Like other Mediterranean-type ecosystems, the California Floristic Province is distinguished more by the endemism of its plants than its animals. Of nearly 3,500 species of vascular plants in the hotspot, more than 61 percent are found nowhere else in the world. About 52 plant genera are also endemic.
The number of vascular plant species found in the California Floristic Province is greater than the total number of species from the central and northeastern United States of America and adjacent parts of Canada, an area ten times larger than this hotspot.
This hotspot is largely within the borders of the United States of America, but a portion extends into Mexico. Four subregions within the hotspot are centers of exceptionally high plant diversity: the Sierra Nevada, the Transverse Ranges in southern California, the Klamath-Siskiyou region in the coastal mountain ranges of California and Oregon, and the Coast Ranges. In addition, serpentine soil habitats occur along fault zones in the Central and North Coast and Cascade ranges, from sea level to an elevation of 2,900 meters. Due to specific chemical and physical characteristics of the soils, these habitats are nutrient-poor, and this has led to the establishment of a highly specialized and diverse flora. It has been estimated that serpentine endemic plant species represent 10 percent of the California Floristic Province's endemics.
The hotspot is also home to two spectacular endemic tree species, the Endangered giant sequoia (Sequoiadendron giganteum) and Endangered coastal redwood (Sequoia sempervirens). The giant sequoia, which remains in 75 groves in the Sierra Nevada range, is the most massive species ever to live on Earth, reaching heights of 75 meters and circumferences of 30 meters in the oldest trees. The closely related redwood is often even taller (sometimes reaching 105 meters), although smaller in circumference.
Although there are less than 10 endemic bird species found in the California Floristic Province, out of a total of more than 340 recorded, more species of birds breed in this region than anywhere else in the United States of America. Mexico's Guadalupe Island is the native range of the Critically Endangered Guadalupe junco (Junco insularis), the now Extinct Guadalupe caracara (Polyborus lutosa) and the Critically Endangered Guadalupe storm-petrel (Hydrobates macrodactylus), last recorded in 1912.
The Critically Endangered California condor (Gymnogyps californianus), the largest North American bird, once ranged across most of the continent; its main stronghold is in this hotspot.
Of the more than 150 native mammal species in the California Floristic Province, about 20 are endemic to the region. Several large mammal species once found in the hotspot have been extirpated from California since the arrival of European settlers. These include the grizzly bear (Ursus arctos), grey wolf (Canis lupus), jaguar (Panthera onca), and bison (Bison bison). Ironically, the grizzly bear appears on the state flag of California and has been the state symbol for more than 150 years. A hunter shot California's last grizzly in 1920. Although there are occasional jaguar sightings reported from southern Arizona, this cat has been driven from most of its U.S. range. The last jaguar in California was shot in Palm Springs in 1860.
Other flagship mammal species occurring in the California Floristic Province are the kit fox (Vulpes macrotis), island fox (Urocyon littoralis), widespread Roosevelt's elk (Cervus elaphus roosevelti) and tule elk (Cervus elaphus nannodes), the largest and smallest of the North American subspecies, respectively. The tule elk was on the verge of extinction at the close of the 1800s, but habitat protection and breeding programs helped re-establish a wild population.
Four of the hotspot's nearly 70 reptiles are endemic. A number of species have fragmented populations or low population numbers, including the coast-patched nose snake (Salvadora hexalepis virgultea), the red-diamond rattlesnake (Crotalus ruber) and the western ringneck snake (Diadophis punctatus).
The highest levels of endemism in the California Floristic Province are found among amphibians, with over half of the nearly 50 species found only in this hotspot. In general, the area is notable for its high endemism of salamander species. The most diverse genus of salamanders is Batrachoseps (nearly endemic to this hotspot), which includes the San Gabriel slender salamander (B. gabrieli), recently discovered in mountains in the Los Angeles metropolitan area. Two representatives of the salamander genus Hydromantes are endemic to this region. This genus is interesting in that it has an unusually disjunct distribution; its only other members are found within the Mediterranean region of southern Italy and France. Other noteworthy salamander species are the arboreal members of the Aneides genus, which ascend to the top of the tallest redwoods, and the endemic California tiger salamander (Ambystoma californiense), listed as Vulnerable on the IUCN Red List for Endangered Species. The Endangered arroyo southwestern toad (Bufo californicus), a stocky upland toad found in the hotspot, is protected under the U.S. Endangered Species Act.
The California Floristic Province has a relatively small number of inland fishes (just over 70 species), because of its isolation from the large eastern North American fish fauna by the western mountains and deserts. One of the most interesting groups is a collection of lamprey species, including a cluster of localized landlocked species in the northern mountains.
The hotspot has impressive invertebrate diversity. The state of California is home to an estimated 28,000 species of insects, about 32 percent of which are endemic. These species represent about 30 percent of all known insects in the United States and Canada.