Pygmy pocket moss (Fissidens exilis) COSEWIC assessment and status report: chapter 2

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COSEWIC Executive Summary

Species information

Fissidens exilis (pygmy pocket moss) is a minute, brown- or dark-green moss of the family Fissidentaceae--a group characterized by a “pocket” in the lower portion of each leaf, enfolding the stem. F. exilis is differentiated from other species by leaf characteristics, including a pocket which is not closed at the upper end, and the presence of an “intramarginal border”--unique rows of elongate cells found several cells in from the lower margin of the pocket region of each leaf.

Distribution

Fissidens exilis is known from the British Isles, central and northern Europe, Scandinavia, Japan, and New Zealand. The first discovery of F. exilis in North America was made in 1947, in Cleveland, Ohio, and since then it has been collected in ten other eastern states. In Canada, Fissidens exilis has been collected in five locales in southern Ontario (two in Essex County and one in each of Kent, Haldimand-Norfolk, and Waterloo Counties) and in two locales in southern Quebec (Gatineau Park and, reportedly, the city of Montréal).

Habitat

Fissidens exilis is found largely on bare, moist, clay soil. It usually occurs in forests, and often on soil disturbed by humans or natural events. It may be associated with ephemeral (extremely short-lived) mosses such as Ephemerum and Micromitrium. F. exilis habitat may be limited climatically to southern latitudes of Ontario and Quebec. Substrate appears not to be limiting, as disturbed clay is frequent in Canada. At least four of the seven Canadian sites where F. exilis has been collected are protected to some degree.

Biology

Very little has been published on the biology of Fissidens exilis. F. exilis disperses as spores and germinates to produce protonemata--filamentous, green precursors to recognizable leafy moss plants (known as ‘gametophytes’)--that permeate and persist in the soil even after mature gametophytes are established. Moisture is required for photosynthesis and growth of the gametophytes and for fertilization of the egg by the sperm, which in F. exilis may occur by self-fertilization or out-crossing. In the absence of repeated disturbance to control moss and herb cover, the exposed soil microhabitats preferred by F. exilis may become increasingly less favourable due to competition by other plants, necessitating dispersal to fresh substrates. Spores, which mature in the winter, are probably the main mode of dispersal.

Population sizes and trends

No Canadian collector of Fissidens exilis prior to 2002 noted the abundance of the species at the time of collection, and indeed, abundance estimates are very difficult because the species cannot be identified with certainty in the field and tends to intermingle with other small Fissidens species. No previously recorded populations were re-discovered during field work carried out in 2002, which may reflect the imprecise location information, the short-term nature of F. exilis’ preferred substrate, or the practical considerations associated with detecting a rare, cryptic species within a sea of similar common congeners. Field work in 2002, nevertheless, resulted in the discovery of one previously unrecorded population, which occurred in three patches sparsely occupying a total area of about 860 cm².

Limiting factors and threats

Factors limiting the distribution and population sizes of Fissidens exilis in Canada may include climate: Canadian populations mark the species’ northern range limit in North America, although the species reaches much farther north in Europe. In Canada F. exilis is found in a heavily populated and developed region in which natural habitats are widely known to be at serious risk. Particularities of habitat preference, which are not well known, may also come into play. F. exilis is minute and inconspicuous, cannot be determined with certainty in the field, and may disperse often to re-establish on exposed substrates as competition excludes it from once-bare soil. These factors make accurate determinations of the species’ distribution and abundance very difficult, and may help to explain the relatively recent discovery of F. exilis in North America despite widespread exploration throughout its North American range.

Special significance of the species

Fissidens exilis is known from very few Canadian sites and reaches its northern North American range limit in southern Ontario and Quebec.

Existing protection and other status designations

No legislation, regulations, customs, or conditions currently protect this species. Globally, the species is listed as G3G4 and in Ontario the moss has a rank of S1.

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 agencies (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 members and the co-chairs of the species specialist and the Aboriginal Traditional Knowledge subcommittees. The Committee meets to consider status reports on candidate species.

Definitions (november 2004)

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)^Footnote1

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)^Footnote2

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

Data Deficient (DD)^Footnote3

A wildlife species for which there is inadequate information to make a direct, or indirect, assessment of its risk of extinction.