Domed Disc (Discus patulus): COSEWIC assessment and status report 2024

Official title: COSEWIC assessment and status report on the Domed Disc (Discus patulus) in Canada

Endangered

2024

COSEWIC assessment summary

Assessment summary – May 2024

Common name: Domed Disc

Scientific name: Discus patulus

Status: Endangered

Reason for designation: This medium-sized land snail (shell diameter about 1 cm) is known to occur in a single 150 ha site within mature Carolinian forests in southern Ontario. It may still occur at a second site that has not been recently searched. The known extant site only contains approximately 20% suitable forested habitat; the snail’s micro-distribution is further restricted because individuals tend to aggregate under fallen, rotting logs. The species has been extirpated from 11 historical occurrences in southern Ontario and continued searches have not found other subpopulations. The main threats are climate change (droughts, changes in frost regimes) and invasive species (for example, earthworms and slugs). The species’ highly restricted distribution, its limited dispersal, and history of extirpation at similar adjacent sites undergoing agricultural expansion are the reasons for the designation.

Occurrence: Ontario

Status history: Designated Endangered in May 2024.

COSEWIC executive summary

Domed Disc

Discus patulus

Wildlife species description and significance

Domed Disc (Discus patulus) is a medium-sized land snail (adult shell width about 1 cm) with a depressed, red-brown shell that has a large hole (umbilicus) on the underside. There is a thin flat layer (callus) at the base of the shell opening and coarse radial ribbing. This Canadian population is part of the unique Carolinian forest fauna and plays a significant role in ecosystem functioning through nutrient cycling. As a range-edge population, it also makes an important contribution to the species’ global conservation. All species are significant and are interconnected and interrelated. There is no species-specific Aboriginal Traditional Knowledge in the report.

Distribution

The species’ global distribution extends from Michigan in the northwest to New York in the northeast, and southward from southern Ontario to Louisiana and Florida. In Canada, the species may still be extant in Middlesex and Essex counties (Co.), southwestern Ontario. The species appears to have been extirpated from about 11 historically occupied southwestern Ontario sites.

Habitat

In North America, Domed Disc inhabits mature and late-successional forests, usually in river ravines with steep slopes, and can be found clustered under rotten logs and in deep leaf litter. Habitat at the only known occupied Canadian site (Middlesex Co.) is surrounded by unsuitable arable land and a river. The habitat at the other possibly occupied site (Essex Co.) appears to be intact, but it is on privately owned land, which was not searched.

Biology

Domed Disc is an egg-laying land snail. Reproduction probably occurs in spring and late summer. Hibernation extends from early October until April in temperate regions. Aestivation may occur only during prolonged drought in summer. Sexual maturity may be reached at 1 year of age and the species may live for 2 to 3 years. The species may feed mainly on decaying wood or fungi in the litter. Active dispersal for colonization of new areas is extremely slow because the species is generally found in sheltered microhabitat (logs, rocks). There is no evidence of transportation by humans.

Population sizes and trends

The species is extirpated from 11 historical occurrences in Canada. One occurrence is considered extant (last collection from 2008), another is uncertain because it is on private land that has not been searched since 1994, and another possible occurrence (collection from 1899) remains unknown (exact coordinates unknown and no public land in the general area). The origin of another record, from drift material along the Thames River, is also unknown. The sole known extant subpopulation is located on 148 ha of protected land, of which about 30 to 40 ha is mature forest and forested slopes. The distribution of individuals in the habitat is patchy because colonies form in association with fallen logs.

Threats and limiting factors

Low dispersal ability and low physiological resistance to fluctuating environmental factors such as temperature and humidity are limiting factors. The main threat to Domed Disc is climate change (droughts, changes in frost regimes) and invasive species, such as earthworms, which are destroying the leaf-litter, or slugs which may be competitors for shelter and food.

Protection, status, and ranks

Domed Disc has no legal conservation status designations. It is ranked by NatureServe as Globally Secure (G5) and Nationally Secure (N5) in the U.S., but is considered imperilled (N2) in Canada and Ontario (S2).

Technical summary

Discus patulus

English common name: Domed Disc

French common name: Bouton voûté

Range of occurrence in Canada: Ontario

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 (2011) is being used)

Approximalely 2 years

Is there an [observed, inferred, or projected] continuing decline in number of mature individuals?

Yes. Inferred past historical decline because of the reduction in number of occupied sites; continuing future decline inferred from continuing threats.

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 a. clearly reversible and b. understood and c. ceased?

1. No
2. Yes
3. No

Are there extreme fluctuations in number of mature individuals?

Unknown

Extent and Occupancy Information

Estimated extent of occurrence (EOO)

4 to 345 km² (including one uncertain occurrence; 3,800 km² if the drift record is included)

Index of area of occupancy (IAO) (Always report 2x2 grid value).

4 to 8 km² (including one uncertain occurrence)

Is the population “severely fragmented” that is, is >50% of its total area of occupancy is in habitat patches that are (a) smaller than would be required to support a viable population, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. Unknown
2. Yes

Number of “locations”^∗ (use plausible range to reflect uncertainty if appropriate)

1 to 2 (including one uncertain occurrence)

Is there an [observed, inferred, or projected] decline in extent of occurrence?

Yes, historical decline observed because of the reduction in number of occupied sites. Continuing decline could be projected due to threats.

Is there an [observed, inferred, or projected] decline in index of area of occupancy?

Yes, historical decline observed because of the reduction in number of occupied sites. Continuing decline could be projected due to threats.

Is there an [observed, inferred, or projected] decline in number of subpopulations?

Yes, historical decline observed because of the reduction in number of occupied sites. Continuing decline could be projected due to threats.

Is there an [observed, inferred, or projected] decline in number of “locations”?

Yes, historical decline observed because of the reduction in number of occupied sites. Continuing decline could be projected due to threats.

Is there an [observed, inferred, or projected] decline in [area, extent and/or quality] of habitat?

Yes, historical loss/degradation observed in area, extent, and quality of habitat, and projected continuing decline in habitat quality due to threats.

Are there extreme fluctuations in number of subpopulations?

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 subpopulation)

Subpopulations (give plausible ranges)

N Mature Individuals

Entire range

Unknown

Quantitative analysis

Is the probability of extinction in the wild at least [20% within 20 years or 5 generations, or 10% within 100 years]?

Unknown, analysis not done

Threats (direct, from highest impact to least, as per IUCN threats calculator)

Was a threats calculator completed for this species? Yes. (Appendix 1)

Overall assigned threat impact: High-Low.

Threats:

Threat 11: Climate Change and Extreme Weather (High-Low impact)

Threat 7: Natural System Modifications (Unknown impact)

Threat 8: Invasive and Other Problematic Species and Genes (Unknown impact)

Threat 9: Pollution (Unknown impact)

What additional limiting factors are relevant?

Low dispersal or migration capacity, low resistance to fluctuating environmental conditions.

