Threaded Vertigo (Vertigo rowellii): COSEWIC status appraisal summary 2022

Official title: COSEWIC Status Appraisal Summary on the Threaded Vertigo (Vertigo rowellii) in Canada

Committee on the Status of Endangered Wildlife in Canada (COSEWIC)

Special concern 2022

COSEWIC assessment summary

Assessment Summary – May 2022

Common name
Threaded Vertigo

Scientific name
Vertigo rowellii

Status
Special Concern

Reason for designation
In Canada, this minute terrestrial snail is at the northern edge of its global range. The species is found in lowland areas around the Strait of Georgia and on southern Vancouver Island with most individuals living on the bark of Bigleaf Maple. The species is presumed to have poor dispersal abilities between trees and sites. Increases in the number of occupied sites and locations since the last assessment are due to increased search effort. Nonetheless, the index of area of occupancy remains below the threshold for Endangered and the extent of occurrence remains below the threshold for Threatened. The primary threats are loss of habitat and habitat degradation due to housing and urban development, logging, and roads and associated infrastructure. The species’ limited distribution and ongoing threats support maintaining a status of Special Concern.

Occurrence
British Columbia

Status history
Designated Special Concern in April 2010. Status re-examined and confirmed in May 2022.

COSEWIC status appraisal summary

Threaded Vertigo
Vertigo à crêtes fines
Vertigo rowellii (formerly Nearctula sp.1)
Range of occurrence in Canada (province/territory/ocean): British Columbia

Wildlife species

SAS 6

Change in eligibility, taxonomy or designatable units:
yes

Explanation:
Nekola et al. (2018) re-examined systematics of snails of the genus Vertigo based on analyses of nuclear DNA (ITS1 and ITS2) and mitochondrial DNA (CytB and 16S), supplemented by investigation of shell features. They resurrected the name Vertigo (Staurodon) rowellii (Newcomb, 1862) to include Nearctula of Turgeon et al. (1998) and Roth and Sadeghian (2003), as well as the unnamed “Hoko” Vertigo (Nearctula new sp. of Burke 2013). The type locality is “near Oakland, California”. MolluscaBase (2020) uses Vertigo rowellii (Newcomb, 1862) with Vertigo (Staurodon) rowellii as an accepted, alternate representation. The change in nomenclature does not affect the eligibility of the Canadian population.

Range

SAS 7

Change in extent of occurrence (EOO):
yes

SAS 8

Change in index of area of occupancy (IAO):
yes

SAS 9

Change in number of known or inferred current locations^[1]:
yes
¹sup> Use the IUCN definition of “location”

SAS 10

Significant new survey information:
yes

Explanation:
As of December 2020, there is a total of 76 records of the species from 42 sites in British Columbia (excluding a historical record with vague locality of “Victoria”). These include 48 new records and 22 new sites documented since the previous COSEWIC (2010) status report (Table 1). The above include six probably valid records from 2010 to 2020 (five from iNaturalist [2020] with photos, and one anecdotal sight record; D. Fraser pers. comm. 2020; Figure 1). Based on all 76 records, the EOO has increased slightly, by 14.5% (from 9,530 km² to 10,915 km², including ocean). The IAO has increased by 127.8% (from 72 km² to 164 km²; Figure 2). These increases reflect a better understanding of the species’ distribution rather than a range expansion. It is likely that there are more undocumented sites.

Concomitant with the increase in known occurrences, the number of threat-based locations has also increased to 42, assuming that each site is subjected to a separate combination of threatening events from residential development, road building, logging and wood cutting, and other site-specific threats. If prolonged and more frequent droughts were deemed the most plausible serious threat, then the number of locations would be smaller, as droughts would affect the entire region. However, habitat heterogeneity is likely to modulate the impacts, especially in moister stands. Furthermore, drought tolerance of the species is unknown. It ranges southward to California, suggesting that it can withstand warmer and drier conditions than presently experienced in British Columbia.

