Pygmy pocket moss (Fissidens exilis) COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
Trends in individual populations of F. exilis could not be determined because previously known populations were neither described in detail at the time of their discovery, nor re-discovered in association with this report. Difficulty in finding previously recorded populations does not necessarily indicate decline or extirpation, because very little detail concerning the population locations was available to direct field searches. Furthermore, the species’ persistent protonemata, which are presumably more common than the shorter-lived mature gametophytes, cannot be reliably identified, even at the family level, and populations lacking distinguishable leafy gametophytes at the time of the field work would not have been recorded. It is also possible that collecting by J. Doubt in 2002 failed to detect mature F. exilis at the sites, because small Fissidens abounds on muddy substrates in southern Ontario and species tend to intermingle. In the absence of characters for field recognition, collectors must gather a representative sample for lab identification. Rare species are less likely to be detected with this somewhat random (but necessary) approach. Finally, a species reliant on bare (disturbed) soil would be expected to disperse and re-establish often in response to increasing competition from plants and plant litter as post-disturbance succession progresses. Long-term persistence at narrowly defined sites is probably not in the nature of F. exilis, although it may be expected to persist in a general area in which patches of suitable substrate predictably recur. Despite these factors, one previously unrecorded population of F. exilis was discovered. This population occurred in three patches sparsely occupying a total area of about 860 cm².
Field work in 2002 was conducted at over thirty southern Ontario and Quebec sites. Field sites represented previously documented sites for F. exilis or for other species (Helodium paludosum, Bryoandersonia illecebra) for which COSEWIC field work was under way, or sites thought likely to support these species. At eleven sites, minute Fissidens species were located and collected for laboratory identification, only one of which proved to represent F. exilis. Additional field work involving extensive sampling throughout the suspected Canadian range of F. exilis is likely to result in the (re)discovery of more populations (Wilf Schofield, personal communication 2004).
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