Part 1: COSSARO candidate species at risk evaluation form – June 2010

Four-leaved Milkweed (Asclepias quadrifolia)

Current designations:

NRANK Canada – N1
COSEWIC – Endangered (April 2010)
SARA – Not listed
General Status Canada – May be at risk (2010)
ESA 2007 – Not listed
General Status Ontario – May be at risk (2010)

Distribution and status outside Ontario:

Four-leaved Milkweed is endemic to eastern North America, occurring in two disjunct regions, separated by a 150 to 400 km wide zone of almost complete absence in the Mississippi River Valley. The western region of occurrence extends from eastern Oklahoma and western Arkansas north to western Illinois and eastern Iowa. The eastern region extends from the southern Appalachian Mountains north to eastern Indiana and southern New Hampshire. The species just extends into Canada in southern Ontario at the eastern end of Lake Ontario in Prince Edward County, and the western end in the vicinity of Niagara Falls, where it is believed extirpated (COSEWIC 2010).

The species has been assessed as Secure (S5), Apparently Secure (S4), or Not Ranked (SNR) by most jurisdictions, except some near the range periphery (NatureServe 2010).

Eligibility criteria

Native status

✔ Yes. There are several Ontario records, including specimens, from the late 1800s. This is not a weedy species and there is no reason to suspect it is not a native component of the Ontario flora.

Taxonomic distinctness

✔ Yes. Taxonomically distinct and considered a valid species in all recent taxonomic treatments. No infraspecific taxa have been described.

Designatable units

Occurs in two regions of Ontario separated by about 190 km, near the western and eastern ends of Lake Ontario. Both populations are in the same ecozone (Mixed Woods Plains). There are no genetic data on Ontario populations (COSEWIC 2010). COSEWIC assessed the species as a single Designatable Unit in their recent (April 2010) status assessment.

Priority-setting criteria

Recent arrival

✔ No. Known in Ontario since the late 1800s (COSEWIC 2010).


✔ No

Primary criteria (rarity and declines)

  1. Global rank

    ✔ Not in any category. G5.

  2. Global decline

    ✔ Not in any category. No evidence of global declines though rare in some peripheral parts of its range.

  3. Northeastern North America ranks

    ✔ Special concern. Highly ranked (S1, S2, SH or SX) in 4 of 12 northeastern jurisdictions in which it occurs natively and is ranked (33%). See Appendix 1.

  4. Northeastern North America decline

    ✔ Not in any category. No evidence of significant declines in northeastern North America.

  5. Ontario occurrences

    ✔ Endangered. Known from two extant Ontario occurrences.

  6. Ontario decline

    ✔ Threatened. There are a minimum of 5 Ontario populations, 3 of which (60%) are considered extirpated (COSEWIC 2010). It is possible that there were additional populations, though locality data are vague for most early records making it difficult to be sure of separate populations.

  7. Ontario’s conservation responsibility

    ✔ Not in any category. Ontario makes up only a small fraction of the species global range (COSEWIC 2010).

Secondary criteria (threats and vulnerability)

  1. Population sustainability

    ✔ Not in any category. No definite evidence of reproductive or recruitment failure in Ontario, though increased shading may be reducing reproductive output in Ontario populations. No Population Viability Analyses have been conducted for the species in Ontario.

  2. Lack of regulatory protection for exploited wild populations

    ✔ Not in any category. No known harvest in Ontario.

  3. Human threats

    ✔ Endangered. Both Ontario populations are threatened by declining habitat quality due to increased shading and the spread of exotic species (particularly Common Buckthorn, Rhamnus cathartica). Potential development threatens one site; the other is in a conservation area.

  4. Specialized life history or habitat-use characteristics

    ✔ Endangered. Restricted to a critically imperiled (S1) habitat type in Ontario, Bur Oak – Shagbark Hickory – Big Bluestem community (COSEWIC 2010).