Rescue effect (immigration from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Pennsylvania (S3), Ohio (SNR), Michigan (S1S2), New York (SNR)

Is immigration known or possible?

Yes, but incidental

Would immigrants be adapted to survive in Canada?

Yes

Is there sufficient habitat for immigrants in Canada?

Yes

Are conditions deteriorating in Canada?

Deterioration is mainly historical; future decline projected due to climate change and other threats.

Are conditions for the source population (that is, outside) deteriorating?

Unknown

Is the Canadian population considered to be a sink?

No

Is rescue from outside populations likely?

No

Data sensitive species

Is this a data sensitive species?

No.

Status history

COSEWIC: Designated Endangered in May 2024.

Status and reasons for designation

Status:

Endangered

Final criteria:

B1ab(iii)+2ab(iii)

Reasons for designation:

This medium-sized land snail (shell diameter about 1 cm) is known to occur in a single 150 ha site within mature Carolinian forests in southern Ontario. It may still occur at a second site that has not been recently searched. The known extant site only contains approximately 20% suitable forested habitat; the snail’s micro-distribution is further restricted because individuals tend to aggregate under fallen, rotting logs. The species has been extirpated from 11 historical occurrences in southern Ontario and continued searches have not found other subpopulations. The main threats are climate change (droughts, changes in frost regimes) and invasive species (for example, earthworms and slugs). The species’ highly restricted distribution, its limited dispersal, and history of extirpation at similar adjacent sites undergoing agricultural expansion are the reasons for the designation.

Applicability of criteria

Criterion A (decline in total number of mature individuals):

Not applicable. The number of mature individuals is unknown. Therefore, insufficient data are available to infer, project, or suspect possible population reduction.

Criterion B (small distribution range and decline or fluctuation):

Meets Endangered, B1ab(iii)+2ab(iii). The EOO and IAO are both below thresholds (< 5,000 km² and 500 km², respectively), there are fewer than 5 locations (a), and there is an observed and projected continuing decline in quality of habitat (b(iii)) caused by a variety of threats.

Criterion C (small and declining number of mature individuals):

Not applicable. Number of mature individuals is unknown.

Criterion D (very small or restricted population):

D1 is not applicable because the number of mature individuals is unknown. Meets Threatened, D2. The very low number of locations is below the typical threshold (5 or fewer) and the species is prone to substantial decline from the effects of human activities or stochastic events within 1 to 2 generations.

Criterion E (quantitative analysis):

Not applicable. Analysis not conducted.

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 (2020)

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.

Wildlife species description and significance

Name and classification

Kingdom: Animalia

Phylum: Mollusca

Class: Gastropoda

Order: Pulmonata

Suborder: Stylommatophora

Family: Discidae

Genus: Discus

Species: Discus patulus

(Deshayes 1830)

Common names:

English: Domed Disc

French: Ponctin vouté (proposed by NGSWG [2022]). A preferred alternative is bouton voûté because the French name used for other species in the genus Discus is “bouton” (MNHN and OFB 2024)

This species was first described under the name Helix perspectiva, but because of homonymy, the next available name, Helix patulus, was given priority. Discus patulus is the currently accepted name for this species (Pilsbry 1948; Turgeon et al. 1998; MolluscaBase 2024). Five species of Discus have been reported from Canada (Grimm et al. 2010), but Angular Disc (Discus catskillensis) and Forest Disc (Discus whitneyi) are synonyms in Canada (Salvador et al. 2020). The non-native Rotund Disc (Discus rotundatus) is now in another genus (Salvador et al. 2023). Thus, Domed Disc is one of three Discus species in Canada.

Morphological description

Domed Disc is a medium-sized land snail. Adults have a 8 to 9 mm (maximum diameter), cinnamon-brown, depressed shell (Pilsbry 1948). The surface of the shell has riblets, although the initial whorls are smooth and lighter coloured. A callus with a round tubercule or small tooth is found on the columellar wall of the aperture (shell opening) and extends slightly into the shell (see cover page photos). The underside of the shell (umbilicus) is largely open, displaying the whorls.

Population spatial structure and variability

One subpopulation is considered extant in southwestern Ontario, in the area called Joany’s Woods, which is currently managed by the Thames Talbot Land Trust. No genetic studies have been done on this species in Canada, but the species was included in broader molecular phylogenies by Salvador et al. (2020, 2023) although no Canadian specimens were included in those studies. Nothing is known about the population structure and variability.

Designatable units

The Canadian population of Domed Disc occurs in COSEWIC’s Great Lake Plains National Ecological Area. There is only one designatable unit in Canada.

Special significance

In Canada, Domed Disc only occurs in the Carolinian forest region. The Canadian range is the northern limit of the species’ global range. Fraser (2000) suggested that range-edge populations can have significance for species’ genetic diversity, long-term survival and evolution, and provide opportunities for human education (for example, cryptic wildlife observations).

Snails and slugs represent 2.5% to 6% (assuming densities of 2 to 38 snails/m²) of the total animal biomass of boreal forest ecosystems (Hawkins et al. 1997b). Snails and slugs generally play important roles in forest ecosystem functioning, specifically by (i) aiding in decomposition, nutrient cycling, and soil building processes (Mason 1970a,b; Jennings and Barkham 1979); (ii) providing food and essential nutrients for wildlife (South 1980; Churchfield 1984; Frest and Johannes 1995; Martin 2000; Nyffeler and Symondson 2001); and (iii) serving as hosts for parasitic worms (for example, Rowley et al. 1987). Graveland et al. (1994) have shown that gastropod declines can have an important impact on population dynamics of forest passerines. Gastropod diversity can also provide an indication of the degree of anthropogenic disturbance (Douglas et al. 2013).

Domed Disc is unknown to most Canadians, has no commercial value, and is not an agricultural or garden pest. Aboriginal Traditional Knowledge is not available.

Distribution

Global range

Domed Disc is distributed across much of eastern North America, with less than 5% of its global range in Canada. The northern limit of its range is southern Ontario. The core east-west distribution in the U.S. is from North Carolina and Virginia to Arkansas, and the north-south distribution is from Michigan and New York to Alabama (Figure 1). See Non-legal status and ranks for the detailed list of U.S. states where the species currently occurs. Some museum records are from the extreme range edges such as Texas, Florida, Maine, and Wisconsin. The Florida Museum of Natural History has a record from Oregon provided by F. Thompson in 1983 (UF69400-Mollusca) that seems doubtful, because it lies well outside of the western range of this species. The labels from records from Montana (FMNH66237 and LI6603075, no date and 1928) were misinterpreted; the state “Mo” actually corresponds to Missouri.

Figure 1. Global distribution of Domed Disc (Discus patulus). Canadian county in orange (for exact possible distribution refer to Figure 2) and U.S. counties in variations of blue following the date of the record: from no date (light blue) and 1880 to 1989 (blue) to 1990 to 2018 (dark blue). Note that the species does not occur throughout the entire county. Occurrences per county are based on the most recent records in collections (see Collections Examined) and literature (Robertson and Blakeslee 1948; Hubricht 1985; Hodges 2016). Map created by A. Nicolai.

