Western skink (Plestiodon skiltonianus): COSEWIC status appraisal summary 2014

Special concern
2014

Table of Contents

List of Figures

List of Tables

 

Document Information

COSEWIC
Committee on the Status
of Endangered Wildlife
in Canada

COSEWIC logo

COSEPAC
Comité sur la situation
des espèces en péril
au Cananda

Committee on the Status of Endangered Wildlife in Canada (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. 2014. COSEWIC status appraisal summary on the Western Skink Plestiodon skiltonianus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xvi pp. (Species at Risk Public Registry website).

Production note:

COSEWIC would like to acknowledge Pamela Rutherford and Drew Hoysak for writing the status appraisal summary on the Western Skink, Plestiodon skiltonianus, in Canada, prepared under contract with Environment Canada. This status appraisal summary was overseen and edited by Kristiina Ovaska, Co-chair of the COSEWIC Amphibians and Reptiles Specialists Subcommittee.

For additional copies contact:

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

Tel.: 819-938-4125
Fax: 819-938-3984
E-mail: COSEWIC E-mail
Website: COSEWIC

Également disponible en français sous le titre Sommaire du statut de l’espèce du COSEPAC sur le Scinque de l'Ouest (Plestiodon skiltonianus) au Canada.

COSEWIC Assessment Summary

Assessment Summary - November 2014

Common name
Western Skink
Scientific name
Plestiodon skiltonianus
Status
Special Concern
Reason for designation
The Canadian distribution of the species is within a densely populated region of the Southern Interior of British Columbia that is undergoing extensive development. Increased survey efforts within the past 10 years have resulted in the discovery of the species at new localities within the known range. Nevertheless, the range remains small and human activities and land use practices continue to threaten skink habitats.
Occurrence
British Columbia
Status history
Designated Special Concern in May 2002. Status re-examined and confirmed in November 2014.

COSEWIC Status Appraisal Summary

Scientific Name:
Plestiodon skiltonianus
English Name:
Western Skink
French Name:
Scinque de l’Ouest
Range of occurrence in Canada:
British Columbia

Status History

COSEWIC:
Designated Special Concern in May 2002. Status re-examined and confirmed in November 2014.

Evidence (indicate as applicable)

Increased survey efforts have resulted in the finding of Western Skinks at new and previously known localities throughout much of the species’ Canadian range in British Columbia. The range remains small and is located within populated areas of the southern interior of British Columbia, where human activities and land uses continue to threaten skink habitats.

Wildlife species:

Change in eligibility, taxonomy or designatable units:
Yes

Explanation:

Western Skinks were formerly known as Eumeces skiltonianus. Griffith et al. (2000), Schmitz et al. (2004), Brandley et al. (2005), and Smith (2005) presented evidence that Eumeces is not a monophyletic group and recommended the adoption of the genus name Plestiodon for all North American species (north of Mexico) formerly referred to as Eumeces. This change has been accepted by authorities (Crother 2012).

Range

Change in extent of occurrence (EO):
Unknown
Change in index of area of occupancy (IAO) :
Yes
Change in number of known or inferred current locations:
Yes
Significant new survey information
Yes

Explanation:

Since the previous status report, the Western Skink has been detected at a number of localities within its known range in British Columbia. Research for a Master’s of Science thesis (Vincer 2012) confirmed the presence of Western Skinks in Vaseux-Bighorn National Wildlife Area south of Penticton. Other surveys have confirmed previous sites and found new ones (e.g., Dulisse 2006). These have been mapped by the B.C. Conservation Data Centre (2014a) and by E-Fauna BC (Rutherford 2013). These sites and others (Rutherford 2002; Hawkes pers. comm. 2014) obtained since 2000 are included in Figure 1. The inclusion of previous records shows a similar core range (Figure 1), but the species may have disappeared from the western and northern extremities of its range (but see below).

There are no recent (since 1993) records of the Western Skink west of the Okanagan drainage (B.C. Western Skink Working Group 2013), and the species might have disappeared from the Similkameen Valley, where it occurred historically. Numerous surveys for snakes in the west Okanagan and Lower Similkameen Valleys over the past two decades have failed to locate Western Skinks, but no systematic surveys targeting this species have been conducted (B.C. Western Skink Working Group 2013). The most northern confirmed locality, by Shuswap Lake, is a sighting of a single lizard in 1969.

