorities Consulted

Robert Forsyth provided data, literature, and the photograph and drawings of shells (Figure 2), and generously shared unpublished distribution records. Nancy Duncan, Tom Burke, and Barry Roth kindly provided information and distribution records of the Threaded Vertigo in the United States. Andreas Hamann provided mapping information for Figure 7. Virgil Hawkes provided information on his gastropod surveys in British Columbia. Alain Filion prepared the map of Canadian distribution and calculated EO and IAO values. Kelly Sendall (Royal British Columbia Museum), Jean‑Marc Gagnon (Canadian Museum of Nature), Maureen Zubowski (Royal Ontario Museum), Timothy Pearce (Carnegie Museum of Natural History), Jochen Gerber (Field Museum of Natural History), and Kevin Roe (Delaware Museum of Natural History) kindly searched their databases for specimen records. This report was improved by comments provided by Barry Roth. Funding for this report was provided by Environment Canada.

The following required contacts were consulted:

  • David Cunnington (Canadian Wildlife Service)
  • Brian Reader and Todd Columbia (Parks Canada)
  • Jennifer Heron (B.C.Ministry of Environment)
  • Lea Gelling (B.C.Conservation Data Centre)
  • Erin Prescott (B.C.Conservation Data Centre)
  • Gloria Goulet (COSEWIC Secretariat – ATK Coordinator)
  • Alain Filion (COSEWIC Secretariat – mapping support)

Top of Page

Information Sources

Applegarth, J.S. and N. Duncan. 2005. Conservation assessment for Vertigo n. sp., Hoko Vertigo. Originally issued as management recommendations in February 1999. Revised by N. Duncan in October 2005. USDA (United States Department of Agriculture) Forest Service Region 6 and USDI (United States Department of the Interior) Bureau of Land Management, Oregon and Washington. Web site: [accessed February 2009].

B.C. Forests and Range. (undated). Tree Book. Learning to recognize trees of British Columbia. Web site: [accessed February 2009].

B.C. Ministry of Environment. State of the Environment Reporting – Environmental Trends 2007. Web site: [accessed February 2009].

Boag, D.A. 1990. On the effectiveness of artificial shelters in the study of population attributes of small terrestrial gastropods. Canadian Journal of Zoology 68:254–262.

Bouchet, P. and J–P. Rocroi. 2005. Classification and nomenclature of gastropod families. Malacologia 47:1–397.

Burke T.E., pers. comm. 2009. Email correspondence to K. Ovaska. February 2009. Biologist, USFS, Entiat, Washington.

Burke T.E., J.S. Applegarth and T.R. Weasma. 1999. Management recommendations for survey and manage terrestrial mollusks. Version 2 (PDF, 880 KB). USDI (United States Department of the Interior) Bureau of Land Management, Salem Oregon. Web site: [accessed February 2009].

Cameron, R.A.D. 1986. Environment and diversities of forest snail faunas from coastal British Columbia. Malacologia 27:341–355.

Clarke, A.H. 1960. Catalogue and illustrations of mollusks described by Wesley Newcomb, with a biographical resumé. Bulletins of American Paleontology 41:135–160.

Coney, C.C. 1988. Systematic significance of shell ultrastructure in the North American genera of Pupillidae. Western Society of Malacology Annual Report 20:32.

Copley, C., pers. comm. 2010. Correspondence to D. Fraser. January 27, 2010. Senior Collections Manager, Entomology, Royal British Columbia Museum, Victoria, B.C.

Cordeiro, J. 2004. Nearctula species 1. Populations/occurrence delineation. inNatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Web site: [accessed February 2009].

CDR (Capital Regional District). 2007.Fact sheet. Demographics – Population. 2006 Census results, Capital Region (PDF, 60 KB). Web site: [accessed January 2009].

Davis, D.S. 1995. Terrestrial molluscs inventory and monitoring study, Kejimkujik National Park, Nova Scotia. Unpublished report, Dartmouth, Nova Scotia. 68 pp.

Digweed, S.C. 1993. Selection of terrestrial gastropod prey by cychrine and pterostichine ground beetles (Coleoptera: Carabidae). The Canadian Entomologist125:463–472.