Figure 2. Canadian distribution of Domed Disc (Discus patulus) in Ontario based on records compiled for this report (Table 1). “Extant” (black dot) means that shells or live individuals have been found in the site within the last 20 years. “Uncertain” (grey dot) means the presence of the species is uncertain, because habitat was not accessible (private land). Because exact historical sites were not always identifiable within the current land use, all searched sites in the vicinity of historical occurrences (Table 1) are indicated as “historical” (orange dots). Map created by A. Nicolai.

Canadian range

Domed Disc is known historically from 14 sites in southern Ontario (Table 1, Figure 2). Oughton (1948) did not find it himself, but cited a few other collectors from the end of the 19th and beginning of the 20th century. Oughton (948: 38) remarked that the species “appears to be sporadic in the province” and is found in the area extending from Thomasburg, Hastings County (Co.) to Port Dover, Norfolk Co., but is not present in Toronto. The species is currently known in Ontario from only one site, Joany’s Woods at the Ausable River in Middlesex County. As recommended by NatureServe (2008), previously known occupied sites are considered historical if there have been no confirmed collections or observations for more than 20 years. The species’ continued presence at another site, White Oak Woods at Leamington, is uncertain because, even though suitable habitat exists, the land, which is privately owned, has not been searched since 1994. Whether the species still occurs at Rose Hill (historical occurrence number 14 from 1899 in Table 1) is unknown; the exact co-ordinates for the record are uncertain. It is uncertain if suitable natural habitat still exists at Rose Hill, and there is currently no public land in the area. In conclusion, the species no longer occurs at 11 historically occupied sites. The Carnegie Museum has a 1916 record from Sioux Lookout (CM104496) which was erroneous (Pearce pers. comm. 2019). The record from Fanshawe Conservation Area in Middlesex Co. (RGF 09.157.2801) only includes drift material from the Thames River (Table 1), which points to the species’ presence upstream and outside of the conservation area, probably on private land. Domed Disc was intentionally introduced to Ottawa but it did not survive (Latchford 1887). There are no records of Domed Disc in iNaturalist (iNaturalist 2023).

Extent of occurrence and area of occupancy

The current known extent of occurrence (EOO) corresponds to one 2 x 2 km index of area of occupancy (IAO) grid cell of 4 km². When the one uncertain record is included, the EOO, as measured by the minimum convex polygon over the full extent of the potential habitat at the two occurrences, increases to 345 km² (Table 1). If the drift material record is included, the EOO increases to 3,800 km², assuming a single point for the drift record occurrence. The EOO consists of mostly unsuitable land. Not including the drift material record, the EOO has declined by about 80% in the last 20 to 100 years. The percent decline would be smaller if the drift record were included in the EOO calculation; however, all that is known about the drift record is that the shell most likely drifted downstream from some unknown site further upstream. Its exact origin is unknown.

The IAO is currently 4 km², based on a 2 km x 2 km grid. If the White Oak Woods record from Leamington (which has not been searched for many years) is included, the IAO is 8 km² (2 grid cells). Given that each of the 14 widespread historical occurrence localities (Table 1) corresponds to one grid cell, there has been a decline in the IAO of about 80% in the last 20 to 100 years, not including the drift material record.

There have been observed historical declines in the species’ EOO, IAO, and number of occurrences, the latter suggesting historical declines in numbers of subpopulations and locations. Habitat loss occurred at multiple historically occupied sites in the past, and while no further habitat loss is expected to occur in Joany’s Woods (see Habitat trends, land management and habitat protection and ownership), a continuing decline in habitat quality can still be projected due to a variety of threats. Similarly, even though the habitat in White Oak Woods appears to be intact, the future fate of this small parcel of land is uncertain (see Habitat trends, land management and habitat protection and ownership).

Because quantitative data are not available for Domed Disc, it is unknown if the species is severely fragmented. The species may still be extant at two sites that are separated by over 100 km of mostly unsuitable agricultural land, but it is unknown if the two sites host viable subpopulations.

Search effort

Domed Disc lives mostly hidden and clustered under logs. Empty shells of gastropods generally remain on the forest floor for some time after the animals have died (> 3 years, Říhová et al. 2018) and may indicate its presence. Thicker shells, such as the shell of Domed Disc, may remain longer than others. Nevertheless, specimens have only rarely been recorded during the last century. Even in recent surveys the species appeared to be rare: in a survey of 60 sites in six counties of Illinois, only 25 Domed Disc were found in comparison to 176 individuals of Shagreen (Inflectarius inflectus; Anderson and Coppolino 2008).

Notable historical surveys in Canada include those conducted by John Oughton between about 1930 and 1940 (Oughton 1948) and by Wayne Grimm between 1970 and the mid-1990s (Grimm 1996). Grimm mainly collected in southern and eastern Ontario. However, none of those surveys resulted in records of Domed Disc.

Surveys between 1991 and 2012 were general land snail searches rather than targeted searches for Domed Disc. There are 2,349 georeferenced collection records from searches by M.J. Oldham between 1991and 2012. A few more surveys were done by J.M. Bowles in 1994, yielding 113 geo-referenced collection records, and by A. Nicolai in 2012, yielding 364 geo-referenced collection records. Only three records resulted from these surveys: one in White Oak Woods in 1994 (a few live individuals were collected), one in Joany’s Woods in 2008 (a few live individuals were collected), and one in the drift material in Fanshawe Conservation Area in 2009 (Table 1).

In total since 2013, 140 localities were searched (in some localities several sites with different habitats were searched) with an effort of 583.5 person-hours (Table 2). Targeted searches for Domed Disc including all known accessible historical sites were conducted in 2017 to 2019 and 2022, with a total search effort of 29 person-hours in 2017, 54 person-hours in 2018, 14 person-hours in 2019, and 4 person-hours in the Niagara sites in 2022 (Table 1). Domed Disc was not found during any of these surveys.

Habitat

Habitat requirements

Baker (1939: 87) defined the preferred habitat for Domed Disc in the U.S. as “woodland and wooded valleys of oak, elm, hickory, sycamore and sometimes pine … in floodplains, ravine hillsides, and limestone cliffs.” Domed Disc seems to prefer mature and late-successional forests, as its preferred microhabitat is rich mould around rotting logs or loose bark (Pilsbry 1948), where it is found in larger colonies consisting of more than 10 individuals (Baker 1939). It is also found in deep pockets of moist leaf litter (Hubricht 1985). In Tennessee, Domed Disc was mainly found in forests without invasive plants (Hodges 2016).

In Canada, Domed Disc lives in a mature, late-successional deciduous forest including a floodplain and a steep slope. The amount of potentially available suitable habitat across southern Ontario has not been quantified. Owing to the extensive tile drainage in southwestern Ontario, most small forest habitats are excessively dry (C. Jones pers. comm. 2024).