The extent of occurrence (EO) of Western Skinks in Canada is 25,086 km², using the minimum convex polygon method and localities confirmed since 2000, as shown in Figure 1. If all records, including those before 2000, are used, then the EO is 37,665 km². Others have reported a much smaller EO (<2000 km² in Ovaska and Engelstoft 2002; 1000-5000 km² in B.C. Conservation Data Centre 2014b), but a different method of calculation was used probably including only suitable habitat. The historical range would have been 33.4% larger, extending farther north (Shuswap Lake) and west (Similkameen Valley). There is uncertainty when the declines occurred or whether the species still persists in pockets of habitat in these areas.

Western Skinks are patchily distributed across the landscape and may occupy only 10% of the EO (B.C. Conservation Data Centre 2014b). In a mark-recapture study in British Columbia, recaptures of tagged individuals were all within 61 m of previous captures (Rutherford 2002), suggesting that home ranges are small and dispersal capabilities may be poor.

The index of area of occupancy (IAO) for Western Skinks is 248 km² (62 2x2 km grid cells), based on occurrence records since 2000, and 400 km², if all records, including those before 2000, are included. There is some justification for using the larger value, as historical sites have not been systematically resurveyed. However, Dulisse (2006) suggested that many historically occupied sites along the west arm of Kootenay Lake no longer have Western Skinks, but the timing of the possible declines is unclear. The smaller value, based on records since 2000, is almost certainly an underestimate, as unreported occurrences most likely exist. Many more than ten threats-based locations are expected because the timing and severity of the various threats depends on landownership. Some locations appear to have been lost based on lack of recent observations from a portion of the range and habitat loss and deterioration, but specific information is available.

Population Information:

Change in number of mature individuals:
Unknown
Change in population trend:
Unknown
Change in severity of population fragmentation:
No
Change in trend in area and/or quality of habitat:
No
Significant new survey information
No

Explanation:

There have been no studies of population size or trends in this species. Quality of habitat continues to decline in areas with human population growth.

Threats:

Change in nature and/or severity of threats:
No

Explanation:

Threats have been clarified, but no new threats have been identified. A detailed assessment of threats was done by the B.C. Western Skink Working Group (2013) and by the BC Conservation Data Centre in 2012 (Gelling pers. comm. 2014) using the IUCN threats calculator (Master et al. 2009) (Appendix 1). The results of the two assessments were similar and identified a number of low-impact threats, which cumulatively resulted in a medium overall threat impact (Appendix 1). The most significant threats are from continued habitat loss due to residential, commercial, and agricultural development. Many skink populations occur on private lands, where development pressure can be intense (B.C. Western Skink Working Group 2013). In addition to having a direct impact on skink populations, these threats may reduce genetic variability, increasing extirpation risk and limiting the ability to adapt to changing environments (Delaney et al. 2013).

Protection:

Change in effective protection:
No

Explanation:

No change.

Rescue Effect:

Change in evidence of rescue effect:
No

Explanation:

Western Skinks appear to be widespread, abundant, and secure in the US states bordering the species’ range in British Columbia: They are ranked S5 in Washington (Hallock and McAllister 2005) and Idaho (Groves et al. 1997). Immigration into Canada is possible, and immigrants would probably be adapted to survive. However, given the small home ranges and limited dispersal ability of these animals, any rescue would be very slow, if it was effective at all.

Quantitative Analysis:

Change in estimated probability of extirpation:
Unknown

Explanation:

No quantitative analyses are available.

Summary and Additional Considerations: 

Since the previous assessment, several new distribution records have been obtained for the Western Skink, but population sizes and trends remain unknown. The species continues to be under threat due to a number of factors, with residential, commercial, and agricultural development being the most important. A management plan that includes an assessment of threats has been prepared (B.C. Western Skink Working Group 2013). The management goal is to maintain stable or increasing populations with three objectives: clarifying distribution and abundance; identifying sites for habitat protection and threat mitigation; and addressing knowledge gaps such as movement patterns, habitat trends, population trends, and habitat suitability. In British Columbia, the BC Wildlife Act prohibits the collection, killing, harassment, and possession of all wildlife without a permit but does not protect habitat. The species is not listed under the provincial Forest and Range Practices Act or in the Oil and Gas Activities Act. It is listed as Special Concern on Schedule 1 of the Species At Risk Act.

