Threaded Vertigo (Nearctula sp.) is a minute land snail in the family Vertiginidae. Snails in this family have high, columnar shells up to 3.5 mm in height at adulthood. The outer layer that covers the shell is dull dark brown and coarsely textured. In adult shells, four white tooth-like protuberances (denticles) are visible inside the shell when viewed from the opening (aperture). The shells need to be examined carefully for positive identification, which usually involves collection of a sample of specimens for microscopic examination.
Threaded Vertigo was assessed as Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) and is listed on Schedule 1 of the federal Species at Risk Act (SARA). In British Columbia, Threaded Vertigo is ranked S2 (Imperiled) by the B.C. Conservation Data Centre and is on the provincial Red list. The B.C. Conservation Framework ranks Threaded Vertigo as a priority 2 under goal 3 (maintain the diversity of native species and ecosystems).
In British Columbia, Threaded Vertigo is at the northern extent of its range in western North America. It is found in lowland areas around the Strait of Georgia and on southern Vancouver Island and from southern Gulf Islands (Saturna Island and Saltspring Island). Most individuals live on the bark of bigleaf maple (Acer macrophyllum) trees. Undocumented sites are likely to exist on private properties and other unsurveyed areas within suitable bigleaf maple forest habitats. Threaded Vertigo appears to have a poor capacity for dispersal between trees and sites. The main threats are from removal of trees and habitat degradation, whether this be from urban expansion, roads and associated infrastructure, forestry, or agriculture.
The management goal is to maintain populations of Threaded Vertigo at all extant sites within the province, including any additional populations that may be identified in the future.
The population and distribution goal for Threaded Vertigo cannot be quantified at present because significant knowledge gaps exist relating to distribution and habitat requirements. The species may qualify for down-listing if: (1) threats to its habitats are managed with demonstrable effectiveness, and/or (2) a substantial number of new subpopulations are found and their habitats are secured.
The management objectives for Threaded Vertigo are:
1. to secure protection (with no loss or degradation of habitat) for extant sites of Threaded Vertigo;
2. to confirm 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; relationships with mosses and other organisms; and reproductive, and other life history features.
Recovery planning for Threaded Vertigo is concurrent with recovery planning approaches for species with similar habitats and threats. Because of the extensive knowledge gaps for this snail, most recovery planning activities focus on inventory, habitat information and mapping, and threat clarification.
British Columbia Ministry of Environment. 2016. Management Plan for Threaded Vertigo (Nearctula sp.) in British Columbia. Province of BC, Ministry of Environment. Management Plan (Species at Risk)