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COSEWIC Assessment and Update Status Report on the Seaside Centipede Lichen (Heterodermia sitchensis) in Canada

Limiting Factors and Threats

Even after an intensive search effort over four years, the known Canadian population of H. sitchensis amounts to only 227 thalli. Sixty-five percent of these are restricted to three localities on Florencia, Wouwer, and the vicinity of Kyuquot. The first two localities are in Pacific Rim National Park Reserve, while the last is located on crown land 130 km distant. All of these localities appear to have been subject to recent or ongoing nitrogen enrichment in connection with sea lion haul-outs or seabird nesting sites. Elsewhere, H. sitchensis averages only about eight thalli per locality, apparently very locally supported by nutrients from perching birds. The small number of thalli present at each site raises the possibility of sudden disappearance from the greater portion of its range, whether as a result, for example, of severe winter storms, decline in perching bird populations, prolonged summer drought, or logging operations.The occurrence of a few of the known populations near trails raises the possibility that branches bearing H. sitchensis could be collected, e.g., for kindling by rain-soaked hikers.

In this report, we present evidence that H. sitchensis is sensitive to prolonged wetting by salt spray as a result of strong winter windstorms. In other parts of the north coast, such storms have been observed to actually destroy entire epiphytic lichen communities over large areas. With the gradual trend to more intense storm events, it seems likely that H. sitchensis could be at risk at least in some parts of its range.

The frequently exclusive association of H. sitchensis with nutrient columns below the roosts of perching birds is especially disquieting: it suggests that this species is not stable at most sites, as for example at the holotype locality, where a population of 12 thalli in 1983 had dwindled to only one when last visited. Though the perch trees themselves may be long-lived, the roosts are likely to be relatively ephemeral.

Apparently the continued existence of H. sitchensis over large portions of its total range depends on an ability to recolonize at rather frequent intervals. Assuming the trend toward global warming (and coastal drying) continues, H. sitchensis could eventually lose this ability at many of the sites currently supporting it. In the long run, therefore, this species can be expected to go into gradual decline. In the short term, however, a recent defoliating event in the Pacific Rimarea, apparently caused by insect outbreak, promises to provide abundant new habitat for this species, at least locally.