The political boundaries of the state of California cover an area of 424,000 square kilometers but the area includes more than the core area of Mediterranean-type climate. These political boundaries include winter rainfall portions of the Sonoran and Mojave Deserts, as well as areas of cold desert habitats east of the Sierra Nevada. The California Floristic Province, generally defined as the core Mediterranean-type ecosystem area, omits these desert regions but adds northwestern Baja California and southern Oregon. Under this definition, the California Floristic Province covers 324,000 square kilometers. Because of the differences between the political and floristic province boundaries of California, some caution must be used in assessing figures on California biodiversity in the literature.

The geomorphic structure of California is complex and topographic diversity within the floristic region is very high. The region encompasses the Coast Ranges, which extend north and south along the state; the broad Central Valley; the Sierra Nevada range; and the Transverse Ranges of Southern California. The Coast Ranges reach elevations as high as 2,700 m, while Mount Whitney in the Sierra Nevada is the highest point in the continental United States at 4,400 m. The Transverse Ranges of Southern California include a number of peaks above 3000 m. A dynamic geologic history of uplift, faulting, and tectonics has produced complex mosaics of soil structure and parent material. Sharp climate shifts over the Quaternary has brought associated glaciation in the Sierra Nevada.

The foothill regions throughout most of California are typically dominated by mosaics of chaparral shrublands and both evergreen and deciduous woodlands with oak species as the typical dominants. These areas commonly receive 400–800 mm annual rainfall. Rainfall is strongly centered on the winter months, and six months without rain is common. Drier areas along the coast and inland at the transition to desert environments support coastal sage scrub dominated by drought deciduous shrubs and a few species of deeply rooted evergreen sclerophylls.

Mountain areas above 1,500 m in northern California and 1,800 m in southern California show a transition to montane conifer forests, subalpine forests, and alpine communities with increasing elevation. Higher rainfall areas along the central and northern coast support mixes of conifer and hardwood forests. These grade into massive coast redwood forests along the northwestern coast. Mean annual rainfall reaches its highest levels above 2500 mm in this region.

The political boundaries of California contain 4,839 native vascular plant species. An estimated 3,488 of these occur within the California Floristic Province. The state flora as a whole includes 99 ferns and fern relatives, 60 gymnosperms (53 conifers), 823 monocots, and 3862 dicots.

The largest family in this flora is the Asteraceae with 627 native species, followed by the Fabaceae with 297 species and the Poaceae with 251species. The largest five genera make up more than 10% of this total and include Carex (131 species, Cyperaceae), Eriogonum (112 species, Polygonaceae), Astragalus (94 species, Fabaceae), Phacelia (93 species, Hydrophyllaceae), and Lupinus (71 species, Fabaceae). All of these genera comprises largely of herbaceous perennial and annual species. Notable speciation has also occurred in two shrub lineages, Arctostaphylos (Ericaceae) and Ceanothus (Rhamnaceae), in response to adaptations to postfire regeneration (Raven and Axelrod, 1978).

Endemism at the species level is relatively high at 61% within the California Floristic Province. Fifty-two genera are strictly endemic to this province. If another 14 genera that extend only slightly outside of the region into Arizona or Baja California are included, then 8.1% of the genera are endemic. This high level of endemism is heavily influenced by the diversity of annual plants that comprise 27.4% of the vascular plants of the California floristic province. For annual species alone, endemism is 65.3%.

The highest species richness of MTEs in California for small plots appears to occur in lightly disturbed grasslands and oak woodlands, where 47–64 species have been reported in 0.1 ha sites. These levels of diversity at this 0.1 ha scale are also matched in postfire stands of chaparral where annual plant diversity is very high. Mature chaparral, however, exhibits very low levels of species diversity.

The terrestrial mammal fauna of California includes about 160 species, with rodents making up more than half of this total. Roughly 30 mammal species are restricted to the desert regions of the state and thus are not considered part of the Mediterranean-climate region fauna. That leaves a total of about 130 terrestrial mammals native to the shrubland, grassland, woodland, and forest regions of California. These range from the mountain beaver, one of the most primitive living rodents, to the bighorn sheep, but also the striped skunk, mule deer, kangaroo rat, and more.

An additional five species have been extirpated from the state in historical times. These include the grizzly bear, wolf, bison, jaguar (only an occasional visitor in the past), and giant deer mouse. Turnover between habitats (beta diversity) accounts for most of the diversity of mammal faunas, with alpha and gamma diversity relatively low.

Counting resident, breeding, and migratory birds, about 350 total species can be found in California. Shore and marine birds make up 39 percent of this number. Passerines form the largest group of birds with 41 percent of the total. There are 21 species of hawks, vultures, and eagles, 13 species of owls, and 12 species of woodpeckers and flickers. Two bird species are endemic to California. These are the yellow-billed magpie and endangered California condor. In terms of passerine birds, the alpha diversity of bird species across landscape gradients peaks in closed woodland and forest habitats, while species turnover between habitats (beta diversity) is greatest in mid-elevation chaparral.

There are 54 species of amphibians and 69 species of reptiles within the political boundaries of California. The salamander fauna is especially notable with 36 species, 24 of which are endemic. The reptiles include two turtles and tortoises, 33 lizards, and 33 snake species. However, 38 reptile and amphibian species are found largely in the state’s desert habitats.

The native freshwater fish fauna of California includes 73 species of fish. Sacramento sucker, tule perch, sturgeon, and golden trout are among the state’s many bony fish. Anadromous species include steelhead trout and salmon,, whose numbers have declined precipitously due to dams and other water diversions.

 

Native Americans entered California 13-15,000 years ago. Although population densities were never high, these peoples nevertheless had significant impacts on their environment. Most notably, they used fire to maintain meadows, clear underbrush, encourage the growth of certain plants, and ensure easy access to a major food source, the acorns of oak trees.  Rapid population growth in California began with the discovery of gold in 1848, and continues today with associated agricultural expansion, land clearance, and urbanization.

Invasive plant species from the Mediterranean Basin have dramatically altered many plant communities.  European grasses, in particular, have supplanted native evergreen bunchgrass communities. Decades of fire suppression has increased undergrowth density in many areas, particularly woodlands. Controlled burns and vegetation management plans are now being implemented in many areas to help reduce fuel loads and encourage native plant communities.