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Plant and Animal Endemism in California
By Susan P. Harrison
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2013 The Regents of the University of California
All rights reserved.
Endemism, or the confinement of species or other taxa to particular geographic areas, can be a slippery concept. Every species is confined to some place; for example, it has been estimated that more than 90 percent of the world's plant species are found in only one floristic province (Kruckeberg and Rabinowitz 1985). So when do species or places become interesting on account of their "endemism"? Islands with unique floras and faunas provide the clearest answer. It is no accident that the Galápagos were instrumental to Darwin's thinking. Long-distance colonization, the curtailment of gene flow with close relatives, adaptation to new biotic and abiotic conditions, and (in some cases) the survival of ancestral forms that have become extinct on mainlands can be seen and studied with exceptional clarity on islands that are rich in species found nowhere else. Similar evolutionary forces may be revealed to operate more subtly in regions and habitats with islandlike qualities. California is a good example of an islandlike area within a continent; it is a region of mediterranean climate completely surrounded by mountains, desert, and ocean hostile to much of its flora and fauna, and the nearest similar "islands" are far away, in Chile and the Mediterranean Basin.
The endemic-rich Californian flora has been an influential living laboratory for the study of plant adaptation and speciation. Two of the founders of modern plant evolutionary biology were G. Ledyard Stebbins (1906–2000; UC Berkeley and UC Davis), who first focused evolutionary theory on the study of plants with his Variation and Evolution in Plants (1950) and whose work called attention to the central roles of hybridization and polyploidy in plant speciation; and Jens Clausen (1891–1969; Carnegie Institution), who is best known for leading interdisciplinary experimental studies of genetic differentiation of plant populations along gradients and who wrote Stages in the Evolution of Plant Species (1951). Since the mid-twentieth century, there has been a flourishing tradition of using endemic-rich Californian genera such as Clarkia, Ceanothus, Limnanthes, Madia, and Mimulus as model systems in evolutionary biology (see Chapter 3).
PROBLEMS IN DEFINING ENDEMISM
Before discussing endemism, or geographic restriction, of species to either the state of California or the California Floristic Province (CFP), let us consider some of the issues that affect its definition.
Relationship to Rarity
In common with many other works, this book uses the term endemism to mean the condition of having a limited geographic range, regardless of whether a species can be considered rare. However, in the literature on the biology of rarity, the term is sometimes used in a narrower sense. For example, in a classic review of endemism in higher plants, Kruckeberg and Rabinowitz (1985) define endemics as species existing as only one or a few populations. They note that such species can nearly always be considered rare in the sense of having very small geographic ranges. Many endemics (as defined by these authors) are also rare in the sense of having narrow niches; the best-known examples are plants specialized on particular soils, often called "edaphic endemics." Endemism is uncorrelated with a third type of rarity, namely, low population density; these authors note that endemics are often locally abundant within their narrow geographic ranges or habitats.
Appropriate Spatial Units
Islands are natural units for defining and measuring endemism, because the boundaries of an island are clearly defined and obviously linked to the evolutionary processes giving rise to unique species. This is less true for almost any other kind of geographic unit. Political boundaries seem especially inappropriate since they are unrelated to biology, yet the majority of the world's biodiversity data are compiled by country, state, province, or other similar unit. In the United States, an important source of data is the Natural Heritage Network, a national program founded by the Nature Conservancy in the mid-1970s and now implemented by each state. Each member of the network—in California's case, its Department of Fish and Wildlife—compiles occurrence records of imperiled species and other conservation elements such as natural communities and makes these records available in an interchangeable format. Analyses of these data (Stein et al. 2000), discussed in Chapters 3 and 4, point to California as the U.S. state with the highest number of total and endemic species, although Hawaii is higher in percentage endemism, as is often true of oceanic islands. The problem with this state-based approach is that it greatly understates the diversity of biogeographic regions that occur across many states. Appalachia is an important U.S. center of biodiversity and endemism that encompasses eight states, none of which ranks particularly high in state-level analyses.
Ecoregions are units defined by biogeographers on the basis of shared climates, vegetation types, and major assemblages of species. Various classifications are used by conservationists (e.g., World Wildlife Fund, Nature Conservancy), resource managers (e.g., U.S. Forest Service, Environmental Protection Agency), and biological databases (e.g., The Jepson Manual [Baldwin et al. 2012]). Analyzing endemism by ecoregions seems more defensible than by states, but it has its pitfalls too, and California is a good example. In a global conservation assessment (Ricketts et al. 1999), California does not register high in either species diversity or endemism; as the authors acknowledge, this is because California is so diverse that it is divided into 13 ecoregions. If California's biological uniqueness results from a heterogeneous landscape, across which a common ancestral pool of species has diverged into many localized endemics, this approach underestimates the "true" diversity of California to the same extent that the state-based approach underestimates Appalachia.
Biogeographic units based on assemblages of related species are another alternative. In the most widely used system, the California Floristic Province forms part of the Madrean Region, which belongs in turn to the Holarctic Kingdom (Table 1; Takhtajan 1986). The majority of authors define the California Floristic Province as including all nondesert parts of the state of California, plus south-central Oregon and northwestern Baja California (Figure 4; see, e.g., Raven and Axelrod 1978; Conservation International 2011; Baldwin et al. 2012). Under a narrower definition, the wetter areas of northwestern California and 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.
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.
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 the exception proves the rule, because California is a sunny but arid region in which water is the limiting factor governing plant productivity. Plant diversity in California is positively related to a remotely sensed index of productivity, which in turn is strongly related to rainfall but not to temperature (Figure 5b; Harrison et al. 2006).
Levels of endemism may follow geographic patterns different from total species diversity. Isolated islands, for example, are often high in endemism but low in total richness. Endemism on continents is harder to explain, but one recent analysis suggests that global patterns in endemism are best explained by climatic stability. Sandel et al. (2011) defined the climate change velocity of any given location as the ratio of climatic change over time at that location since the last glacial maximum, 22,000 years ago, to the average change in climate over space at the same location at present. This velocity represents how fast an organism has had to shift its distribution to keep pace with postglacial warming. It is slow, for example, in mild maritime climates that have undergone less change in temperature over time and in rugged regions where present-day temperatures vary sharply over short distances (e.g., from low to high elevations). Globally, animal endemism is higher where climate change velocity is lower, and this effect is stronger for sedentary amphibians than for mobile mammals and birds, suggesting that stable climates have promoted the persistence of sedentary species with small geographic ranges (Figure 6; Sandel et al. 2011).
Excerpted from Plant and Animal Endemism in California by Susan P. Harrison. Copyright © 2013 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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