Since the COSEWIC (2010) assessment, awareness of the species by conservation organizations and the public has led to new records. The vast majority are from Salt Spring Island, obtained during Species at Risk surveys and additional site visits by Salt Spring Island Conservancy and other non-profit conservation groups operating on the island (L. Matthias pers. comm. 2020). On southern Vancouver Island, as part of Habitat Acquisition Trust’s Species at Risk program, Biolinx Environmental Research Ltd. conducted annual surveys in 2010–2017 for terrestrial gastropods in 15 Capital Regional District (CRD) parks. Threaded Vertigo was detected at two parks in 2010 (one and two individuals, respectively; Ovaska and Sopuck 2010), but not thereafter. The focus was Blue-grey Taildropper (Prophysaon coeruleum) with overlapping habitat with Threaded Vertigo, but the survey method of artificial cover-objects was not well suited for detecting the largely arboreal Threaded Vertigo. Several other terrestrial gastropod surveys have been conducted within the CRD since 2009, including a multi-year study on Observatory Hill (Ovaska et al. 2014 – 2018), field verification surveys for Blue-grey Taildropper status report update (COSEWIC 2016), and various surveys on Department of National Defence lands associated with environmental assessments (Ovaska and Sopuck unpubl. data 2015 – 2020). Threaded Vertigo was not specifically targeted or detected during these surveys.

Population Information

SAS 11

Change in number of mature individuals:
unknown

SAS 12

Change in population trend:
unknown

SAS 13

Change in severity of population fragmentation:
no

SAS 14

Change in trend in area and/or quality of habitat:
no

SAS 15

Significant new survey information:
no

Explanation:
Efforts have focused on obtaining information on the species’ distribution, and very little is known on its abundance or demography. At one site (Kemp Lake, Sooke), up to 87 snails were detected on daily counts on the trunks of five Bigleaf Maple (Acer macrophyllum) that were monitored in April 2021 (R. Jorna pers. comm. 2021). The snails’ small size and largely arboreal habits make abundance estimates very difficult to obtain.

Threats

SAS 16

Change in nature and/or severity of threats:
no

Explanation:
Threats have been clarified. As part of the preparation of the provincial recovery plan for the species (B.C. Ministry of Environment 2016), threats were assessed using the IUCN–CMP (International Union for Conservation of Nature–Conservation Measures Partnership) threats calculator across the distribution of the species, including unsurveyed habitats. The overall threat impact was assessed as “low” based on three “low” impact threats. The greatest impact was deemed to be from residential development (IUCN threat 1.1). There is potential for development of subdivisions and small-scale housing projects across the species’ Canadian range. Other threats were from road building and improvements (IUCN threat 4.1) resulting in habitat loss and edge effects (leading to habitat fragmentation and isolation of subpopulations), and from logging (IUCN threat 5.3) on private commercial forestry lands and from small-scale wood cutting on private lands. While Bigleaf Maple, favoured by the snails, may be retained during logging operations, more open stand creates drier, unfavourable conditions. Ecosystem modification by fire and fire suppression (IUCN threat 7.1), invasive, non-native invertebrate predators (IUCN threat (8.1), pollution (IUCN threat 9.3, 9.5), and climate change (IUCN threat 11.1, 11.2) were all assessed as potential threats with unknown impacts.

A new threat—decline of Bigleaf Maple—may be applicable. Betzen (2018) attributed increases in mortality of Bigleaf Maple in Washington State, first documented beginning in 2011, to the effects of climate change (higher summer temperatures, vapour pressure deficits, decreased precipitation) and land development. However, it is uncertain if a similar decline is occurring in the range of Bigleaf Maple in Canada.

Protection

SAS 17

Change in effective protection:
no

Explanation:
Many known occurrences are from parks and protected areas, reflecting survey bias towards these managed areas (COSEWIC 2010; B.C. Ministry of Environment 2016). A multi-species action plan has been prepared for the Gulf Islands National Park (Parks Canada Agency 2018). It contains no specific measures for Threaded Vertigo. The general actions for all species at risk include documenting opportunistic observations and increasing visitor awareness. Since the previous COSEWIC assessment, Threaded Vertigo has been recorded from two additional CRD regional parks (Mt. Wells, Bearhill) and a provincial park (Gowlland Tod) on southern Vancouver Island and from parks and protected areas, including Tuam Special Management Area, on Salt Spring Island. Within these areas, the species’ habitat is protected from development, but other activities, such as hazardous tree removal and infrastructure development, may still take place. There is no effective protection for known and potential occurrences on private lands, apart from voluntary stewardship by landowners working with local conservation organizations.

Rescue effect

SAS 18

Change in evidence of rescue effect:
no

Explanation:
Immigration remains undocumented but wind dispersal, more likely within than between Bigleaf Maple stands, is possible for this and other microsnails. However, rescue from the US continues to be unlikely.