Habitat trends

Climate change

Even though the species is at the northern edge of its range in Canada, climate change will not necessarily result in conditions more like those found in the core of its range in the United States. Hydrological regimes, snow cover, and temperatures can all influence survival at different times in the lifecycle. An increase in the frequency of extreme weather events, such as storms, freeze-thaw cycles, and droughts, has been seen in northern parts of the range; however, the species may not experience the same conditions further south. Some climate change projections derived using the prediction model available on the Ontario climate change data portal for the period 1960 to 1990 to 2015 to 2045 (PRECIS model under A1B emissions scenario, Wang and Huang 2013) are described below.

Average winter temperatures will increase by 3.4°C in southwestern Ontario (from ‑4.4°C in 1960 to 1990 to ‑1.0°C in 2015 to 2045). A mean temperature close to 0°C increases the chances of increased frequency of freeze-thaw cycles in fall/winter (Nicolai and Ansart 2017) and more spring frosts (Augspurger 2013).

There would also be an increase in storm frequency and magnitude, leading to increased flooding risks and longer periods between rainfall events with a greater risk of droughts, especially in mid-continental regions (Meehl et al. 2007). Average summer precipitation is also predicted to decrease (from 280 mm in 1960 to 1990 to 278 mm in 2015 to 2045.

Under climate change scenarios, changes to average and extreme temperatures will alter microhabitat conditions. Both beneficial and adverse effects may ensue, but the overall effects are difficult to predict. In addition, anthropogenic activity influences microhabitat structure, although the link between habitat choice and physiology is poorly understood (Deutsch et al. 2008).

Land management

Parts of Joany’s Woods, which is located in the Ausable River Valley, consist of former agricultural land that was planted mainly to soybeans. Some areas of the property were restored to woodland in the 1980s. Habitat trends for the Ausable River watershed are summarized here from Nelson et al. (2003) and Coleman et al. (2018). Prior to European settlement, 80% of the basin was covered in forest, 19% in lowland vegetation, and 1% in marsh. By 1983, 85% of the land area was used for agriculture and over 70% of the basin had some form of tile drainage. Other parts of Joany’s Woods were logged and converted to pine plantations, which are still present. Currently, about 30 to 40 ha consist of late-successional forest harbouring some endangered plant species, such as Butternut (Juglans cinerea; COSEWIC 2003) and Eastern Flowering Dogwood (Cornus florida; COSEWIC 2007). Two trails for hiking and nature appreciation are open to the general public (dogs allowed on leash) during daylight hours. However, visitor frequency is extremely low compared to the situation in national and provincial parks. Recreational use by motorized vehicles and activities such as hunting are not allowed. Management is aimed at erosion mitigation and endangered species preservation.

In general, Joany’s Woods is protected land, and management is aimed at preservation of rare species and their specific habitat requirements. There is uncertainty around how climate change will affect habitat quality there (see Threats, Threat 11: Climate Change and Severe Weather). White Oak Woods near Leamington appears to have intact habitat but its future is uncertain given that the land is privately owned. There is nonetheless some level of protection.

White Oak Woods near Leamington in Essex Co. has been recognized as an environmentally significant area (ESA) by the Essex Regional Conservation Authority (ERCA 2024). Less than 3% of the land in Essex County is in forest cover and scrubland, the lowest percentage of any Ontario county or regional municipality. Less than 2.5% of the original wetlands are still in existence (Oldham 1983). Privately owned ESAs are identified as “Natural Environment Overlay” in Schedule A of the Official Plan for the Corporation of the Municipality of Leamington (Municipality of Leamington 2008). With regard to lands identified as Natural Environment Overlay, Policy 3.8.2(g) of the Official Plan states that a goal of the Plan is “that the preservation of natural heritage features and areas is encouraged. Development and site alteration shall not be permitted in areas identified as “Natural Environment Overlay” unless it has been demonstrated, to the satisfaction of the approval authority, in consultation with the relevant Conservation Authority, that there will be no negative impacts on the natural features or the ecological functions for which the area was identified.” Oldham (1983: 246) states the following in his evaluation and recommendations for the site: “Reduction in the size of the woodlot or cutting within the woodlot are undesirable, and should be discouraged. The Leamington White Oak Woods is one of the finest Carolinian woodlots remaining in Essex County.”

Biology

Little information is available about the biology of Domed Disc. General aspects of terrestrial snail biology are discussed in Barker (2001). Some information is available on other members of the family Discidae or Discus spp.. However, this information could give misleading conclusions about the capacity of Domed Disc to survive or to adjust to specific conditions, because the family Discidae includes common species that are not of conservation concern.

Life cycle and reproduction

Domed Disc is an air-breathing (pulmonate) snail that is a simultaneous hermaphrodite (concurrently possesses both male and female reproductive organs) and lays eggs (Pilsbry 1948). In general, both members of a mating pair exchange sperm and produce eggs. In most snail species, larger individuals lay more eggs than smaller ones (Heller 2001). In temperate regions, reproduction usually occurs in spring and late summer, and egg clutches are deposited in shallow holes excavated in moist soil (Barker 2001). The clutch size of Domed Disc is unknown.

In general, most terrestrial gastropods are crepuscular or nocturnal, and sympatric species often have different activity patterns (Asami 1993). During 2013 to 2019 and 2022 surveys, most snail species observed were mainly active in the morning or after rain. However, Domed Disc was only observed under logs, not moving. The hibernation period probably extends from early October until mid-April. The exact timing is expected to vary depending on conditions in particular years. Typical hibernation sites for other species consist of shallow depressions in the forest floor (at depths of 5 to 10 cm) that are covered with leaf litter or soil (Pearce and Örstan 2006) and insulated by snow. Snails in regions with prolonged periods of drought and heat generally aestivate in buffered refuges, such as in soil or leaf litter, and under logs, and seal their shell opening with a calcified epiphragm to avoid evaporation (Barker 2001; Pearce and Örstan 2006). Larger Discidae species (for example, Eastern Banded Tigersnail [Anguispira kochi kochi]) in aestivation with epiphragms were observed in August during 2017 to 2019 surveys, while active species of similar size to the Domed Disc (for example, Angular Disc) were found under logs.

In general, and in other species, growth occurs only during periods of activity (spring to fall; Nicolai 2010; Nicolai et al. 2010; Charrier et al. 2013). Adult shell size (approximalely 8 to 9 mm in width) can be reached after 1 year, with sexual maturity being reached at the same time. Domed Disc may live 2 to 3 years. The estimated generation time is somewhere between the age at sexual maturity and maximum lifespan, probably 2 years.

Physiology and adaptability

Physiological responses to environmental factors and their plasticity and adaptability have not been studied intensively in Discidae. Details of diet and feeding behaviour are unknown. Like other litter-dwelling species such as Broad-banded Forestsnail (Allogona profunda; COSEWIC 2014), Eastern Banded Tigersnail (COSEWIC 2017), or Striped Whitelip (Webbhelix multilineata; COSEWIC 2018), Domed Disc may eat decaying plants (everywhere in the leaf litter) or microfungi on rotten logs.

