Lichens are a unique and very interesting form of "plant" life, actually a combination of lichenizing (lichen-forming) fungi and an algal photobiont.   This combination of two species growing together benefiting each other is called "symbiosis".   Some argue that the trapping of the algal photobiont by the fungi is not always sybiotic, as the algae can survive independently without the fungal host, the algae usually benefits overall by the protection (house) provided by the fungal component.   There are about 25 genera of green algae, a few golden algae, one brown alga, and 12 genera of cyanobacteria (blue-green algae) that are known to serve as photobionts for lichenizing fungi.   Most of the fungi species that form lichens are sac fungi (Ascomycetes) of 13 orders.
Lichens come in three basic forms: crustose (like a crust), foliose (leaf-like), and fruticose (shrub-like, but in miniature), but have been grouped into up to seven artificial groups.   The taxonomy of lichens is very interesting, based primarily on the types of chemical compounds formed by the lichen, mostly acids, generally referred to as "lichen substances".
There is thought to be over 14,000 species of lichens worldwide.   Many lichen species have very wide (worldwide) range or distribution, while others are somewhat to highly restricted in range or habitat.   There is still much research needed on lichen taxonomy, with new species being described every year.   Currently, there is only one lichen species that is formally listed as endangered by the U.S. Fish and Wildlife Service under the federal Endangered Species Act.   There are no lichens listed under the California Endangered Species Act.   However, there are many species of lichens that are truly endangered and should be listed by the USFWS and several that should also be listed as threatened or endangered by the California Fish and Game Commission.   Listing petitions are currently being drafted for several rare lichens found in California.
Identification for many species requires chemical tests and examination of tiny spores, making field identification next to impossible for many species, particularly the crustose species.   Lichens grow on just about every substrate: bark, soil, rocks, other lichens, mosses, with variations or stratification of such habitats.   Soil lichens are called terricolous.   Rock lichens are called saxicolous.   Lichens growing on bark (that of tree trunks to small twigs) are called corticolous.   Lichens growing on non-living wood [lignum] are called lignicolous.   Epiphtytic lichens are called phytocolous, epiphytes.
Terricolous and saxicolous lichens help control erosion by holding soil and rock particles together.   Lichens also provide habitat and food for a number of wildlife, mostly invertebrates, such as tartegrades.   Many native peoples use lichens for sources of dye, including my Swedish ancestors who used them to color fabrics they wove on their looms.
Below are some information and photographs of various lichens, almost all unidentified at this point, that occur in a wide range of habitats.   They range in color wildly, and contribute to the biodiversity.   Some photos will have more than one lichen species, so look carefully.
One of the best relatively recent publications on lichens is Irwin Brodo and Sylvia and Stephen Sharnoff's 2001 "Lichens of North America", published by Yale University Press.   It provides basic information about lichens, and basic information, maps, and beautiful photographs of all the common lichens on North America.   Identification keys are also provided.
There are many websites about lichens.   One of them (associated with the book above) has a nice page on lichen vocabulary, which can be viewed by clicking here.
I developed an annotated checklist of lichens of Ventura County. At this point, I have found records/reports of 137 lichen taxa in Ventura County.   See if you can add to this list.   I am sure there are many more species to be found and documented.
All photos copyrighted by David L. Magney 2003-2014, unless otherwise noted.