Seaside centipede lichen (Heterodermia sitchensis) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Search effort

From 2001 to 2004, Parks Canada supported a considerable search effort for H. sitchensis. A total of 30 person days of search time has been dedicated to this species each summer, with a total of 60 days devoted to Pacific Rim National Park and 30 each to Gwaii Haanas (South Moresby Island) and northern Vancouver Island. To these more recent search efforts can be added several earlier field studies concentrated in coastal B.C., especially those of Dr. Irwin Brodo in the Queen Charlotte Islands in 2000 (I.M. Brodo, pers. comm.). See Appendix 1. Other search efforts within the known range of H. sitchensis are summarized in Table 2 and mapped in Figure 3.

Abundance

Two hundred and twenty-seven thalli of H. sitchensis in twelve localities have been recorded to date, including two localities that no longer support this species. Sixty-five percent of these thalli, however, were noted at only three of the existing localities (Table 3).

Fluctuations and trends

What little is known about population trends in H. sitchensis is based on observations made at two localities. The first of these is the holotype locality at Schooner Cove where in 1983 a single Sitka spruce branch bore at least a dozen thalli. When this same branch was re-examined twenty years later, in 2002, it was found to support only a single, rather moribund thallus of H. sitchensis. Also lacking on this branch was Physcia tenella, which had been reported two decades earlier (Goward 1984). The fact that both H. sitchensis and P. tenella are nitrophiles could suggest that their decline here is related to a gradual decline in nutrient status. In 1983 the host branch was apparently situated within the nutrient column of a bird perch no longer in use 20 years later. According to this scenario, potential host branches for H. sitchensis are likely to be available to this species for a short time only, depending on the spatial configuration of nutrient supply, suitable substrate (slow-growing Sitka spruce twigs), and diaspore availability.

One of the most productive sites for H. sitchensis is a stretch of shoreline on WouwerIsland in the Broken Group Islands. Here, in 2001, a dozen thalli were noted among the lower branches of a stunted Sitka spruce tree exposed to northwest winds. When the same tree was visited in 2002, only eight thalli were found, one of which was apparently healthy, the others dead. A careful survey of the upper canopy revealed an additional twelve thalli, of which only four were alive. The high incidence of dead thalli points to a die-back event during the winter of 2001/2002. In fact, such a die-back had been predicted by Barry Campbell (pers. comm.), on the basis of exceptionally strong northwest winds in April of 2002. The branches of the host tree appear to have been repeatedly drenched with salt water, as suggested by the greenish algal layer covering the basal portions of the lichens. Epiphytic mosses, too, had been partly smothered by algae. Damage in the upper canopy was less extensive than in the lower canopy. Discoloured lichen thalli were noted in many other localities in 2002, especially in sites exposed to northwest winds.

Rescue effect

At the present time, H. sitchensis is an almost exclusively Canadian species, with only a single, presumably relictual population known from the United States. Because of this, diaspores originating from points south of the border are extremely unlikely to remedy future population losses in Canada.