Acknowledgements and Authorities Contacted

Jakob Dulisse and Virgil C. Hawkes kindly supplied location data and information about Western Skinks in B.C. Jenny Wu provided EO and IAO calculations and prepared the distribution map. Kristiina Ovaska provided numerous useful comments.

Information Sources

B.C. Conservation Data Centre. 2014a. BC Species and Ecosystems Explorer. B.C. Ministry of Environment. Victoria, British Columbia. [accessed March 2014].

B.C. Conservation Data Centre. 2014b. Conservation Status Report: Plestiodon skiltonianus. B.C. Ministry of Environment. [accessed March 2014].

B.C. Western Skink Working Group. 2013. Management Plan for the Western Skink (Plestiodon skiltonianus) in British Columbia. Unpubl. report prepared for the B.C. Ministry of Environment, Victoria, British Columbia. 28 pp.

Brandley, M., A. Schmitz, and T. Reeder. 2005. Partitioned Bayesian analyses, partition choice, and the phylogenetic relationships of scincid lizards. Systematic Biology 54(3):373–390.

Conservation Measures Partnership (CMP). 2010. Threats Taxonomy. [accessed May 2014].

COSEWIC. 2002. COSEWIC assessment and status report on the western skink Eumeces skiltonianus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 19 pp.

Crother, B.I. (ed). 2012. Scientific and Standard English Names of Amphibians and Reptiles of North America North of Mexico, with Comments Regarding Confidence in Our Understanding 7th ed. Society for the Study of Amphibians and Reptiles.

Delaney, K.S., S.P.D. Riley, and R.N. Fisher. 2010. A rapid, strong, and convergent genetic response to urban habitat fragmentation in four divergent and widespread vertebrates. PLoS ONE 5(9):e12767. doi:10.1371/journal.pone.0012767

Dulisse, J. 2006. Columbia Basin Western Skink (Eumeces skiltonianus) Inventory and Assessment. Columbia Basin Fish and Wildlife Compensation Program. Nelson, B.C. iii + 30 pp.

Fitch, H.S. 1936. Amphibians and reptiles of the Range River Basin, Oregon. Amer. Midland Nat. 17:643.

Gelling, L. 2014. Email correspondence to K. Ovaska. May 2014. Zoologist, BC Conservation Data Centre, Victoria, British Columbia.

Griffith, H., A. Ngo., and R. W. Murphy. 2000. A cladistic evaluation of the cosmopolitan genus Eumeces Wiegmann (Reptilia, Squamata, Scincidae). Russian Journal of Herpetology 7(1):1–16.

Groves, C.R., B. Butterfield, A. Lippincott, B. Csuti, and J.M. Scott. 1997. Atlas of Idaho’s Wildlife. Idaho Department of Fish and Game Nongame and Endangered Wildlife Program, Boise, Idaho. Xiii + 372 pp.

Hallock, L.A., and K.R. McAllister. 2005. Western Skink. Washington Herp Atlas. [accessed May 2014].

Hawkes, V.C., pers. comm. 2014. Email correspondence to D. Hoysak. February 2014. Vice-President & Senior Wildlife Biologist LGL Limited environmental research associates, Sidney, British Columbia.

Master, L., D. Faber-Langendoen, R. Bittman, G.A. Hammerson, B. Heidel, J. Nichols, L. Ramsay, and A. Tomaino. 2009. NatureServe conservation status assessments: factors for assessing extinction risk. NatureServe, Arlington, Virginia.

MLFNRO (B.C. Ministry of Forests, Lands and Natural Resource Operations). 2011. Sightings databases presented in ArcMap. Penticton, BC. Cited in B.C. Western Skink Working Group. 2013. Management Plan for the Western Skink (Plestiodon skiltonianus) in British Columbia. Unpubl. report prepared for the B.C. Ministry of Environment, Victoria, British Columbia. 28 pp.

Snow, K., Teucher, A., and Tomaino, A. 2012. NatureServe conservation status assessments: factors for evaluating species and ecosystem risk. NatureServe, Arlington, Virginia. viii + 64 pp.

Ovaska, K.E., and C. Engelstoft. 2002. COSEWIC status report on the western skink Eumeces skiltonianus in Canada, in COSEWIC assessment and status report on the western skink Eumeces skiltonianus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 19 pp.