Drake, R.J. 1963. The history of nonmarine malacology in British Columbia. pp. 1–16, in Contributions to Canadian Zoology, 1962. National Museum of Canada, Bulletin 185.

Duncan, N., pers. comm. 2008. Email correspondence to K. Ovaska. May 2008. Biologist, USDA Bureau of Land Management, Roseburg, Oregon.

Forsyth, R.G. 2004. Land Snails of British Columbia. Royal British Columbia Museum Handbook. Royal British Columbia Museum, Victoria, B.C.188 pp.

Forsyth, R.G., pers. comm. 2008. Email correspondence to K. Ovaska.November 2008. Research Associate, Royal British Columbia Museum, Victoria, B.C.

Frest, T.J. and E.J. Johannes. 2000. Baseline mollusk survey of southwestern Oregon, with emphasis on the Rogue and Umpqua River Drainages. Unpublished report prepared for Oregon Natural Heritage Program, Portland, Oregon. 403 pp.

Hamann, A. Email correspondence to K. Ovaska, February 2009. Professor, Department of Renewable Resources, University of Alberta, Edmonton.

Hawkins, J.W., M.W. Lankester and R.R.A. Nelson. 1998. Sampling terrestrial gastropods using cardboard sheets. Malacologia 39:1–9.

Kirchner, Ch., R. Krätzner and F.W. Welter–Schultes. 1997. Flying snails--how far can Truncatellina (Pulmonata: Vertiginidae) be blown over the sea? Journal of Molluscan Studies 63:479–487.

Meidinger, D. and J. Pojar. 1991. Ecosystems of British Columbia. B.C Ministry of Forests, Victoria, British Columbia. Special Report Series 6: xii + 330 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Web site: [accessed February 2009].

Ovaska, K. and L. Sopuck. 2000. Evaluation of the potential of terrestrial gastropods (slugs and snails) for monitoring ecological effects of logging practices on forest–floor conditions on Vancouver Island, British Columbia. A pilot study, October November 1999. Unpublished report prepared by Biolinx Environmental Research Ltd. for Weyerhaeuser Company Ltd., Nanaimo, British Columbia (now Western Forest Products, Campbell River, British Columbia). 44 pp.

Ovaska, K. and L. Sopuck. 2001. Potential of terrestrial gastropods and salamanders as indicators for monitoring ecological effects of variable–retention logging practices. A pilot study in the North Island Timberlands, May – November 2000. Unpublished report prepared by Biolinx Environmental Research Ltd. for Weyerhaeuser Company Ltd., Nanaimo, British Columbia (now Western Forest Products, Campbell River, British Columbia). 105 pp.

Ovaska, K. and L. Sopuck. 2002. Surveys for terrestrial and freshwater molluscs on DND lands near Victoria, Vancouver Island, British Columbia. Unpublished report prepared by Biolinx Environmental Research Ltd.for CFS/CFB (Canadian Forest Service/Canadian Forces Base) Esquimalt Natural Resources Management Program, CFB Esquimalt, Victoria, British Columbia. 56 pp.

Ovaska, K. and L. Sopuck. 2008. Surveys for the Blue–grey Taildropper and other gastropods at risk within CDR Parks and Trails System in 2008. Unpublished report prepared by Biolinx Environmental Research Ltd. for CDR (Capital Regional District) Parks, Victoria, British Columbia. 61 pp.

Ovaska, K. and L. Sopuck. 2009. Surveys for the Blue–grey Taildropper slug

(Prophysaon coeruleum) on federal lands on southern Vancouver Island, BC,

autumn 2008. Unpublished report prepared by Biolinx Environmental Research Ltd. for CFS/CFB (Canadian Forest Service/Canadian Forces Base) Esquimalt Natural Resources Program, Victoria, B.C. 124 pp.

Peterson, E.B., N.M. Peterson, P.G. Comeau and K.D. Thomas. 1999. Bigleaf Maple Managers’ Handbook for British Columbia (PDF, 2156 KB). B.C. Ministry of Forests Research Program. Web site: [accessed February 2009].

