Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Substrate and ecology: wide ranging on rock, from the immediate coast to many kilometers inland, and has soil or sand populations in central Baja California and on a few of the Channel Islands (San Clemente and San Nicholas) and the islands adjacent western Baja California World distribution: northern California to Baja California Sur Sonoran distributions: north of San Francisco south Baja California Sur, Guadalupe Island and other islands off western Baja California. Notes: Niebla homalea is one of the most conspicuous, richly polymorphic species on rock. On the islands along the Baja California coastline (and islands) and on several of the Channel Islands (San Clemente and San Nicholas), soil populations have developed, fomring a tumble-weed like, rounded thallus, perhaps a morphology that enhances trapping moisture from fog. The ground populations reproduce primarily through fragmentation and are often sterile. Morphological plasticity has allowed both N. homalea and N. josecuervoi to colonize extensively soil in the absence of rock substrates, and serves as a dispersal bridge for many kilometers inland. Similar formation of sterile ground populations by switching substrates is known from other species, such as Teloschistes californicus (shrub and soil populations) and in several Ramalina species. The ground populations of N. homalea closely resemble Teloschistes capensis in growth form, and both are dependent upon harvesting moisture at ground level from coastal fog. Niebla homalea has a rich chemistry including the depsides sekikaic or divaricatic acids (barbatic acid has also been reported), or a race lacking either depsidones or depsides. Thalli with the depsidone secondary metabolite pathway producing hypoprotocetraric, salazinic, or protocetraric acids are designated Niebla josecuervoi. Thalli with the depside line (N. homalea sense. str.) are usually distinguishable on a morphological basis from the depsidone lineage (N. josecuervoi), but in mixed soil populations, thalli of the two species usually cannot be separated. If one follows a strict chemotaxonomic species concept, then additional species would have to be recognized representing the divaricatic (N. homalea), sekikaic, protocetraric, salazinic, and hypoprotocetraric acids as well as an acid deficient species. However, as with other chemically rich groups in the Ramalinaceae, like R. farinacea and R. siliquosa, that approach is not generally accepted because the "races" cannot be separated morphologically throughout their ranges. Both rock and ground populations are known from most of the chemical combinations. The blades are highly variable, and thalli can be short and stubby, broad, or long and pendulous, depending upon exposure. Many stands appear wind-trimmed in sites exposed to wind, while thalli are strap-like and pendulous at adjacent protected sites. Often the blades, particularly in ground populations, have marginal proliferations (branchlets). The depside containing races of N. homalea are allopatric throughout the species Distribution. The depsidone producing races of N. josecuervoi occur from Colonet southward along the coast of Baja California and on the adjacent islands. Like the R. siliquosa and R. farinacea complexes, in places there is limited habitat selection among several of the thallus chemistries (divaricatic and sekikaic acids, for example), but there is no clear-cut or consistent pattern. The divaricatic acid race appears to have the broadest ecological amplitude, ranging from shoreline rocks to the highest elevations and the greatest distance inland from the ocean, while the sekikaic acid race appears to be more common at mid-elevations on the immediate coast. The blades range from slender branches a few mm wide to larger ones several centimeters in width, and are flattened and compressed, angular, or occasionally subcylindrical. The cortical surface may be cracked, ridged, smooth or plated and may be dull or shiny. The branches are green to greenish yellow, or pale yellow, particularly in exposed soil populations. Apothecia are terminal to subterminal and are present in most large thalli on rocks. Because they both have flattened blades, Niebla homalea and N. laevigata appear superficially similar. However, the smoother, often shiny cortex of N. laevigata and its lack of the filament-like medullary strands separate it from N. homalea. Even for N. laevigata specimens that are cortically ridged, the lack o