Prairie-dock (Silphium terebinthinaceum): COSEWIC assessment and status report 2024

Official title: COSEWIC assessment and status report on the Prairie-dock Silphium terebinthinaceum in Canada

Special Concern

2024

COSEWIC assessment summary

Assessment summary - May 2024

Common name: Prairie-dock

Scientific name: Silphium terebinthinaceum

Status: Special Concern

Reason for designation: This showy long-lived plant is associated with tallgrass prairie habitat and reaches the northern edge of its range in southwestern Ontario where it occurs in nine subpopulations. Although the number of plants likely is greater than 10,000, only a small percentage are known to reproduce through seeds. Changes in its native habitat through competition with native and exotic plants, compounded by fire suppression, are believed to be the greatest threats, although, housing, commercial, and road construction, as well as herbicides, also threaten the species. Its persistence will likely require ongoing monitoring and management activities.

Occurrence: Ontario

Status history: Designated Special Concern in May 2024.

COSEWIC executive summary

Prairie-dock

Silphium terebinthinaceum

Wildlife species description and significance

Prairie-dock (Silphium terebinthinaceum) is a forb in the Asteraceae family associated with tallgrass prairie habitat. The species has a deep taproot, a mass of large basal leaves and yellow composite flowers atop a stem that can reach more than 2.5 m in height. Two varieties are recognized, with all Canadian plants being of the variety terebinthinaceum.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no species-specific ATK in the report.

Distribution

Prairie-dock occurs throughout the eastern United States from Wisconsin to Arkansas in the west and Virginia to Georgia in the east. Its core range includes southern Wisconsin, Illinois, and Missouri. The Canadian distribution of the species is restricted to the Carolinian Zone in southwestern Ontario. There are nine extant native subpopulations of Prairie-dock in Canada, specifically in Essex and Lambton counties. One subpopulation in Brantford and another near Townsend could not be relocated in 2023 and are presumed extirpated. These two subpopulations are of uncertain origin. Three other subpopulations are considered extirpated: River Canard and Knapps Island, which are considered native, and Paris, which is of uncertain origin. Prairie-dock has been introduced at some sites by seeding and transplanting plants, and the species is occasionally included in restoration or prairie creation plantings beyond its native range.

Habitat

At Canadian sites, Prairie-dock is found in wet-mesic to mesic tallgrass prairie, oak savannah, and marsh. Many of the extant sites along rail lines, hydro corridors, and roadsides support assemblages of tallgrass prairie species and many are considered remnant prairie habitat. In the United States, some plants also occur in prairie fen communities. The sites where Prairie-dock occurs typically have limited tree and shrub cover with calcareous soil.

Biology

Prairie-dock is a long-lived species that can reach maturity at three to five years in a cultivated setting but may take longer to mature in the wild. Individual plants can live for more than 25 years and form a large taproot that supports the tall flowering stem. The taproot stores energy and allows plants to withstand drought, fire, mowing and declining habitat conditions. In full sunlight, the leaves of Prairie-dock orient in a north-south direction to minimize evapotranspiration. Mature Prairie-dock plants can enter a prolonged state of reproductive suppression when conditions become unsuitable due to competing vegetation, periods of drought, or a combination of these factors.

The composite flowers are pollinated by long-tongued bees, bee flies, and hummingbirds, and are self-incompatible. The plants reproduce from seed that is viable for one to two years but is selectively predated by small mammals and finches. The seeds have no specialized adaptations for long-distance dispersal.

Population sizes and trends

Of the 12,803 plants counted and estimated in 2022, only 133 were flowering, representing just over 1% of the Canadian population. It is unknown how many of the remaining plants are likely to reproduce in the future. The remaining number likely constitutes plants that are immature or otherwise reproductively suppressed. Individual plants are typically very long-lived and can persist without flowering for years.

Previous estimates of mature plants do not exist from any of the native subpopulations. The surveys conducted in 2022 inferred a decline in flowering plants at three sites that previously contained dozens or hundreds of flowering plants within the last ten years.

Threats

The main threats to Prairie-dock in Canada affecting all native subpopulations are fire suppression and the alteration of habitat by invasive non-native species, namely European Common Reed and Autumn Olive. Five of the extant subpopulations are also threatened by road and railroad management, while three subpopulations are threatened by imminent residential and commercial development. Herbicide use may also threaten plants that occur along railways, utility corridors and roadsides.

Protection, status, and recovery activities

Prairie-dock is not currently afforded legal protection in Canada or the United States. The NatureServe global conservation range rank for the species is G4 (Apparently Secure). It is ranked N1 (Critically Imperilled) in Canada, S1 in Ontario, and has conservation rankings of S1 to S3 (Critically Imperilled to Vulnerable) in six southern United States jurisdictions. It is not ranked in nine U.S. jurisdictions and is considered SU (Data Deficient) in Iowa. Three of the extant Canadian subpopulations occur in a provincial nature reserve, city parkland, and a recreational trail owned and operated by a conservation authority. A fourth subpopulation occurs on Walpole Island First Nation lands. The remaining extant subpopulations occur on private lands.

Technical summary

Silphium terebinthinaceum

Prairie-dock

Silphe térébenthine

Range of occurrence in Canada: Ontario

Demographic information

Generation time (usually average age of parents in the population)

At least 25 years

No studies have been completed that identify a specific generation time. It is a slow-to-establish plant that may not flower until at least the second or third year. The species is very long-lived and therefore a minimum average age of 25; that is, years for mature plants is a reasonable estimate.

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

Yes

A decline in mature individuals has been inferred for the Ojibway Prairie Complex subpopulation based on 2022 survey results, the surveyor’s recollection of sites in previous years, and input provided by local naturalists. Census data are unavailable for Walpole Island. Data for other subpopulations is descriptive only (for example, “locally common”) or is unknown.

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Unknown

There are no data to calculate percentages.

Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Unknown

There are no data to calculate percentages.

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Unknown

There are no data to calculate percentages.

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

There are no data to calculate percentages. The threat calculation projects a 3% to 70% decline.

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

There are no data to calculate percentages.

Are the causes of the decline clearly reversible?

Unknown

Decline due to invasive non-native species and fire suppression are reversible. Reproductively suppressed plants have shown to respond positively to removal of brush and to prescribed burns. It appears that the Ojibway Prairie Complex subpopulation may also be experiencing a decline on account of drought and/or water table fluctuation. This may not be reversible but may be temporary.

Are the causes of the decline clearly understood?

Yes

The species requires open conditions. Encroachment of trees and shrubs results in a decline in mature plants and recruitment. Any influence of drought or water table fluctuation is inferred and not clearly understood.

Are the causes of the decline clearly ceased?

No

The most prevalent invasive species that pose a threat to Prairie-dock, European Common Reed and, to a lesser extent, Autumn Olive, are increasing at most subpopulations where they are present.

Are there extreme fluctuations in number of mature individuals

No

This is a long-lived plant that is not expected to undergo population fluctuations.

Extent and occupancy information

Estimated extent of occurrence (EOO)

531 km²

Based on the range of all extant or presumed extant (Historical) subpopulations

Index of area of occupancy (IAO), reported as 2x2 km grid value

60 km²

Based on extant or presumed extant (Historical) subpopulations

Is the population “severely fragmented, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. Yes

With the exception of the “Near Victory Park” subpopulation, which is only 2 plants and may not be viable, all extant subpopulations contain sufficient mature and immature plants to support viable subpopulations. The distance between known Canadian subpopulations ranges from 1.2 km to 190 km, and the seeds are large and likely regularly disperse only metres from mature plants.

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

16 to 18

There are nine native extant subpopulations of Prairie-dock in Canada. There are also two subpopulations that are presumed extirpated. The number of locations is based on land ownership because the effects of threats are expected to differ at each site based on land management practices. Of the nine extant subpopulations, separate sites within the Ojibway Prairie Complex represent five locations, while separate sites at South Cameron are considered four locations. Intra-limital manipulated subpopulations, if included, would add additional locations.

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

Yes

There is an observed decline in the extent of occurrence. Historical range including all extirpated subpopulations is 9,839 km² (94.6% decrease); a decline from 8,767 km² (93.9%) based on the loss since 1975 (< 3 generations).

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

Yes

There is an observed decline in the area of occupancy as the Knapps Island, River Canard, and Paris subpopulations are extirpated. Surveys of the Brantford and Townsend subpopulations failed to locate plants. The historical IAO including all extirpated subpopulations is 84 km² (31% decline). There has been a decline from 80 km² (decline of 25%) since 1975 (< 3 generations).

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

Yes

There is an observed decline in the number of subpopulations as the Knapps Island, River Canard, and Paris subpopulations are extirpated, with the Brantford and Townsend subpopulations presumed to be extirpated.

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

Yes

There is an observed decline in the number of locations as the Knapps Island, River Canard, and Paris locations are extirpated, with the Brantford and Townsend locations presumed to be extirpated.

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

Yes

There is an observed decline in the area and quality of habitat for the species. The encroachment of invasive non-native species as well as succession occurring in the absence of fire is reducing the area and quality of habitat each year. A single site within the Ojibway Prairie Complex has undergone periodic management of invasive shrubs. Regular maintenance of hydro line and rail corridors acts as a surrogate for fire to control woody vegetation but generally does not result in a reduction in invasive species presence.

Are there extreme fluctuations in number of subpopulations?

No

The species is long-lived and can persist for some time as reproductively suppressed plants when conditions become unsuitable.

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

The known subpopulations are long-established and new subpopulations are not likely to establish due to limited seed dispersal ability.

Are there extreme fluctuations in index of area of occupancy?

No

The known subpopulations are long-established and new subpopulations are not likely to establish due to limited seed dispersal ability. As the potential for subpopulation extirpation is largely associated with degradation of suitable habitat over a long period, extirpation would occur slowly over time.

Number of mature individuals (by subpopulation)

Ojibway Prairie Complex: 24 (3,674)

South Cameron: 29 (91)

St. Clair College: 9 (217)

Tecumseh Road West: 35 (4,488)

Northwood Street: 12 (34)

Central Avenue 7 (47)

near Victory Park: 0 (2)

Upper Big Creek Woods: 17 (4,250)

Brantford: 0 (0)

Townsend: 0 (0)

Walpole Island: Unknown

Based on NRSI field surveys (2022 to 2023). The number of flowering plants is followed by the total number of plants in parentheses.

This species is sometimes planted in conservation plantings, and 12 intra-limital plantings were examined for possible inclusion as subpopulations. However, none of them met guidelines for inclusion as subpopulations, for reasons explained in the report.