Pilsbry, H.A. 1948. Land Mollusca of North America (north of Mexico). The Academy of Natural Sciences of Philadelphia, Monograph 3, 2(1):521–1113, i–xlvii.

Pokryszko, B.M. 1987. On the aphally in the Vertiginidae (Gastropoda: Pulmonata: Orthurethra). Journal of Conchology 32:365–375.

Roth, B., pers. comm. 2008, 2009. Email correspondence to K. Ovaska. June, October, November 2008; February 2009. Malacologist, Santa Barbara Museum of Natural History, Santa Barbara, California.

Roth, B. 1993. Critical review of terrestrial mollusks associated with late successional and old–growth forests in the range of the Northern Spotted Owl. Report prepared for the Forest Ecosystem Management Working Group, USDA (United States Department of the Interior) Forest Service, Portland, Oregon, 42 pp. Cited in Burke et al. 1999 (Vertigo n. sp., p. 6).

Roth, B. and P.S. Sadeghian. 2003. Checklist of the land snails and slugs of California. Santa Barbara Museum of Natural History Contributions in Science 3:1–81.

Roth, B. and P.S. Sadeghian. 2006 . Checklist of the land snails and slugs of California. 2nd edition. Santa Barbara Museum of Natural History Contributions in Science 3:1–82.

Schileyko, A.A. 1998. Treatise on recent terrestrial pulmonate mollusks. Part 2. Ruthenica (Supplement 2): 134 pp.

SCORE. 2006. Website created by Urban–Eco Consultants Ltd. and maintained by Gibsons Community Initiatives Association. Web site: [accessed January 2009].

Smith, A.G., W.B. Miller, C.C. Christensen and B. Roth. 1990. Land Mollusca of Baja California, Mexico. Proceedings of the California Academy of Sciences 47:96–158.

Sopuck, L. and K. Ovaska 2009. Winter Cove EIS Risk assessment for gastropods on the Winter Cove property of Gulf Islands National Park Reserve, March 2009. Unpublished report prepared by Biolinx Environmental Research Ltd. for Parks Canada, Sidney, British Columbia. 13 pp.

Tompa, A.S. 1979. Oviparity, egg retention and ovoviviparity in pulmonates. Journal of Molluscan Studies 45:15–60.

Tompa, A.S. 1984. Land snails (Stylommatophora). pp. 47–140 in A.S. Tompa, N.H. Verdonk, and J.A.M. van den Biggelaar (eds). The Mollusca, 7: Reproduction Academic Press, London & New York.

Turgeon, D.D., J.F. Quinn, A.E. Bogan, E.V. Coan, F.G. Hochberg, W.G. Lyons, P.M. Mikkelsen, R.J. Neves, C.F.E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione and J.D. Williams. 1998. Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks. 2nd edition. American Fisheries Society Special Publication 26: ix + 526 pp.

Zeev A., S. Goldenberg and J. Heller. 1998. Short– and long–term resistance to desiccation in a minute litter–dwelling land snail Lauria cylindracea (Pulmonata: Pupillidae). Journal of Zoology 246:75–81.

Top of Page

Biographical Summary of Report Writers

Kristiina Ovaska, Ph.D., M.Sc., received her doctoral degree in biology from the University of Victoria in 1987, after which she completed two post–doctoral studies in animal behaviour and population biology. Presently, she is a partner in Biolinx Environmental Research Ltd. and a research associate at the Department of Forest Sciences, University of British Columbia. Her experience with terrestrial gastropods includes research into effects of forestry practices, studies on patterns of abundance and distribution of species at risk, and numerous surveys in different parts of British Columbia. She has prepared status reports, recovery strategies, a multi–species action plan, and best management practices guidelines for terrestrial gastropods. Her photographs of gastropods appeared in the Royal B.C. Museum Handbook “Land Snails of British Columbia” by R. Forsyth. She is the author of more than 40 publications in the refereed scientific literature, including several papers on terrestrial gastropods.

