California Floristic Province

The California Floristic Province is one of the five Mediterranean-type hotspots and the only hotspot that occurs largely within the borders of the United States of America. Stretching nearly 1 800 km along the western coast of North America, most of its 293 804 km² are found within the State of California, with extensions into southwestern Oregon and northwestern Baja California, Mexico. All areas west of the peaks of the Cascade and Sierra Nevada ranges are included within its boundaries, as are a handful of islands off California's southwestern coast, including the Channel Islands (913 km²), and Isla Guadalupe (264 km²), located some 300 km west of Baja California.

As its name implies, the California Floristic Province is an ecological construct based on plant species composition, a unique mixture of northern temperate and southern xeric elements fostered by a Mediterranean climate of hot, dry summers and cool, wet winters. Four other hotspots share this climate: Central Chile, the Cape Floristic Region, Southwestern Australia, and the Mediterranean Basin (Barbour et al. 1993; Dallmann 1998).

Four subregions within the Province stand out as centers of exceptionally high plant diversity: the Sierra Nevada, Transverse Ranges, Klamath-Siskiyou region, and Coast Ranges (Stebbins 1978; Davis et al. 1997). Rare plant communities of the southern Sierra Nevada include the giant sequoia (Sequoiadendron giganteum) forest, piute cypress woodland, and lone manzanita shrubland (Grossman et al. 1994). The Transverse Ranges are a narrow strip that runs east to west in southern California, separating the Coast Ranges to the north from the Peninsular Ranges to the south. At least 10 rare plant communities have been recorded from this region. The Klamath-Siskiyou region bridges the coastal mountain ranges of California and Oregon, and is home to approximately 20 rare plant communities, including the most diverse temperate coniferous tree community in the world (Vance-Borland et al. 1995–1996). This region also represents the contact zone between the Pacific Northwest Floristic Province and the California Floristic Province. The Coast Ranges comprise a wide variety of habitats, including coastal dune, coastal salt marsh, maritime chaparral, coastal cypress forest, redwood forest, mixed evergreen forest, mixed hardwoodredwood forest, northern yellow pine forest, southern oak forest, Calocedrus forest, mixed hardwoods, valley oak savanna, coastal prairie scrub, vernal pools, and freshwater marshes, within which at least a dozen rare plant communities can be found (Steinhart 1994; Davis et al. 1997). The unusually high plant diversity in the Coast and Transverse ranges, coupled with the disproportionate human habitat preference for lowlands and foothills near the coast, puts these regions at special risk (Seabloom et al. 2002).

Some of the highest levels of plant diversity within the California Floristic Province are found in the southern part of the Sierra Nevada Range and in the Klamath-Siskiyou region (Davis et al. 1997). 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 m. 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 (Davis et al. 1997). It has been estimated that serpentine endemic plant species represent 10% of the California Floristic Province's endemics (Kruckeberg 1984).

High levels of species diversity and endemism have developed within this region due to its varied topography, climate zones, geology, and soils. Plant diversity is exceptional, with 3 488 native vascular species, including 2 124 endemics. Fifty-two of the region's plant genera are also unique. The total number of plant species is greater than that for the central and northeastern United States and adjacent portions of Canada, an area almost 10 times as large (Raven and Axelrod 1978; Raven 1988; Davis et al. 1997).