Total: 133 to 12,803+

It is uncertain how many of the non-flowering plants may be mature. There are thought to be more than 10,000 mature individuals.

Quantitative analysis

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

Not completed

Analysis not conducted

Threats:

Was a threats calculator completed for this species?

Yes (Appendix 4).

Overall estimated threat impact: High to Medium (2023)

Key threats were identified as:

1. Other ecosystem modifications (7.3) – high-medium impact
2. Fire and fire suppression (7.1) – medium impact
3. Housing and urban areas (1.1) – low impact
4. Commercial and industrial areas (1.2) – low impact
5. Roads and railroads (4.1) – low impact
6. Industrial and military effluents (9.2) – low impact

What limiting factors are relevant?

• At most subpopulations, only a very small proportion of plants flower in a given year
• Prairie-dock is pollinated primarily by long-tongued bees, of which some species are in decline
• Poor seed dispersal in a fragmented landscape limits the spread of plants
• The large seeds are favoured by some seed-eating birds, and rodents selectively consume seeds

Rescue effect (from outside Canada)

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

Unknown

Prairie-dock is not ranked (SNR) in Michigan and Ohio and occurs throughout the western end of Lake Erie from north of Detroit toward Cleveland in the east.

Is immigration known or possible?

No

Natural seed dispersal is localized and immigration therefore is unlikely. The nearest extant U.S. subpopulations are in the vicinity of the River Raisin in Monroe County south of Detroit, at least 30 km from Canada and are separated by the west end of Lake Erie and the Detroit River. Cultivated or otherwise introduced plants are present in both Detroit and Niagara Falls, New York; similarly, these are unlikely to immigrate.

Would immigrants be adapted to survive in Canada?

Yes

Immigrants from states at similar latitudes would likely be cold-adapted to survive in Canada.

Is there sufficient habitat for immigrants in Canada?

No

There is not sufficient habitat for immigrants in Canada.

Are conditions deteriorating in Canada?

Yes

Suitable natural habitat conditions are deteriorating in Canada due to habitat loss and habitat degradation caused by European Common Reed, woody species succession, and habitat declines associated with development. Human-maintained habitats, including hydro corridors and rail lines, where regular vegetation management occurs, contribute to maintaining suitable conditions for existing stands of plants.

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

Unknown

Tallgrass prairie habitat in the adjacent states of Michigan and Ohio faces similar threats to Ontario prairies. Many U.S. conservation agencies have dedicated funding for prescribed burns and invasive species management programs.

Is the Canadian population considered to be a sink?

No

There is no evidence of immigration from the United States.

Is rescue from outside Canada likely, such that it could lead to a change in status?

No

Recue is unlikely to change status.

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Status history

COSEWIC

Designated Special Concern in May 2024.

Status and reasons for designation

Status: Special Concern

Alpha-numeric codes: Not applicable.

Reason for change in status: Not applicable

Reasons for designation: This showy long-lived plant is associated with tallgrass prairie habitat and reaches the northern edge of its range in southwestern Ontario where it occurs in nine subpopulations. Although the number of plants likely is greater than 10,000, only a small percentage are known to reproduce through seeds. Changes in its native habitat through competition with native and exotic plants, compounded by fire suppression, are believed to be the greatest threats, although, housing, commercial, and road construction, and herbicides, also threaten the species. Its persistence will likely require ongoing monitoring and management activities.

Applicability of criteria

A: Decline in total number of mature individuals:

Not applicable.

Insufficient data to reliably infer, project, or suspect population trends.

B: Small range and decline or fluctuation

Not applicable.

The EOO of 531 km² and the IAO of 60 km² are below the thresholds for Endangered, and there is an observed continuing decline in the EOO, IAO, the area and quality of habitat, the number of subpopulations, and locations and an inferred continuing decline in the number of mature individuals. The population is not severely fragmented, occurs at > 10 locations, and does not experience extreme fluctuations.

C: Small and declining number of mature individuals

Not applicable.

Number of mature individuals is likely below the threshold of 10,000 for Threatened, and there is an inferred continuing decline; however, there are believed to be more than 1,000 mature individuals in at least three subpopulations.

D: Very small or restricted population

Not applicable.

Number of mature individuals is unknown but believed to exceed the threshold of 1,000 mature individuals; and the population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

(b) the Wildlife Species may become Threatened if factors suspected of negatively influencing the persistence of the Wildlife Species are neither reversed nor managed with demonstrable effectiveness; or

(c) the Wildlife Species is near to qualifying, under any criterion, for Threatened status;

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

2024

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.

*   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.

Wildlife species description and significance

Name and classification

Current classification: Silphium terebinthinaceum Jacquin

Class: Equisetopsida

Order: Asterales

Family: Asteraceae

Genus: Silphium

Species: Silphium terebinthinaceum

Subspecies in Canada: var. terebinthinaceum

Common names: Brouillet et al. 2010+, NatureServe 2024

English: Prairie-dock, Prairie Dock, Prairie Rosinweed, Rosin-plant

French: Silphe térébenthine, Rhubarbe de Louisiane

Synonyms and notes:

Synonyms include Silphium rumicifolium, S. terebinthinaceum var. lucy-brauniae (Clevinger 2006), S. terebinthinaceum var. sinuatum, and S. chickamaugense (POWO 2022). Clevinger and Panero (2000) maintain the placement of Silphium terebinthinaceum in section Composita proposed by Small (1933).

Plants with pinnately-lobed leaves are considered S. t. var. pinnatifidum (Clevinger 2006), sometimes recognized as S. pinnatifidum (Perry 1937; NatureServe 2024). This variety occurs within the core range of Prairie-dock from Ohio (iNaturalist 2023) and Indiana to Alabama and Georgia (NatureServe 2024). Clevinger’s (2006) reports of S. t. var. pinnatifidum from Michigan have not been substantiated (Michigan Flora 2011+; NatureServe 2024). Hybrids of Prairie-dock and Compass Plant (S. laciniatum) have been reported where the two species co-occur (Fisher 1959, 1966; Michigan Flora 2011+). Further studies that include sampling a chloroplast marker to determine if hybridization has been involved in the evolution of S. t. var. pinnatifidum are necessary (Clevinger and Panero 2000).

A separate species, Whole-leaved Rosinweed (Silphium integrifolium), is also known as Prairie Rosinweed. This species does not occur naturally in Ontario but has been introduced in seed mixtures at prairie creation sites and has been found along a railway in Sarnia (Oldham pers. comm. 2024). It is easily distinguished from S. terebinthinaceum by the sessile opposite leaves on the stem of the plant.

Description of wildlife species

Prairie-dock (see cover photo) is a long-lived perennial forb in the Aster family (Asteraceae). The composite flowers are 4-7 cm across, each including up to 140 bright yellow disc petals. Several to many flowers are borne in a broad panicle atop a tall stem that can reach a height of more than 2.5 m and up to 3.5 m (Fernald 1950; Clevinger 2006; Michigan Flora 2011+). In Ontario, an individual plant can flower continuously between early July and late September (iNaturalist 2023; Deacon pers. obs.). The large, flattened seeds are narrowly winged with two teeth at the apex (Britton and Brown 1970). The smooth green to purple stem may include several rudimentary leaves, but often lacks foliage altogether. Well-established plants are characterized by a dense mass of leaves that are 30-60 cm in length and arise from the base of the plant on slender petioles (Fernald 1950). The heart-shaped leaves are leathery with a rough texture and persist into early winter. Plants with glabrous upper leaf surfaces have been documented in southern Ohio (Steyermark 1951) but are not considered to be a distinct variety (Clevinger 2006). First-year and reproductively suppressed plants are composed of one to several small basal leaves. The plants form a deep, woody taproot that can measure up to 8 cm wide at the root collar (Deacon pers. obs. 2008; Woodliffe pers. comm. 2022) and may extend to a depth of more than 3.5 m (Hilty 2020). Damaged portions of the stem exude a resinous substance (USDA 2022).

Designatable units

The Canadian population of Prairie-dock represents one Designatable Unit within the Great Lakes Plains Ecological Area (COSEWIC 2018b). The native subpopulations are not discrete in that they occupy similar habitats, and although the native Canadian subpopulations are separated from one another by as much as 190 km, there is no evidence of heritable traits or markers that clearly distinguish the putative DU from other DUs. It is unknown whether sufficient time has passed that either natural selection or genetic drift are likely to have produced discrete units. There is no evidence of evolutionary significance as the species is not known to harbour heritable adaptive traits or an evolutionary history not found elsewhere in Canada.

Special significance

On account of its immense size and unique foliage, Prairie-dock is a well-recognized component of wet to mesic tallgrass prairie habitat. The Canadian population constitutes the northeastern range limit of the species and is representative of native lowland tallgrass prairie communities that were largely lost due to land conversion beginning in the 1800s (Rodger 1994). The specific epithet terebinthinaceum is derived from the Greek “like turpentine” in reference to the odour of the resinous sap produced by the plant (USDA 2022). Prairie-dock is used in horticulture and as a species used in prairie restoration.

Aboriginal (Indigenous) knowledge

Aboriginal Traditional Knowledge (ATK) is relationship-based. It involves information on ecological relationships between humans and their environment, including characteristics of species, habitats, and locations. Laws and protocols for human relationships with the environment are passed on through teachings and stories, and Indigenous languages, and can be based on long-term observations. Place names provide information about harvesting areas, ecological processes, spiritual significance or the products of harvest. ATK can identify life history characteristics of a species or distinct differences between similar species.

Cultural significance to Indigenous peoples

There is no species-specific ATK in the report. However, Prairie-dock is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

Prairie-dock occurs from Michigan, Wisconsin, and Iowa in the northwest to Ontario and Ohio in the northeast with a southern limit that includes Arkansas in the southwest and Georgia in the southeast (NatureServe 2024) (Figure 1) with the most southern occurrence being in Alabama (GBIF 2024). The core of its range is centred on Illinois where it is known from 91 of 102 counties (Kartesz 2022). To the west of Ontario, the species is present in many counties in the southern half of Wisconsin and Michigan (Kartesz 2022). Two records from the Upper Peninsula of Michigan, at the same latitude as Manitoulin Island in Canada, occur along rail lines (Michigan Flora 2011+); at least one of these is considered to be likely a recent introduction (Oldham pers. comm. 2024).

A report from Nebraska in the Flora of North America (Clevinger 2006) is not considered to be native (Helzer pers. comm. 2022; Rolfsmeier pers. comm. 2022). Similarly, the species was reported from Niagara Falls, New York in 1915 (Werier et al. 2022), but is considered introduced in New York State, and has not naturalized there (Werier pers. comm. 2022).

