Plant and Animal Endemism in California. Susan Harrison
southern Oregon may be considered part of the Rocky Mountain Province (Takhtajan 1986). The California Floristic Province broadly coincides with the mediterranean climate or mediterranean biome, as defined by rainy winters, dry summers, annual precipitation of 25 to 100 centimeters, and sclerophyllous vegetation (Dallman 1998). (Again, by some definitions, northwestern California and southern Oregon are too rainy, the Sierra Nevada too snowy, and parts of the Central Valley too arid to be considered mediterranean.) Under the broad definition, which is consistent with a floristic analysis of the West Coast (Peinado et al. 2009), there is 70 percent geographic overlap between the state of California and the California Floristic Province (Conservation International 2011). Thus it is reasonable to speak of endemism in California as a natural phenomenon and not just the product of a political boundary. This book uses the broad definition of the California Floristic Province (see Figure 4), in accordance with major works on the flora (Raven and Axelrod 1978; Baldwin et al. 2012), and a novel effort is made to compile data on its animal endemism.
TABLE 1FLORISTIC KINGDOMS, REGIONS, AND PROVINCES
Data on endemism in the state of California were generally obtained from published sources (plants, Baldwin et al. 2012; mammals, CDFW 2003; birds, Shuford and Gardali 2008; reptiles and amphibians, Jennings and Hayes 1994; fish, Moyle 2002; butterflies, Pelham 2008); these lists were updated for taxonomic and distributional changes by consultation with experts. Data on endemism in the California Floristic Province were harder to obtain. Remarkably, for plants there is currently no database from which the thousands of species endemic to the Floristic Province can easily be counted or identified, but a preliminary attempt is made in this book (see Appendix). For animals, the modest lists of Floristic Province endemics were obtained by visually interpreting range maps in atlases and by asking experts on each group.
FIGURE 4. The California Floristic Province, under a broad definition that includes the Sierra Nevada, the northern Coast Ranges, and parts of the Central Valley (regions sometimes excluded from the CFP as being too cold, wet, or dry to be “mediterranean”).
Spatial and Taxonomic Scales
Systematic biases in the estimation of endemism arise from both spatial and taxonomic scales. Larger geographic units will tend to have more endemics than smaller ones. Converting numbers of species to species density (species/area), as is sometimes done, is not a valid correction for this bias because the expected number of species (S) does not increase linearly with area (A). Instead, it follows a logarithmic relationship, S = cAz, where the exponent z is typically 0.15–0.35 among islands or other units that share some of their species. A tenfold increase in area therefore results in only an approximate doubling of species, and species density (S/A) has a strong bias toward being higher on small islands. Among continents or other units sharing relatively few species, z may approach 1.0, reducing the bias in species density (Rosenzweig 1995). Still, the best way to correct diversity for variation in area is to use S/Az, where z is estimated from regressing ln(S) on ln(A). Another solution is to calculate diversity and endemism from species range maps that have been converted to equal-area polygons (e.g., Stein et al. 2000; CDFW 2003), as long as the underlying data are accurate enough.
With regard to taxonomic scale, some data sources report endemism based on all named taxa (species, subspecies, and varieties); others report only full species. Logically, endemism in a given geographic area will always be higher among taxa of lower rank (Kruckeberg and Rabinowitz 1985). Taxa below the species level are described more often and on the basis of smaller differences in vertebrates than invertebrates, and in showier invertebrates (butterflies) than inconspicuous ones (most others). Examples from California suggest this leads to considerable bias. In kangaroo rats, 23 subspecies but only 5 full species are endemic to the state (Goldingay et al. 1997). In birds, 64 named taxa but only 2 full species are state endemics (Shuford and Gardali 2008). In plants, however, endemism is 34 percent for all named taxa and 28 percent for full species (Chapter 3, Table 3). Grasshoppers show endemism of 53 percent for full species plus subspecies and 51 percent for full species only (Chapter 4). The much smaller disparities for plants and grasshoppers than for kangaroo rats and birds suggests that subspecies and varieties are less often described in plants and invertebrates than in vertebrates. In the majority of invertebrates, in fact, surveys are too incomplete for even crude estimates of species-level endemism (Chapter 4). Full species are the focus of this book because of the extra subjectivity and bias introduced by subspecies.
Defining species remains a perennial source of debate in both plant and animal systematics (Mallet 2001). Traditionally, most taxonomists have sought consistent breakpoints in the variation of multiple traits, presumably reflecting a lack of gene flow, as a way to define the boundaries between related species (e.g., Oliver and Shapiro 2007). As molecular data have become increasingly available, one alternative that has gained popularity is that any unique trait can define a lineage as a species (Mallet 2001). In practice, these diagnostic traits are often variations in mitochondrial DNA, which evolves relatively fast in animals. Many existing species can be split up into multiple, small-ranged, and morphologically nearly identical new species under this concept (Agapow et al. 2004). The California raven, for example, could be its own species based on molecular variation, even though it does not differ in appearance or behavior from other North American ravens (Omland et al. 2000). Species numbers would more than double in plants and nearly double in most groups of animals under this “phylogenetic” or “diagnostic” species concept (Agapow et al. 2004), leading to even more substantial increases in endemism. This book accepts and includes all species that have been formally described by any method but does not deal with proposed new species of unclear status, nearly all of which are subdivisions of existing species.
Relative versus Absolute Values
Endemism may be reasonably expressed and compared either in percentages or numbers of species. It is worth remembering that percentages are more meaningful the greater the diversity as well as the higher the taxonomic rank of the group being examined. Thus 50 percent Californian endemism in the grasshopper family Acrididae (with 186 species in the state) means more for the state’s biotic uniqueness than 50 percent endemism in the grasshopper families Eumastacidae and Tanaoceridae (4 and 2 species in the state), or even than 86 percent endemism in the 21 species of Timema (a genus of walking stick insects). Throughout this book, endemism is expressed in both numbers and percentages, in the belief that they provide complementary information.
Comparative information from other geographic regions is essential to characterizing and explaining Californian endemism. Acridid grasshoppers are one of the most endemic-rich groups in California, but they may be equally so in other parts of the mountainous western United States (Knowles and Otte 2000). Whether or not it is remarkable that 5 of 23 kangaroo rats (Dipodomys) or 21 of 22 slender salamanders (Batrachoseps) are endemic to California depends on whether ecologically similar groups are just as diverse in neighboring regions. It is challenging to find, for almost any group, either comparative data or interpretive analyses that place endemism in California in a larger context. This book relies on comparisons with other states and the other four mediterranean climate regions to provide a context for Californian endemism.
LARGE-SCALE PATTERNS IN SPECIES RICHNESS AND ENDEMISM
One of the best predictors of species richness at a global scale is plant productivity, which is determined at large scales by the abundance of water and solar energy. At low latitudes water exerts stronger control, whereas at high latitudes solar energy is a stronger limitation. There are consistently more species of plants and animals in the warm and wet parts of the world than the colder or drier ones, regardless of whether the latitude is tropical or nontropical (Figure 5a; Hawkins et al. 2003). Within the United States as a whole, plant and vertebrate animal diversity is higher in the warmer southerly states (Stein et al. 2000). Within California, in contrast, the diversities of plants, birds, mammals, and amphibians (although not reptiles) are highest in the rainier north (CDFW 2003). However, this is a case where