Quantitative analysis

SAS 19

Change in estimated probability of extirpation:
no

Details:
Insufficient data for analysis

Summary and Additional Considerations [e.g., recovery efforts; summarize exactly what has changed since the previous assessment]
Since the previous assessment (COSEWIC 2010), the species has been listed as Special Concern on Schedule 1 of the Species at Risk Act. The taxonomy of the snails has been re-examined and the scientific name of the species changed from Nearctula sp.1 to Vertigo rowellii; the eligibility of the species remains unchanged. Awareness of the species since the assessment has led to an increase in the number of known occurrences, especially on Salt Spring Island, but systematic targeted surveys have not been conducted. Consequently, the known EOO has increased by 14.5% and the IAO by 127.8%. Additional undocumented occurrences most likely exist within the approximate area of the EOO. Threats have been clarified through the application of the IUCN–CMP threats calculator (B.C. Ministry of Environment 2016), with one possible new threat since identified.

Provincial (B.C. Ministry of Environment 2016) and federal (ECCC 2019) management plans have been completed. The provincial management plan (p. 20) listed the following management objectives:

1. “to secure protection (with no loss or degradation of habitat) for the extant sites of Threaded Vertigo
2. to clarify the distribution of Threaded Vertigo within its range in British Columbia
3. to assess and mitigate threats to extant sites and more broadly to low-elevation bigleaf maple dominated stands within the range of Threaded Vertigo; and
4. to address knowledge gaps, including, but not limited to: habitat requirements at the landscape, stand, and microhabitat scales; associations and roles within epiphytic communities, and relationships with mosses and other organisms; and reproductive and other life history features”

Recommended management actions are provided but have yet to be implemented. The federal management plan includes the provincial management plan (Part 2) and an addition, consisting of a section of Effects on the Environment and Other Species (Part 2).

Acknowledgements

Robert Forsyth and Laura Matthias generously shared their respective data on distribution records for Threaded Vertigo. Heidi Gartner provided records of the species in the Royal British Columbia Museum’s collection. Lea Gelling and Katrina Stipec provided records from the British Columbia Conservation Data Centre database. Rosemary Jorna shared her observations of the snails on her property. Sydney Allen (COSEWIC Secretariat) prepared the distribution maps and EOO and IAO calculations for this report. Dwayne Lepitzki, Co-chair of COSEWIC Mollusc Species Specialist Subcommittee and anonymous reviewers provided helpful comments on an earlier draft.

Authorities contacted

• Canadian Wildlife Service: Rhonda Millikin
• Natural Resources Canada: Andrea Schiller, Federal Lands Natural Resources Specialist, Victoria, British Columbia
• Parks Canada: Pippa Shepherd, Species at Risk Coordinator; Pippi Lawn, CoRe Project Manager, Gulf Islands National Park Reserve of Canada
• Province of British Columbia: Jennifer Heron, Invertebrate Specialist and Recovery Team, Vancouver, British Columbia; Lindsay Anderson, Rare and Endangered Species Biologist, Nelson, British Columbia; Lea Gelling and Katrina Stipec, British Columbia Conservation Data Centre, Victoria, British Columbia
• Royal British Columbia Museum: Heidi Gartner, Collections Manager, Molluscs. Victoria, British Columbia
• COSEWIC Secretariat: Sonia Schnobb and Jenny Wu (ATK); Sydney Allen (mapping)
• Other relevant contacts: Robert Forsyth, independent gastropod expert, Kamloops, British Columbia; David Fraser, COSEWIC non-government science member and landowner, Victoria, British Columbia; Laura Matthias, independent biologist, Nashville, Tennessee

Information sources

B.C. Ministry of Environment. 2016. Management plan for Threaded Vertigo (Nearctula sp.) in British Columbia. B.C. Ministry of Environment, Victoria, British Columbia. 42 pp.

Betzen, J.J. 2018. Bigleaf Maple decline in western Washington. M.Sc. thesis, University of Washington, Seattle, Washington. 118 pp.

Burke, T.E. 2013. Land Snails and Slugs of the Pacific Northwest. Oregon State University Press, Corvallis, Oregon. 344 pp.

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

COSEWIC (Committee on the Status of Endangered Wildlife in Canada). 2016. COSEWIC assessment and status report on the Blue-grey Taildropper Prophysaon coeruleum in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xii + 50 pp.

