Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 4

Technical Summary

Allogona townsendiana

Oregon Forestsnail

Escargot-forestier de Townsend

Range of occurrence in Canada (province/territory/ocean): British Columbia

Demographic Information

Generation time (usually average age of parents in the population) 4-6 yrs, average at least 5 years
Is there an observed, inferred, or projected continuing decline in number of mature individuals? Yes, inferred due to habitat loss
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 in total number of mature individuals over the last 10 years or 3 generations. Yes inferred, due to cumulative habitat loss but magnitude unknown
Projected or suspected percent reduction in total number of mature individuals over the next 10 years or 3 generations]. Magnitude of suspected percent decline due to habitat loss uncertain
Observed, estimated, inferred or suspected percent reduction 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. Magnitude of inferred decline due to cumulative habitat loss uncertain
Are the causes of the decline clearly reversible and understood and ceased? Causes understood but not reversible and not ceased.
Are there extreme fluctuations in number of mature individuals? No

Extent and Occupancy Information

Estimated extent of occurrence 3,313 km2 (including Vancouver Island site)
Index of area of occupancy (IAO)
(Always report 2x2 grid value).
300km2 based on known sites, likely larger

Is the total population severely fragmented?

52 of the 66 known sites (78.8%) and 50 of the 75 known occupied 2 km x 2 km grid squares (66.7%) were assessed as fragmented and isolated. At least 50 sites are less than 5 ha in size and/or subject to urban development within the next 10 years. These sites are isolated by distances of more than 1 km of unsuitable habitat as a result of past land development and most likely will not be able to maintain a viable snail population in the future.

Yes.

Number of locations[*]
The number of locations is estimated to be much greater than 10 if each occupied site with a different land owner is a location.

Number of locations in the last ten years is likely to have declined with the loss of at least 17 sites in the previous ten years.

>>> 10
Is there an observed, inferred, or projectedcontinuing decline in extent of occurrence? yes, decline is observed and is expected to continue due to habitat conversion.
Is there an observed, inferred, or projectedcontinuing decline in index of area of occupancy? yes, decline is observed and is expected to continue due to habitat conversion.
Is there an observed, inferred, or projectedcontinuing decline in number of populations? yes, decline is observed and is expected to continue due to habitat conversion.
Is there an observed, inferred, or projected continuing decline in number of locations? yes, decline is observed and is expected to continue due to habitat conversion.
Is there an observed, inferred, or projectedcontinuing decline in area, extent and/or quality of habitat? yes declines in area, extent and quality of habitat are observed and are expected to continue
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)

Population N Mature Individuals
Steensma et al. (2009) provide population estimates among the four study areas within the TWU-ESA ranging from seven to 47 snails in four 24-m2 sampling sites with an overall mean population density of 1.0 snail/m2. N/A
Chilliwack site: (Hawkes and Gatten 2011) estimated density of Oregon Forestsnail was highest in riparian habitats (0.14 snail/m2) and second-growth mixed deciduous forests (0.13 snail/m2). These data were not gathered in the breeding season, which is considered ideal timing; however, they were collected in the wet fall when snails are known to be active and visible. Until a survey is repeated in spring mating season, the Chilliwack results should be treated with uncertainty. N/A
Total
The respective density estimates above were multiplied by the total biological area (m2) of mapped occurrences (~ 3,278,300 m2) (see Table 1; BC Conservation Data Centre 2013) and yield a range of 426,000 to 3.3 million individuals.
100,000s of individuals

Quantitative Analysis

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

Not calculated

Threats (actual or imminent, to populations or habitats)

The predominant threat to Oregon Forestsnail is ongoing habitat loss from residential and commercial development. Habitat conversion of riparian areas and low elevation forest habitats combined with a clumped natural distribution pattern suggest that populations are becoming more fragmented. Additional major threats include the increased spread of invasive non-native/alien species at all sites and ongoing recreational activities at many of the sites within parks and/or adjacent to more urban areas.

Rescue Effect (immigration from outside Canada)

Status of outside population(s)?
Global conservation status rank is G3G4 (vulnerable-apparently secure)
Washington State S3S4 (vulnerable-apparently secure)
Oregon State SNR (status not ranked).

Is immigration known or possible?
Not known.

Would immigrants be adapted to survive in Canada?
Yes.

Is there sufficient habitat for immigrants in Canada?
Canadian habitat is under severe threat of loss and decline in quality from residential and commercial development, invasive species, and recreational activities.

Unlikely.

Is rescue from outside populations likely?
Yes, but minimal at natural areas along the US border

Data-Sensitive Species

Is this a data-sensitive species?
Yes.

Status History

Designated Endangered in November 2002. Status re-examined and confirmed in May 2013.

Status and Reasons for Designation

Status:
Endangered

Alpha-numeric code:
B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v)

Reasons for designation:
This large land snail is endemic to western North America. In Canada it occurs mainly in the Lower Fraser Valley, the most densely populated and highly fragmented region of British Columbia. It also has been found at one site on Vancouver Island. Habitat loss due to residential and commercial development continues to fragment and isolate remaining populations.

Applicability of Criteria

Criterion A
(Decline in Total Number of Mature Individuals):
Not applicable. While continuing loss of mature individuals due to declines in IAO, EO, and quality of habitat is expected in the future both within the next 10 years and 3 generations and beyond, the rate of loss cannot be quantified using available data.

Criterion B
(Small Distribution Range and Decline or Fluctuation):
Meets Endangered B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v) where both the EO (3,313 km2) and IAO (300 km2) of known sites are below the thresholds for Endangered (<5,000 and <500 km2, respectively). While the number of locations based on the threat of urban and commercial development as well as land ownership exceeds the threshold (10 or fewer locations), nearly 80% of the known sites are not viable. Therefore the species is considered severely fragmented (subcriterion a) and all subcriteria under (b) are applicable: continued loss and fragmentation of habitat will lead to further declines in EO, IAO, area, extent and quality of habitat, number of locations and populations, and ultimately to number of mature individuals.

Criterion C
(Small and Declining Number of Mature Individuals):
Not applicable. The actual number of mature individuals is unknown and while the number of mature individuals is expected to decrease in the future, the rate cannot be quantified at this time.

Criterion D
(Very Small or Restricted Total Population):
Not applicable. The actual number of mature individuals is unknown but probably exceeds the thresholds (< 1000 for Threatened); the IAO and number of locations also exceed the typical thresholds.

Criterion E
(Quantitative Analysis):
Not applicable. No population viability analyses have been done.

*See Definitions and Abbreviations on COSEWIC Web site and IUCN 2010 for more information on this term.

*See Definitions and Abbreviations on COSEWIC Web site and IUCN 2010 for more information on this term.

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