In general, snails require calcium for shell formation. Soil and bedrock calcium availability influence snail species richness (that is, number of species) in an area (Nekola 2005) along with physiological processes such as heat resistance in eggs (Nicolai et al. 2013). Heavy metals and pesticides present in the soil and plants accumulate in the tissues of snails (Barker 2001; Notten et al. 2005) and have been associated with decreases in food consumption, growth, and fecundity (Laskowski and Hopkin 1996). However, Domed Disc seems to be tolerant to some metals, such as iron. Anderson and Coppolino (2008) found this species in high abundances on iron-rich soils where gastropod diversity was relatively low.

In temperate regions, many species only aestivate in extreme summer conditions for a short period. They have developed biochemical stress responses that protect cellular architecture and processes such as membrane fluidity, osmoregulation, and enzyme activity, which help maintain survival mechanisms. Exceptionally long, hot, and dry periods with unusual timing can increase mortality, for instance, up to 70% mortality has been observed in Roman Snail (Helix pomatia), right after arousal from hibernation (Nicolai et al. 2011).

Snails are susceptible to freezing in winter. Various, somewhat plastic, strategies have evolved to enable survival at subzero temperatures (see review by Ansart and Vernon 2003). Species in the same family have evolved different strategies which may be disadvantageous to some species taking into account the effects of climate change and human-driven microhabitat degradation (Nicolai and Ansart 2017). Mortality during hibernation is around 40% in some species and drives population dynamics (Peake 1978; Cain 1983). Snails in temperate regions typically hibernate in buffered microsites that are additionally insulated by snow (Nicolai et al. 2011). Burch and Pearce (1990) indicated that the availability of refuges with buffered environmental conditions, such as temperature and humidity, may be the most important factor limiting terrestrial snail abundance.

Many terrestrial gastropods can be reared in captivity with relative ease (see Ansart et al. 2014 for a study involving short-term rearing of over 30 different species). The long-term success of rearing depends on knowledge of a species’ specific requirements; this aspect has not yet been studied in Domed Disc.

Dispersal and migration

Active movement distances are negligible in Discus sp., and snails mainly stay within a few centimeters in the leaf litter (Boag 1985). While other, larger snails have been found to move distances of between 120 and 220 cm per day within a home range of 80 to 800 m² (Pearce 1990), Domed Disc has been observed to form colonies and to remain under logs. Therefore, its maximum dispersal distance may be even less than 32.2 m over 3 years, as observed in Oregon Forestsnail (Allogona townsendiana; Edworthy et al. 2012). Eggs and immature stages are not known to be dispersed by wind. However, some snails can survive short periods in water, in hypoxic conditions (Nicolai and Ansart 2017), and they can survive the passage through birds’ intestines (Wada et al. 2012). Other snails have been found to be dispersed by migrating birds (Kawakami et al. 2008). In the case of riparian populations in particular, snails may be dispersed by rafting on floating objects (Vagvolgyi 1975) or by fish (Altaba 2015). While the likelihood of aerial or aquatic transport of Domed Disc is unknown, it is probably small, because shells have been found in drift material. It is uncertain whether the snail shells found in drift material were already dead or were alive when dispersal by water began.

In Ontario, the likelihood of natural dispersal from the U.S. is unlikely given the snails’ poor dispersal capabilities (see Population spatial structure and variability and Rescue effect). The potential for northward expansion of the peripheral Canadian population of Domed Disc could be largely negated by historical and current habitat loss and degradation, which are important factors to consider for peripheral species under climate warming (Gibson et al. 2009). Because Domed Disc does not actively search for fresh plant material as food at its habitat edges (agricultural fields, gardens), it is unlikely that it would be transported through human activity, for example, on horticultural or agricultural products, and then be introduced to new habitats (Robinson 1999; Robinson and Slapcinsky 2005).

Interspecific interactions

Although no information is available on parasites of Domed Disc, trematodes (Barger and Hnida 2008; Barger 2011) and free-swimming or attached flagellates have been observed in other snail species (Current 2007). Parasitic mites are also common in snails in general, with infection rates in a population typically ranging from 45% to 75%. Depending on the mite species, infestations can cause high mortality, reproductive perturbations, and reduced cold hardiness in some snail species (Baur and Baur 2005). Nematodes can also infect a snail population and increase mortality among juveniles (Morand et al. 2004). In snails reared in the laboratory, thus in a confined space, nematodes can cause extremely high mortality (Örstan 2006). However, nematodes were not found to be efficient in controlling pest gastropods in an urban green space (Fredon Inc. unpubl. data).

Predation can be a source of mortality for land snails. Potential predators are described in a literature review by Jordan and Black (2012: 7):

“Gastropods are an important food source to a vast number of species, including salamanders, frogs, toads, turtles, snakes, lizards, birds, shrews, voles, moles, rats, mice, chipmunks, and squirrels. Invertebrate predators of terrestrial mollusks include sciomyzid fly larvae, firefly larvae, parasitic wasp larvae, carabid and staphylinid beetles, ants, spiders, and harvestmen.”

Among carnivorous gastropods, Dark-bodied Glass-snail (Oxychilus draparnaudi) includes forest snails in its diet, such as Striped Whitelip, Bristled Slitmouth (Stenotrema barbatum), and Flamed Tigersnail (Anguispira alternata; often observed in the same sites, Örstan 2006). Garlic Glass-snail (Oxychilus alliarius), an invasive predatory snail, is negatively affecting native Hawaiian land snails (Curry et al. 2016). Introduced predators or an increase in the abundance of native predators due to ecological disturbance can increase mortality due to predation.

Competition for food with other terrestrial gastropods, including exotic species, may affect native Discidae in southwestern Ontario, but this has not been documented. The gastropod community in Joany’s Woods in 2019 was composed of 10 native species (Flamed Tigersnail, Glossy Pillar [Cochlicopa lubrica], Upland Pillsnail [Euchemotrema fraternum], Downland Pillsnail [Euchemotrema leaii], White-lip Globe [Mesodon thyroidus], Whitelip [Neohelix albolabris], Northern Three-tooth [Triodopsis tridentata], Dished Three-tooth [Triodopsis vulgata], and Black Gloss [Zonitoides nitidus]). It also included three introduced species: Strawberry Snail (Trochulus striolatus), Arion sp. slugs, and Grovesnail (Cepaea nemoralis). Introduced exotic gastropods, such as Grovesnail and various species of slugs, mainly Grey Fieldslug (Deroceras reticulatum) or Dusky Arion (Arion fuscus/subfuscus), are present in many natural areas in Ontario (A. Nicolai pers. obs.), and might compete directly with Domed Disc for food, as they mainly eat decaying plant material or fungi.

