Plant and Animal Endemism in California. Susan Harrison
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Natural History of the Islands of California, Peter Dallman’s Plant Life in the World’s Mediterranean Climates, and the entire California Natural History Guides series published by UC Press. Key resources for data on the state’s species and habitats are the Department of Fish and Wildlife (www.dfg.ca.gov/biogeodata), the Native Plant Society (www.rareplants.cnps.org), and the Calflora project (www.calflora.org).
The botanical bias of this book has to be admitted at the outset. Reasons for this are probably obvious: the long history of studying endemism in California and even of treating California as a biogeographic unit has been largely the work of plant-oriented scientists. Not coincidentally, as Chapters 3 and 4 discuss, Californian endemism is more pronounced in plants than in most animal groups. It is no coincidence, then, that a plant person should attempt a book on Californian endemism. Your author freely admits to having studied Californian plant diversity for the past fifteen years, although my degrees are in zoology, ecology, and biology, and my master’s and PhD work was on insect ecology. Please don’t close the cover, animal lovers; you will find here never before compiled material on the state’s winged, finned, and four- to eight-legged inhabitants. Contemplating the contrasts between plant and animal endemism has been an enjoyable exercise that I hope will interest you as well.
This book was made possible by the generosity of many people. Expert knowledge and data came from Bruce Baldwin, Roxanne Bittman, David Bunn, Frank Davis, Tom Gardali, Terry Griswold, Brenda Johnson, Doug Kelt, Lynn Kimsey, Tim Manolis, Richard Moe, Peter Moyle, Paul Opler, Monica Parisi, Jerry Powell, Greg Pauly, Gordon Pratt, Jim Quinn, Steve Schoenig, Nathan Seavy, Art Shapiro, Aaron Sims, Robert Thomson, James Thorne, Robbin Thorp, Darrell Ubick, Dirk Van Vuren, Phil Ward, David Wake, and David Weissman. The new list of California Floristic Province endemic plants was generously created by Dylan Burge, and the Baja California data were kindly updated by Bart O’Brien. Artwork (or data for artwork) was generously provided by Bruce Baldwin, Ron Blakey, Dylan Burge, Paul Fine, Kathy Keatley Garvey, Brad Hawkins, Lynn Kutner, Ryan O’Dell, Brody Sandel, Cristina Sandoval, Aaron Schusteff, Zack Steel, David Wake, David Weissman, Darrell Ubick, Joseph Vondracek, and James Zachos. Assistance with data processing was given by Brian Anacker, Erica Case, and Brandon Sepp, and illustration help was provided by Steven Oerding and Marko Spasojevic. Kind yet helpful comments on early drafts were given by Howard Cornell, Peter Moyle, Philip Rundel, Mark Stromberg, and John Thompson. Chuck Crumly and Lynn Meinhardt at UC Press shepherded this book through its many stages. Finally, this book is dedicated to all those who have worked to understand and conserve the “Californian” (in the broad sense) flora and fauna . . . you know who you are!
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