Lennart Sopuck, M.Sc., RPBio, has studied a wide variety of wildlife species over the past 25 years. His expertise includes assessing and mitigating effects of various human activities on wildlife, including species at risk. Together with Dr. Ovaska, he is a partner of Biolinx Environmental Research Ltd. and has conducted numerous survey and research projects on terrestrial gastropods of British Columbia. He is co‑author of several status reports, recovery strategies, a multi–species action plan, and management documents for terrestrial gastropod species.

Top of Page

Collections Examined

The following institutional collections were consulted. Only the Royal British Columbia Museum (one record) and the Field Museum of Natural History (three records) had specimens from British Columbia.

  • Canadian Museum of Nature, POBox 3443, Stn. D, Ottawa, ONK1P 6P4
  • Carnegie Museum of Natural History, 4400 Forbes Ave, Pittsburgh, PA 15213–4080
  • Delaware Museum of Natural History, 4840 Kennett Pike, PO Box 3937, Wilmington, DE 19807–0937
  • The Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, IL 60605‑2496. Identifications verified by R. Forsyth.
  • Royal British Columbia Museum, 675 Belleville Street, Victoria, BC V8V 1X4. Identifications verified by R. Forsyth.
  • Royal Ontario Museum, 100 Queen’s Park, Toronto, Ottawa, ON M5S 2G6

Top of Page

Appendix 1. Distribution records of the Threaded Vertigo, Nearctula species, from Canada.  CDC EO# – B.C. Conservation Data Centre Element of Occurrence number; FMNH – Field Museum of Natural History; RBCM – Royal British Columbia Museum.
Coll. dateNo. of snailsCollectorLocation descriptionElev. (m)Habitat (microhabitat)Database, collection, or source
Undated6UnknownVancouver Island, BC: "Quamichan Lake, Duncan"  FMNH (52665) "Ex ROM Zoology"
Undated11UnknownVancouver Island, BC: "Victoria"  FMNH (132500) "Ex Alan Solem Collection"
Undated17UnknownVancouver Island, BC: "Wellington (Departure Bay)"  FMNH (132267) "Ex Alan Solem Collection"
14–17 Aug–19845R.A.D. CameronVancouver Island, BC: Lake Horne Second–growth mixed–wood forest with Bigleaf Maple; damp mull type litter; Sword Fern & Thimbleberry in understory (presumably within litter)Cameron (1986)
10–Nov–961R. Forsyth, T. ForsythSechelt Peninsula, BC: near Egmont At side of the road on flattened shoulder; grass, Sword Ferns; a few Maiden–hair Ferns (on maple leaves on ground)Robert Forsyth personal collection (96.159.4271); CDC EO #4
13–Nov–963T. Forsyth, R. ForsythVancouver Island, BC: Union Bay80–100Alder, Fir and Salmonberry (on leaf litter on ground)Robert Forsyth personal collection (96.167.4330); CDC EO #5
08–Nov–992Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck, S. Beauchesne)Vancouver Island, BC: Chemainus Lake90Mature mixed–wood stand with Red Alder, Douglas–fir, Western Hemlock, and Western Redcedar; 80% canopy cover; sparse understory; moderately deep leaf/needle litter layer (within litter)RBCM (000–00046–005; verified by R. Forsyth in 2000); Ovaska and Sopuck 2000; CDC EO #6
30–Mar–024Biolinx Environmental Research Ltd. (K. Ovaska, L. Sopuck)Vancouver Island, BC: Saanich (Maple transect 3)80–100Mixed–wood stand with large Bigleaf Maples (>60% canopy coverage) and Douglas–fir, Grand Fir, and Western Redcedar; understory with dense Sword Ferns (within maple leaf litter)K. Ovaska personal collection (#S1); Ovaska & Sopuck 2002; CDC EO #3
25–Oct–021Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island, BC: Metchosin (Alder transect 1)30–35Mature Red Alder with a minor component of Bigleaf Maple, Douglas–fir, Grand Fir, and Cottonwood; understory of Sword Fern, Salmonberry, and Salal; sedges and Skunk Cabbage in wet areas (within litter)K. Ovaska personal collection (#234); Ovaska & Sopuck 2002; CDC EO #1
25–Oct–021Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island, BC: Metchosin30–35Mature Red Alder with a minor component of Bigleaf Maple, Douglas–fir, Grand Fir, and Cottonwood; understory of Sword Fern, Salmonberry, and Salal; sedges and Skunk Cabbage in wet areas (on fern frond)K. Ovaska personal collection (#119); Ovaska & Sopuck 2002; CDC EO #1
13–Nov–027Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck, L. Matthias)Vancouver Island, BC: Metchosin (Maple transect 2)30–40Mixed–wood stand dominated by mature Bigleaf Maple, interspersed with Douglas–fir, Red Alder, Grand Fir, and Cottonwood; understory of Sword Fern, Salal, and Salmonberry (within litter (6 snails); on fern frond (1 snail))K. Ovaska personal collection (#2, #31, #122; 1 per sample); Ovaska & Sopuck 2002; CDCEO #2
20–Nov–021Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)BC: Vancouver Island: Metchosin Maple transect 3)30–40Mixed–wood stand dominated by mature Bigleaf Maple; some Douglas–fir, Red Alder, Grand Fir, and Cottonwood present; understory of Sword Fern, Salal, and Salmonberry (within litter)K. Ovaska personal collection (#1055); Ovaska & Sopuck 2002; CDC EO #1
13–Jun–083Robert ForsythVancouver Island, BC: Metchosin37On Bigleaf Maple trunks, to 2 m above ground. Exposed, on bark, or in crevices; no moss or a thin network of mossR. Forsyth, personal collection (08.052.1795)
13–Jun–0811Robert ForsythVancouver Island, BC: Sooke20On Bigleaf Maple trunk, to 2 m above ground. Sunny side, no moss; snails exposedR. Forsyth, personal collection (08.053.1798)
13–Jun–085R. Forsyth, L. Ramsay, L. GellingVancouver Island, BC: Metchosin27On Bigleaf Maple tree trunks; in crevices of bark and in moss, 1–2 m above the ground; in leaf litterR. Forsyth, personal collection (08.048.1803 )
13–Oct–081Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island, BC: Thetis Lake Regional Park (Plot 2)80Woodland with Douglas–fir, Garry Oak, and Bigleaf Maple; dense shrub understory with Oregon Grape and Snowberry (under cardboard cover–object)K. Ovaska, personal collection; Ovaska & Sopuck 2008
14–Oct–081Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island, BC Metchosin< 50Moist depression with abundant Sword Fern and some Salal, surrounded by Douglas–fir (about 80 years old) and some alder/willow (under cardboard cover–object)K. Ovaska, personal collection; Ovaska & Sopuck 2009
15–Oct–089Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island, BC: Sooke82Forested trail right–of–way with Bigleaf Maple, Western Redcedar, Western Hemlock, and Douglas–fir; understory of ferns and some shrubs; very moist site (maple trunk, in grooves of bark (most snails) or on smooth bark (1 snail))K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
       