The region's impressive endemism is a mixture of outstanding relicts and newly derived species (Stebbins and Major 1965). Many of the relicts once had much larger ranges in the Tertiary, but during the cooling and drying trends that accompanied the ice age in North America, these species became restricted to the California region's relatively mild climate. Because many of these relicts have no extant close relatives, they are particularly important botanically. The most famous relicts that best symbolize the region are the giant sequoia and coastal redwood (Sequoia sempervirens). The giant sequoia, known to reach more than 75 m in height and 30 m in circumference, is believed to be the largest species that ever lived. Although coastal redwoods are among the world's tallest trees, with record specimens surpassing 105 m, they are far less massive than their relatives. Other noteworthy relict trees include the ironwood trees (Lyonothamnus floribundus) endemic to Santa Cruz and Santa Catalina islands, and the California bay laurel (Umbellularia californica). The region's newly produced species result from recent outbursts of speciation within certain genera —primarily annual herbaceous dicots— in response to climate fluctuations since the Middle Pliocene in areas increasingly diverse geographically (Raven and Axelrod 1978). Genera such as Clarkia, Lasthenia, and Phacelia have undergone remarkable radiations. The above-mentioned serpentine endemic species represent examples of such newly produced endemics.

While plant endemism is impressive, vertebrate diversity and endemism are less so. Of approximately 341 resident, breeding, and migrant bird species, only eight are endemic, as are 18 of the 151 native terrestrial mammals. Four of the 69 native reptiles are endemic, including two species found only on Cedros Island, 12.9 km off the Baja California Peninsula: the Cedros Island diamond rattlesnake (Crotalus exsul) and Cedros Island horned lizard (Phrynosoma cerroense).

The highest levels of endemism are found among the amphibians, with 25 endemic species out of a total of 54. The salamander fauna of California is especially noteworthy for the high degree of endemism and its uniqueness: 38 described species, 24 of which are endemic. The region contains one of the two bestunderstood complexes, the ensatina (Ensatina eschscholtzii), which demonstrate stages in the pattern and process of species formation. It also contains the arboreal salamanders (genus Aneides, with three endemic species), which ascend to the tops of the tallest redwoods. The slender salamanders of the genus Batrachoseps are especially diverse, and only one of the 18 currently recognized species does not occur in the hotspot. Included in this genus are the only truly desertadapted fully terrestrial salamanders (two species), as well as the distinctive San Gabriel Mountains slender salamander (B. gabrieli), discovered recently in mountains in the Los Angeles Metropolitan Area.

Isolated from the large eastern North American fish fauna by the western mountains and deserts, the California Floristic Province Hotspot has a relatively small fauna of inland fishes comprising 73 native species. One of the most distinctive elements is a concentration of lamprey species that includes a cluster of localized landlocked species in the northern mountains. Lampreys represent the deepest offshoot of living vertebrates, and eight of the world's 43 surviving species occur in this hotspot. Together with two sturgeons that represent another of the deepest vertebrate lineages, these species cause the hotspot to rank with Japan as global leaders in phyletic rarity for fishes.

Among vertebrates, there are some important flagship species. The grizzly bear (Ursus arctos) has been the California state symbol for more than 150 years, and is immortalized in the names of creeks, mountains, peaks, and valleys throughout California. Some might consider it a classic flagship species for this hotspot, despite the fact that it has been extirpated; the last California grizzly was shot sometime in the 1920s (Wirka 1994). Also, of the world's 13 subspecies of elk or wapiti, two —Roosevelt's elk (Cervus elaphus roosevelti) and the tule elk (C. e. nannodes), the largest and the smallest of the North American subspecies, respectively—are native to the California Floristic Province and served as flagships for conservation throughout the last century. One species that has received a great deal of attention is the endemic California tiger salamander (Ambystoma californiense, VU), currently a major point of contention in rapidly growing Sonoma and Santa Barbara counties. Animals perhaps less well-known, but very effective symbols for local action, include the San Francisco garter snake (Thamnophis sirtalis tetrataenia), California gnatcatcher (Polioptila californica), and island fox (Urocyon littoralis), the latter with six subspecies all confined to the six largest of the eight California Channel Islands. In salt marshes around San Francisco Bay, the salt-marsh harvest mouse (Reithrodontomys raviventris, VU) faces dual pressures from encroaching development and feral house cats.

