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Introduction

Endemism or biological uniqueness is woven into what most people think of when they hear “California.” Along with Hollywood, the Golden Gate Bridge, and wineries, even nonbiologists might think of coastal redwoods, condors, and fields of orange poppies in their imaginings of the state. Almost anything famously Californian has some connection to the wealth of unique species. The pleasantly winter-wet / summer-dry (mediterranean) climate, for example, is found in only five places in the world; it invariably means not only excellent wines and dense human populations, but an abundance of native plant species adapted to the long dry season. Hollywood is named after an endemic plant with the characteristic mediterranean climate trait of thick evergreen leaves (toyon, Heteromeles californica, one of over 300 plants with the species name californica or californicus). The fog shrouding the coastal redwoods hints at an ancient, wetter, less seasonal climate that also figures importantly in explaining the state’s biological riches. Then there’s geology; an active plate tectonic margin laid the groundwork, literally, for California’s frequent earthquakes and history-making gold rush and the granite pinnacles of Yosemite. Geologic forces also created the array of past and present barriers, including the Golden Gate, Monterey Bay, mountain ranges, and deserts, that not only gave the state spectacular scenery but also split ancestral plant and animal lineages into today’s diverse suites of species. Geologic upheaval also gave rise to the state’s dramatic variety in climates, bedrock, and Soils, producing the most diverse agricultural region in the world and creating equally diverse habitats for native species.

Some characteristically Californian strains of intertwined nature and culture are found in the state’s problems and conflicts, as well as its attractions. Invasive non-native species, threatened and endangered species, and rare plants with disputed taxonomic status are more numerous in California than in most other world regions. Wildfires have become more frequent and severe in Southern California in recent decades, intensifying the challenges of conserving natural habitats in urbanizing landscapes. Water wars, a perennial feature of California politics, have intensified as endemic fish have crashed from fantastic abundance to near-extinction and have been officially listed as endangered. Climate change has led to proposals for alternative energy, mass public transportation, and water storage that threaten some of the state’s best remaining natural areas and longest-held conservation priorities. None of these problems is unique to California, of course. However, their sheer intensity stems from the state’s rich biological diversity combined with its dense human population and, in turn, from its particular blend of climate, topography, and geology.

Focusing on endemism is not the only approach to understanding the origins, distribution, or conservation of biological diversity, but endemism offers an alluring pathway into these questions. Evolutionary biologists have often studied newly evolved endemics to understand the origin of species, and biogeographers use ancient relict endemics as clues to past environments. For ecologists such as me, a biologically distinct region such as California offers the chance to explore how large-scale evolutionary and historical forces (climate change, plate tectonics) interact with local ecological processes (dispersal, competition, disturbance) to assemble ecological communities. For conservation biologists, analyses of geographic concentrations, or hotspots, of endemism have been an important aspect of designing effective strategies. In the policy realm, no law has been more important for biological conservation than the U.S. Endangered Species Act (ESA), and the sheer abundance of ESA-listed species in California (around half of them restricted to the state) has motivated many attempts to fine-tune and supplement this cornerstone law.

Why are there so many endemic species in California? From a scientific perspective, the problem is not so much to find a plausible explanation for California’s richness of endemics as to choose among too many compelling explanations. One often reads that California’s biological richness is a result of its tremendous heterogeneity, in other words, its broad spans of elevation, latitude, and coastal-to-interior climates, the soil 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).