Population sizes and trends

Sampling effort and methods

The objective of the fieldwork conducted in 2017 to 2019 and 2022 was to search historically occupied sites and other suitable habitats within the previously known range, in order to measure abundance and demography, and to better understand the distribution and ecology of the species. The method used was a visual search under logs and in the leaf litter on a wandering transect across different habitats per site.

Abundance, fluctuations, and trends

Body size distribution and species’ densities could not be analyzed because neither live individuals nor shells were found in 2017 to 2019 or 2022. The species is currently known to occur in Joany’s Woods (based on the live individuals collected in 2008), a 148-ha parcel of protected land, of which about 30 to 40 ha is mature forest and forested slopes. The distribution of individuals within the habitat is suspected to be patchy because colonies form under logs. There is no available information on fluctuations or trends in numbers of mature individuals.

Rescue effect

Although snails have some capacity for passive dispersal (see Dispersal and migration), rescue from outside Canada is unlikely due to barriers and population disjunction. The closest U.S. subpopulations, specifically in New York, Ohio, and Michigan, are separated by large water bodies, such as Lake Erie and the Niagara and Detroit rivers, and surrounded mainly by habitat unsuitable for Domed Disc (Figures 1 and 2).

Threats and limiting factors

Threats

Domed Disc is vulnerable to the cumulative effects of various threats, especially climate change. The nature, scope, and severity of these threats is described in Appendix 1, following the IUCN-CMP (International Union for Conservation of Nature – Conservation Measures Partnership) unified threats classification system (see Salafsky et al. 2008 for definitions and Master et al. 2012 for guidelines). The threat assessment process consists of assessing impacts for each of 11 main categories of threats and their subcategories, based on the scope (proportion of population exposed to the threat over the next 10-year period), severity (predicted population decline within the scope during the next 10 years or 3 generations, whichever is longer, up to approximalely 100 years), and timing of each threat. The overall threat impact is calculated by taking into account the separate impacts of all threat categories and can be adjusted by the species experts participating in the threats evaluation. The overall calculated threat impact for Domed Disc is HIGH-LOW (Appendix 1). The threats are listed below according to their calculated level of impact, from highest to lowest impact. Only those threats that scored as Unknown or Low or higher are discussed. The numbering of threats corresponds to the categories and subcategories of the threat calculator.

Climate change and severe weather (IUCN threat 11) – high – low impact

Using the framework for assessing species’ vulnerability to climate change developed by Foden et al. (2013), Domed Disc can be considered highly vulnerable because (i) it is exposed to climate change (spring frosts, absence of snow cover, droughts), (ii) it is sensitive (specific microhabitat conditions), and (iii) it has a low adaptive capacity (low intrinsic dispersal capacity).

Threat 11.2: droughts; and threat 11.3: temperature extremes (high – low impact)

Climate change models project an increase in extreme weather events, including droughts, floods, and temperature extremes, for southwestern Ontario(Varrin et al. 2007). Snails may be vulnerable to increasing average temperatures accompanied by increased incidence of drought (Pearce and Paustian 2013). With increasing average temperatures, spring frost is occurring more frequently (Augspurger 2013), which can cause spring mortality in snails when snow cover is absent (for example, up to 90% in Grovesnail; Nicolai unpubl. data). However, medium-sized snails are less susceptible to freezing but still rely on snow cover and temperature-buffered micro-habitat shelter (Ansart et al. 2014). Droughts can cause high mortality in some species depending on the presence of shelter (for example, 75% in Roman Snail [Helix pomatia], Nicolai et al. 2011).

Threat 11.4: storms and flooding; and threat 11.1: habitat shifting and alteration (low impact)

As the frequency of storms and the intensity of rainfall increases, flooding will also increase and affect larger parts of the habitat in the future. While flooding can be a large mortality factor, it is also important for riparian species’ dispersal (Altaba 2015). However, with the projected increase in the frequency and severity of floods, erosion of the riparian forest is an additional threat to this species.

Threat 7: natural system modifications – unknown impact

Threat 7.3: other ecosystem modifications (unknown impact)

There are several highly invasive plants in southern Ontario, including Garlic Mustard (Alliaria petiolata), in Joany’s Woods. Garlic Mustard has been found to displace native vegetation and alter soil nutrient cycles, thereby slowing restoration (Catling et al. 2015). Although a positive impact of an invasive plant on land snail diversity has been documented in western Pennsylvania (Utz et al. 2018), invasive plants can also lead to a decrease in the abundance of endangered snails, as shown in Europe (Stoll et al. 2012).

Non-native earthworms have invaded parts of Canada relatively recently and have altered forest floor habitats by reducing or eliminating the natural leaf litter layer and digging up and mixing the mineral soil with the organic surface layer (CABI 2016). While direct evidence of the effects of exotic earthworms on terrestrial gastropods is lacking, Norden (2010) and Forsyth et al. (2016) suggested that invasive earthworms could indirectly alter terrestrial snail communities. Earthworms, such as the Asian genus Amynthas, which removes the surface leaf litter (Qiu and Turner 2017) where snails live, would pose a particular threat to Domed Disc (see also Dobson 2017 and Lee 2017 for photographs of the effects of exotic earthworms on soil duff layers). Other indirect effects could result from earthworms feeding on forest plant seeds (Cassin and Kotanen 2016) or altering plant-fungi mutualism (Paudel et al. 2016). This would affect understorey vegetation composition (Drouin et al. 2016) and potentially reduce available food plants. Such a change in forest floor structure would profoundly affect plant- and litter-dwelling invertebrate communities (Addison 2009; Dobson and Blossey 2015) as well as bird abundance and nesting success (Loss et al. 2012). Invasive earthworms are present on the north shore of Lake Erie (Evers et al. 2012) and elsewhere in southern Ontario (Reynolds 2014).

Threat 8: invasive and other problematic species and genes – unknown impact

Threat 8.1: invasive non-native/aliens species (unknown impact)

Competition with exotic terrestrial gastropods is also a potential threat (Whitson 2005; Grimm et al. 2010) through aggression (Kimura and Chiba 2010), density effects, and/or food competition (Baur and Baur 1990). Exotic gastropods can compete for resources and shelter with native species. Arion sp. and Grovesnail are present in Joany’s Woods. Carnivorous snails such as glass snails (Oxychilus spp.) were found elsewhere in southwestern Ontario during surveys conducted in 2013 to 2017 and may directly affect native species (Mahlfeld 2000).

Threat 9: pollution – unknown impact

Threat 9.3: agriculture and forestry effluents (unknown impact)

The impacts of pesticides on terrestrial gastropods are poorly known. Population-level impacts of herbicides on terrestrial snails and slugs were not detected in agricultural (Roy et al. 2003) or forested (Hawkins et al. 1997a) landscapes, but laboratory studies have shown that exposure to some herbicides increases mortality of some snail species (Koprivnikar and Walker 2011) and could affect reproduction (Druart et al. 2011). Neonicotinoid insecticides, which are increasingly being used as a coating on soybean and maize seeds (Douglas and Tooker 2015), were not harmful to Grey Field Slug but were harmful to arthropod predators of molluscs (Douglas et al. 2015). It is currently unknown how these pesticides act on native gastropod species. The close proximity of agricultural land just above the wooded slopes of Joany’s Woods (soybeans in August 2018/2019) and around White Oak Woods may also expose snails to pesticide drift.