The most important flagship, however, remains the California condor (Gymnogyps californianus, CR), North America's largest bird, and a strong symbol for wildlife conservation in the western part of the continent. Native Americans considered the condor a sacred animal with supernatural powers, but westward-moving pioneers shot these birds for target practice, collected their eggs, and poisoned the animals upon which they fed. The species was a hair's breadth away from extinction. Today, however, thanks to a collaborative captivebreeding and reintroduction effort involving the U.S. Fish and Wildlife Service, California Department of Fish and Game, National Audubon Society, Zoological Society of San Diego, Los Angeles Zoo, and The Peregrine Fund, captive-bred California condors are being returned to the wild.

California is the most populated (estimated at 35 million people in 2002; U.S. Census Bureau) and fastest growing state in the United States, and this severely impacts biodiversity within the hotspot (Jensen et al. 1993). Urbanization, air pollution, agricultural expansion, logging, strip mining, oil extraction, road construction, livestock grazing, the spread of non-native plants, an increasing use of off-road vehicles, and the suppression of natural fires all pose significant threats. Human population pressures have rendered California one of the four most ecologically degraded states in the country, with all or part of the nation's eight most threatened ecosystems represented: beach and coastal strand, southern California coastal sage scrub, large streams and rivers, California riparian forests and wetlands, California native grasslands, old-growth ponderosa pine forests, cave and karst systems, and the ancient forests of the Pacific Northwest, which include the coastal redwoods (Noss 1994; Noss and Peters 1995).

Today, native grasslands and vernal pool habitats have been reduced to perhaps 1% of their original extent (Holland and Jain 1990), and wetlands, riparian woodlands, and southern maritime sage scrub to 10% or less. Saltwater and freshwater wetlands once covered almost two million hectares in California, but have since declined by about 90% due largely to landfilling and the diversion of river systems. As a result, so have significant shellfish, fish, and waterfowl populations (Barbour et al. 1993). Along California's lowland rivers, riparian forests provide essential wildlife habitat. However, the 4 000 km2 or more of riparian forests found two centuries ago in California's Central Valley have been reduced by 90% due to logging, grazing, and industrial development. Along the coast, redwoods evolved to thrive in a mild, foggy maritime climate until logging began in earnest in the mid-nineteenth century, largely to supply construction needs in San Francisco. Mechanized timber extraction has effectively reduced original stands by 85% since cutting began, although, as Sequoia is one of the only stump-sprouting conifers, many of these stands have regenerated. In total, we estimate that no more than 25% of the California Floristic Province remains in pristine condition.

California has a long history of conservation. In 1864, U.S. President Abraham Lincoln established California's Yosemite Valley and the Mariposa Grove of Giant Sequoias as the first national area to receive protection explicitly for public use (Turner 1991). Together with Sequoia National Park, it offers protection for outstanding biodiversity within the southern Sierra Nevada. Redwood National Park, officially established in 1968 (and expanded in 1978), has evolved over more than 35 years of intensive conservation effort. By contrast, the 1 010-km² Channel Islands National Park provides protection for nesting colonies of seabirds and breeding populations of seals and sea lions. In total, approximately 37% of this hotspot has some form of official protection, although protected areas offering a stricter level of protection (IUCN categories I to IV) cover only 10.2% of the hotspot's surface area. The creation of many protected areas was brought about largely through the actions of groups such as the Sierra Club, Wilderness Society, and The Nature Conservancy. Unfortunately, while a considerable degree of protection has resulted for the highelevation flora, much less, if any, protection has been accomplished for the serpentine or vernal pool subsets of the endemic flora.

In conclusion, while the California Floristic Province lies largely within one of the world's richest nations and contains some of Earth's most famous and most popular national parks, it suffers from threats similar to those operating in hotspots found within countries that are much more disadvantaged economically. Furthermore, a great deal remains to be done in order to ensure that the unique and threatened biodiversity of this hotspot is adequately safeguarded in suitable protected areas. Biodiversity loss clearly is not a problem unique to developing tropical nations.

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