[Insert image]

Figure 1. Distribution of Prairie-dock in North America. Adapted from Kartesz (2022). Dark green indicates presence of the species in that state or province. Light green indicates US counties where the species is not rare for the state. The approximate localities of Canadian subpopulations are shown with red dots.

Canadian range

Prairie-dock is restricted to the Carolinian Zone and does not naturally occur elsewhere in Canada (Soper 1962). The Carolinian Zone includes an extension of the Prairie Peninsula represented by tallgrass habitats present at Windsor, Walpole Island, Grand Bend, and the Norfolk Sand Plain among other sites (Bakowsky and Riley 1994). During his time in the area in the late 19^th century, botanist John Macoun described the prairies at Windsor as the eastern extension of the prairie flora (Macoun 1893) and Macoun (1894) described Prairie-dock range in Canada as "Open woods and grassy banks. Cayuga and Malden, Ont. (Maclagan.) Along the Great Western Railway, east of Paris, Ont. (Geo. Prescott.)". Approximately 5000 years ago during the hypsithermal interval, a drier and warmer environment facilitated the eastward expansion of prairie vegetation (Pratt 1979). These frontier prairie communities were maintained as openings among temperate forest by a combination of fire, soil type, and moisture regime (Whitford 1970; Maycock 1973). The western-most Canadian subpopulation is more than 20 km from the nearest United States (USA) subpopulations with urban and agricultural land use fragmenting the natural cover throughout southern Ontario and southern Michigan. The proportion of the global range and population of Prairie-dock that occurs in Canada is estimated to be less than 1%.

Nine native extant subpopulations are currently known from Ontario; eight are located in Essex County between Windsor and Amherstburg and one is located at Walpole Island First Nation in Lambton County (Table 1, Figure 2, Appendix 1). Two extirpated native subpopulations are also known from Essex County: Knapps Island and River Canard. These combined sites represent the western and southern extent of Prairie-dock in Canada. Some subpopulations have been newly documented in the past 30 years: Victory Park (2019), Brantford (1999), South Cameron (1994), Tecumseh Road West (1992), and Northwood Street (1992). All these subpopulations appear to be long-established and are not the result of recent dispersal.

Figure 2. Distribution of Prairie-dock subpopulations in Ontario, Canada (created by R. Mulder).

Two subpopulations occur in Brant County, but whether they are native or temporarily naturalized from an outside source (adventive) is uncertain (see below). The northeastern-most occurrence is an extirpated subpopulation known from Paris, where it was the third collection in Canada in 1894. The locality was referenced and included in the collector’s list of Compositae of Galt Ontario and the vicinity (Herriot 1910) and by Macoun (1894). A subpopulation from the city of Brantford is considered presumed extirpated (species is believed to be extirpated at this site, has not been located despite intensive search effort, and there is virtually no likelihood that it will be rediscovered) (NatureServe 2024). This presumption is due to the absence of plants in the exact locality where plants were last observed 18 years earlier. The site is next to a constructed berm amongst native prairie flora at the toe-of-slope/woodland edge. The habitat was very disturbed with many non-native species present and might have been seeded/planted (Oldham pers. comm. 2024). This site is periodically mowed and although no plants could be detected in 2022 or 2023, some habitat remains suitable. An additional subpopulation was known from near Townsend in Norfolk County that represents the eastern-most Canadian occurrence. A survey in 2023 did not observe any plants to be present at the Townsend subpopulation and a portion of the habitat where plants were observed in 1987 has become unsuitable due to tree establishment. All three of these subpopulations are approximately 190 km east of Walpole Island and 240 km northeast of the Essex County subpopulations.

The Brant and Norfolk county subpopulations, included in the range by Soper (1962) who stated, "The Prairie Dock is now confined to the westernmost edge of the Carolinian Region but two specimens have been seen from along the Grand River collected near the end of the last century." These reports have previously been discounted as possibly adventive and the result of casual introductions (Argus et al.1982 to 1987; Sutherland 1987). During the preparation of the Atlas of Rare Vascular Plants of Ontario (ARVPO), collections from the eastern subpopulations were omitted. The ARVPO entry for Prairie-dock reads: “Occasionally cultivated in gardens and adventive. Collections from Brant and Haldimand counties may represent such casual introductions (Argus et al. 1982 to 1987). The Ontario Natural Heritage Information Centre (NHIC) considers all sites outside of the Essex and Lambton counties introduced and not a conservation target (Brinker pers. comm. 2022). A recent publication noted the species was historically recorded from Brant and Haldimand counties as non-native, but outside of cultivation (Oldham and Brinker 2009). Although rail corridors may harbour introduced species that arrived by way of cattle cars or shipments of prairie hay from the USA, the assemblage of prairie species at some sites suggests the presence of a long-established and naturally-occurring community. At the Paris subpopulation, the sustained presence of other prairie species including American Columbo (Frasera caroliniensis) and Hoary Puccoon (Lithospermum canescens) indicates that Prairie-dock may have been part of the native local flora when observed in 1894 (Bakowsky pers. comm. 2022), 47 years after the rail line was constructed (Zadro and Delamere 2009). Although the Townsend subpopulation does not include a substantial prairie component, the locality of the plants in a creek floodplain in the center of the concession would seem to rule out an intentional introduction (Sutherland pers. comm. 2022), but could have washed downstream from an upstream natural or planted subpopulation (Oldham pers. comm. 2024). During a survey of the site in 2023, tallgrass prairie species were noted from the nearby abandoned rail corridor and the floodplain vegetation community was typical of intact riparian meadow habitats in the area and showed no evidence of intentionally introduced species. Collectively, these three records align with a peninsula of tallgrass prairie and oak savannah remnants that extend from the vicinity of Paris southwest to Lake Erie. Because there is no conclusive evidence that the eastern subpopulations represent introductions, and could be the remnants of a once larger range (Tallgrass Ontario 2024), these sites are being included as part of the full potential range for this assessment (Figure 3). There is no evidence or reports of the Prairie-dock or appropriate habitat from the intervening area between Windsor-LaSalle and the Grand River drainage (Brinker pers. comm. 2024; Brownell pers. comm. 2024).

Figure 3. Range based on all natural occurrences and ones of uncertain origin recently extant within 3 generations EOO 8767 km² IAO 68 km².

The Canadian range of Prairie-dock may have been larger prior to European settlement as surveyor notes identified “fine open plains” “extensive natural meadows” and “prairie” totaling roughly 400 km² in Essex, Kent, and portions of Lambton counties (McNiff 1791; Rankin 1847; LaJeunesse 1960; Bakowsky and Riley 1994; Tallgrass Ontario 2024). These habitats were largely destroyed as a result of drainage and vegetation clearing that occurred through the late-1800s to facilitate agricultural land use.

Manipulated subpopulations

The subpopulations identified in Table 2 include 12 intra-limital manipulated subpopulations that were established for the purpose of species conservation, and reviewed to determine if each fulfills the COSEWIC Manipulated Wildlife Species Guidelines (COSEWIC 2018a). In consideration of Guideline 4 for the inclusion of manipulated populations in the assessment criteria, intra-limital reintroductions that have a net positive impact on the species are to be included in the assessment. These subpopulations vary from source-identified ex-situ propagation of plants originating from naturally occurring source subpopulations, to plantings with no record of seed progeny but still contributing to species conservation in the broad sense. Throughout southern Ontario, Prairie-dock is frequently included in naturalization planting seed mixtures, is sold by native plant nurseries, and is planted in gardens (Deacon pers. obs.). Some of these plantings likely use imported seed from the USA or Canadian-grown seed of USA progeny. Salvaged plants and seed from the Spring Garden Prairie in Windsor are now propagated ex-situ at a nursery in Norfolk County (Gartshore pers. comm. 2022). Inquiries with other native plant nurseries regarding the progeny of their Prairie-dock seed were not returned. It is likely that the seed used by Ontario NativeScape, located in Lambton County, was initially sourced from the Windsor area or Walpole Island (Lamb pers. comm. 2023).

None of the intra-limital sites are considered to meet the criteria for inclusion as manipulated subpopulations. All of the sites lack detailed information concerning reproduction. Although intra-limital in range, the manipulated sites were not established within natural prairie habitat, and never contained prairie drivers, such as soil composition, and ecological processes required to maintain this species. As such, without ongoing management, these sites are likely to revert to forest that would not support this species (Brinker pers. comm. 2024; Brownell pers. comm. 2024; Oldham pers. comm. 2024). In addition, four intra-limital subpopulations are not included following Guideline 2 in the assessment, as they are cultivated for commercial purposes (Table 2).

Many of the plantings in Ontario that include Prairie-dock are beyond the natural or potential range (Figure 2, Appendix 1) of the species including naturalization plantings in Toronto, Kitchener, and Sarnia as well as dozens of plantings throughout the Carolinian Zone (Appendix 2). Following Guideline #5: COSEWIC will generally only include populations resulting from benign extra-limital introductions as part of the Wildlife Species being assessed if suitable habitat remaining within the natural range of the Wildlife Species in Canada no longer exists, or is limited to the extent that long-term viability of the Wildlife Species is uncertain. Sixteen extra-limital subpopulations are not included in this assessment as they occur beyond the accepted or potential natural range of Prairie-dock (Figure 2; Appendices 1 and 2). Although tallgrass prairie habitat is more extensive in Ontario (Tallgrass Ontario 2024) there is no evidence that Prairie-dock extended beyond the known range.

Many of these manipulated subpopulations were identified using the citizen science platform iNaturalist (2023). Although this species is often reported at publicly-accessible restoration plantings, it is probable that many other private or otherwise undocumented sites occur throughout southern Ontario or beyond. To date, no reintroductions of Prairie-dock have occurred at sites that were known to support the species historically.

Population structure

In this report, population refers to the sum of all Prairie-dock individuals naturally occurring in Canada. Subpopulations are defined as geographically or otherwise distinct groups in the population between which there is likely to be little demographic or genetic exchange (typically one successful migrant individual or gamete per year or less; COSEWIC 2023). Subpopulation corresponds to the habitat-based plant element occurrence (EO) delimitation standards (NatureServe 2020) where a subpopulation is defined as a group of occurrences that are separated by less than 1 km; or if separated by 1 to 3 km, with no break in suitable habitat between them exceeding 1 km; or if separated by 3 to 10 km but connected by linear water flow and having no break in suitable habitat between them exceeding 3 km. A site is a group of plants within a subpopulation.

The current, native Canadian population of Prairie-dock is concentrated in Essex County where plants are generally associated with the glacial lake sand deposits along the Detroit River and in Lambton County on the delta islands of Lake St. Clair. This group of sites is generally accepted as the extent of naturally-occurring plants in Canada (Argus et al. 1982 to 1987). The origin of the Paris, Brantford, and Townsend sites is uncertain. Located approximately 190 to 240 km to the northeast of the widely accepted native subpopulations, these sites may represent naturally-occurring subpopulations that occur as an isolated portion of the Canadian population. To date, no research or spatial analyses has been conducted to examine spatial discontinuities.

Subpopulations are fragmented by urbanization with seven of the nine extant subpopulations occurring within the City of Windsor and Town of LaSalle (Figure 4; Appendix 1). The distance between these subpopulations ranges from 1.2 km to 4 km of urban land use. The other two extant subpopulations, Walpole Island, and Upper Big Creek Woods, are 14 to 55 km from the Windsor-LaSalle group and are surrounded by natural cover and agricultural land use.

There is limited opportunity for genetic mixing between the Canadian population and populations in the United States as they are separated by large areas of land that are unsuitable for the species, including lakes Erie and St. Clair and the Detroit and St. Clair rivers.

Figure 4. Confirmed extent of occurrence excluding historical 531 km² and IAO 60 km². Map and calculation by Alain Filion of the COSEWIC Secretariat (March 2024).

Extent of occurrence and area of occupancy

The extent of occurrence (EOO) for all currently known extant Canadian subpopulations of Prairie-dock is 531 km² (Figure 4) The index of area of occupancy (IAO), which includes all extirpated and presumed extirpated subpopulations, is 84 km² (Figure 3). Excluding extirpated and presumed extirpated subpopulations, the IAO is 60 km² (Figure 4; GeoCAT 2023).

The EOO is the total area covered by a minimum convex polygon connecting the peripheral sites of the species range in Canada. The IAO was calculated by adding the area of any grid square that is overlapped by an extant or historical record. The determination of the IAO assumes that locational data provided for each Element Occurrence are accurate.

Based on the 2022 and 2023 surveys, a total of nine native subpopulations are considered extant and viable (Figure 4). The EOO and IAO calculations include these subpopulations (Table 2). Three subpopulations (Paris [1894], River Canard [1975], and Knapps Island [1984]) are extirpated. A site within the St. Clair College subpopulation is extirpated due to expansion of Highway 401 and removal of the edge of the woodlot where plants occurred up until recently. These have been excluded from the EOO and IAO calculations. The River Canard subpopulation was immediately adjacent to recent vegetation clearing at the roadside and the site was subsequently re-visited and confirmed to have been destroyed (Catling pers. comm. 2022). The Paris subpopulation has been searched by various botanists in recent years and although prairie indicator species remain, no Prairie-dock plants have been observed (Buck pers. comm. 2022). A search of the Knapps Island site in 2022 did not observe any plants and invasive species have greatly altered the habitat.

Fluctuations and trends in distribution

The current EOO of 531 km² (Figure 4), and IAO of 60 km² are based on the inclusion of the Essex and Lambton county subpopulations and the exclusion of presumed extirpated Brant and Norfolk counties. A loss from 94.6% of its historical range including extirpated and presumed extirpated subpopulations is 9,839 km² (Figure 3). When only considering losses in less than the last three generations (since 1975), the decline is from 8,767 km², a loss of 93.9% (Figure 2).. The IAO declined by 25% within the last three generations with the loss of the subpopulations at Knapps Island, River Canard, Brantford, Townsend, the site within the St. Clair College subpopulation, and possibly a decline of 29% as the date of the Paris extirpation is unknown.

Biology and habitat use

Life cycle and reproduction

Prairie-dock is a long-lived perennial that reproduces exclusively from seed. One study that examined the closely related Compass Plant seedling establishment estimated that 1% of the seeds produced in a given year germinated and established as seedlings (Pleasants and Jurik 1992). A study of seed viability following exposure to fire found that a substantial proportion (32%) of Prairie-dock seed remains viable following exposure to heat (150°C) but viability decreases substantially above 175°C (Hahn and Orrock 2014). Upon germination, plants typically grow a single leaf while a taproot establishes over several years. A flowering stem may be produced within two to three years (Missouri Botanical Garden 2022). Plants that were propagated from Ontario seed in a garden setting reached maturity (produced a flowering stem) at approximately three to five years, depending on sunlight availability and soil moisture conditions (Deacon pers. obs.). In the wild it may take much longer to reach maturity. Cultivated plants can survive for at least 25 years and it is reasonable to assume that wild plants exhibit this same longevity, with an individual possibly growing past 50 years (Lamb pers. comm. 2023). As no studies have been completed relating to generation time and the species is very long-lived, minimum average age of 25 years for mature plants is being used as an estimate.

In Ontario, flowers are produced continuously between early July and late September (NHIC 2022b; iNaturalist 2023). No formal studies of the relationship between Prairie-dock and pollinators have been completed to date (Speer 1966; Clevinger pers. comm. 2023); however, anecdotal observation has noted flowers being visited by long-tongued bees, bee flies, and hummingbirds (Hilty 2020). Within the genus Silphium, each flowering head contains hundreds of florets but only produces 15 to 27 seeds, with most florets in the centre of the flower being staminate (Pleasants and Jurik 1992; Van Tassel and DeHaan 2023). Silphium species are thought to be self-incompatible (Reinert et al. 2020) and rely on successful out-crossing to produce viable seed. A hybrid of Prairie-dock and Compass Plant has been reported (Fisher 1959; Michigan Flora 2011+). In Canada, the two species are known to co-occur in at least a couple of sites. They have been seen growing together at Marthaville Habitat Management Area (St. Clair Region Conservation Authority; Oldham pers. comm. 2023), with no hybrids observed. There is also at least one Ontario iNaturalist record that shows the two species growing together (Oldham pers. comm. 2024). In late fall, the elevated seedheads break apart and the large seeds, about the size of a dime, fall to the ground near the plant. It is likely that those seeds that become covered by thatch and are less exposed to desiccation and consumed by rodents are more likely to reach germination the following spring.

The large taproot provides the plant with access to soil moisture and acts as an energy reserve that makes it tolerant to periods of prolonged drought and occasional fire. In comparing previous observations of plants in the vicinity of the Ojibway Prairie Complex, the 2022 surveys suggest that the species can enter a reproductively suppressed state (see Physiological, Behavioural, and Other Adaptations; Deacon pers. obs.). This transition may be a response to drought or a mechanism for survival when woody species encroachment makes the habitat unsuitable. The ability of Prairie-dock to senesce and survive on energy reserves, potentially for many years, makes it resilient during periods of unfavorable conditions.

Habitat requirements

The habitat for Prairie-dock includes mesic to wet-mesic prairies, fens, roadsides, railway embankments, thickets, and dry woods (Argus et al. 1982 to 1987; Clevinger 2006; Michigan Flora 2011+). The Ontario sites include wet-mesic to mesic prairie, marsh, sparse thicket and anthropogenic sites such as rail, utility, and road corridors (Bakowsky 1988; Oldham and Brinker 2009). The natural communities reflect Ecological Land Classification (ELC) types that include: Fresh – Moist Tallgrass Prairie Type (TPO2-1), Mineral Meadow Marsh Ecosite (MAM2), Mineral Cultural Meadow Ecosite (CUM1), and Mineral Cultural Thicket Ecosite (CUT1) (Lee et al. 1998). ELC communities with higher tree cover including woodlands and savannahs may also support Prairie-dock, in particular within canopy gaps within these communities. Although plants can tolerate a range of soil moisture regimes, the wet-mesic prairie that occurs at the Ojibway Prairie Complex and Walpole Island First Nation subpopulations represents typical conditions (Bakowsky pers. comm. 2024). Common native associate species observed in the mesic to wet-mesic prairie community during the 2022 surveys included Bluejoint Reedgrass (Calamagrostis canadensis), Yellow False-sorghum (Sorghastrum nutans), Big Bluestem (Andropogon gerardi), Swamp Agrimony (Agrimonia parviflora), Virginia Mountain-mint (Pycnanthemum virginianum), Canada Goldenrod (Solidago canadensis), Giant Sunflower (Helianthus giganteus), Tall Coreopsis (Coreopsis tripteris), Field Thistle (Cirsium discolor), and Missouri Ironweed (Vernonia missurica) (Deacon pers. obs.). Both Willow-leaved Aster (Symphyotrichum praealtum) and Dense Blazing-star (Liatris spicata) also occur in the same wet-mesic prairie habitat and are Species at Risk (MECP 2022; Government of Canada 2023). The Upper Big Creek Woods site is situated on well-drained sand alongside Stiff Goldenrod (Solidago rigida), Fragrant Sumac (Rhus aromatica), and other species associated with dry prairie. Prairie-dock plants that occur in marsh communities in the Ojibway Prairie Complex are associated with stands of Spotted Joe Pye Weed (Eutrochium maculatum), Bluejoint Reedgrass, and Swamp Milkweed (Asclepias incarnata).

During the 2022 surveys, tree canopy was generally absent where Prairie-dock occurred with some sites located in the openings of oak savannah or at the edge of treed features where plants are partially shaded. Plants at the edge of woodlands, forests, and among dense shrub thickets showed reduced vigour and fewer flowering stems.

The prairies and savannahs of Windsor-LaSalle occur on poorly drained sandy loam (Granby sand) with areas of more well-drained soil (Plainfield sand and Berrien sand) also present (Richards et al. 1949). Approximately 1-2.3 m of sandy loam was deposited in the vicinity of the Ojibway Prairie as a result of glacial Lake Rouge (Hoffman 1975; Guiton 1976). The sandy soil overlies a clay lens, which impedes drainage and prolongs the wet conditions through the spring (Pratt 1979; Bakowsky pers. comm. 2022). At Ojibway Prairie, soils are seasonally saturated, and portions of the site can flood in March and April. During the summer months the water table drops 1-2 m to the depth of the underlying clay (Pratt 1979). The Upper Big Creek Woods subpopulation is similarly situated on sandy soil atop less permeable Perth clay (Richards et al. 1949). At Walpole Island, the hydrology of the low-lying prairies and savannahs is closely related to the elevation of the St. Clair River. The Tuscola sandy loam becomes saturated when water levels are high and dries out when water levels are low (Bakowsky pers. comm. 2022). The subpopulation in Brantford occurred on poorly-drained alluvial soil while the Norfolk subpopulation occurs on well-drained Burford loam containing fluvial gravel and sand (Presant and Acton 1984; Acton 1989). Each of the Brant and Norfolk county sites have tallgrass prairie indicator species present, but the habitats are predominantly composed of meadow and floodplain thicket species (Buck pers. comm. 2022; Gartshore pers. comm. 2022).

Movements, migration, and dispersal

It is hypothesized that Prairie-dock arrived in Canada approximately 5000 years ago during the hypsithermal interval which allowed a suite of prairie species to expand eastward from the core of the Prairie Peninsula (Soper 1962; Pratt 1979). Prior to European settlement and the conversion of land, this migration was facilitated by the large, unbroken expanses of tallgrass prairie that were present between the present-day USA Midwest and Ontario.

The seed of Prairie-dock is not easily dispersed across large distances by natural means. Each seed is approximately the size of a dime and lacks a pappus or barbs that would increase its mobility by wind or wildlife. Bird species such as American Goldfinch (Spinus tristis) selectively feed on the seed in the fall (Pleasants and Jurik 1992; Hilty 2020; Deacon pers. obs.), but the large seeds are consumed at or near the plant and are not carried large distances. With the seeds held atop a 2-3 m tall stem, wind dispersal within one generation is estimated to be a distance of 5 m or less. Compass Plant is similar in form and seed size and a study examining seedling establishment surrounding reproducing plants identified a median dispersal distance of 1 m with almost all seedlings within 3 m of a reproducing plant (Pleasants and Jurik 1992). Based on propagation of seed from cultivated plants (Deacon pers. obs.) viable seed can float on water; however, in Canada the only subpopulations near open water include the extirpated Knapps Island subpopulation and the Townsend subpopulation along the Catfish Creek floodplain.

The Ojibway Prairie Complex and the habitat on Walpole Island is a mosaic of prairie openings, thicket, oak savannah, and forest. Treed habitats present barriers for spread of Prairie-dock as these areas lack sufficient sunlight to support recruitment. Similar barriers exist for plants along utility or rail corridors and roads whereby their potential for successful establishment is confined to a managed linear habitat. Native Canadian subpopulations known from these habitats include three of the Windsor subpopulations, Upper Big Creek Woods, and the extirpated site at Paris. The subpopulations along rail lines in Windsor and Amherstburg occur among a series of prairie remnants and are therefore assumed to be relic plants that pre-date European settlement and have persisted due to the periodic control of trees and shrubs along the corridor.

Prairie-dock is often included in naturalization seed mixtures and seed can be purchased through seed suppliers based in Canada and the U.S., many of which are not distributing source-identified material. Seed is occasionally available at horticultural group “seed swap” events. Well-established subpopulations are present in conservation areas, along highways and on private land. Manipulated subpopulations are known from at least nine counties in southern Ontario (Deacon pers. obs.; NHIC 2022a,b; iNaturalist 2023; Table 2; Figure 2). Plants at conservation areas and along highway corridors have the potential to spread by intentional seed scattering or on mowing equipment in the late fall. During the recent Windsor-Essex Parkway project, prairie that was being removed as part of the project was partially salvaged by relocating prairie sod and by collecting and scattering seed (Snyder pers. comm. 2022). This included relocating Prairie-dock material to a restoration site on Chappus Street, north of the Ojibway Prairie Provincial Nature Reserve. Based on a brief survey of the restoration site in 2022, it appears that these plants did not survive.

Interspecific interactions

Predators and competitors

The large seeds of Prairie-dock are consumed by birds, particularly American Goldfinch that perch on the tall stems in late fall to forage on the seedheads (Pleasants and Jurik 1992; Hilty 2020; Deacon pers. obs.). There is no evidence that birds disperse the seeds, which would be difficult to transport large distances due the seed’s large size. It is likely that seed on the ground would be consumed by small rodents including mice and voles. A study examining rodent granivory of 16 prairie forbs seeded into plots, including Whole-leaved Rosinweed, identified that Meadow Vole (Microtus pennsylvanicus) selectively depleted the seeds of the rosinweed by 59% over the winter period while other species were avoided (Howe and Brown 2000). In a similar study, the granivory of 12 prairie forbs by small mammals and arthropods found both Prairie-dock and Cup Plant (Silphium perfoliatum) to be preferentially consumed by small mammals at much higher proportions than the other species that were used in the study (Johnson and Zettlemoyer 2022). Potentially as a result of the resinous substance excreted by the plant, herbivory by White-tailed Deer (Odocoileus virginianus) is not widespread and no evidence of browse was noted during the 2022 surveys.

Host/parasite/disease interactions

Silphium Rust (Puccinia silphii) has been observed on at least one plant in Canada (Campbell pers. comm. 2022) and has been observed on plants throughout the USA (iNaturalist 2023). This disease is not known to harm the plants.

Other interactions

Prairie-dock flowers attract long-tongued bees including bumblebees, miner bees, and introduced European Honey Bee (Apis mellifera) (Hilty 2020). They are also visited by halictid bees, bee flies, and hummingbirds (Hilty 2020). The large, hollow stems may be used by insects for nesting or over-wintering. The Silphium Weevil (Haplorhynchites aeneus), ranked S1S2 (Critically Imperilled to Imperilled) in Ontario and N3N4 (Vulnerable to Apparently Secure) in Canada, is known from the prairies in Windsor (Bright 1993). The adult weevils mate on the flowers of Prairie-dock, among other sunflower species, and the larvae feed on the flowers (NHIC 2023).

Physiological, behavioural, and other adaptations

Prairie-dock is adapted to fire and benefits from the open conditions that are maintained by periodic fires. To some extent, regular mowing replicates this disturbance regime but does not cycle nutrients or expose bare mineral soil as fire does. Although the large leaves are flammable during March and April, above-ground growth is relatively delayed with the preliminary leaves emerging in late April into May (Deacon pers. obs.). The root crown is somewhat insulated from the radiant heat caused by fire by the mass of hardened leaf stalks and early-emerging foliage. Any damage to early leaves caused by a spring fire would not significantly harm the plant as many additional leaves are produced through late spring (Deacon pers. obs.). At large colonies of plants, where prairie grasses are generally excluded, the fine fuel load is relatively low and fire intensity would be lower within these sites.

Prairie-dock plants are tolerant of periods of drought and can enter a reproductively suppressed state when conditions become unfavourable. The summer of 2022 was very dry between June and August when plants would typically be producing a flowering stem. No aborted stems or flowers were observed in 2022 and a flowering stem is usually evident among the basal leaves by late May (Deacon pers. obs.). As the Ojibway Prairie Complex is situated over a perched water table, the drought in 2022 and dry years that preceded it may have reduced the number of Prairie-dock that flowered despite the tolerance afforded by the deep taproot (Woodliffe pers. comm. 2022). The soil moisture conditions within the Ojibway Prairie Complex have been dry for several consecutive years and flowering Prairie-dock plants are now only reaching 1-2 m tall (Cedar pers. comm. 2022). This response may indicate that prolonged periods of drought and drier spring conditions due to reductions in snow melt or precipitation are having an impact on the plants beyond their tolerance.

Well-established plants are tolerant of disturbance (Swink and Wilhelm 1979) including mowing or scarification of the soil outside the growing season. As observed with cultivated plants, the root crown can be severed several centimetres below the soil surface and the plant is able to regenerate new leaves within a short period (Deacon pers. obs.). Prairie-dock plants are hardy and seedlings can be established in degraded old fields without controlling competing vegetation (Kirt and Durnberger 1990).

A study of leaf orientation (Smith and Ullberg 1989) found that the foliage of mature plants has a strong preference for north-south orientation with most leaves erect and less than 30 degrees from vertical. The study inferred that the leaf orientation maximizes gas exchange and photosynthesis and minimizes water loss allowing the plant to survive drought stress that can be profound in tallgrass prairie communities. The leaf orientation becomes obscured through the growing season as the rosette enlarges and old leaves are forced outward by new leaves (Smith and Ullberg 1989).

Limiting factors

Limiting factors are generally not human-induced and include intrinsic characteristics that make the species less likely to respond to conservation efforts. Limiting factors may become threats if they result in population decline. The main limiting factors for Prairie-dock are:

Limited flowering

As observed during the 2022 surveys, less than 1% of the plants had flowering stems at each of the four largest sites, which range from 257 to approximately 4400 individuals. The ability of Prairie-dock to enter a state of dormancy when the habitat becomes unsuitable, or if the weather conditions do not support flowering, is beneficial for the long-term survival of individual plants but reduces the potential for recruitment of new plants at a site.

Pollination

Prairie-dock is thought to be self-incompatible and it is presumed that successful pollination would require a stand of mature plants. At subpopulations where there are very few mature individuals, either due to senescence or due to a low number of plants overall, these subpopulations may be at a disadvantage for successful pollination. Due to the elongated corolla tube of the ray and disc flowers of Prairie-dock (>1 cm), secondary pollination may rely on long-tongued animals including bees, bee flies, and hummingbirds (see Interspecific interactions). Many of the subpopulations are situated in sizable natural areas with connections between habitat patches; however, the urban landscape of Windsor and LaSalle is likely to limit the widespread movement of pollinators.

Limited dispersal

The localized dispersal of seed, estimated to be 5 m in a single generation (Deacon pers. obs.), presents a challenge for the long-term viability of Prairie-dock stands. A patch of plants that is essentially static within a larger habitat is more vulnerable to stochastic events. The formation of dense colonies of plants may provide the species with competitive advantages over other grasses and forbs and facilitates cross pollination but in the absence of outlier sites, all plants can quickly be lost to a threat.

Seed characteristics

Although a mature plant will often produce dozens of seed heads, a seed head will contain only 15 to 20 seeds around the rim of the floret (Van Tassel and DeHaan 2023). In cultivated plants, a portion of the seeds are typically not viable and seed predation has been observed in both cultivated and wild plants (Deacon pers. obs.). Coupled with limited dispersal, the nature of the seeds presents challenges for recruitment.

Population sizes and trends

Data sources, methodologies, and uncertainties

The field surveys conducted in fall 2022 and summer 2023 comprise the only comprehensive abundance data collected for Prairie-dock in Canada. These data do not include counts of the subpopulations at Walpole Island.

The 2022 and 2023 survey effort adequately assessed the known sites for Prairie-dock in Essex, Brant, and Norfolk counties. As Prairie-dock occurs primarily in the well-studied remnant tallgrass prairie habitats of southern Ontario, the likelihood of new sites being documented in this habitat is low. Potentially, new sites native in origin may be found along rail, utility, and road corridors where plants have persisted.

Surveys commenced by relocating known sites. The surveyor walked about the vicinity to determine the rough distribution of plants in the immediate area. Using a handheld GPS unit with a track function, equally-spaced transects were walked throughout the stand and a dot-tally was compiled to count all plants. A separate dot-tally was maintained to document flowering stems. In some instances, a flowering stem from a previous year was observed on a plant that did not flower in 2022 and these were included in the count. Where multiple stems arose from a cluster of basal leaves, the root crown at the surface of the soil was closely examined to determine if the stems belonged to one plant or multiple plants in close proximity. Notes regarding plant vigour, apparent threats, associated species, and the vegetation community structure were also recorded. Upon completion of the count, the surveyor inspected adjacent areas of suitable habitat for potential outlier plants and other sites.

The survey effort includes several limitations. A large proportion of plants at many sites were composed of basal leaves only during the 2022 surveys. In considering past observations and photographs, there are records of larger numbers of flowering stems at a number of sites in past years. This presents a challenge in defining a mature plant for a species that may live for decades but is inconsistent in producing flowering stems. It is inferred that annual recruitment of Prairie-dock plants is low and some sites may see few or no new plants establish in a given year. No studies have been completed that examine recruitment of Prairie-dock. It is also inferred that large, non-flowering plants have the potential to reproduce if they occur in suitable open habitat. These plants, as well as flowering plants, would be considered mature individuals by IUCN definition. It was estimated that most plants (up to 90%) that were not flowering would do so in the future (Clevinger pers. comm. 2023), particularly those reproductively suppressed if conditions improved by reducing competition. However, because it is difficult to distinguish immature plants and also individuals that may be reproductively suppressed due to unsuitable habitat conditions, the total number of flowering stems observed in 2022 is considered to be the minimum number of mature individuals (Table 3).

^a Brantford was surveyed in both 2022 and 2023. Townsend was surveyed in 2023.

The Essex County surveys were conducted between November from 4-11^th at which time nearly all plants had been reduced to curled and desiccated brown leaves. Although the large leaves remained quite visible, their lax appearance contrasts the erect stature of live plants during the growing season. Individual plants could be separated by visualizing all of the common root crowns among the downed leaves; however, the completion of a count would have been easier during the growing season, when showy flowers would be present. In addition, the drought conditions of 2022 and the inferred negative impact it had on the number of flowering stems should be taken into consideration. The total counts of flowering plants seemed quite low to the report writer and others with knowledge of the sites (Cedar pers. comm. 2022; Pratt pers. comm. 2022; Preney pers. comm. 2022; Woodliffe pers. comm. 2022).

Observation data provided by the NHIC (2022 a,b) were reviewed to identify sites where a plant count or estimate had been conducted. The current data set includes 97 records that range from the 1892 collection by John Macoun through to 49 observations made using the iNaturalist platform as recently as 2023. A small number of the iNaturalist observations include photographs that allow for an estimate of the number of plants present including flowering plants. Determining the abundance of plants based on the iNaturalist observation is limited because a single plant can produce multiple stems and it is not always clear if the photograph captures the entire stand of plants. Approximately 15 of the provided observations included an exact count of plants or provide a general estimation of numbers or use relative terms such as “rare” or “locally abundant”. Although a subset of these observations makes note of flowering plants being present, the counts or estimates do not differentiate between flowering and vegetative plants.

Herbarium specimen labels were also reviewed, two of these included relative abundance. Although one of the specimens was included in the NHIC data set the 1894 collection from Paris, the 1975 collection from River Canard and a 1977 collection from the Windsor Raceway property were not included. Each was cross-referenced with other observations or further information was obtained from the observer (Catling pers. comm. 2022). A 1945 collection from “Cayuga” in Haldimand County was omitted from the ARVPO project and was not considered as a subpopulation in this report. It is suspected that the herbarium sheet label contains an error with respect to the locality and the plant may not have been collected in Haldimand County (Sutherland pers. comm. 2022).

In preparing this report, the report writer inquired with local naturalists. Many were able to provide accounts of past site conditions and an estimate of the stem count within a general range of years and how the site has changed. All of the stem count estimates provided in Appendix 3 are considered to be coarse with some site recollections dating back more than 40 years.

Abundance

Three of the subpopulations contain a relatively large number of plants including Ojibway Prairie Complex, the Tecumseh Road West subpopulation, and Upper Big Creek Woods near Amherstburg. In 2022, less than 1% of the plants were flowering at those sites. The size of the the Walpole Island First Nation subpopulation is unknown. The Walpole Island First Nation and Ojibway Prairie Complex sites are characterized by high-quality tallgrass prairie while the other sites are associated with disturbed areas along rail corridors (Table 3; Appendix 3).

A total of 12,803 plants were counted at eight of the nine extant native subpopulations in 2022 (Table 3). There are no current data for Walpole Island. Only 133 of the plants had flowering stems; these were largely from 2022 but also included a small number of persistent stems from 2021. The number of flowering stems at the subpopulations ranged from less than 1% to 35% of the overall count of plants. The drought conditions of 2022 and the inferred negative impact it had on the number of flowering stems should be taken into consideration. Several naturalists report seeing substantially higher numbers of flowering plants at some sites in recent years including hundreds of flowering plants in the northern extent of Ojibway Prairie Provincial Nature Reserve and hundreds at “The Prairie Dock Field” within Spring Garden Natural Area (Cedar pers. comm. 2022; Pratt pers. comm. 2022; Preney pers. comm. 2022; Woodliffe pers. comm. 2022).

The Walpole Island subpopulation was not surveyed but may include a sizable number of plants in wet tallgrass prairie habitat based on observations made by botanists who visited the island throughout the early-2000s (Oldham pers. obs. 2004; Obbard pers. obs. 2009; Deacon and Korol pers. obs. 2015). Both the Brantford and Townsend subpopulations included few plants when they were last observed (Appendix 3). Due to the time that has elapsed since the last observation, and no plants being observed at either subpopulation in 2022 and 2023, these subpopulations are considered presumably extirpated. The LaSalle subpopulation near Victory Park was composed of only two vegetative plants in 2022 and is situated along a narrow strip of forest edge. Development of an agricultural field as a subdivision immediately adjacent to the treed feature is likely to continue a trend of habitat degradation for these plants as succession occurs. In the absence of any substantial amount of prairie or meadow in the vicinity, this subpopulation may be at risk including a low potential for cross pollination and the production of viable seed.. All other subpopulations are well established. It is estimated that the current Canadian population of Prairie-dock could be as high as 20,000 plants when including the potential number of plants on Walpole Island, but the number is very uncertain. The number of mature individuals is uncertain with known plants being between 133 and12,803+.

Fluctuations and trends

Continuing decline^{Footnote 1} in number of mature individuals

Of the 11 extant or historical subpopulations, only the Ojibway Prairie Complex and Walpole Island subpopulations have numerous observations made over the course of many years. The documented presence of Prairie-dock at other sites, without reference to a plant count or habitat conditions, limits the ability to assess fluctuations and trends at those sites. Census data were not obtained for Walpole Island in the preparation of this report and as such, a detailed comparison over time is not possible. As the main threats; invasive non-native species and fire suppression, are relevant to all subpopulations, the inferred trends from the Ojibway Prairie Complex may be applicable to the other sites at a high-level. For the Ojibway Prairie Complex subpopulation, it is estimated that a decline in mature individuals is continuing. The loss of 93.9% of the species range since 1975 is not believed to represent a >30% population decline as the eastern portion of the species range is believed to represent a small proportion of the Canadian population. It is possible that the habitat losses including those in the Windsor – LaSalle area may have resulted in a >30% loss in the last 75 years (3 generations) as the subpopulation sizes were the largest at the western portion of range.

Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years)

In the Preliminary Life Science Inventory for Ojibway Prairie Complex (Pratt 1979), Prairie-dock was noted to be present at several study sites. Three wet-mesic forb prairie stands were studied at unknown localities within the complex. Two stands contained Prairie-dock with a quadrat frequency of 32%. Information for one of these stands, the St. Clair College site is provided. The data show that in 1975, Maycock surveyed the St. Clair College stand (Stand 17) using quadrat frequency sampling where a one hectare plot containing 20 quadrats was established. Data included in the Life Science Inventory notes that Prairie-dock had a quadrat frequency of 90% (Pratt 1979). It is inferred that Stand 17 overlaps the same prairie remnant found on the campus today, and contained 217 plants of which nine were flowering plants. The study methodology, mapping, and additional quantitative data that informed this summary in the Life Science Inventory were not obtained. If the 1975 plot overlaps with the site where plants were tallied in 2022, it is likely that the cover of Prairie-dock has decreased at least moderately from the 90% quadrat frequency Maycock observed. Although there are currently 217 plants present at this site, their distribution is largely restricted to the northern portion of the prairie with plants sparse in the central portion of the site and absent in the south.

Although the 2022 surveys documented a total of 3674 Prairie-dock plants within the Ojibway Prairie Complex subpopulation across 12 sites, only 24 flowering plants were observed. While discussion with naturalists at the City of Windsor identified a recent decrease in flowering plants that may be a fluctuation related to drought, comments were also made about the decrease in total number of plants over the last several decades.

The Life Science Inventory for the Ojibway Prairie Complex (Pratt 1979) does not provide additional information relating to Prairie-dock frequency within the Complex and inferences relating to trends at the Provincial Nature Reserve rely on a series of observation notes often associated with specimen collections, photographs, or recollections of site conditions.

Ojibway prairie provincial nature reserve

Prairie Dock was first documented in the vicinity of Sandwich (now known as Windsor) by John Macoun in 1892 with a second collection made in 1901 that references “Windsor; near the race course”. These specimens are the first record of the species from the subpopulation but provide no indication of plant abundance. In 1975, Allen Woodliffe collected a plant from “800 yards SE of Windsor Raceway” in wet prairie. It is possible that this patch of plants was the same as the patch observed by Macoun 74 years prior. In 1985, Paul Pratt photographed a robust patch of hundreds of flowering stems from the northwest corner of the Provincial Nature Reserve. Allen Woodliffe recalls numerous small patches in the Provincial Nature Reserve along the east side of Matchette Road in an area known as the “old fields” (Woodliffe pers. comm. 2022). A return to this area in 2022 did not locate any plants and European Common Reed (Phragmites australis australis) is now dense in that area. In 2008, E. Sanders counted 19 plants from a georeferenced locality that appears to be north of the 1975 site. No mention is made of flowering stems. The report writer visited this same patch in the following year in 2009 and noted approximately 10 plants. In 2022, this patch was re-visited and 21 vegetative plants were present. According to park naturalists, Prairie-dock used to be much more prevalent (dozens or hundreds of flowering stems) in the far northwest of the Provincial Nature Reserve (Pratt pers. comm. 2022; Woodliffe pers. comm. 2022). In 2022, few plants were observed in this area and European Common Reed had invaded both sides of Titcombe Road where plants had been observed in the past. Pratt recalls more than 2000 plants being present in the northwest, near Matchette Road, in the 1980s and mentions that this area used to be burned regularly, but has become overgrown with European Common Reed (Pratt pers. comm. 2022). He adds that during a site walk in that area in October 2022 he did not notice any Prairie-dock plants but notes that some may be persisting and obscured among the dense European Common Reed.

Spring Garden Prairie

Several collections make reference to “Spring Garden Prairie” and it is unclear if they are referring to a specific area of prairie just off of Spring Garden Road near a church, or the larger natural area, within which there are several sites for Prairie-dock. In 1982, Botham and Oldham documented plants at “Spring Garden Prairie” with Oldham revisiting the site in 1984 with Gary Allen. No counts or details relating to flowering or habitat were noted on the herbarium label. In 2006, Oldham photographed a patch of approximately 20 flowering stems near the church. Oldham returned to the site in 2007 with Jane Bowles and Samuel Brinker and noted roughly 80-100 plants, many in flower, in open woods. Brinker returned to this site in 2011 and photographed a stand of approximately 25 flowering stems (see cover photograph), the full count of flowering stems at the site may have been 100 or more (Brinker pers. comm. 2022). In 2022, the report writer surveyed this area and counted 75 plants including two flowering stems. Based on these coarse estimates between 2006 and 2022, it appears that the flowering stem count has fluctuated over the past 16 years. The number of vegetative plants has likely stayed the same over this period and photographs indicate that the habitat has remained similar with small amounts of Gray Dogwood (Cornus racemosa) encroachment noted in 2022.

“The Prairie Dock Field”

A site in the south of Spring Garden Natural Area was photographed by Russ Jones and Tom Preney in 2017. The large clearing contained approximately 100 flowering stems and thousands of vegetative plants. In 2022, Preney re-visited the site with the report writer when approximately 3300 plants were noted of which 17 were flowering. Autumn Olive (Elaeagnus umbellata) is overtaking this clearing with thousands of plants present including fruiting shrubs and seedlings. Management work has occurred with mixed results but Autumn Olive continues to threaten the Prairie-dock patch (Preney pers. comm. 2022).

Population fluctuations, including extreme fluctuations

The population of mature individuals is believed to be relatively stable year to year with annual recruitment inferred to be low and existing mature plants very long-lived. The number of subpopulations has remained relatively stable for many years with little change to the EOO or IAO (see Fluctuations and trends in Distribution).

Severe fragmentation

The population of Prairie-dock is not considered severely fragmented. Most of the nine extant native subpopulations of Prairie-dock occur in habitat patches that are thought to be large enough to support a viable subpopulation. Although the sites in Amherstburg and Walpole Island are isolated from the core group of sites in Windsor and LaSalle, the colonies are viable on their own without genetic exchange between subpopulations. The LaSalle subpopulation near Victory Park was composed of only two vegetative plants in 2022 and is situated along a narrow strip of forest edge and is likely not viable in the long term. Development of an agricultural field as a subdivision immediately adjacent to the treed feature is likely to continue a trend of habitat degradation for these plants as succession occurs. In the absence of any substantial amount of prairie or meadow in the vicinity, this subpopulation may be at risk including a low potential for cross pollination and the production of viable seed.

Rescue effect

It is unlikely that adjacent subpopulations of Prairie-dock in Michigan or Ohio would naturally immigrate to suitable habitat in Canada. Despite the heavily urbanized landscape, there are numerous subpopulations in protected areas and along roadsides and rail corridors throughout the adjacent area.

Although birds forage on the large seeds, most seeds are consumed on or near the plant (Deacon pers. obs. 2022). Given that the distance between Canada and the nearest USA subpopulations is 15 km or more, separated by the City of Detroit and Lake Erie, natural migration by birds will not result in meaningful rescue. The seeds are not conducive to dispersal by wind or water and deteriorating conditions in neighbouring and Canadian prairie habitats are likely further limiting dispersal rates and the ability of unassisted establishment.

Threats

Historical, long-term, and continuing habitat trends

The extent and quality of tallgrass prairie habitat in Canada has diminished substantially since the onset of European settlement and the conversion of prairie to arable land. Referring to early surveyor notes for southern Ontario, prairie, oak savannah, and woodland covered 530 km² of the landscape at the time of the surveys through the 19^th century as a minimum estimate (Bakowsky and Riley 1994). Many of these habitats were cleared, ploughed under, or drained over the past 200 years. It is estimated that the current extent of these habitats in Ontario is less than 0.5% of the original extent and includes remnant habitat at both Walpole Island and Ojibway Prairie (Bakowsky and Riley 1994).

At Walpole Island and Ojibway Prairie, Prairie-dock is found within the Moist – Fresh Tallgrass Prairie Type (TPO2-1) plant community which is ranked G2 (Imperilled globally) and S1 (Critically Imperilled in Ontario) (NHIC 2023). Few remnants of this vegetation community greater than two hectares persist in Ontario today with approximately 6-20 element occurrences known (Rodger 1998). Early surveyor notes describe a massive area of prairie that bordered almost the entire eastern fringe of Lake St. Clair and extended inland for some distance (McNiff 1791).

Soil mapping in the vicinity of Windsor and LaSalle (Richards et al. 1949) indicates that approximately 60 km² of the former Sandwich West Township is situated on sandy loam soil that likely once supported a mosaic of tallgrass prairie, marsh, and oak savannah similar to the habitats that remain present in Ojibway Prairie today. Pratt (1979) notes that McNiff’s 1790 to 1791 survey described the area extending from the south of the current day Ojibway Prairie Complex to River Canard as “a sandy barren plain” (McNiff 1791). Although the Ojibway Prairie was spared from planned development following the purchase of lands by the U.S. Steel Corporation in 1908 (Pratt 1979), a large portion of natural cover has been progressively replaced with urban development as the urban areas of Windsor and LaSalle have expanded. South of LaSalle is a combination of residential lots and specialty crop agriculture with few woodlots and riparian areas remaining.

Several City of Windsor naturalists have noted that several consecutive years of dry conditions appear to be impacting Prairie-dock within the Ojibway Prairie Complex (Cedar, Pratt, Preney pers. comm. 2022). Plants that are not directly impacted by other threats such as invasive species or woody succession, and which occur in high-quality tallgrass prairie, appear to be producing fewer flowering stems and not growing as large as they typically would. Fluctuation in the shallow perched water table is suspected to be the reason behind this recent change. A hydrological study of the Ojibway Prairie (Guiton 1976) found that the vertical movement of water through the underlying clay was negligible and as such, precipitation and evapotranspiration regulate the hydrology of the prairie (Pratt 1979). The habitat at the Walpole Island subpopulation is similarly influenced by a high water table. The alteration of groundwater levels and spring melt conditions is a threat to Prairie-dock at these subpopulations where plants are adapted to wet-mesic prairie conditions.

Current and projected future threats

The threat assessment included only the native subpopulations and did not consider threats to manipulated subpopulations. Prairie-dock is vulnerable to the cumulative effects of various threats, especially ecosystem modifications resulting from invasive non-native species and fire suppression. The nature, scope, and severity of these threats have been described in Appendix 3, following the IUCN-CMP (International Union for the 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 approximately 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 threat impact for Prairie-dock is considered to be high to medium, corresponding to an anticipated further decline of between 3 and 70% over the next 75 years (three generations). These values are to be interpreted with caution, as they may be based on subjective information, such as expert opinion, although efforts have been made to corroborate the scores with available studies and quantitative data.

Natural system modifications (IUCN 7.0; overall threat impact high-medium)

Other ecosystem modifications (invasive species) (IUCN 7.3; overall threat impact high-medium)

European Common Reed occurs at a number of Prairie-dock sites, often as very dense stands reaching 3 to 4 m in height. This species was present during the surveys for the Life Science Inventory (Pratt 1979) but has become much more prevalent, and along with fire suppression, presents the largest threat to Prairie-dock (Pratt pers. comm. 2022). This species is pervasive along utility corridors and roadsides throughout the Ojibway Prairie Complex. Sparse stands are also present within core areas of tallgrass prairie habitat within the Ojibway Prairie Provincial Nature Reserve. During the 2022 surveys, Prairie-dock plants located among European Common Reed were consistently limited to sparse patches of depauperate basal leaves. Mature Prairie-dock plants appear to be restricted to the edges of maintenance trails and footpaths that pass through European Common Reed where competition is reduced.

Autumn Olive is prevalent at “The Prairie Dock Field” within the Ojibway Prairie subpopulation with thousands of plants present and suppressing Prairie-dock. The City of Windsor has attempted to control this invasive shrub using both cut stump and foliar applications of herbicide with mixed results (Preney pers. comm. 2022).

The Upper Big Creek Woods subpopulation is declining due to invasive shrubs that include Glossy Buckthorn (Frangula alnus) as well as privet (Ligustrum sp.) and Autumn Olive to a lesser extent. The well-drained soil at this site appears to limit the vigour of the shrubs, but a patchy thicket is reducing that quality of the prairie habitat along the rail line.

Other non-native invasive species that are present in small numbers at some subpopulations, but have the potential to increase, include Tree-of-heaven (Ailanthus altissima), Scot’s Pine (Pinus sylvestris), White Mulberry (Morus alba), Multiflora Rose (Rosa multiflora), Amur Honeysuckle (Lonicera maackii), Smooth Brome (Bromus inermis), Reed Canary Grass (Phalaris arundinacea), Japanese Honeysuckle (Lonicera japonica), and Wild Parsnip (Pastinaca sativa).

Fire and fire suppression (IUCN 7.1; overall threat impact medium)

The suppression of fire is allowing tree and shrub succession to occur, resulting in rapid structural and compositional changes to the habitat of Prairie-dock. As wildfire is essentially non-existent in southern Ontario under current conditions, the outcome that was once achieved by fire is now largely the result of other factors such as the grubbing of woody vegetation in utility corridors. Nowicki and Abrams (2008) term the phrase “mesophication” to describe the positive feedback cycle whereby cool, damp, shaded conditions caused by woody species encroachment detract from the flammability of a site while promoting the growth of fire-intolerant species. This threat applies to all subpopulations where fire intolerant species are encroaching on prairie and meadow where Prairie-dock occurs. The problematic species include Eastern Cottonwood (Populus deltoides), Trembling Aspen (P. tremuloides), Pin Oak (Quercus palustris), Black Walnut (Juglans nigra), ashes (Fraxinus spp.), White Elm (Ulmus americana), White Pine (Pinus strobus), Sassafras (Sassafras albidum), Eastern Red Cedar (Juniperus virginiana), Staghorn Sumac (Rhus typhina), Gray Dogwood and Northern Prickly-ash (Zanthoxylum americanum). A regular fire return interval is critical for reducing woody species encroachment and allowing grasses and forbs to thrive. Prescribed burning appears to be a necessary tool in any form of modern-day prairie management (Pratt 1979).

Most prescribed burning of tallgrass prairie sites in the Windsor area occurs in March or early April with late spring and fall burns being less common due to increased smoke caused by the “green up” of vegetation and the presence of snake species at risk which are typically dormant in the early spring. The timing of the burn has been identified as a potential threat to Prairie-dock because fire early in the season promoting forbs, whereas fires later in the season promoting the tall grass species (Woodliffe pers. comm. 2022). A late spring fire can be beneficial for the habitat to reduce non-native species, but it also promotes the growth of the warm season prairie grasses that can compete with young Prairie-dock plants.

Residential and commercial development (IUCN 1.0; overall threat impact low)

Housing and urban areas (IUCN 1.1; overall threat impact low)

Several stands of Prairie-dock within the South Cameron Woodlot are under immediate threat of removal due to residential development. Three of the four sites at South Cameron occur on private land that includes recently constructed curb and gutter infrastructure and cleared road alignments suggesting imminent vegetation removal. The proposed development of this area has been a contentious issue for many years and recent revisions to wetland protections within the province of Ontario have weakened the protections afforded to the woodlot.

A small number of Prairie-dock plants are present on the former Windsor Raceway property which has been proposed for residential and commercial development. Development of this parcel would threaten the small patch of plants and would increase the constraint level for the completion of prescribed burns within the Ojibway Prairie Provincial Nature Reserve to the immediate east. A private lot that is slated for residential development is also present in the south of the Ojibway Prairie Complex subpopulation and although situated on the bank of a watercourse; development may threaten the long-term survival of these plants.

Commercial and industrial areas (IUCN 1.2; overall threat impact low)

The St. Clair College subpopulation is located on one of the last undeveloped areas on the Windsor campus. The adjacent parking lot was recently developed as a student residence which further separates the prairie remnant from the nearby Saint Clair Prairie Environmentally Significant Area and casts a late-day shadow on the plants. Future expansion of the campus infrastructure could directly threaten the entire subpopulation.

Transportation and service corridors (IUCN 4.0; overall threat impact low)

Roads and railroads (IUCN 4.1; overall threat impact low)

Maintenance of rail lines and road right-of-ways poses a threat to four subpopulations including the Ojibway Prairie Complex, Tecumseh Road West, Northwood Street, and Central Avenue. Prairie-dock plants at these sites benefit from the periodic removal of brush but may be impacted by localized works that result in ground disturbance. The frequency of vegetation management within these corridors can also threaten plants as flowering stems could be cut and prolonged periods of unmanaged woody vegetation growth could result in declines in mature plants and the overall health of the stand. The construction of spur lines, outbuildings, new towers, natural gas substations and culvert replacements associated with these corridors may all result in the loss of Prairie-dock plants and suitable habitat.

Pollution (IUCN 9.0; overall threat impact low)

Industrial and military effluents (IUCN 9.2; overall threat impact low)

Plants that occur along railroads, utility lines and roadsides may be exposed to herbicide as part of vegetation management along these corridors. Herbicide application may affect plants within the Ojibway Prairie subpopulation as well as the Tecumseh Road West, Northwood Street, and Central Avenue subpopulations. The impact would depend on the timing of spraying relative to the leaf-out and exposure of Prairie-dock plants.

Number of threat locations

There are 16 to 18 locations for Prairie-dock, based on the ownership of lands that reflects the individual potential to manage threats (Table 4). Each of the nine extant and two historical subpopulations vary in habitat as well as threats that could rapidly affect all plants at a given subpopulation. Of the nine viable extant subpopulations, separate sites within the Ojibway Prairie Complex represent five locations while separate sites at South Cameron are considered four locations.

Protection, status, and recovery activities

Legal protection and status

Prairie-dock is not listed under the Ontario Endangered Species Act, 2007 or the Canadian Species at Risk Act. It is not listed as an Endangered Species in the USA at the federal level or within any state where it occurs.

Non-legal status and ranks

Prairie-dock is considered Secure or Apparently Secure globally (G4 last reviewed in 2024) and is Critically Imperilled in Canada (N1) and Ontario (S1). The species has a conservation rank of S1 in Virginia and South Carolina, is Imperilled (S2) in Tennessee and North Carolina, and Vulnerable (S3) in Georgia. In Mississippi, Prairie-dock is Vulnerable to Apparently Secure (S3S4) and is Data Deficient (SU) in Iowa. A conservation status rank has not been assigned (SNR) throughout the midwestern and southern states where it occurs (Alabama, Arkansas, Illinois, Indiana, Kentucky, Missouri, and Wisconsin); this includes the adjacent states of Michigan and Ohio (NatureServe 2024). Prairie-dock has not been assessed by the IUCN (2023).

Land tenure and ownership

Of the nine subpopulations verified as extant in 2022, three occur entirely on public land, two occur in a combination of public and private land, three occur entirely on private land, and one is located on a First Nations reserve. Only those plants located in the Ojibway Prairie Provincial Nature Reserve are afforded formal protection. Prairie-dock plants are also present on lands owned by the City of Windsor and Essex Region Conservation Authority; however, the recreational nature of these properties does not provide formal protection for the species. The remaining public sites are located within City of Windsor road right-of-ways. Sites on private land include railway properties, hydro corridors, private lots, and land owned by St. Clair College. It is unknown if the subpopulation on Walpole Island, the traditional territory of the Bkejwanong Walpole Island First Nation, is located on lands under the jurisdiction of the Walpole Island First Nation or on other lands under private ownership.

The subpopulations that occur on public conservation lands require ongoing management to control invasive species to ensure the long-term survival of Prairie-dock in Ontario. Approximately one third of the population occurs along railway and hydro corridors where plants persist largely due to periodic clearing of woody vegetation and many of these subpopulations do not provide adequate protection for the species. Few, if any of the subpopulations that occur on private land are likely to be secured for conservation purposes.

Information sources

References cited

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Collections examined

In preparing this status report, nine digitally imaged specimens were examined from the collection held at the Canadian Museum of Nature (CAN), Ottawa, Ontario. Six specimens were examined from the University of Guelph (OAC), Guelph, Ontario.

Authorities contacted

Acknowledgements

Funding for the preparation of this report was provided by Environment and Climate Change Canada. The authorities listed above provided valuable data and/or advice. Mapping and the calculation of IAO/EOO were completed by Randi Mulder, and Bruce Bennett, Co-chair of the Vascular Plant Specialist Subcommittee was most helpful in providing technical guidance and editorial comments throughout the drafting of this report. The report writer would like to thank all those that shared their expertise (see Authorities Contacted) and personal observations, especially Wasyl Bakowsky, Graham Buck, Xander Campbell, Brad McLeod, Michael Oldham, Paul Pratt, Don Sutherland, and Allen Woodliffe for their input regarding the occurrences. Samuel Brinker, and Tom Preney provided assistance with field surveys and shared their knowledge of site trends, threats and management that has occurred. Jennifer Doubt and Lyndsey Sharp at the Canadian Museum of Nature (CAN) were helpful in providing digital imaging of the specimens in the collection at CAN. Del Meidinger, Vivian Brownell, Samuel Brinker, Anna Hargreaves, Mannfred Masahiro Asada Boehm, Danna Leaman, Sean Blaney (VPSSC), Barbara Frazer (ATK, VP SSC), Gina Schalk, Holly Bickerton, Marie Archambault, Karolyne Pickett, CWS Ontario Region (ECCC) provided comments on earlier drafts.

Biographical summary of report writer

Patrick W. Deacon is a consulting biologist with over 15 years of experience conducting botanical inventories throughout Ontario and western Canada. He holds a Bachelor of Environmental Studies in Environment and Resource Studies from the University of Waterloo. Patrick has served as a director for Tallgrass Ontario, the Waterloo Stewardship Council and the Field Botanists of Ontario. He spends much of his free time exploring tallgrass prairie remnants throughout southern Ontario and is involved in ongoing stewardship efforts. Patrick recently wrote the COSEWIC status Report update for Kentucky Coffee-tree (Gymnocladus dioicus) (2021) and the status report for Cleland’s Evening-primrose (Oenothera clelandii) (2023).

Appendix 1. Summary of sites included in figure 2.

Appendix 2. Excluded sites

Appendix 3. Summary of Prairie-dock subpopulations in Canada

¹Unless noted otherwise, a count or estimate of plants or flowering stems was not recorded.

Appendix 4. Threat calculator for Prairie-dock

Species scientific name: Silphium terebinthinaceum - Prairie-dock

Element ID: 168708

Elcode: PDAST8L0G0

Date: 125/2023/

Assessor(s): Patrick Deacon (report writer), Bruce Bennett (VPSSC Co-chair, facilitator), Del Meidinger (VPSSC co-chair), Vivian Brownell, Sam Brinker, Anna Hargreaves (VPSSC), Barbara Frazer (ATK, VP SSC), Holly Bickerton, Lingfei Li (ECCC)

References:: Draft COSEWIC status Report on Prairie-dock (2022) by Patrick Deacon

Assigned overall threat impact: BC = High - Medium

Impact adjustment reasons: Two low threat impacts (housing, roads) were at the low range for Scope, so reduced assigned impact to High-Medium.

Overall threat comments: Various Ontario experts that were contacted highlighted 1) Invasive Species 2) Fire Suppression and 3) Residential Development as the major threats to the species. Under the comment boxes, the bracketed numbers, for example, (1) are intended to highlight specifics at a given subpopulation. The numbers do not associate consistently with the same subpopulation and just used as separators for a given threat. A generation time of 25 years has been assumed with three generations totalling 75 years.

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