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Plant Endemism in California

Patterns and Causes

California’s endemic plants have long been the objects of attention from evolutionary biologists, beginning with classic mid-twentieth-century biosystematics studies of Ceanothus, Layia, and other endemic-rich groups that played an influential role in the modern synthesis of evolution and genetics (Stebbins 1950; see also Smocovitis 1997). Early quantifications of neoendemism and paleoendemism (Stebbins and Major 1965), insular endemism (Raven 1967), and edaphic endemism (Kruckeberg 1954, 1984) in Californian plants gave rise to basic concepts about plant endemism that remain in wide use today. As molecular and phylogenetic techniques have become more prominent, Californian endemic plants have been examined in studies of diversification (e.g., Clarkia, Gottlieb 2003; Mimulus, Beardsley et al. 2004; Layia, Baldwin 2006; Hesperolinon, Springer 2009; Ceanothus, Burge et al. 2011), adaptation to novel habitats (e.g. Mimulus, Wu et al. 2007; Linanthus, Kay et al. 2011), genetic consequences of narrow relictual distributions (e.g., Pinus torreyana, Ledig and Conkle 1983; Pinus radiata, Millar 1999), and genetic and evolutionary processes in island populations (e.g., chaparral shrubs, Bowen and Van Vuren 1997; Camissionia and Cryptantha, Helenurm and Hall 2005; Deinandra, Baldwin 2007).

This chapter evaluates classic and modern evidence on Californian plant endemism: geographic patterns of species and genetic diversity within the state, genetic and evolutionary analyses, fossil evidence, and comparisons between the Californian flora and flora in the other mediterranean climate regions. Together, this makes it possible to reevaluate the classic story told in the previous chapter and to arrive at some conclusions about the relative importance of modern climate, climatic history, geographic barriers, and other factors.

ESTIMATING PLANT ENDEMISM

Raven and Axelrod’s (1978) figures for Californian endemism are comprehensive and remain widely cited. For the state, they enumerated 1,517 endemics (30%) out of 5,046 total species and 26 (3%) endemics out of 878 genera. For the Floristic Province, the corresponding figures are 2,125 (48%) endemics out of 4,452 species and 50 (6%) endemics of 795 genera. They noted that southwestern Oregon contributes 40 endemics and 90 total species, and northwestern Baja 107 endemics and 227 total species to the Floristic Province that are not found in the state. Other sources have cited state endemism as 1,416 species (30%) (Stein et al. 2000) and Floristic Province endemism as 2,124 species (60.9%) (Conservation International 2011), with reasons for the discrepancies being unclear. A recent analysis of collection and literature data finds 106 species endemic to the mediterranean climate coastal corridor of Baja California, as well as another 66 species endemic to the Sierra San Pedro Mártir at the northern end of the peninsula (Riemann and Ezcurra 2007).

For the state of California, new endemism figures accounting for recent taxonomic and distributional revisions are given in Table 3. Note that the figures in Table 3 under the heading “State Endemics Found within the California Floristic Province” are not the same as, nor are they comparable to, “all species endemic to the California Floristic Province.” Rather, they represent species that are restricted to the state and that occur within its Floristic Province regions. A preliminary updated list of Floristic Province endemic plants, including those in Oregon and Baja California, is given in the Appendix.

California’s endemic plant richness easily exceeds that of any other U.S. state (e.g., Texas, 251 species, 6%; Florida, 155 species, 5%), although Hawaii’s proportional endemism is higher (1,048 species, 87%) (Stein et al. 2000). On a global level, the California Floristic Province is similar to Japan (1,371, 34%) and New Zealand (1,618, 81%). Although not rich in endemic genera, it exceeds Hawaii, Japan, and New Zealand (31 [12%], 17 [2%], 39 [10%], respectively) (Raven and Axelrod 1978).

When ecoregional endemism is considered, California is less impressive because it is divided among 13 ecoregions. The Colorado Plateau Shrublands and the Great Basin Shrubsteppe ecoregions have more endemic plants (290 and 151, respectively) than any of California’s 13 ecoregions, of which the richest is interior chaparral and woodland (150) (Ricketts et al. 1999). Other rich floras divided among multiple states are found in Appalachia, the southeastern coastal plain, the rock outcrops or “glades” within the eastern deciduous forest, and the “sky islands” of the southwestern deserts (Ricketts et al. 1999; Stein et al. 2000).

TABLE 3NUMBERS OF PLANTS STRICTLY ENDEMIC TO THE STATE OF CALIFORNIA