Cumulative effects

In general, human activities such as recreation and the establishment of second-growth forest increase the abundance of invasive plants (Calinger et al. 2015) in natural areas. Climate change and forest disturbance may also facilitate the spread of introduced species in Canada, with largely unknown and untracked, but potentially serious, impacts on native gastropod faunas.

Limiting factors

In Canada, Domed Disc may continue to exist at the northern limits of its global distribution, and northward expansion is probably limited by harsh winters. Human-caused fragmentation and habitat loss are likely to play a more significant role in this regard (Gibson et al. 2009) along with physical barriers such as extensive bodies of water. Low dispersal ability, together with low physiological resistance to fluctuating environmental factors such as temperature and humidity, restrict gene flow among subpopulations. At the microhabitat scale, availability of moist refuges that buffer environmental fluctuations is probably a limiting factor for population growth and persistence of land snails in general at particular sites (Burch and Pearce 1990).

Number of locations

The most serious and plausible threat is climate change. Given the small size of the one site where Domed Disc was last observed in Canada (Figure 2), a single threatening event could rapidly affect all individuals of the species that are present (IUCN 2018). Therefore, at a minimum, there is one location. If the one uncertain occurrence at White Oak Woods is included, there are probably two locations because climate change could have different impacts at these two sites, which are separated by a considerable distance (Figure 2).

Protection, status, and ranks

Legal protection and status

Domed Disc is not protected by any legislation, regulations, customs, or conditions under any Ontario act. It is not listed on the IUCN Red List (IUCN 2023), under the U.S. Endangered Species Act (US FWS 2023), or under any provincial act. It is not listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES 2023).

Non-legal status and ranks

NatureServe (2024) provides the following ranks for Domed Disc for the US and CESCC (2022) for Canada:

Global Rank: G5 – secure (last reviewed 22 Oct 2009)

National Rank (U.S.): N5 - secure

National Rank (Canada): N2 – Imperilled

Subnational ranks (S-ranks) as provided by NatureServe (2024) for the U.S., and by CESCC (2022) for Canada, are as follows:

SU: Wisconsin

SNR: Alabama, Florida, Georgia, Illinois, Indiana, Iowa, Louisiana, Maryland, Mississippi, Missouri, New York, Ohio, South Carolina

S5: Tennessee, West Virginia

S4S5: Kentucky

S4: Virginia

S3S4: North Carolina

S3: Pennsylvania

S1S2: Michigan

S2: Ontario

Habitat protection and ownership

The currently known occupied habitat in Ontario is owned by Thames Talbot Land Trust, which is dedicated to protecting species and ecosystems. The Leamington White Oak Woods site is privately owned.

Acknowledgements and authorities contacted

Many thanks to Valérie Briand, who assisted with fieldwork, and Robert Forsyth, who assisted with fieldwork, identification, databasing, curation of Ontario specimens, and provided useful information. Thames Talbot Land Trust (TTLT) granted us permission to access their property. Several conservation authorities, including Grand River, Moira River, and Niagara Peninsula, as well as Nature Conservancy Canada provided information and granted access to historical sites for the survey. In Hamilton, Dundas Valley Conservation Area and the conservation land of the Royal Botanical Garden (RBG) were searched in 2017. Special thanks to Sarah Richer from the RBG for her enthusiasm and commitment to continue gastropod monitoring. Michael J. Oldham from the Natural Heritage Information Centre, Ontario Ministry of Natural Resources and Forestry, assisted with field surveys and provided information about the historical records. The status report writers would also like to thank Northern Bioscience Inc. for assisting with field surveys in 2013. Valérie Briand (University Rennes 1) compiled the information sources. Funding for fieldwork in Ontario and the preparation of this status report came from Environment and Climate Change Canada.

Authorities contacted are listed below:

• Canadian Wildlife Service:
  • Ontario Region (13 March 2018)
• Museums:
  • Royal Ontario Museum (visit in August 2015, record query in May 2018)
  • Canadian Museum of Nature (via Robert Forsyth who visited the collection)
  • Carnegie Museum of Natural History, Pittsburgh (July 2018, January 2019)
  • University of Michigan, Museum of Zoology (July 2018, January 2019)
• Parks:
  • Parks Canada (2013 to 2018)
  • Ontario Parks (2013 to 2018)
• Provincial / territorial representatives:
  • ON (8 December 2017)
• Conservation Data Centres or Natural Heritage Information Centres:
  • ON: Natural Heritage Information Centre (2013 to 2019)
• COSEWIC Secretariat:
  • ATK (7 June 2017, 8 December 2017)
• Conservation organizations:
  • NCC (2013 to 2018)

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Biographical summary of report writers

Annegret Nicolai is a biologist at the UMR CNRS 6553 EcoBio/OSUR of the University Rennes 1, France. She has a Ph.D. from the University of Bremen in Germany and from the University Rennes 1 in France. Her research involves investigating ecophysiological questions related to terrestrial snails, specifically the impact of climate change and resource availability on the physiology of and reproduction in endangered and invasive species. She has very specific knowledge about the biology, anatomy, physiology, and ecology of terrestrial gastropods. In Germany she developed a captive-breeding program for the protected species Helix pomatia, and in France she co-authored the National Action Plan for the Conservation of Tyrrhenaria ceratina in Corsica. In the Sinclair lab at Western University, Ontario, she investigated the overwintering strategy of the invasive species Cepaea nemoralis. Since 2012 she has been surveying terrestrial gastropods in Ontario and participating in the “barcoding of life” project at the University of Guelph. She became a member of the Mollusc subcommittee of COSEWIC in 2014.

