Seaside centipede lichen (Heterodermia sitchensis) COSEWIC assessment and status report: chapter 6
Biology
Life cycle and reproduction
Mature lobes of H. sitchensis invariably bear apothecia (sexual fruiting structures) in urn-shaped outgrowths near the lobe tips. Notwithstanding their frequency, the apothecia are not directly involved with reproduction in H. sitchensis. Rather, their spores are apparently vestigial, failing to reach maturity. Instead, reproduction in this species takes place vegetatively, through the release of powdery soredia, borne on the inner walls of the urns. Because soredia contain both the fungal and algal partners of the lichen, they are effectively "instant lichens," establishing directly, with no need to acquire the opposing partner in order to reestablish the lichen symbiosis from its separate partners.
Herbivory/predation
No signs of herbivory on H. sitchensis have been noted.
Physiology
With the exception of its presumed requirement for nitrogen enrichment (Goward & Fredeen, in prep.), little is known about the physiological needs of H. sitchensis. Still, its narrow occurrence in hyperoceanic localities suggests a requirement for cool temperatures and high atmospheric humidity, a pattern supported by the absence in this species of a protective lower cortex. These and other observations led Goward (1994) to propose that H. sitchensis may have evolved as a species of equatorial cloud forests.
Dispersal/migration
Though soredia are produced in great abundance in H. sitchensis, they are nevertheless mostly confined to the inner surface of "urns" that invariably form near the lobe tips. If only for this reason, it seems unlikely that these propagules would be an efficient means of dispersal. One possibility (highly speculative) is that twig-feeding forest birds – especially Chestnut-backed Chickadees and Golden-crowned Kinglets – incidentally peck at the urns while foraging for invertebrates. Later, having flown to new trees, they disperse the soredia to new twigs.
Interspecific interactions
As mentioned above, H. sitchensis is a pioneer species, that is, it colonizes early in the development of its host twig. At this stage, the twig is usually devoid of competing epiphytes, allowing H. sitchensis to establish without direct interaction with other species. Within about a decade, however, other epiphytes – mosses, liverworts, and other lichens – begin to colonize the host twig, and H. sitchensis gradually goes into decline. Possibly the high nitrogen concentrations characteristic of twigs supporting H. sitchensis may actually promote the same heavy moss loadings that ultimately displace it.
Adaptability
Compared to other epiphytic lichens, H. sitchensis exhibits little adaptability in its choice of habitat. Indeed, careful searching has turned up populations only on nitrogen-enriched twigs on old Sitkaspruce trees growing at seaside. Clearly, H. sitchensis is much more strict in its ecological requirements than any of the lichen species with which it occurs, including the related H. leucomela.
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