FIGURE 2. Seasonal distribution of rainfall and temperature in mediterranean climate (Redding, CA); north-temperate climate (Detroit, MI); desert climate (Yuma, AZ); and tropical climate (Hilo, HI).
This book aims to examine all these factors—environmental heterogeneity, barriers, contemporary climate, and climate history—as explanations for California’s endemic richness. Instead of being satisfied with the conclusion that they are all important, an attempt is made to evaluate them critically, using many sources of evidence: comparisons of California with other parts of North America, comparisons of the five mediterranean climate regions to one another and the rest of the world, comparisons of species richness and endemism in different regions within California, and evidence from evolutionary studies of Californian plants and animals.
FIGURE 3. World distribution of mediterranean climates.
The stage is set in Chapter 1 by considering the meaning of endemism, the finer points and pitfalls of measuring endemism, broad global patterns of species diversity and endemism, and the general modes by which species become endemics. The physical history of California and the classic story of the origins of its endemic-rich flora are reviewed in Chapter 2. In Chapter 3, the questions are posed, for plants, Does the classic story hold up under new evidence? What are the relative roles of physical heterogeneity, the novel mediterranean climate, internal barriers, and long-term climatic stability in producing plant endemism in California? Animals are the subject of Chapter 4, which asks what levels of endemism are found in various animal groups in California and whether the explanations relevant to plants also hold up for animals. Chapter 5 examines the unique challenges of conservation in an endemic-rich region and how these are being met in California. The book closes with an attempt to synthesize the answers (Chapter 6).
1
Biotic Uniqueness
An Overview
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