Rutherford, P. (2002). Costs of reproduction in a temperate-zone lizard, Elgaria coerulea. Ph.D. dissertation, Department of Biology, University of Victoria, British Columbia. 138 pp.

Rutherford, P. 2012. E-Fauna BC Atlas Page: Plestiodon skiltonianus. In Klinkenberg, B. (ed.). 2012. E-Fauna BC: Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. [accessed March 2014].

Schmitz, A., P. Mausfeld, and D. Embert. 2004. Molecular studies on the genus Eumeces Wiegmann, 1834: phylogenetic relationships and taxonomic implications. Hamadryad-Madras 28:73–89.

Smith, H.M. 1946. Handbook of Lizards. Lizards of the United States and of Canada. Comstock Publishing Associates, Cornell University Press, Ithaca, New York.

Smith, H. 2005. Plestiodon: a replacement name for most members of the genus Eumeces in North America. Journal of Kansas Herpetology 14:15–16.

Tanner, W.W. 1957. A taxonomic and ecological study of the western skink. Great Basin Naturalist 17:59-94.

Vincer, E. 2012. Microhabitat selection of the western skink (Plestiodon skiltonianus) in the Okanagan region of British Columbia. M.Sc. Thesis, Simon Fraser University, Vancouver, British Columbia. 55 pp.

Technical Summary

Scientific Name:
Plestiodon skiltonianus
English Name:
Western Skink
French Name:
Scinque de l’Ouest
Range of occurrence:
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).

    From Ovaska and Engelstoft (2002)

    • 5 yrs
  • Is there an inferred continuing decline in number of mature individuals?

    Inferred decline due to reduction in habitat. Some formerly occupied sites appear to no longer have Western Skinks, but the timing of declines is unclear.

    • Yes
  • Estimated percent of continuing decline in total number of mature individuals within 2 generations

    • Unknown
  • Estimated percent reduction in total number of mature individuals over the last 3 generations.

    • Unknown
  • Suspected percent reduction or in total number of mature individuals over the next 3 generations.

    • Unknown
  • Estimated percent reduction in total number of mature individuals over any 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?

    Understood but not clearly reversible or ceased

    • No
  • Are there extreme fluctuations in number of mature individuals?

    Several studies in the US have reported large annual fluctuations in population size, attributed to habitat change and vegetation succession (Fitch 1936; Smith 1946;, Tanner 1957). However, the changes were not an order in magnitude and hence would not be considered extreme. Fluctuations in Canadian populations are unknown.

    • Unknown

Extent and Occupancy Information

  • Estimated extent of occurrence

    The smaller value is based on records since 2000, while the larger value includes all records, including historical records.

    • 25,086 – 37,665 km²
  • Index of area of occupancy (IAO)

    (Always report 2x2 grid value).

    The smaller value is based on records since 2000 (62 grid cells), while the larger value includes all records, including historical records (100 grid cells).

    • 246 – 400 km²
  • Is the population severely fragmented?

    • Unknown
  • Number of locations

    >> 10 locations are expected because the timing and severity of the various threats depends on landownership.

    • Unknown; >>10
  • Is there an inferred continuing decline in extent of occurrence?

    Apparent 33.4% decline based on lack of recent records from the northern and western extremity of the species’ range. The timing of the decline is unknown and may be historical and not within past 15 years. Furthermore, historical localities have not been surveyed systematically.

    • Yes
  • Is there an observed continuing decline in index of area of occupancy?

    Apparent decline of 38% (from 100 to 62 grid cells) may at least partially represent paucity of survey effort, as historical localities have not been surveyed systematically. The species appears to have disappeared from many sites along the West Arm of Kootenay Lake.

    • Yes
  • Is there an inferred continuing decline in number of populations?

    • Unknown
  • Is there an inferred continuing decline in number of locations?

    • Unknown
  • Is there an inferred continuing decline in area, extent and quality of habitat?

    • Yes
  • Are there extreme fluctuations in number of populations?

    • No
  • Are there extreme fluctuations in number of locations?

    • No
  • Are there extreme fluctuations in extent of occurrence?

    • No
  • Are there extreme fluctuations in index of area of occupancy?

