This species has a relatively small Area of Occupancy (92 km²) and throughout much of its range its potential habitat has been fragmented and has declined in extent owing to fire and development, and has declined in quality owing to air pollution from the Central Valley.
This species is endemic to southern California and known from only a couple dozen localities.
Rhizoplaca marginalis is endemic to southern California with a single outlying subpopulation in southern Nevada. Its Extent of Occurance (EOO) is 108,000 km², and its Area of Occupancy (AOO) is 92 km².
Rhizoplaca marginalis is a conspicuous lichen easily recognized at a glance in the field, so the small number of known localities it has been found in is strong evidence of its rarity. However, it tends to be locally abundant, so overall population size is probably relatively large despite the small number of known localities.
Population Trend: Stable
Rhizoplaca marginalis occurs on shaded vertical to overhanging siliceous rocks at relatively low elevation. It has been found in canyons and on large outcrops in open woodlands. It is most common in the foothills and lower canyons of the southern Sierra Nevada.
The strongest threat to Rhizoplaca marginalis is posed by increasing frequency and intensity of forest fire in southern California, especial subpopulations in woodlands and chaparral.
Land development including road building, housing developments, dams and mining are also concerns because subpopulations tend to be restricted to fairly small areas, so even small-scale disturbance can easily affect an entire subpopulation.
Lichens in general are well known to be sensitive to air pollution. Subpopulations in the Sierra Nevada foothills are particularly subject to pollution from widespread agriculture activities in the Central Valley. However the sensitivity of Rhizoplaca marginalis to various types of pollution is not currently known.
Two subpopulations of Rhizoplaca marginalis occur in national parks; one in Sequoia National Park, another in Sequoia National Park. Most populations are at low elevations and therefore occur outside parks.
Known populations should be monitored and protected from development wherever possible.
Undiscovered subpopulations of Rhizoplaca marginalis are likely to exist. The foothills and east side of the Sierra Nevada are relatively poorly studied by lichenologists. Surveys of likely habitat will probably turn up additional localities.
The effects of agricultural pollution on subpopulations of Rhizoplaca marginalis in the foothills of the Sierra Nevada should be investigated.
Rhizoplaca marginalis has no known commercial or cultural uses.
Ryan, B.D. 2002. Rhizoplaca. In: T.H. Nash, III, B.D. Ryan, C. Gries & F. Bungartz (eds.). Lichen Flora of the Greater Sonoran Desert Region. Volume I. Lichens Unlimited, Arizona State University, Tempe, Arizona. pp. 442–448.