ECCC (Environment and Climate Change Canada). 2019. Management Plan for the Threaded Vertigo (Nearctula sp.) in Canada. Species at Risk Act Management Plan Series. Environment and Climate Change Canada, Ottawa. 2 parts, 4 pp. + 42 pp.

Fraser, D., pers. comm. 2020. Email correspondence to K. Ovaska. October 2020. Biologist, Victoria, British Columbia.

iNaturalist. 2020. Observations: Vertigo Snails (Vertigo). Website: https://inaturalist.ca/observations?place_id=6712&taxon_id=92785 [accessed December 2020].

Jorna, R., pers. comm. 2020. Email correspondence to K. Ovaska. May 2021. Landowner, Sooke, British Columbia.

Matthias, L., pers. comm. 2020. Email correspondence with K. Ovaska in October 2020. Independent biologist, Nashville, Tennessee.

MolluscaBase. 2020. MolluscaBase. Vertigo rowellii (Newcomb, 1860). Website: http://www.molluscabase.org/aphia.php?p=taxdetails&id=1430564 [accessed 23 December 2020].

NatureServe. 2021. NatureServe Explorer: an online encyclopedia of life [web application]. NatureServe, Arlington, Virginia. Website: https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.114128/Nearctula_sp_1 [accessed 4 June 2021].

Nekola, J.C., S. Chiba, B.F. Coles, C.A. Drost, T. von Proschwitz, and M. Horsák. 2018. A Phylogenetic overview of the genus Vertigo O. F. Müller, 1773 (Gastropoda: Pulmonata: Pupillidae: Vertigininae). Malacologia 62:21-161. https://doi.org/10.4002/040.062.0104

Ovaska, K., and L. Sopuck. 2010. Surveys for the Blue-grey Taildropper and other gastropods at risk with focus on Capital Regional District Parks, fall 2010. Unpubl. report prepared by Biolinx Environmental Research Ltd. for Habitat Acquisition Trust, Victoria, British Columbia. 31 pp.

Ovaska. K., and L. Sopuck, unpubl. data. 2015 – 2020. Terrestrial gastropod surveys on DND lands at Colwood and Rocky Point, Vancouver Island, British Columbia.

Ovaska, K., L. Sopuck, and C. Engelstoft. 2014 – 2018. Distribution and habitats of the Blue-grey Taildropper at Observatory Hill on southern Vancouver Island. Annual reports covering surveys from April 2013 to April 2018 prepared for National Research Council of Canada, Ottawa, Ontario.

Parks Canada Agency. 2018. Multi-species Action Plan for Gulf Islands National Park Reserve of Canada. Species at Risk Act Action Plan Series. Parks Canada Agency, Ottawa. v + 27 pp.

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.

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. Second edition. American Fisheries Society Special Publication 26: ix + 526 pp.

Writer of SAS

Kristiina Ovaska

Technical summary

Vertigo rowellii
Threaded Vertigo
Vertigo à crêtes fines
Range of occurrence in Canada (province/territory/ocean): British Columbia

Applicability of criteria

Criterion A (Decline in total number of mature individuals):
Not applicable. Insufficient data to reliably infer, project, or suspect population trends.

Criterion B (Small distribution range and decline or fluctuation):
Not applicable. While the EOO of 10,915 km² is below the threshold for Threatened and IAO (164 km²) of known occurrences is well below the threshold for Endangered, the population is not known to be severely fragmented, occurs at well over 10 locations, and is not known to experience extreme fluctuations even though the area, extent, and quality of habitat are inferred and projected to continue to decline.

Criterion C (Small and declining number of mature individuals):
Not applicable. Insufficient data to determine number of mature individuals.

Criterion D (Very small or restricted population):
Not applicable. Number of mature individuals is unknown and typical thresholds for D2 Threatened are exceeded (> 5 locations and IAO > 20 km²).

Criterion E (Quantitative analysis):
Not applicable. Analysis not conducted.

Figure 1. Distribution of Threaded Vertigo (Vertigo rowellii) in Canada. Lack of recent records from large areas of the distribution north of the Capital Regional District on Vancouver Island and from the Sunshine Coast on the mainland reflect paucity of search effort. Map prepared by Sydney Allen (COSEWIC Secretariat).

Figure 2. Extent of Occurrence (EOO) and Area of Occupancy (IAO) for Threaded Vertigo (Vertigo rowellii) in Canada, based on all records, as compiled in 2020. Map prepared by Sydney Allen (COSEWIC Secretariat).

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 (2022)

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 and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.