    • No

Number of Mature Individuals (in each population)

  • Total SLE Population:

    Total: Population size unknown but could be as low as 2500 (B.C. Conservation Data Centre. 2014b)

    • Unknown; possibly <10,000

Quantitative Analysis

  • Probability of extinction in the wild is at least 20% within 20 years or 5 generations.

    • Not done due to lack of data

Threats (actual or imminent, to populations or habitats)

Residential and commercial development, agriculture, energy production and mining, transportation development

Rescue Effect (immigration from outside Canada)

  • Status of outside population(s)?

    • Stable
  • Is immigration known or possible?

    • Possible
  • Would immigrants be adapted to survive in Canada?

    • Yes
  • Is there sufficient habitat for immigrants in Canada?

    • Unknown
  • Is rescue from outside populations likely?

    • No

Data-Sensitive Species

  • Is this a data-sensitive species?
    • No

Status History

  • COSEWIC: Designated Special Concern in May 2002. Status re-examined and confirmed in November 2014.

Status and Reasons for Designation:

Status:
Special Concern
Alpha-numeric code:
Not Applicable
Reasons for designation:
The Canadian distribution of the species is within a densely populated region of the Southern Interior of British Columbia that is undergoing extensive development. Increased survey efforts within the past 10 years have resulted in the discovery of the species at new localities within the known range. Nevertheless, the range remains small and human activities and land use practices continue to threaten skink habitats.

Applicability of Criteria

Criterion A (Decline in Total Number of Mature Individuals):
Not applicable. Declines are suspected based on habitat trends and threats, but their magnitude is unknown.
Criterion B (Small Distribution Range and Decline or Fluctuation):
Not applicable. IAO based on known localities is below the threshold value for endangered, but sub-criteria “a” and “c” do not apply, as there are more than 10 locations, the population is not severely fragmented, and there are no extreme fluctuations, as far as it is known.
Criterion C (Small and Declining Number of Mature Individuals):
Not applicable. Population size is unknown.
Criterion D (Very Small or Restricted Population):
Not applicable. Population size is unknown.
Criterion E(Quantitative Analysis):
Not available due to lack of data.
Figure 1. Distribution of Plestiodon skiltonianus in British Columbia. Map prepared by Jenny Wu, COSEWIC Secretariat.
Map of distribution of Plestiodon skiltonianus
Long description for Figure 1

Map showing the distribution of the Western Skink in the southern interior of British Columbia. Different symbols are used to distinguish pre-2000 records from 2000 to 2013 records and suggest the species may have disappeared from the western and northern extremities of its range.

 

Appendix 1. Threat assessment for the Western Skink using the IUCN threats calculator (Masters et al. 2009). Threat categories that do not apply to the species are omitted. Overall threat impact is “medium”. Reproduced from B.C. Western Skink Working Group (2013; pp. 13-14).
Threat # Threat description ImpactNoteaof Table 1 ScopeNotebof Table 1 SeverityNotecof Table 1 TimingNotedof Table 1
1 Residential and commercial development Low Small Extreme High
1.1 Housing and urban areas Low Small Extreme High
1.2 Commercial and industrial development Low Small Extreme High
1.3 Tourism and recreation areas Low Small Extreme High
2 Agriculture and aquaculture Low SmallNoteeof Table 1 ExtremeNoteeof Table 1 High
2.1 Annual and perennial non-perennial crops Low Small Extreme High
2.3 Livestock farming and ranching Low Large Slight High
3 Energy production and mining Low Small Extreme High
3.2 Mining and quarrying Low Small Extreme High
4 Transportation and service corridors Low Small Extreme High
4.1 Roads and railroads Low Small Extreme High
4.2 Utility and service lines Low Small Slight High
5 Biological resource use Negligible Negligible Not Scored High
7 Natural system modifications Low Small Slight High
7.1 Fire and fire suppression Low Small Slight High
7.2 Dams and water management Low Small Slight Moderate
8 Invasive and other problematic species and genes Unknown Small Unknown High
8.1 Invasive non-native/alien species Unknown Small Unknown High
11 Climate change and severe weather Unknown Pervasive Unknown Moderate
11.2 Droughts Unknown Pervasive Unknown Moderate

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 (2014)

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)
(Note: Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.)
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)
(Note: Formerly described as “Not In Any Category”, or “No Designation Required.”)
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.
Data Deficient (DD)
(Note: 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.)
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.

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

Page details

Date modified: