Cleland’s Evening-primrose (Oenothera clelandii): COSEWIC assessment and status report 2023

Official title: COSEWIC assessment and status report on the Cleland’s Evening-primrose (Oenothera clelandii) in Canada

Committee on the status of Endangered Wildlife in Canada (COSEWIC)

Endangered

2023

COSEWIC assessment summary

Assessment Summary – December 2023

Common name

Cleland’s Evening-primrose

Scientific name

Oenothera clelandii

Status

Endangered

Reason for designation

This relic of dry tallgrass prairie communities occurs from mid-western United States to Ontario. In Canada, this prairie wildflower is known from four isolated sites in southwestern Ontario. While no plants have been seen since 2001, it may still be present within the seedbank. Its decline in Canada is thought to be due to habitat loss and degradation through development, fire suppression, and competition with both exotic and native terrestrial plants.

Occurrence

Ontario

Status history

Designated Endangered in December 2023.

COSEWIC executive summary

Cleland’s Evening-primrose

Oenothera clelandii

Wildlife species description and significance

Cleland’s Evening-primrose (Oenothera clelandii) is primarily a biennial forb that produces spikes of yellow flowers between the months of July and August in Ontario. The species may be a relic of the tallgrass prairie communities that once extended from the U.S. Midwest into Ontario and that comprised a large portion of the southern Ontario landscape following the retreat of the Laurentide Ice Sheet.

The Canadian population of Cleland’s Evening-primrose is restricted to the Carolinian Zone in southwestern Ontario, where it is part of a nationally significant suite of species of conservation concern at the northern edge of their ranges.

Aboriginal (Indigenous) knowledge

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

Distribution

The range of Cleland’s Evening-primrose reaches its northern extent in Canada, where it has been documented in four subpopulations within the Carolinian life zone of southwestern Ontario. In the United States, the species’ distribution extends from Wisconsin east to New Jersey and south to Arkansas.

Habitat

Cleland’s Evening-primrose grows on dry sandy soils in fields, prairies, sand barrens, and savannahs, as well as along roadsides and rail corridors. Within these habitats, the species often favours full sunlight and areas where soil disturbance has resulted in a patchy cover of bare substrates and herbaceous vegetation. In Ontario, the species is known from dry tallgrass prairie habitats and open sandy areas along rail corridors and roads. Throughout much of its range, the quality of its habitat is generally in decline due to competition from invasive species and the lack of natural disturbance, including fire suppression.

Biology

Cleland’s Evening-primrose is a mainly biennial forb that reproduces by seed. Most plants require a two-year life cycle and produce a basal rosette of leaves in the first year, followed by a flowering stem and seeds in the second year. Plants are self-compatible and capable of self-fertilization. The night-flowering blooms, which include a relatively long floral tube, are pollinated by long-tongued insects, including sphinx moths (Sphingidae) as well as other moths, bees, butterflies, flies, and hummingbirds. Evening‑primrose seed can remain viable for up to several decades and may remain dormant in the seed bank until soil disturbance occurs and creates favourable conditions for the species to germinate.

Population sizes and trends

All known subpopulations in Canada appear to be possibly extirpated, based on targeted surveys at the sites of all four subpopulations in 2022. A stem count estimate was not provided for the Jaffa subpopulation in 1975, but the species was noted to be sparse and rare at that time, and the subpopulation is assumed to have been composed of only a small number of mature individuals. Plants in the Mississauga subpopulation were noted to be uncommon, with approximately 30 individuals in flower in 1985. The London subpopulation contained only a few plants in 1993. All three of these subpopulations have undergone substantial habitat changes due to the proliferation of invasive species. The Komoka subpopulation contained as many as 200 to 300 plants in a localized area in the 1990s, and the number of plants was described as abundant in the early 2000s. The last observation of the species in Canada was at the Komoka site in 2001, prior to a development that likely extirpated or severely reduced that subpopulation. According to the estimated stem counts in each subpopulation, unsuccessful search efforts in the intervening years, and changes that have occurred at each site, it is likely that all mature plants disappeared prior to 2012. In the absence of mature plants, the continued persistence of the species in all subpopulations now relies on the germination of seed bank material, if present.

Threats and limiting factors

The main threats to the species include the alteration of soil disturbance and fire regimes and the establishment of competitive vegetation, including invasive species, which have resulted in habitat degradation. Residential development is also a main threat to one subpopulation. The increased competition from invasive species noted in all four subpopulations is largely the result of fire suppression efforts, which threaten the persistence of dry prairie habitats. Ongoing site development continues to threaten the rail corridor sites in London and Mississauga.

Protection, status, and recovery activities

Currently, Cleland’s Evening-primrose has no legal protection in Canada. It is protected in the state of Ohio, where it is listed as Endangered. It is ranked S1 (Critically Imperiled) in Arkansas and S2 (Imperiled) in Missouri. The Global NatureServe conservation rank for the species is G3G5 (Vulnerable to Secure). It is ranked N1 (Critically Imperiled) in Canada, S1 in Ontario, and Unranked in the United States at the national level. Three of the four documented Canadian subpopulations are located on private land, including one on conservation authority land. The fourth site is on land owned by the Government of Ontario.

Technical summary

Oenothera clelandii

Cleland’s Evening-primrose

Onagre de Cleland

Range of occurrence in Canada: Ontario

Demographic information:

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

12 to 22 years

Based on the plant being biennial. The lifespan of the seed bank is unknown, but it is expected to persist for decades. The three-generation period is estimated at 36 to 66 years.

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

Yes

No plants were observed at any of the sites of the four subpopulations during the 2022 surveys. A decline has been observed, based on a comparison of 2022 search results with historical specimens and sight records.

[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

It is estimated that the four known subpopulations of the species have been extirpated, based on surveys conducted in 2022.

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

It is estimated that the four known subpopulations of the species have been extirpated, based on surveys conducted in 2022.

[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]

100% reduction over 47 years (1975 to 2022)

The Jaffa and Mississauga subpopulations are suspected to have disappeared prior to 2012, as the suitable habitat is very limited and the species faced long-established competition. It is estimated that the Komoka and London subpopulations may have disappeared prior to 2012, due to golf course construction and a small number of plants, respectively.

[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

Surveys conducted in 2022 indicate that the species may already be extirpated from all four known subpopulation sites or may only be persisting in a seed bank.

[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)

100% reduction

The Komoka subpopulation, which represents the only occurrence with a sizeable number of individuals (200 to 300 in the 1990s, species described as “abundant” in 2001), was apparently lost to development in the early 2000s. If a seed bank persists, seed may remain viable for decades, but seed viability is likely constantly declining.

Are the causes of the decline clearly reversible?

Yes

The restoration of periodically disturbed dry prairie near the subpopulations could reverse the decline, if a viable seed bank or re-introduced plant material were present.

Are the causes of the decline clearly understood?

Yes

The construction of a golf course and/or the cessation of soil disturbance at the Komoka site led to the decline of that subpopulation. The three other subpopulations were vulnerable due to low numbers, small areas of suitable habitat, cessation of disturbance, and habitat changes caused by native and non-native species.

Are the causes of the decline clearly ceased?

No

Suitable habitat continues to decline in quality and size.

Are there extreme fluctuations in number of mature individuals

Unknown

Based on limited observation data (nine observations in four subpopulations over 27 years), it is unknown whether extreme fluctuations in the number of mature individuals have occurred.

Extent and occupancy information:

Estimated extent of occurrence (EOO)

2,775 km² if the range of historical subpopulations is used.

This value was calculated by Natural Heritage Information Centre (NHIC) staff by creating a minimum convex polygon that includes the four historical subpopulations, using available locational data. The EOO value includes observations spanning from 1975 to 2001.

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

16 km² if the range of historical subpopulations is used.

Each of the four subpopulations fits within a 2 x 2 km grid square. The IAO value includes observations spanning from 1975 to 2001.

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

Cleland’s Evening-primrose is self-compatible and autogamous, suggesting that a very small subpopulation can be viable for reproduction. The distance between known Canadian subpopulations ranges from 20 km to 150 km, and the seeds are not wind dispersed.

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

0-4

The four subpopulations are geographically distinct and are separated by unsuitable habitat. All four subpopulations face a similar main threat (Natural System Modifications); however, this threat likely occurs independently in each subpopulation.

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

Yes

Observed, based on surveys conducted in 2022 that did not document any extant plants; however, a seed bank may be present and declining.

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

Yes

Inferred, based on the apparently unsuitable conditions in the vicinity of the known subpopulations and the observed prevalence of invasive species, which have resulted, continue to result, or likely will result, in a decline in the area of occupancy.

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

Yes

Observed, based on the presence of plants in four subpopulations between 1975 and 2001 and the absence of any plants in these same subpopulations during the 2022 surveys.

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

Yes

Observed, based on the presence of plants at the four subpopulation sites between 1975 and 2001 and the absence of any plants at the same sites during the 2022 surveys.

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

Yes

Observed decline in area, extent and quality of habitat in all subpopulations based on the construction of a golf course, planting of a conifer plantation, and spread of invasive and other species, namely Dog-strangling Vine, Staghorn Sumac, and Trembling Aspen.

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of Mature Individuals (by subpopulation):

Jaffa 0

Komoka 0

London 0

Mississauga 0

Total 0

Based on NRSI field surveys (2022)

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 Done

Analysis not conducted

Threats:

Was a threats calculator completed for this species?

Yes (see Appendix 2)

Overall assigned threat impact: Very high to High (2023)

Key threats were identified as:

1. other ecosystem modifications (7.3) (very high to high impact)
2. housing and urban areas (1.1) (very high to medium impact)
3. tourism and recreation areas (1.3) (high to medium impact)
4. agricultural and forestry effluents (9.3) (medium to low impact)
5. fire and fire suppression (7.1) (low impact)
6. problematic native species/diseases (8.2) (unknown impact)

What limiting factors are relevant?

• Potential declines in pollinators
• Absence of natural disturbances at sites of subpopulations contributes to reduced seed germination
• Periods of drought may be required to maintain suitable habitat

Rescue effect (from outside Canada):

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

S1 (Critically Imperiled) in Ohio, S4S5 (Apparently Secure) in Michigan, SNR (Not Ranked) in New York

may be historical in New York. Known from a single subpopulation in Ohio. In Michigan, restricted to the southern portions of the Lower Peninsula, but more frequent in the west.

Is immigration known or possible?

Possible but unlikely

The nearest known U.S. subpopulations are over 50 km away from Windsor (Essex County) and separated from Canada by unsuitable habitat. Seed ingested by birds may result in long-range dispersal. The seed is not wind dispersed.

Would immigrants be adapted to survive in Canada?

Yes

The species is known from subpopulations in the U.S. located at the same latitude as Manitoulin Island. Soil pH and moisture preferences may differ between subpopulations.

Is there sufficient habitat for immigrants in Canada?

Yes but limited

There is limited habitat for immigrants in Canada. Many of the U.S. subpopulations occur in sand prairies or sand barrens, habitat types that are very rare and geographically restricted in Ontario. Rail corridors and sandy road edges are present throughout southwestern Ontario.

Are conditions deteriorating in Canada?

Yes

Suitable non-anthropogenic habitats (for example, sand barrens and dry prairie ecosystems) are deteriorating in Canada due to development, fire suppression, and establishment of invasive species. Anthropogenically maintained habitats (for example, railroad and road corridors) are likely maintaining suitable habitat for the species.

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

Unknown

Sand barren and dry prairie habitats are threatened in the U.S. in a similar way to those in Canada, but conservation efforts through sand barren and prairie habitat stewardship are common throughout the U.S.

Is the Canadian population considered to be a sink?

No

The Canadian population would be able to sustain itself regardless of its isolation from U.S. subpopulations.

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

No

Very low probability of immigrants from U.S. states, as nearest source is very small and >50 km from Canadian border with intervening unsuitable habitat, and suitable habitat may no longer be available in Canada

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 Endangered in December 2023.

Status and reasons for designation:

Status

Endangered

Alpha-numeric codes

A2ace; B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v); C2a(i); D1

Reason for change in status

Not applicable

Reasons for designation

This relic of dry tallgrass prairie communities occurs from mid-western United States to Ontario. In Canada, this prairie wildflower is known from four isolated sites in southwestern Ontario. While no plants have been seen since 2001, it may still be present within the seedbank. Its decline in Canada is thought to be due to habitat loss and degradation through development, fire suppression, and competition with both exotic and native terrestrial plants.

Applicability of criteria

A: Decline in total number of mature individuals

Meets Endangered, A2ace.

There is an observed 100% decline in the number of mature individuals over the past three generations (66 years), based on direct observations; a decline in the index of area of occupancy, extent of occurrence, and quality of habitat; and the effects of introduced taxa and competitors. The causes of the reduction are understood and may be reversible, but have not ceased.

B: Small range and decline or fluctuation

Meets Endangered, B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v).

Assuming it persists as seed banks at some or all historical localities, the EOO and IAO are at most 2,275 km² and 16 km², respectively, and the number of locations is at most 4. The species is experiencing an observed continuing decline in (i) the extent of occurrence; (ii) index of area of occupancy; (iii) area, extent and quality of habitat; (iv) number of locations or subpopulations; and (v) the number of mature individuals.

C: Small and declining number of mature individuals

Meets Endangered, C2a(i).

There is an observed decline of 100% in the number of mature individuals, and there is a projected continuing decline in the seed bank. Number of mature individuals is 0, with fewer than 50 mature individuals in any single subpopulation.

D: Very small or restricted population

Meets Endangered D1.

Number of mature individuals is fewer than 250.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2019)

Wildlife species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special concern (SC)
(Note: Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.)

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at risk (NAR)
(Note: Formerly described as “Not In Any Category”, or “No Designation Required.”)

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data deficient (DD)
(Note: Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” [insufficient scientific information on which to base a designation] prior to 1994. Definition of the [DD] category revised in 2006.)

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

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

Wildlife species description and significance

Name and classification

Current classification: Oenothera clelandii W. Dietrich, P.H. Raven and W.L. Wagner

Class: Equisetopsida

Order: Myrtales

Family: Onagraceae

Section: Oenothera, subsection Candela

Genus: Oenothera

Species: Oenothera clelandii

Common names: Brouillet et al. 2022, NatureServe 2022

English: Cleland’s Evening-primrose, Lesser Four-point Evening-primrose

French: Onagre de Cleland

Indigenous: None known

Synonyms and notes: Prior to 1983, Cleland’s Evening-primrose was included within Four-point Evening-primrose (O. rhombipetala), and was split off as a distinct species on account of its smaller flowers and its stigma being at the same level as the anthers (Wagner 1983). Cleland’s Evening-primrose is self-compatible, while Four-point Evening-primrose is not.

Four-point Evening-primrose is primarily a central plains species that has scattered localities in the United States Midwest to Illinois, Michigan, and Wisconsin, and barely enters the easternmost parts of Colorado and New Mexico (Wagner 2021). It is not known or reported in Canada (Brouillet et al. 2022).

Description of wildlife species

Cleland’s Evening-primrose is a biennial herbaceous plant in the Onagraceae family. Its stem reaches 20 to 100 cm in height and includes one or more dense spikes of yellow flowers that first develop from the apex of the spike and gradually open toward the base of the spike (Wagner 2021; cover photo). Each flower includes four petals that are usually pointed and a long floral tube (Hilty 2020). In Ontario, the species flowers in July and August, with the flowers open through the night and closed during the midday (vespertine), typically wilting within one to several days (Hall et al. 1988; Hilty 2020; Wagner 2021; Reznicek pers. comm. 2022). The seed capsules are slender and short in comparison to similar evening-primrose species that occur in southern Ontario (Deacon pers. obs. 2022).

Designatable units

The Canadian population of Cleland’s Evening-primrose represents one designatable unit entirely contained within the Great Lakes Plains Ecological Area (COSEWIC 2018). Although the subpopulations are separated from one another by as much as 150 km, they occupy similar habitats within a region that once contained extensive occurrences of these habitat types. There is no evidence that this species should be split into further designatable units.

Special significance

The Canadian population of Cleland’s Evening-primrose is restricted to the Carolinian Zone in southwestern Ontario, where it is part of a nationally significant suite of species of conservation concern at the northern edge of their ranges. As a species often associated with remnant sand barren and dry prairie habitats, the presence of Cleland’s Evening-primrose is representative of southern Ontario’s rich natural heritage and particularly of tallgrass communities that were largely lost due to land conversion beginning in the 1800s.

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, Cleland’s Evening-primrose is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

Cleland’s Evening-primrose is known from the U.S. Midwest and southern Canada. Its core range includes Michigan, Illinois, Wisconsin, and Minnesota, with subpopulations as far south as Arkansas and Kentucky (Figure 1). There is a single extant subpopulation in Ohio (Gardner pers. comm. 2022; Wagner pers. comm. 2022). The species is considered exotic (SNA) in New Jersey and South Carolina (NatureServe 2022). Subpopulations in eastern New York are believed to be native (Wagner pers. comm. 2022), but only historical collections are known, with the most recent collections occurring in the 1940s (Northern Great Plains Herbaria 2022). The New York Flora Atlas (Werier et al. 2023) lists Cleland’s Evening-primrose as non-native in New York. Subpopulations near Duluth, Minnesota and the northern Lower Peninsula of Michigan represent the northernmost extent of the range (Figure 1). Reports from West Virginia and Maryland are not considered to represent established subpopulations (Frye pers. comm. 2022; Knapp pers. comm. 2022).

Figure 1. Distribution of Cleland’s Evening-primrose in North America. Adapted from Kartesz (2022). Dark green indicates presence of the species in that state or province. Light green indicates U.S. counties where the species is not rare for the state. Yellow indicates counties where the species is rare for the state. Teal indicates that the species is adventive in the state. The approximate localities of Canadian subpopulations, where the species is considered native (Brouillet et al. 2010; NatureServe 2022), are shown with red dots.

Canadian range

The Canadian population of Cleland’s Evening-primrose is restricted to the Carolinian life zone of southwestern Ontario. The proportion of the global range and population of Cleland’s Evening-primrose that occurs in Canada is estimated to be less than 1%.

Four subpopulations represent the northern limit of the species’ range in Canada. Two subpopulations are situated in Middlesex County within the City of London and in the vicinity of Komoka. The Komoka site represents the western-most subpopulation in Canada. A subpopulation in the Regional Municipality of Peel, in the Lorne Park neighbourhood of Mississauga, represents the northern and eastern-most subpopulation. A subpopulation near Jaffa in Elgin County represents the southern-most subpopulation (Figure 2). The City of London subpopulation, first documented in 1993, is the most recently discovered subpopulation. No new observations of the species have been made in Canada for 30 years and no plants have been observed at the known sites for more than 20 years. The species was last observed in Canada in 2001 at Komoka (Appendix 1).

Figure 2. Distribution of Cleland’s Evening-primrose historical subpopulations in Ontario, Canada (map produced by Alain Filion, COSEWIC Secretariat, August 2023).

The Canadian distribution of Cleland’s Evening-primrose may have been more widespread prior to European settlement, as surveyor notes and remnant habitats suggest the historical presence of larger dry prairie and sand barren communities between Windsor east toward the Rice Lake Plains and extending north toward Barrie (Bakowsky and Riley 1994). The western-most Canadian subpopulation is more than 150 km from the nearest known extant U.S. subpopulations, with agricultural and urban land use fragmenting the natural cover throughout southern Ontario and southern Michigan.

Although all four subpopulations occur in tallgrass prairie remnants or scattered indicators of this habitat type, all of the sites are associated with anthropogenic disturbance. Both the London and Mississauga subpopulations occur in the immediate vicinity of a rail corridor, where plant material may have potentially been introduced by rail cars or machinery used to maintain the rail corridor. The Jaffa site, purchased by the Catfish Creek Conservation Authority in 1963, previously consisted of agricultural fields and a forest that was harvested in the early 1900s and was planted with conifer trees in the 1980s (Catfish Creek Conservation Authority 2021). The site of the Komoka subpopulation operated as an aggregate extraction pit until the late 1990s (Buck pers. comm. 2022), and equipment and trucks operating in the pit may have moved plant materials. The Ontario subpopulations of Cleland’s Evening-primrose are situated between native occurrences to the west and a combination of native and adventive occurrences to the east (see Global range). Given that the core distribution of the species is surrounded by isolated native subpopulations in Iowa, Missouri, Arkansas, Illinois, Indiana, and Kentucky (Kartesz 2022), the southern Ontario subpopulations similarly reflect an eastern extension beyond the core range. The species is treated as native in Ontario (Brouillet et al. 2010; NatureServe 2022).

The localities of all four historical subpopulations were surveyed in 2022 during the expected peak bloom period, based on flower phenology notes from a subset of the nine Ontario observation records and the bloom period observed in southern Michigan. In 2022, southwestern Ontario sustained drought-like conditions throughout the late spring and summer (Deacon pers. obs. 2022), which may have affected plant vigour and the results of the surveys. A total of 21 hours of search effort occurred over the course of five days. In 2022, the report writer also spent time conducting vascular plant surveys at several sites that could be considered suitable habitat for the species in Essex, Lambton, Middlesex, Elgin, Norfolk, and Northumberland counties.

A number of other areas that may provide suitable habitat for the species are generally considered to be well-botanized (Bakowsky pers. comm. 2022). These sites include eastern Ontario (Catling and Catling 1993), Ojibway Prairie, Walpole Island, Port Franks, and the Norfolk Sand Plain. Other areas that may have received less search effort include private land in the vicinity of Kingsville and Wasaga Beach and in the Bothwell Sand Plain and the Rice Lake Plains. Small prairie remnants along rail lines near Komoka and Mount Brydges may also have potential habitat to support Cleland’s Evening-primrose (Buck pers. comm. 2022), and other sections of Ontario rail corridor networks may contain habitat that has not been thoroughly searched.

Population structure

The Canadian population of Cleland’s Evening-primrose is limited to a few isolated sites in southwestern Ontario, due mainly to historical land clearing and habitat fragmentation. Owing to the geographic separation of the sites, which ranges from 20 km to 150 km, and the very limited opportunity for genetic exchange, each site is considered a unique subpopulation.

There is limited opportunity for genetic mixing between the subpopulations in Canada and the U.S., as they are separated by large areas of land that are unsuitable for the species, as well as bodies of water that include lakes Erie, St. Clair, and Huron, and the Detroit and St. Clair rivers. Cleland’s Evening-primrose is not commercially sold as seed or live plants in the nursery trade, and introduced subpopulations are not known to occur in Canada.

When it was recognized as a species, Cleland’s Evening-primrose was described as a complex structural heterozygote^{Footnote 1} with a ring of 14 chromosomes formed at meiotic metaphase^{Footnote 2} and with the pollen only about 50% fertile (Wagner 1983). Genetic barcoding information has been collected from herbarium specimens from the Komoka, Mississauga, and Jaffa subpopulations (EMBL-EBI 2022; IBOL 2022). To date, no research has been conducted to examine genetic discontinuities.

Extent of occurrence and area of occupancy

The extent of occurrence (EOO) for all known extant Canadian subpopulations of Cleland’s Evening-primrose is 2,775 km², calculated using a minimum convex polygon for all historical occurrences. The index of area of occupancy (IAO), which includes all historical subpopulations, is 16 km² (Figure 2).

The EOO and IAO were calculated by plotting the historical observations and element occurrences (EO; NHIC 2022a,b) spanning 27 years (1975 to 2001) on a 2 x 2 km grid using the Canada Albers Equal Area Conic projection. EOO is the total area covered by the minimum convex polygon connecting the peripheral subpopulations within the species’ range in Canada (Figure 2). The IAO is calculated by adding the area of any grid square that is overlapped by an extant or historical occurrence. The determination of the IAO assumes that the locational data provided for each extant occurrence are accurate.

As no prior EOO and IAO assessment was completed, a comparison has not been made between the historical and present-day spatial data.

Fluctuations and trends in distribution

As no plants have been found since 2001, the EOO and IAO are both considered to be 0 km². This represents a decline of 2,775 km² in the EOO as shown on Figure 2, and of 16 km² in the IAO, based on the inclusion of all four subpopulations. Although the available data do not allow for the assessment of fluctuations and trends in distribution, they do suggest a significant decline in the area of occupancy over the last five decades, leading up to the present situation, where the species’ presence in Canada is severely diminished and the species is possibly extirpated.

Biology and habitat use

Life cycle and reproduction

Cleland’s Evening-primrose is short lived, with most plants maturing, producing seed, and then dying within two years (Wagner 2021). Although most form a basal rosette of leaves in the first year, followed by a flowering stem in the second year, some plants will flower and set seed within one year (Dietrich et al. 1997; Hilty 2020). IUCN guidelines (2022) state “for plants with seed banks, use juvenile period + either the half-life of seeds in the seed bank or the median time to germination, whichever is known more precisely.” Although there are no studies documenting seed bank longevity in Cleland’s Evening-primrose, the seeds of other species of evening-primroses may remain viable in the seed bank for a number of decades, with Biennial Evening-primrose, for example, having seed that is viable after being buried for 80 years (Telewski and Zeevaart 2002). The generation time, based on the plant being biennial and the lifespan of the seed bank, is estimated to be from 12 to 22 years. Three generations were estimated at 36 to 66 years.

The species is self-compatible, which may provide a genetic advantage by allowing it to avoid the challenges of outcrossing in ecologically marginal areas (Dietrich and Wagner 1988). Self-pollination is favoured, as the pollen begins to shed directly onto the stigma within the convolute petals while the flower is in bud (Dietrich and Wagner 1988; Hall et al. 1988).

Animal-mediated pollination of the species’ elongated vespertine flowers is mainly carried out by long-tongued nocturnal insects, particularly sphinx moths (Sphingidae) (Hilty 2020). Up to several flowers open each day throughout the July–August bloom period and wilt within one to several days (Hall et al. 1988; Deacon pers. obs. 2022), depending on weather conditions. As the species is not wind pollinated and the Canadian subpopulations are separated by distances of 20 km or more, cross pollination between subpopulations is unlikely to occur. Other common species, including Biennial Evening-primrose (Oenothera biennis) and Small-flowered Evening-primrose (O. parviflora), are present in some of the subpopulations; however, hybrids of these species with Cleland’s Evening-primrose have not been reported. Cleland’s Evening-primrose is easily distinguished from these species by its sharply pointed petals (Wagner 1983; Reznicek et al. 2011; Hilty 2020). Fruiting plants differ by having relatively narrow and short seed capsules, which are approximately half the size of those of similar species (Deacon pers. obs. 2022). The rosettes of Cleland’s Evening-primrose (Figure 3) can be differentiated from those of similar species in Ontario by the moderately to deeply divided and almost pinnatifid leaves, as opposed to the coarsely undulate-toothed, or entire along the leaf edge, leaves of other species (Reznicek pers. comm. 2022). Although this difference in leaf morphology can assist in the identification of Cleland’s Evening-primrose, an experienced botanist with no previous knowledge of the species could easily misidentify the basal rosette as one of the more common species in Ontario. All four Canadian subpopulations included flowering plants at the time of the last observation. See Appendix 1.

Figure 3. First-year basal rosette leaves of Cleland’s Evening-primrose in Toledo, Ohio. Photograph by P. Deacon

Reproduction occurs exclusively by seed (Wagner 1983). Despite their affinity for dry soil, evening-primrose plants have an elevated demand for water when a flowering stem is developing, and sufficient soil moisture is also necessary for seed germination (Greiner and Köhl 2014). Typically, dozens of capsules on a single plant will produce thousands of seeds, which mature through the late fall and scatter once the capsules dehisce (Hall et al. 1988; Deacon pers. ob. 2022). The total number of seeds produced by a single Biennial Evening-primrose capsule can range from 180 to 500 (Hall et al. 1988; Dietrich et al. 1997) with an entire plant producing approximately 25,000 to 120,000 seeds (Ridley 1930; Stevens 1932; Gross 1980). Despite having smaller capsules, a Cleland’s Evening-primrose plant is inferred to produce many thousands of seeds as well. Viable evening-primrose seeds germinate readily at or near the surface of the soil and in full sun, after a brief period of cold-moist stratification (Greiner and Köhl 2014; Deacon pers. obs. 2022). Seeds of other species in the genus are known to remain viable for an extended period of time, with successful germination from herbarium specimens occurring up to several decades after the collection date (Wagner pers. comm. 2022). The long-term seed viability of Cleland’s Evening-primrose has not been studied. A long-term seed viability experiment that began in 1879 included Biennial Evening-primrose seed stored in moderately moist sand below grade, and germination of that species was observed until 1960, 80 years after the seed was collected and stored (Telewski and Zeevaart 2002). The longevity of evening-primrose seed has implications for the potential persistence and viability of Cleland’s Evening-primrose occurrences in Canada, which may not have produced mature individuals in recent years or decades.

Habitat requirements

The Canadian subpopulations of Cleland’s Evening-primrose occur in a range of relatively dry, open habitats. The common habitat characteristics of the various communities that support the species include well-drained soil, periodic disturbance of substrates, and full sun conditions.

The species is found in natural and anthropogenic habitats throughout its range, including sand prairies, sand dunes, upland sandy savannahs, abandoned sandy fields, sandy paths and their verges, railroad beds, and embankments (Reznicek et al. 2011; Hilty 2020; Wagner 2021; Oldham pers. comm. 2022). The sites of each of the Canadian subpopulations support, or previously supported, tallgrass prairie flora. Dry prairie communities historically occurred at various localities in southern Ontario (Bakowsky and Riley 1994), and some remnants persist today at a handful of small, isolated sites. Fire, which is an important component of tallgrass prairie ecosystems, was once more common due to First Nations land use or practices, lightning strikes, and lack of fire suppression. In recent years, the ignition of fires along rail corridors and the management of vegetation through grubbing (clearing of trees and shrubs to their stumps) have helped to maintain tallgrass prairie sites in southern Ontario (Bakowsky pers. comm. 2022; Buck pers. comm. 2022). A more detailed description of the current habitat conditions at the site of each subpopulation is provided in Appendix 1.

The report writer visited two Cleland’s Evening-primrose sites in Michigan and Ohio where the habitat is described as sand barren. Associate species at these sites included Spotted Beebalm (Monarda punctata), Virginia Goat’s-rue (Tephrosia virginiana), Rough Blazing-star (Liatris aspera), Hop Flatsedge (Cyperus lupulinus), Eastern Prickly-pear (Opuntia cespitosa), Round-headed Bush-clover (Lespedeza capitata), Showy Goldenrod (Solidago speciosa), Partridge Pea (Chamaecrista fasciculata), Arrowfeather Threeawn Grass (Aristida purpurascens), Fall Witchgrass (Digitaria cognata), and Sweet-fern (Comptonia peregrina). These sites varied from 0% to 10% canopy cover, with herbaceous groundcover ranging from an estimated 25% to 50%, interspersed with patches of bare sandy soil. The remaining intact prairie habitat at the Komoka site differs from the Michigan and Ohio habitats in the nature of its substrate, which is gravelly rather than the much sandier soil at the U.S. sites. The habitat at Komoka is composed of dry ridges dominated by Little Bluestem (Schizachyrium scoparium), with Big Bluestem (Andropogon gerardii) scattered throughout. Among the grass cover, forbs include prairie species such as Flowering Spurge (Euphorbia corollata), Canada Tick-trefoil (Desmodium canadense), Butterfly Milkweed (Asclepias tuberosa), Smooth Aster (Symphyotrichum laeve), Round-headed Bush-clover (Lespedeza capitata), and small numbers of introduced species such as Spotted Knapweed (Centaurea stoebe) and White Sweet-clover (Melilotus albus). The dry ridges are interspersed with wet depressions containing Smooth Twig-rush (Cladium mariscoides) and Ohio Goldenrod (Solidago ohioensis), which do not constitute suitable habitat for the species. This difference in substrates may be a reflection of the history of the Komoka site being used for aggregate extraction, which may have altered the soil profile.

Two of the Cleland’s Evening-primrose subpopulations in Canada included plants growing in anthropogenic habitats, reflecting the species’ preference for light soil disturbance. Habitat notes associated with herbarium specimens include “sandy, gravelly open ground in abandoned gravel pit,” “railway gravel of abandoned RR (railroad) yard,” and “waste ground adjacent to railway tracks.” These sites act as surrogate habitat for the species, in that the substrates are well drained and the vegetation cover is generally limited, as they would be in sand barren or dry prairie habitat. From a habitat quality perspective, these anthropogenic sites are not optimal, but can provide the disturbance regime required to maintain suitable conditions for the species.

Movements, migration, and dispersal

The seed produced by Cleland’s Evening-primrose does not easily disperse across large distances by natural means. Birds such as American Goldfinch (Spinus tristis) feed on evening-primrose seed (Hilty 2020; Deacon pers. obs. 2022), and their excretion of intact seeds could make long-range dispersal possible (Hall et al. 1988). However, the potential for dispersal of the species’ seed to other suitable habitats in a highly fragmented landscape of varying soil types is very low. Material that adheres to vehicle tires or machinery may also facilitate the movement of seed (Hall et al. 1988). Most of the seed produced by a plant will fall onto the ground in its immediate vicinity, with germination likely limited by a suitable substrate, soil moisture, and sunlight (Greiner and Köhl 2014). It is estimated that plants could migrate as far as 5 m in one generation by seeds expelling from the capsules when struck by other vegetation stems or passing mammals (Deacon pers. obs. 2022). Within the present-day southern Ontario landscape, the combination of agricultural fields and large areas of unsuitable clay-rich soil would act as barriers to the broad-scale movement of Cleland’s Evening-primrose.

Some of the southern and eastern populations in the U.S. (for example, Arkansas, New York, New Jersey, and South Carolina) are thought to have been introduced (Wagner 1983; Bradley pers. comm. 2022; Kartesz 2022; Snyder pers. comm. 2022; Ring pers. comm. 2023; Werier 2023). Two of the Canadian subpopulations are located along the Great Western Railway line, which is now operated by the Canadian National Railway Company (CN). This rail line was constructed in the 1850s (Zadro and Delamere 2009). There are varying opinions on the likelihood of certain U.S. Midwest prairie species arriving in Ontario on rail cars or livestock. The London and Mississauga subpopulations, although located adjacent to rail lines, are associated with areas that contain a diverse assemblage of native tallgrass prairie species, which suggests that these plants may be a naturally occurring part of a relict prairie (Bakowsky pers. comm. 2022; Buck pers. comm. 2022). Despite the presence of large numbers of Big Bluestem, Yellow Indiangrass (Sorghastrum nutans) and Butterfly Milkweed at the site of the London subpopulation, the plants observed there were in a very disturbed portion of the gravel rail yard, and it is possible that they were introduced via rail cars (Oldham pers. comm. 2022).

Interspecific interactions

Predators and competitors:

During the 2022 surveys, a small number of Biennial Evening-primrose plants in the Komoka and London subpopulations showed evidence of deer browsing, with severed flower spikes and regrowth evident (Deacon pers. obs. 2022). It is likely that Cleland’s Evening-primrose would face similar browse pressure.

Other interactions:

Members of the genus Oenothera are pollinated by sphinx moths and may also be visited by various other species of moths, bees (Halictidae, Anthophoridae, and Bombus spp.), butterflies, syrphid flies, and hummingbirds (Trochilidae) (Gregory 1963). The larval stage of Primrose Moth (Schinia florida) feeds on evening-primrose and adults were observed on other evening-primrose species during the 2022 surveys (Deacon pers. obs. 2022).

Physiological, behavioural, and other adaptations

Cleland’s Evening-primrose employs several adaptations that allow it to persist under adverse conditions. The species is adapted to anthropogenic settings, as evidenced by U.S. subpopulations that thrive along rail lines and laneways, and at aggregate extraction sites. The apparent long-term viability of evening-primrose seed is another physiological adaptation that allows seed to stay dormant in the soil for prolonged periods of time, with germination occurring when conditions are suitable. The sequential blooming of a small proportion of the total number of flowers over a number of weeks or months facilitates pollination occurring over an extended period of times, which subjects the plant to a variety of weather conditions and potential pollinator species. By blooming in the evening and through the night, the flowers are protected from hot and dry daytime conditions and are favoured by nocturnal insects when other forbs may not be available for nectaring.

Limiting factors

Soil disturbance regime

Cleland’s Evening-primrose appears to benefit from substrate disturbance. At the sites of the Canadian subpopulations, periodic anthropogenic soil disturbance has included aggregate extraction, vegetation management along a rail corridor and in a rail yard, and potentially grading and mowing along a road margin. Historically, fires were set intentionally or occurred as a result of lightning strikes or rail car sparks, and would have reduced the vegetation cover and exposed the mineral soil. Drought and the movement of soil particles by wind may also have maintained the dynamic substrate favoured by Cleland’s Evening-primrose. The prolonged absence of soil disturbance at the sites of the Canadian subpopulations has likely made these habitats less suitable for recruitment.

Pollination

In addition to being able to self-pollinate (see Life cycle and reproduction), Cleland’s Evening-primrose is a night-flowering species that is cross-pollinated by long-tongued moths, including but not limited to sphinx moths (see Interspecific interactions). It is inferred that the urban setting surrounding the London and Mississauga subpopulations would not deter the presence of sphinx moths. Since Cleland’s Evening-primrose is a biennial, the failure to have mature plants pollinated in a given year could result in rapid and potentially detrimental declines in subpopulation numbers.

Population size and trends

Data sources, methodologies, and uncertainties

The Natural Heritage Information Centre provided observation data for three of the four subpopulations (NHIC 2022a,b). Information pertaining to a fourth subpopulation at Jaffa (Elgin County) was obtained through a re-determination of a Small-flowered Evening-primrose specimen collected by W.G. Stewart, which Brunton later determined to be Cleland’s Evening-primrose. Additional details relating to this collection and subsequent search efforts were provided by Brinker (pers. comm. 2022) and Oldham (pers. comm. 2022). Examination of a digital scan of the specimen (Doubt and Blades 2022) provided coarse locational data and a general description of the habitat and abundance at the time of the 1975 observation. On the basis of the limited number of Canadian observations, only two of which provided coarse estimates of stem counts, it is not possible to determine the historical abundance and trends that occurred from 1975 onward.

Abundance

The surveys conducted in 2022 (see Appendix 1) did not locate any Cleland’s Evening-primrose specimens, including basal rosettes and flowering stems, at the sites of the four subpopulations. However, since the active rail line areas associated with the London and Mississauga subpopulations were not searched, there is a possibility of plants being present among the rail ballast. All four subpopulations may have seed present in the soil where plants had been previously observed. Seed and first-year rosettes that are obscured by other vegetation or are poorly developed due to competition or drought could have been overlooked.

Fluctuations and trends

The last observation of the species in Canada was in 2001. Since then, all known subpopulations have been potentially lost as no individuals were detected during focused surveys in 2022. This represents a complete loss from as many as 200 to 300 plants at the Komoka site, at least 30 plants at the Mississauga site, and an inferred small number of plants at the London and Jaffa sites. The plants at both the London and Jaffa sites were only observed on one occasion, and stem count estimates were only obtained for the Komoka and Mississauga subpopulations on one occasion. All four subpopulations have undergone substantial habitat changes over the past 20 to 30 years.

Plants in the Komoka subpopulation were scattered across an area of approximately 15 ha in an abandoned aggregate pit when last observed in 2001 (Buck pers. comm. 2022). This habitat was contiguous with a prairie remnant in a nearby rail corridor that was also searched in 2022, but no plants have ever been documented from this prairie remnant. Although the golf course development in 2003 was designed in a manner that would preserve the highest quality habitat within the site (Buck pers. comm. 2022), the suitable habitat for Cleland’s Evening-primrose was reduced to approximately 5 ha of isolated patches separated by golf course fairways and cart paths.

The exact locality and area occupied by plants at the rail yard in London is unknown, but was presumably a small, localized area based on the herbarium label indicating “rare” and observer recollections of the site (Oldham pers. comm. 2022). During the 2022 survey of this subpopulation, approximately 6 ha of suitable habitat was found, including disturbed areas of sand and gravel, a prairie remnant, and a section of rail line and ballast that bisects the sandy site. The amount of suitable habitat observed in 2022 is likely reduced slightly from what was observed in 1993, as tree cover has increased and a pedestrian trail and a rail line overpass were constructed in 2017.

The plants in the Jaffa subpopulation were noted to occur in a grassy opening and be “sparse and rare” when observed in 1975. Although the subsequent search in 1989 documented species associated with dry prairies or sand barrens, the opening had been filled in with tree and shrub cover and is no longer evident on aerial imagery (Oldham pers. comm. 2022). The area occupied by the plants in 1975 is estimated to be less than 500 m², and is now part of a section planted with Red Pine (Pinus resinosa). Except for a mowed sandy embankment within the road right-of-way, no suitable habitat remained in 2022.

The 1984 and 1985 observations of plants in the Mississauga subpopulation occurred along the rail corridor, both in a relict prairie and on waste ground beside the rail line. On the basis of the very limited distribution of prairie grasses observed in 2022 and the absence of any discernable relict prairie, it is assumed that the relict prairie habitat observed in the 1980s was composed of one or several very small, linear patches of prairie along the sandy embankments that now exist as thicket. This habitat likely totalled less than 500 m² in the 1980s, and is now gone. The area occupied by the plants in 1985 on the waste ground adjacent to the rail line is unknown but is presumably also small and localized. At this locality, approximately 11 km of the rail corridor runs through a large sand deposit at the mouth of the Credit River (Hoffman and Richards 1953), and likely includes much of the area that could provide suitable conditions for the species. The area of waste ground is free of vegetation and is likely maintained on a regular basis as part of rail line maintenance.

A combination of factors likely contributed to the decline of these subpopulations, including development, the cessation of fire and soil disturbance, competition from invasive species, and the establishment of a conifer plantation, which altered site conditions to the extent that Cleland’s Evening-primrose could not persist.

Continuing decline^{Footnote 3} in the number of mature individuals:

No plants were observed at any of the sites of the four subpopulations during the 2022 surveys. A potentially severe decline has been observed, based on a comparison of the 2022 search results with historical herbarium specimens and sight records: Komoka: “ca. 200 to 300 plants, local” (1990), “abundant” (2001); London: “rare in railway gravel of abandoned RR yard” (1993); Jaffa: “sparse and rare” (1975); Mississauga: “uncommon, 30+/-” (1985).

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

According to the results of fieldwork conducted in 2022, it is suspected that the species may have been extirpated at the four known subpopulations. With no mature, seed-producing plants present in 2022, the only potential for the sustained presence of the species at the subpopulation sites is through seed bank germination. Since evening-primrose seed may remain viable in the seed bank for a number of decades (Telewski and Zeevaart 2002), there is the potential for the decline at a given subpopulation to be reversed if seed bank germination occurs.

Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years):

See text in section above.

Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify):

It is suspected that the small subpopulations at Jaffa and Mississauga disappeared prior to 2012, as the suitable habitat at these sites is very limited and subject to encroachment from competitive vegetation.

Aggregate extraction had ceased at the site of the Komoka subpopulation when the 200 to 300 plants were first observed in 1990, and the borrow pit may have been idle for a number of years before that. The 2001 observation noted plants to be abundant, and the pit remained idle until that time (Buck pers. comm. 2022). During this period, soil disturbance was presumably minimal and settling of the substrates was likely occurring. Given the abundance of plants noted in 2001, the conversion of a portion of the site to a golf course in 2003, and the absence of plants in 2022, it is inferred that this subpopulation declined during the early 2000s and had disappeared by 2012 or earlier.

The Jaffa subpopulation likely disappeared between 1975 and 1988, since the search conducted in 1989 did not locate any plants and the surveyors noted that the once open habitat had been almost entirely filled in with trees, including planted Red Pine, and shrubs (Oldham pers. comm. 2022). The small number of plants in 1975 would have made this site vulnerable to a stochastic event such as deer browsing on the last flowering plants or plant mortality due to drought.

As no targeted surveys were conducted at the site of the London subpopulation between 1993 and 2022, it is difficult to estimate when the plants disappeared from this site. The few plants observed in 1993 may have sustained a small subpopulation for the years that followed or may have declined rapidly due to plant mortality or failure to produce viable seed.

The plants at the Mississauga site likely disappeared between 1985 and 1999, as a search of the area in 2000 did not locate any plants (Varga pers. comm. 2022). In 2022, few prairie grasses remained and the relict prairie observed in 1984 and 1985 appeared to have succeeded to thicket. The 1985 observation by Oldham and Webber noted plants growing in waste ground adjacent to the railway tracks, which may have been a different portion of the site than the relict prairie. This waste ground remains, although the 2022 surveys did not observe any mature plants in flower along the rail corridor and, due to limited access, the rail ballast could not be inspected thoroughly for basal rosettes.

Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years):

Recent site surveys suggest that all four known subpopulations may already be extirpated. If a seed bank persists, the seed may remain viable for decades, but its viability would be constantly declining. In the event that some of these subpopulations remain extant, continued decline could be expected given the observed deterioration of suitable habitat from fire suppression, lack of soil disturbance, and encroachment of competitive vegetation. The likelihood of rescue from U.S. subpopulations is extremely low.

Extinction risk based on quantitative analysis:

No quantitative analysis was conducted.

Long-term trends:

On the basis of the limited number of Canadian observations, only two of which provided approximate stem counts, it is not possible to determine historical abundance and trends that occurred from 1975 onward.

Population fluctuations, including extreme fluctuations:

On the basis of limited observation data (nine observations at four sites in 27 years), it is unknown whether extreme fluctuations in the number of mature individuals have occurred.

Severe fragmentation

The four Cleland’s Evening-primrose subpopulations are separated by distances of 20 km to 150 km, in a highly fragmented landscape of agricultural land and urban areas. The amount and quality of suitable habitat has declined at all four sites since the most recent observations were made. Ongoing vegetation succession, the lack of soil disturbance, and conversion of portions of the sites to a golf course and a conifer plantation have all contributed to these declines. The persistence of localized sandy or gravelly soil at all four sites provides habitat that is likely large enough to support a viable population. Cleland’s Evening-primrose is self-compatible and autogamous, suggesting that a very small subpopulation can be viable for reproduction. Therefore, the criteria for severe fragmentation, as defined by COSEWIC, are not met.

Rescue effect

Rescue from subpopulations in states adjacent to southern Ontario, particularly Michigan and Ohio, is possible but unlikely as it would require long-distance seed dispersal to areas of suitable habitat. The nearest known U.S. subpopulations are more than 50 km from the Ontario border and are separated from Ontario by metropolitan areas, agricultural lands, and the St. Clair and Detroit rivers.

Southwestern Ontario still holds suitable habitats that would support potential immigrants from U.S. subpopulations, including scattered but rare sand barrens and dry prairies, and more common anthropogenic habitats such as rail and road corridors, and borrow pits. Some of the nearest U.S. subpopulations occur at sites where stewardship activities are maintaining sand barrens that support healthy subpopulations of Cleland’s Evening-primrose (for example, Pinckney Recreation Area, Oak Openings Region) (Reznicek pers. comm. 2022).

Threats

Cleland’s Evening-primrose is vulnerable to the cumulative effects of various threats, especially ecosystem modifications (that is, fire suppression, cessation of soil disturbance, and the proliferation of invasive species). The nature, scope, and severity of these threats are described in Appendix 2, 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 ~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 Cleland’s Evening-primrose is considered Very high to High. This finding is to be interpreted with caution, as it 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.

Current and projected future threats

Other ecosystem modifications (7.3, impact: very high – high)

Cleland’s Evening-primrose has an affinity for disturbed sandy soil, which may occur as a result of anthropogenic activities or a combination of natural landscape processes (fire, drought, wind erosion and deposition of sand, or soil scarification by large mammals). With no mature plants observed in 2022, the disturbance of substrates at all four sites may be critical in facilitating the germination of seed bank material. The lack of soil disturbance and fire disturbance enables succession to progress to taller, denser vegetation. This is the most widespread and urgent threat, and may be the cause of most of the observed declines. Succession is a widespread and urgent threat even in protected areas. In 2022, the sites of all four subpopulations had very limited to no substrate disturbance.

When the Komoka site was used for aggregate extraction, the conditions for Cleland’s Evening-primrose were maintained through equipment operation and the frequent disturbance of substrates. The dry prairie that is present to the south of the golf course suggests that well-drained and sparsely vegetated areas of sand and gravel may have been present prior to aggregate extraction operations and these areas would have supported Cleland’s Evening-primrose plants. It is estimated that, prior to the development of the golf course in 2003, 15 ha of prairie existed in the parcel (Buck pers. comm. 2022), which was reduced to approximately 5 ha following the development. During the 2022 surveys, approximately 5% of the prairie area in the golf course was composed of patchy vegetation with the exposed substrates favoured by Cleland’s Evening-primrose, with the remainder of the habitat having a moderately dense cover of grasses and forbs. The conditions for Cleland’s Evening-primrose may have deteriorated as the soils settled and allowed other herbaceous species, notably Little Bluestem, to become well established. The proliferation of herbaceous vegetation on the stable substrates has resulted in the accumulation of thatch and a hardened soil surface that is not conducive to a disturbance-dependent species like Cleland’s Evening-primrose.

The opening that had existed at the Jaffa site in 1975 was progressively lost due to a combination of succession and the planting of conifer trees. As the conifers matured, a dense canopy formed and conditions were no longer suitable for species that require full sunlight and exposed substrates. The conifer plantations at Springwater Conservation Area have resulted in stable soil that is covered by a layer of pine needle duff and likely has a greater moisture content than when the site was more open.

At the London and Mississauga sites, disturbance along the rail corridor is likely to occur intermittently and in localized areas. A laneway used by vehicles and machinery is present on the south side of the rail corridor at both sites, consisting of two tire tracks with an exposed substrate, with a patch of vegetation in the middle. Localized areas of disturbance at these sites also include gravel pads surrounding the infrastructure, locations where rail ties are piled up, and areas where the pivots of tracked machinery scarify the soil. In 2022, both the London and Mississauga sites appeared relatively inactive, with no signs of recent soil disturbance.

The Komoka site still contains large patches of dry, stony prairie, but encroachment by woody species is reducing the area of suitable habitat. The site supports several invasive species that can be controlled by fire (see Fire and Fire Suppression), including Glossy Buckthorn, European Common Reed (Phragmites australis ssp. australis), White Sweet-clover (Melilotus albus), and Spotted Knapweed (Centaurea stoebe). The area around the Mississauga site is blanketed by swaths of Dog-strangling Vine (Vincetoxicum rossicum), with Staghorn Sumac (Rhus typhina), Manitoba Maple (Acer negundo), Black Locust (Robinia pseudoacacia), European Buckthorn (Rhamnus cathartica), and Bouncing-bet (Saponaria officinalis) also present sporadically along the rail line. The London site has stands of Trembling Aspen (Populus tremuloides) and localized patches of Spotted Knapweed scattered across the areas of suitable habitat in the rail yard. The area around the Jaffa site contains small numbers of Multiflora Rose (Rosa multiflora) and Autumn Olive (Elaeagnus umbellata) along the edge of the conifer plantation.

Housing and urban areas (1.1, impact: very high – medium)

The golf course at the Komoka site is zoned as a combination of Natural Environment and Recreation, and is identified as a Special Policy Area in the municipality’s official plan (Middlesex Centre 2022). The Special Policy Area designation limits any new development of the site to golf course and accessory uses and requires that an environmental impact study be prepared and undergo agency review prior to approval, providing the remaining habitat with some degree of protection. Although currently zoned for recreation, the golf course is immediately adjacent to the Komoka settlement area boundary, and future zoning changes to allow residential development are possible. During the 2022 survey, the lands immediately north of the golf course were being graded in preparation for the development of a subdivision. The development limit is approximately 50 m from where Cleland’s Evening-primrose was known to have occurred (Buck pers. comm. 2022).

The three other subpopulations are not exposed to the Housing and Urban Areas threat.

Tourism and recreation areas (1.3, impact: high – medium)

The Komoka subpopulation could be affected by alterations to the golf course layout or the creation of new golf cart paths in areas that are currently tallgrass prairie habitat. Since the Komoka subpopulation had the greatest number of mature plants spread across a large area (Buck pers. comm. 2022), this site is most likely to contain viable seed in the seed bank. The quantity of seed produced by a single plant and the long-term viability of evening-primrose seed (see Life cycle and reproduction) suggest that thousands of seeds may persist in the soil for a number of years. As of 2022, there was no formal conservation easement covering the prairie habitat in the out-of-play areas of the golf course.

According to the Springwater Master Plan (Catfish Creek Conservation Authority 2021), the campsites in the area where Cleland’s Evening-primrose plants are thought to have occurred in 1975 will be upgraded with electricity and water service between 2022 and 2025. The document also identifies an area of proposed permanent roofed accommodations along the Conservation Line road, as funding permits. It is unclear if this development would include the removal of some or all of the planted conifer trees. The development of campsites could result in increased sunlight and soil disturbance, which would stimulate the seed bank, but would also present threats related to the trampling of plants, mowing, and the displacement of suitable habitat by permanent structures.

Agricultural and forestry effluents (9.3, impact: medium – low)

At the Komoka site, the prairie habitat where Cleland’s Evening-primrose was observed has been fragmented by a series of golf course fairways. Fertilizer and herbicide are applied to the manicured areas to maintain the turfgrass. The open, rolling topography allows surface runoff and herbicide drift to reach the areas of prairie, which may result in elevated nitrogen levels and harm prairie vegetation that comes into contact with herbicides.

During the 2022 surveys, the application of herbicide along the rail corridor was observed at the London site, and is also likely to occur at the Mississauga site as part of vegetation management activities. Spraying at the London site involved the two active rail lines and extended approximately two metres beyond the rail ballast. Although the plants observed at this site occurred in the rail yard, where herbicide application is less likely to occur, one of the Mississauga observations was on waste ground adjacent to the rail line in an area that is likely sprayed periodically.

Fire and fire suppression (7.1, impact: low)

Cleland’s Evening-primrose requires open habitat that is free of tree and shrub cover (see Habitat requirements). The long-term suppression of fire at the sites of all four subpopulations has allowed both native and non-native species to become established in areas that previously provided suitable open habitat for the species. Although the species and its natural sand barren habitat are maintained largely by drought, all the Ontario sites contain elements of tallgrass prairie communities that, historically, were maintained by periodic fires. The species is associated with sand barrens throughout its U.S. range, where fire serves to reduce the encroachment of competing species capable of colonizing this habitat (or other types of dry clearings). Although the species favours areas with low competition and a low fuel load, which reduce the intensity of heat and flame, plants growing in close proximity to accumulated fuel could be damaged or killed by fire.

Many of the invasive species that occur at the sites of the subpopulations are contributing to mesophication and a shift toward more closed-canopy habitats, with the resulting accumulations of leaf litter and increased humidity making intense fire less likely over time. Certain invasive species can react positively to low-intensity fire through increased seed bank germination or rhizome growth, which can increase the cover of competing vegetation in the vicinity of Cleland’s Evening-primrose plants. Additionally, all four subpopulations are located near urban areas or infrastructure, which would result in any fires being quickly extinguished. Within the heavily settled landscape of southern Ontario, the likelihood of naturally occurring or inadvertent fires at the sites is very low.

At the present time, the reinstatement of fire through prescribed burning may only be feasible at the Komoka site. However, the owner of this property has indicated that burning within the town limit is not currently supported by the fire department.

Problematic native species/diseases (8.2, impact: unknown)

Evidence of browsing by White-tailed Deer (Odocoileus virginianus) on other evening-primrose species was observed at the Komoka site, and all the Cleland’s Evening-primrose subpopulations would likely be subject to similar browsing pressure. Although browsing may not uproot plants, the removal of the upper portion of developing fertile stems could eliminate the potential for viable seed production. The browsing of this short-lived species in the small subpopulations in which it occurs could reduce the number of mature plants and seed bank size.

Roads and railroads (4.1, impact: unknown)

The London subpopulation occurred at a CN rail yard used for staging rail cars. In addition, maintenance equipment is stored in several outbuildings, and the area is surrounded by remnant tallgrass prairie and disturbed substrates that have been colonized by tallgrass prairie species. Extensive works in the rail yard, such as the construction of a new rail line, new building or a paved laneway, could destroy suitable habitat. Light to moderate disturbance, such as the grading of a gravel laneway, could have a positive or negative impact on the species. The Mississauga subpopulation is situated along a very active, heavily used section of the Metrolinx corridor, and infrastructure may be expanded to support higher volumes of train traffic.

Number of threat locations

The number of subpopulations is used as an approximation of the number of locations. There are zero to four locations, corresponding to the four potentially extirpated subpopulations. The main threats to the species include the lack of regular substrate disturbance, and habitat loss due to residential development. All four subpopulations face a similar main threat (Natural System Modifications, Threat 7), but this threat likely occurs independently in each subpopulation.

Protection, status, and recovery activities

Legal protection and status

Cleland’s Evening-primrose is not listed under the Ontario Endangered Species Act, 2007 or the federal Species at Risk Act. It is designated Endangered in Ohio (Ohio Department of Natural Resources 2020), providing protection provisions in that state.

Non-legal status and ranks

Cleland’s Evening-primrose has been assigned a global conservation status rank of G3G5 (last assessed in 1994), which is a range rank that indicates uncertainty about the species’ exact status (NatureServe 2022).

The species has been assigned a national NatureServe rank of N1 (Critically Imperiled). In Ontario, the species has been given a subnational rank of S1 (Critically Imperiled), which reflects and serves as the basis for the national rank of N1.

In the states bordering Ontario that support Cleland’s Evening-primrose, it has been given a subnational rank of S1 (Critically Imperiled) in Ohio, but is not ranked in Minnesota, Michigan, or New York (SNR; NatureServe 2022). At the far southwestern extent of the species’ range in the states of Missouri and Arkansas, subnational ranks of S2 (Imperiled) and S1 have been assigned, respectively. Cleland’s Evening-primrose is considered exotic (SNA) in New Jersey, South Carolina (NatureServe 2022), and possibly New York (Werier et al. 2023). In Illinois, the species was recently ranked as S4 (Apparently Secure; Cox pers. comm. 2022).

Although not ranked (SNR) in Indiana, Iowa, Michigan, Minnesota and Wisconsin, local experts suggested the following ranks for their jurisdictions:

• Michigan S4S5 (Bassett pers. comm. 2022)
• Indiana S4 (Namestnik pers. comm. 2022)
• Wisconsin S4 (Doyle pers. comm. 2022)
• Minnesota S3S4 (Anderson pers. comm. 2022)
• Iowa S3 (Pearson pers. comm. 2022)

In Kentucky, where the species remains unranked (SNR), three potential records require confirmation (Littlefield pers. comm. 2022).

Land tenure and ownership

Two of the four subpopulations (Komoka and London) occur entirely on privately owned land (See Appendix 1). At the Komoka site, there is potential for a non-governmental conservation agency to negotiate a land-tenure agreement covering the remnant tallgrass prairie habitat around the golf course. A rail line parcel immediately south of the golf course is owned by CN and contains a 3.5-ha tallgrass prairie remnant. The habitat on the CN lands in London is not likely to be protected and would be of minimal interest to private conservation organizations. The Jaffa and Mississauga subpopulations are located on publicly owned land. The Jaffa subpopulation was in the northern section of the Springwater Conservation Area, which is privately owned by the Catfish Creek Conservation Authority. The rail corridor where the Mississauga subpopulation occurred was previously owned by CN but was purchased by Metrolinx, a transport agency of the Government of Ontario. All documented plants at this site were confined to the rail corridor parcel.

Information sources

References cited

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Collections examined

In preparing this status report, specimens were examined from the Canadian Museum of Nature (CAN), Ottawa, Ontario. Western University (UWO) confirmed that they have a duplicate collection from Komoka in 1990 by M.J. Oldham, but no other material in their collection. It was confirmed that that no new O. clelandii specimens are currently held by the herbaria at Agriculture and Agri-food Canada (DAO), Ottawa, Ontario; the University of Guelph (OAC), Guelph, Ontario; Royal Ontario Museum (TRT), Toronto, Ontario; Erindale College, University of Toronto (TRTE), Toronto, Ontario; or Western University (UWO), London, 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 Alain Filion at the COSEWIC Secretariat. Bruce Bennett, co-chair of the Vascular Plant Specialist Subcommittee (VPSSC), 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 who shared their expertise (see Authorities Contacted) and personal observations, especially Wasyl Bakowsky, Sam Brinker, Graham Buck, Mike Oldham, and Steve Varga for their input regarding occurrences and their assistance with surveys. Tony Reznicek provided valuable commentary on the species’ habitat and accompanied the report writer to extant subpopulations in Michigan and Ohio. Herbaria staff were helpful in reviewing their collections for additional records: Jennifer Doubt (CAN); Shannon Asencio (DAO), Carole Ann Lacroix (OAC), and Kris Mendola (UWO). Amanda Cosentino of FireRock Golf Club generously allowed access to the property for the completion of surveys. Many reviewers contributed to improving this report including Syd Cannings, Holly Bickerton, Marie Archambault, Rebecca Sutherland, Lingfei Li, and Karolyne Pickett of Environment and Climate Change Canada; Vivian Brownell, David Mazerolle, and Cary Hamel of the Vascular Plants Specialist Subcommittee (VPSSC). Del Meidinger and Anna Hargreaves of the VPSSC assisted with the threats call.

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 of Tallgrass Ontario, the Waterloo Stewardship Council and the Field Botanists of Ontario. He spends much of his free time exploring tallgrass remnants throughout southern Ontario and is involved in ongoing stewardship efforts. Patrick recently co-wrote the COSEWIC status Report update for Kentucky Coffee-tree (Gymnocladus dioicus) (2021).

Appendix 1. Cleland’s evening-primrose subpopulations, status, observation data, and land tenure in Canada

Appendix 2. Threat calculator for cleland’s evening-primrose

Threats assessment worksheet

Species or Ecosystem Scientific name

Cleland’s Evening-primrose; Oenothera clelandii

Element ID

1056644

Elcode

PDONA0C1T0

Date:

2023-02-21

Assessor(s):

Bruce Bennett and Del Meidinger (Co-chairs); Syd Cannings, Jennifer Thompson and Holly Bickerton (ECCC); Vivian Brownell, Anna Hargreaves, David Mazzerole, Sam Brinker (SSC members); Pat Deacon (writer); Graham Buck (MNRF); Alyssa Pogson (Secretariat)

Assigned Overall threat impact:

AB = Very high - High

Overall threat comments

Biennial; generation time 2 plus years (plus due to seed bank); perhaps 12 to 22 years, so three generations is about 36 to 66 years; no mature individuals; viability of seed bank unknown but could remain viable for decades awaiting a disturbance; Biennial Evening-primrose had viable seed after decades (up to 80 years); last observation of any plant is 2001; golf course at Komoka developed in 2003; golf course development attempted to leave at-risk plant habitat; area was an old aggregate pit; London site owned by CN Rail; Mississauga site owned by Metrolinx (Province of Ontario); Jaffa site owned by Catfish Creek Conservation Authority.

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