19–Oct–086Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Sechelt Peninsula, BC: near Egmont87Second–growth stand with Western Redcedar, Western Hemlock, and large Bigleaf Maples; understory of Sword Fern and some shrubs (maple trunk, in crevices & under moss)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
19–Oct–085Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Sunshine Coast, BC: SE of Sechelt123Old stand of mixed–wood forest with Western Redcedar, Western Hemlock, Douglas–fir, and scattered Bigleaf Maple; understory of Sword Fern and some shrubs (trunk of large maple with grooved bark)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
19–Oct–087Biolinx Environmental  Ovaska, L. Sopuck)Sunshine Coast, BC: Skookumchuck Narrows110Riparian forest along small creek with Bigleaf Maple, Western Hemlock, and Western Redcedar; understory of ferns and some shrubs (maple trunk (2 snails); alder trunk (1 snail); fern (1 snail)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
19–Oct–088Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Sunshine Coast, BC near Ruby Lake100Grove of large Bigleaf Maples in a depression within disturbed landscape (maple trunk, on bare bark or under moss)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
25–Oct–0813Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)BC: Vancouver Island: North Saanich160Mature forest with Douglas–fir, Bigleaf Maple, Western Redcedar, Red Alder, and Grand Fir; moist site with large maples (maple trunk, large tree with grooved bark)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
26–Oct–0810Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)BC: Vancouver Island: Metchosin<50Riparian forest with large Bigleaf Maples, Douglas–fir, and Red Alder; dense understory of shrubs (maple trunk, in grooves of bark in one large tree)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
28–Oct–0812Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)BC: Vancouver Island: Metchosin<50Riparian forest with large Bigleaf Maples, Cottonwoods, and Red Alders; dense understory of mostly Sword Fern; very moist site (maple trunk (7 snails); cottonwood trunk (3 snails); alder trunk (1 snail))K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
03–Nov–0815Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)BC, Vancouver Island: south of Courtenay36Riparian forest with Douglas–fir, abundant Bigleaf Maple, Western Hemlock, Western Redcedar, and Red Alder; moist, rich site (maple trunk; most snails in crevices or furrowed bark or under moss of large trees)K. Ovaska, personal collection; Fieldwork associated with COSEWIC status report
22–Nov–081Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Vancouver Island: Saanich80–100Older, mainly coniferous forest with Western Redcedar and Grand Fir; few scattered Bigleaf Maples; understory of Sword Fern (under cardboard cover–object)K. Ovaska, personal collection; Ovaska & Sopuck 2009
20–Mar–0912Biolinx Environmental Research Ltd.(K. Ovaska, L. Sopuck)Saturna Island: Gulf Islands National Park Reserve10Trunks of large Bigleaf Maples in second–growth stand with Western Redcedar, Western Hemlock, and large Bigleaf Maples; understory of Sword Ferns, nettles and some shrubs (maple trunks)Risk assessment for gastropods on the Winter Cove property of Gulf Islands National Park Reserve, March 2009
(Sopuck and Ovaska 2009)

