Porsild's bryum (Haplodontium macrocarpum) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Sites here are defined by the existence of a group of M. macrocarpacolonies that is discontinuous from any other such nearby groups. All areas in between sites that are less than 15 km apart have been thoroughly searched by Natalie Cleavitt (in 1997-2002) for any additional colonies. Sites measured in Alberta tend to be quite small (0.71 m² ± 0.31), although site coverage is fairly high for a moss (0.15%) (Cleavitt 2002a). Colonies have a mean size of 22 cm² (± 27) and the number of colonies for those populations counted varied from 3 to 260 colonies at a site. The Newfoundland populations tended to be smaller and more patchily distributed than Alberta populations (see Appendix 1). Hedderson (2002, pers. com.) had previously remarked that the only large population of the four known in northern Newfoundland was the Straitsview population and this population now consists of only nine colonies (see Appendix 1). However, Hedderson (pers. com.) later noted that he had seen the size of this population fluctuate in the past. Currently the largest population documented in Newfoundland was a newly discovered population at White Cape (see Appendix 1). The total gain in population size from work leading to this report was at least 117 colonies.

Based on label information for Mielichhoferia macrocarpa specimens that indicate collections taken from the same site (and presumably population) over time (three sites total) the long-term trend has been population stability. Three recently studied (1997-2000) populations near Cadomin have documentation dating from 1966, 1977 and 1984 (see Collections Examined). The one site in Kananaskis has been known since 1982. All four of these sites had healthy populations as of 2000. The Montana site at Silver Gate in Park County has been collected in 1948, 1953, 1973 and 1992 (Brassard & Hedderson 1983; see Collections Examined). Brassard and Hedderson (1983) further noted the stability of a population on Disko Island in Greenland where the species was collected in 1898 and 1973 (75 years apart). However, as mentioned previously under Habitat Trends, the Newfoundland and Alberta populations were observed to decline over the winter of 2001-2 (an estimated 296 colonies). This decline represents a 15% decrease in known population size. Population monitoring would help to establish how well this species recovers from severe population reduction at a site.