Collections examined

The collections of the Canadian Museum of Nature, the Royal Museum of Ontario, the Academy of Natural Sciences, Philadelphia, the Carnegie Museum of Natural History, Pittsburgh, the University of Michigan Museum, and occurrence data from the Natural Heritage Information Centre of Ontario were examined by contacting curators (see Acknowledgements and authorities contacted). A global survey of museum records was searched through the Global Biodiversity Information Facility (GBIF 2018). This made it possible to check a wide range of museum records:

• Honey L, Kutner L (2017). NatureServe Network Species Occurrence Data. Version 8.1. NatureServe. Occurrence dataset https://doi.org/10.15468/lysaex accessed via GBIF.org on 2018-07-02. 10.15468/lysaex
• Shorthouse D (2018). Canadian Museum of Nature Mollusc Collection. Version 1.11. Canadian Museum of Nature. Occurrence dataset https://doi.org/10.15468/esfi97 accessed via GBIF.org on 2018-07-02. 10.15468/esfi97
• Harvard University, Morris P J (2018). Museum of Comparative Zoology, Harvard University. Version 162.107. Museum of Comparative Zoology, Harvard University. Occurrence dataset https://doi.org/10.15468/p5rupv accessed via GBIF.org on 2018-07-02., doi:10.15468/p5rupv 10.15468/p5rupv
• Grant S, Jones J (2018). Field Museum of Natural History (Zoology) Invertebrate Collection. Version 18.11. Field Museum. Occurrence dataset https://doi.org/10.15468/6q5vuc accessed via GBIF.org on 2018-07-02. 10.15468/6q5vuc
• Paulay G, Brown W (2018). UF Invertebrate Zoology. Florida Museum of Natural History. Occurrence dataset https://doi.org/10.15468/sm6qo6 accessed via GBIF.org on 2018-07-02. 10.15468/sm6qo6
• Yamazaki J (2018). Mollusca specimens of Okinawa Prefectural Museum and Art Museum. National Museum of Nature and Science, Japan. Occurrence dataset https://doi.org/10.15468/jinrhm accessed via GBIF.org on 2018-07-02. 10.15468/jinrhm
• Leal J (2018). BMSM Bailey-Matthews National Shell Museum. Version 1.22. Bailey-Matthews National Shell Museum. Occurrence dataset https://doi.org/10.15468/49s45k accessed via GBIF.org on 2018-07-02. 10.15468/49s45k
• Gall L (2018). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset https://doi.org/10.15468/0lkr3w accessed via GBIF.org on 2018-07-02. 10.15468/0lkr3w
• Menard K, King P (2018). Recent Invertebrates Specimens. Version 101.62. Sam Noble Oklahoma Museum of Natural History. Occurrence dataset https://doi.org/10.15468/glxcep accessed via GBIF.org on 2018-07-02. 10.15468/glxcep
• Royal Belgian Institute of Natural Sciences (2017). RBINS DaRWIN. Occurrence dataset https://doi.org/10.15468/qxy4mc accessed via GBIF.org on 2018-07-02. 10.15468/qxy4mc
• Senckenberg. Collection Malakologie - SNSD. Occurrence dataset https://doi.org/10.15468/dmitnd accessed via GBIF.org on 2018-07-02. 10.15468/dmitnd
• Roberts D (2018). CHAS Malacology Collection (Arctos). Version 13.13. Chicago Academy of Sciences. Occurrence dataset https://doi.org/10.15468/tk35ga accessed via GBIF.org on 2018-07-02. 10.15468/tk35ga
• Museums Victoria (2018). Museums Victoria provider for OZCAM. Occurrence dataset https://doi.org/10.15468/lp1ctu accessed via GBIF.org on 2018-07-02. 10.15468/lp1ctu
• Fautin D (2014). KUBI Invertebrate Zoology Collection. University of Kansas Biodiversity Institute. Occurrence dataset https://doi.org/10.15468/ubq6lh accessed via GBIF.org on 2018-07-02. 10.15468/ubq6lh
• Creuwels J (2017). Naturalis Biodiversity Center (Netherlands) - Museum collection digitized at storage unit level. Naturalis Biodiversity Center. Occurrence dataset https://doi.org/10.15468/17e8en accessed via GBIF.org on 2018-07-02. 10.15468/17e8en
• Orrell T, Hollowell T (2018). NMNH Extant Specimen Records. Version 1.18. National Museum of Natural History, Smithsonian Institution. Occurrence dataset https://doi.org/10.15468/hnhrg3 accessed via GBIF.org on 2018-07-02. 10.15468/hnhrg3
• Creuwels J (2018). Naturalis Biodiversity Center (NL) - Mollusca. Naturalis Biodiversity Center. Occurrence dataset https://doi.org/10.15468/yefvnk accessed via GBIF.org on 2018-07-02. 10.15468/yefvnk
• Slieker F (2018). Natural History Museum Rotterdam (Netherlands) - Mollusca collection. Version 17.28. Natural History Museum Rotterdam. Occurrence dataset https://doi.org/10.15468/tgt9dj accessed via GBIF.org on 2018-07-02. 10.15468/tgt9dj
• iNaturalist.org (2018). iNaturalist Research-grade observations. Occurrence dataset https://doi.org/10.15468/ab3s5x accessed via GBIF.org on 2018-07-02. 10.15468/ab3s5x
• Academy of Natural Sciences. MAL. Occurrence dataset https://doi.org/10.15468/xp1dhx accessed via GBIF.org on 2018-07-02. 10.15468/xp1dhx
• Biologiezentrum Linz Oberoesterreich. Biologiezentrum Linz. Occurrence dataset https://doi.org/10.15468/ynjblx accessed via GBIF.org on 2018-07-02. 10.15468/ynjblx
• Darrigran G, Damborenea C (2017). Zoología Invertebrados - Malacología. Version 1.1. Facultad de Ciencias Naturales y Museo - U.N.L.P. Occurrence dataset https://doi.org/10.15468/3h7xpk accessed via GBIF.org on 2018-07-02. 10.15468/3h7xpk
• Norton B (2018). NCSM Mollusks Collection. Version 1.2. North Carolina State Museum of Natural Sciences. Occurrence dataset https://doi.org/10.15468/unormg accessed via GBIF.org on 2018-07-02. 10.15468/unormg
• Tablado A, Rodríguez D (2018). Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" (MACN). Invertebrates National Collection (MACNIn). Museo Argentino de Ciencias Naturales. Occurrence dataset https://doi.org/10.15468/uuz636 accessed via GBIF.org on 2018-07-02. 10.15468/uuz636

Appendix 1. Threat assessment for Domed Disc (Discus patulus)

Threats assessment worksheet

Species or ecosystem scientific name: Discus patulus (Domed Disc)

Date: 8/21/2023

Assessor(s): Dwayne Lepitzki (Facilitator and responsible Co-chair), Todd Morris (Co-chair), Brent Patterson (ON COSEWIC member); SSC members: Andrew Hebda, Cam Goater, Kelly McNichols O'Rourke, Olwyn Friesen, Suzanne Dufour; Secretariat: Dean Whitehead, Rosie Soares. Report writer provided post-call comments.

References: Draft threats assessment provided with draft status report; threats assessment occurred while 6-month provisional was undergoing COSEWIC review.

 

Assigned overall threat impact:

BD = High - Low

Impact adjustment reasons:

No adjustment required.

Overall threat comments:

Large range of ranks for the overall threat impact is a consequence of the uncertainty regarding scores of severity for climate change. Generation time is around 2 years, therefore time frame for severity and timing is 10 years. The current extant subpopulation, in Joany's Woods, is managed by Thames Talbot Land Trust (TTLT). A second uncertain occurrence (White Oak Woods, Leamington, record from 1994) is on privately owned land and was not searched but does have intact natural habitat. However, its future is uncertain.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).