COSSARO criteria met (primary/secondary)

  • Endangered – 1/2
  • Threatened – 1/0
  • Special concern – 1/0


Four-leaved Milkweed (Asclepias quadrifolia) is a widespread eastern North American species known in Canada from two small populations in Prince Edward County near the eastern end of Lake Ontario. Historically the species was also present at several populations in the vicinity of the Niagara River, though there are no records in more than 50 years from this area. It occurs in open Bur Oak – Shagbark Hickory woodland on shallow soil over limestone, a provincially rare habitat type, where it is threatened by increased shading due to habitat succession and invasive species. Fewer than 200 mature individuals are known in Ontario.

Information sources

COSEWIC. 2010. COSEWIC Status Report on Four-leaved Milkweed Asclepias quadrifolia. Committee on the Status of Endangered Wildlife in Canada, Ottawa. Two-month Interim Report (Feb. 2010). 30 pp.

NatureServe. 2010. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. (Accessed: May 9, 2010).

Appendix 1

Northeastern North America rank, status and decline

CTSNR (not ranked; NatureServe 2010)
DESH (possibly extirpated; NatureServe 2010)
ILSNR (not ranked; NatureServe 2010)
INSNR (not ranked; NatureServe 2010)
IAS3 (vulnerable; NatureServe 2010)
KYS4 (apparently secure; NatureServe 2010)
MASNR (not ranked; NatureServe 2010)
MBNot present (NatureServe 2010)
MDSNR (not ranked; NatureServe 2010)
MENot present (NatureServe 2010)
MINot present (NatureServe 2010)
MNSNR (not ranked; NatureServe 2010); considered not present for this evaluation since the species is not reliably known from Minnesota (Welby Smith, Minnesota DNR, pers. comm. May 2010).
NBNot present (NatureServe 2010)
NHS2 (imperiled; NatureServe 2010)
NJS3 (apparently secure; NatureServe 2010)
NSNot present (NatureServe 2010)
NYS5 (secure; NatureServe 2010)
OHSNR (not ranked; NatureServe 2010)
ONS1 (critically imperiled; NatureServe 2010)
PAS5 (secure; NatureServe 2010)
PENot present (NatureServe 2010)
QCNot present (NatureServe 2010)
RIS1 (critically imperiled; NatureServe 2010)
VAS5 (secure; NatureServe 2010)
VTS3 (vulnerable; NatureServe 2010)
WINot present (NatureServe 2010)
WVS5 (secure; NatureServe 2010)

Occurs as a native species in 18 of 27 northeastern jurisdictions SRANK or equivalent information available for 12 of 18 jurisdictions = (67%) S1, S2, SH, or SX in 4 of 12 = (33%)

Part 2: Ontario evaluation using COSEWIC criteria

Regional (Ontario) COSEWIC criteria assessment

Criterion A – declining population

No. Historical population decline is beyond the time frame to qualify for this criterion.

Criterion B – small distribution and decline or fluctuation

Endangered B1ab(iii,v)+2ab(iii,v). Meets Endangered B1ab(iii,v)+2ab(iii,v) based on very small Extent of Occurrence and Index of Area of Occupancy and presence at only two locations with a continuing decline in the quality of habitat and inferred loss of individuals due to the spread of exotic species, especially Common Buckthorn, a major invasive shrub.

Criterion C – small population size and decline

Endangered C2a(i). Meets Endangered under C2a(i) based on an inferred future decline in mature individuals that will continue with the spread of major invasive exotics at the sites; neither of the two populations has >250 mature individuals.

Criterion D – very small or restricted

Endangered D1. Meets Endangered D1 with a total of <250 mature individuals.

Criterion E – quantitative analysis

No. No quantitative analysis available.

Rescue effect

No. Unlikely to be rescued from adjacent jurisdictions since the species is restricted to a rare habitat and separated from U.S. populations by significant water bodies (Lake Ontario, Niagara River).