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COSEWIC assessment and status report on the incurved grizzled moss in Canada

COSEWIC Executive Summary

Incurved Grizzled Moss

Ptychomitrium incurvum

Species information

Ptychomitrium incurvum (Schwägr.) Spruce, a small (2-6 mm high) moss species, grows in yellow-brown to blackish-green tufts on rocky substrates.  It was originally described by C.F. Schwägrichen in 1823 as Weissia incurva, and belongs to the moss (division Bryophyta, subdivision Musci) class Bryopsida, order Grimmiales, and family Ptychomitriaceae. Although there appears to be no recent controversy surrounding the taxonomic status of this species, the only known Canadian specimen is an isotype of Grimmia hookeri Drumm., which is presently considered a synonym of P. incurvum.


Ptychomitrium incurvum is temperate in global distribution, with populations both east and west of the Atlantic Ocean.  It is relatively widespread in eastern North America but its distribution is concentrated in the southern United States.  In the northeastern United States, it is known from relatively few, historical locations, but more research is required before the existence of a declining trend in the states bordering Canada can be confirmed or rejected. 

A single historical record (1828) of this species constitutes the only known Canadian collection of this species.  The herbarium label for this collection bears no more detail than “On a rock, near the Falls of Niagara, Ontario”.  The precise location is uncertain and no extant populations are known.  Given the fact that almost 200 years have passed without the species being re-discovered despite active collection in the region, doubt exists as to whether or not Ptychomitrium incurvum is currently part of the Canadian flora.


Ptychomitrium incurvum is commonly found in hardwood forests, inhabiting surfaces or tiny crevices of exposed or protected rocks of variable chemistry (calcareous or non-calcareous).  It occurs rarely at the bases of trees or on logs.  Although human disturbance, particularly forest destruction and fragmentation, is known to have affected many eastern deciduous forest plant species in southern Ontario, it is not likely that it accounts for the absence of P.  incurvum there.  The fact that P. incurvum can inhabit a variety of anthropogenic and natural rock substrates suggests that substrate availability does not limit its distribution; its southerly distribution may indicate, rather, a climatic limitation.


Little is known, specifically, of the biology of Ptychomitrium incurvum.  Like most mosses, it is dispersed as spores, although it is occasionally known to produce gemmae (asexual propagules).  P. incurvum is autoicous (and therefore can presumably self-fertilize), and spore production is very common.  The only known Canadian specimen bears abundant sporophytes. Establishment may present particular challenges to mosses like P. incurvum: the rocky habitats preferred by P. incurvum are hostile in that they do not retain moisture well and in that they often offer little protection from the elements.

Cushion-forming mosses like Ptychomitrium incurvum are less adapted to vegetative proliferation than the more branched, spreading mat-forming species.  Species such as P. incurvum, which occur in rock crevices, also have limited space in which to expand before dispersal over longer distances becomes necessary. This preference for discrete habitats necessitates dispersal through unfavourable habitats, which cannot be accomplished through vegetative expansion of colonies.

Population sizes and trends

As no extant populations of Ptychomitrium incurvum are known, and as the historical collector of the species did not make note of the species’ abundance on the herbarium label, population sizes and trends cannot be assessed.

Limiting factors and threats

As noted above, climate appears to limit Ptychomitrium incurvum’s northern extent in eastern North America.  Human activity (pollution, habitat destruction) may also contribute to the species absence from southern Ontario.  Since no living Canadian populations are known, threats cannot be assessed.

Special significance of the species

The Ontario occurrence of Ptychomitrium incurvum is significant in that it represents the only Canadian population of the species and in that it helps to delineate the northernmost occurrence of the species in North America.  P. incurvum ranks among a large suite of Carolinian plants that are endangered in Canada.



The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) determines the national status of wild species, subspecies, varieties, and nationally significant populations that are considered to be at risk in Canada. Designations are made on all native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fish, lepidopterans, molluscs, vascular plants, lichens, and mosses.


COSEWIC comprises representatives from each provincial and territorial government wildlife agency, four federal agencies (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership), three nonjurisdictional members and the co-chairs of the species specialist groups. The committee meets to consider status reports on candidate species.



Species: Any indigenous species, subspecies, variety, or geographically defined population of wild fauna and flora.

Extinct (X) : A species that no longer exists.

Extirpated (XT) : A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E) : A species facing imminent extirpation or extinction.

Threatened (T) : A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)* : A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)* : A species that has been evaluated and found to be not at risk.

Data Deficient (DD)*** : A species for which there is insufficient scientific information to support status designation.


* : Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.

** : Formerly described as “Not In Any Category”, or “No Designation Required.”

*** : Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994.


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.


Environment Canada          Environnement Canada

Canadian Wildlife Service          Service canadien de la faune

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