Endemics arise through the shrinkage of widespread geographic ranges (paleoendemism) and the evolution of new species (neoendemism). Pathways to neoendemism include allopatric divergence, ecological speciation, peripheral isolate formation, and hybridization, all of which are well known in the Californian flora. There is not yet a predictive understanding of the traits that make lineages likely to diversify in particular environments such as California’s.
2
A Brief History of California
Plant and animal diversity in California are clearly linked to the rich complexity of the contemporary landscape, including its rugged topography, (see Figure 1), many climatic zones, varied geologic substrates, and resulting tapestry of vegetation types. This ecological variety has been well described in many places; see, for example, Barbour et al., Terrestrial Vegetation of California (2007), for plant communities and vegetation; and CDFW, Atlas of the Biodiversity of California (2003), for a pictorial overview of plant and animal diversity in relation to the landscape. This chapter presents a brief overview of how California’s ecological landscape evolved (for other accounts, see Edwards 2004; Minnich 2007; Millar 2012). The monograph by P. H. Raven and D. I. Axelrod, The Origins and Relationships of the Californian Flora (1978), is central to this discussion. Raven and Axelrod’s account is exceptional in its sweeping, yet detailed view of the history of a regional flora. Although it has been critiqued on various counts, it has not yet been replaced. This chapter uses the Raven and Axelrod story as an essential starting point. Chapter 3 reconsiders the classic story in light of more recent ideas and evidence.
GEOLOGIC HISTORY
During much of evolutionary history, ocean existed where California is today, and the coastline has gradually grown westward through a series of plate tectonic events (Figure 9; Table 2). Just over 200 million years ago (Ma), the supercontinent Pangaea broke up and North America began colliding at its western edge with smaller oceanic plates known as terranes, causing subduction (movement of one plate beneath another) and accretion (addition of one plate to another). By 140 million years ago, the edge of the continent had reached the present-day location of the Sierra Nevada and its western foothills. Then the same processes shifted farther westward and built the Coast Ranges. Subduction ceased around 28 million years ago when the zone where it was occurring collided with the East Pacific Rise (the midocean trench or spreading center). The subduction zone gave way to a mostly horizontally moving plate boundary fault, namely, the complex of northwest-to-southeast faults known as the San Andreas system, along which the motion is relatively northwest on the west side and southeast on the east side. This change in plate motion ultimately led to the rise of the Coast, Peninsular, and Transverse Ranges.
FIGURE 9. Paleogeography of California in (a) mid-Eocene, 50 Ma; (b) Oligocene, 35 Ma; (c) mid-Miocene, 20 Ma; and (d) late Miocene, 10 Ma. (Source: Ron Blakey, Colorado Plateau Geosystems)
TABLE 2THE GEOLOGIC TIME SCALE (IN MILLIONS OF YEARS)
The Sierras were first uplifted beginning about 80 million years ago and subsequently eroded into an undulating plain that rose gradually 50 million years ago to Tibet-like heights in east-central Nevada. The present Sierra Nevada includes remnants of this old surface as well as younger granitic rocks that intruded as subduction occurred in the Coast Ranges. A second period of more rapid uplift of the Sierra Nevada began around 3 million years ago. The Klamaths either represent the northern end of the Sierra offset westward by a fault 130 million years ago or are the remains of oceanic terranes lying west of the northern continuation of the Sierra Nevada. During much of the Eocene, 56 to 34 million years ago, the Klamath region was an island with a stable land surface that eroded in a tropical climate, and much of this land surface still exists. The uplift of the Coast Ranges began in Southern California around 30 million years ago and migrated northward with the north end of the San Andreas fault and the Mendocino Triple Junction, where it currently continues. The Coast Ranges began as an offshore submerged subduction complex but were fully joined to the continent by 5 million years ago. The east-west Transverse Ranges arose at around that time because of a bend in the San Andreas fault that resulted in compression during the northward movement of the Pacific plate along the fault.
The Central Valley is one of the largest and flattest valleys in the world. It is believed to have been created when a large slab of oceanic plate was thrust over the North American continental margin during the collison of North America with a west-dipping subduction zone. A new east-dipping subduction zone formed west of this slab in the modern Coast Ranges, leaving a broad gap in between. It lay beneath ocean until about 5 to 15 million years ago depending on location. Between about 5 and 2 million years ago, mountain uplift created a marine embayment in the Central Valley encircled by mountains and draining to the ocean from the southern valley near Monterey Bay. Continued mountain uplift later blocked this seaway, and by about 600,000 years ago the Central Valley was a freshwater basin draining through the region of present-day San Francisco Bay (Harden 2004).
Deserts in southeasternmost California contain ancient continental rocks that attest to their having been part of ancient North America rather than accreting onto the edge, as did most of the west coast. The deserts also harbor more extensive fossils of ancient terrestrial life than the rest of the state, including Eocene terrestrial vertebrates. However, their history as deserts is quite recent (see below).
The California Channel Islands are seafloor ridges, transported northward, rotated, and uplifted by movement on the San Andreas fault beginning 28 million years ago. Although the four smaller islands were inundated at various times in the Pleistocene, beginning 1.8 million years ago, parts of the four largest islands have been continuously exposed for at least 600,000 years, and it is even possible that they had ancient (Miocene) connections to the continent. During the lower sea levels of the Pleistocene, the four northern islands were united into the superisland of Santa Rosa lying only 6 kilometers from the mainland. Santa Cruz, the closest island today, is now 30 kilometers from shore.
CLIMATIC HISTORY
The global climate has generally cooled and dried over the past 50 million years, and plate tectonics have played an important role by altering oceanic circulation and atmospheric composition. Major climate-changing events have included the opening and widening of Antarctic ocean passageways, the uplift of the Himalayas and consequent decline in atmospheric CO2 due to chemical weathering, and the arrival from the west of Panama and closure of the Central American seaway. Superimposed on these longer-term trends are oscillations on the order of tens to a few hundreds of thousands of years, caused by variation in the eccentricity, obliquity, and precession of the earth’s orbit around the sun (Milankovitch cycles). The interaction of these forces is complex. In particular, it appears that the tectonically driven changes have increased the sensitivity of the climate system to orbital forcing, leading to increasingly rapid and extreme climatic fluctuations toward the present day (Zachos et al. 2001).
The earth’s transition “from greenhouse to icehouse” has been reconstructed mainly from the carbon and oxygen isotopic composition of foraminiferan shells recovered from Antarctic deep-sea drilling (Figure 10). Warming trends from the mid-Paleocene (59 Ma) to early Eocene (52–50 Ma) produced the Eocene climatic optimum, a time when much of the earth experienced climates resembling today’s wet tropics, except for caps of temperate climate at the poles. This was followed by a long period of cooling, leading to the formation of Antarctic ice sheets by the early Oligocene (34 Ma). Moderate warming from the late Oligocene (26 Ma) to the middle Miocene (15 Ma) reduced the extent of oceanic ice, although temperatures never regained their Eocene levels. Cooling resumed from the middle Miocene to the middle Pliocene (6 Ma). The Northern Hemisphere Glaciation,