* Approximate location; no coordinates given; identification confirmed by R. Forsyth in 2002; collected from mid–1890s to early 1920s
^ Sites 27–28 for Vertigo rowellii in Cameron (1986). No coordinates given, but map indicates that both sites are at the western tip of Lake Horne.

Top of Page

Appendix 2. Sites where the Threaded Vertigo was not detected during field verification surveys associated with the preparation of this status report by Biolinx Environmental Research Ltd. Nine sites where the species was found are shown in Appendix 1.
DateSite descriptionElev. (m)Latitude NorthLongitude WestMacrohabitat
19–Jun–08BC, Vancouver Island: South end of Quamichan Lake (Kinsmen Park)30 48° 47.264'123° 40.279'Riparian area with scattered Cottonwood and Bigleaf Maple; dense patches of shrub
19–Jun–08BC, Vancouver Island: NE end of Quamichan Lake at Elkington Creek (at Osprey Place) 9048° 48.547'123° 37.455'Strip of mixed–wood forest with Red Alder, Bigleaf Maple, and Douglas–fir amid residential development; dense understory of shrubs and herbs
19–Jun–08BC, Vancouver Island: Departure Bay, Beach Estate Park (Nature Trail along Northfield Creek)40 49° 11.466'123° 57.960'Remnant strip of forest along creek surrounded by residential areas; Western Redcedar and Bigleaf Maple dominant trees with variable understory
19–Jun–08BC, Vancouver Island: Union Bay (woodlot off Green Road)100 49° 34.458'124° 53.949'Selectively logged second–growth stand with Douglas–fir, Red Alder, and scattered Bigleaf Maple; dense shrub understory
23–Oct–08BC, Vancouver Island: Fort Rodd National Heritage Site, Metchosin (CDR)<5048° 25.965'123° 27.264'Mixed–wood forest with Douglas–fir, Grand Fir, Bigleaf Maple, and Arbutus; dense shrub understory
03–Nov–08BC, Vancouver Island: Somenos Road, north of Duncan4748° 50.930'123° 43.3841'Moist riparian forest with Bigleaf Maple, Red Alder, Western Redcedar, and Grand Fir on steep slope; dense Sword Fern understory
03–Nov–08BC, Vancouver Island: Englishman River Falls Provincial Park21149° 14.864’124° 21.140'Mostly coniferous forest with scattered Bigleaf Maples
03–Nov–08BC, Vancouver Island: Fuller Lake6548° 54.279'123° 43.300'Mostly coniferous stand with scattered maples