variation caused by its complex geologic structure, and the resulting rich diversity of vegetation types. Though environmental variability is certainly one explanation for high endemism, there are others. California is also rich in internal barriers to dispersal, including mountain ranges, waterways, and offshore islands that have appeared and (in some cases) disappeared over the past 50 million years, leaving detectable imprints on today’s species and genetic diversity (Figure 1).
FIGURE 1. Physiography of California.
The mediterranean climate seems almost indisputably linked with California’s botanical richness. In this odd climate, as in no other, the two things that plants need most—rainfall and warm growing temperatures—are almost completely decoupled from one another in the course of the year (Figure 2). A few fleeting weeks of ideal growing conditions in spring are bracketed by cool, rainy winters and fiercely long, dry summers. Plants adapt in varied ways. Many herbs grow slowly or not at all in winter, mature rapidly and flower in spring, and survive summer as dormant seeds, bulbs, or roots. Lacking these options, trees and shrubs endure summer drought by having tough evergreen leaves or by shedding leaves in the summer. Hot, dry summers followed by windy falls generate intense fires, and plants either resprout or regenerate from dormant seeds. These strategies have evolved in the floras of all five of the world’s mediterranean climate regions, all of which are rich in endemic plants (Figure 3). But climatic history may be as important as today’s climate in explaining California’s biotic uniqueness. The region has remained somewhat equable throughout the global cooling and drying of the past 50 million years (see Chapter 2), avoiding the extremes of glaciation and desertification that have affected much of the earth’s terrestrial surface.
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
