Cover Image

Larger ImageView Larger

E-BOOK

Amphibians and Reptiles of Baja California, Including Its Pacific Islands and the Islands in the Sea of Cortés

L. Lee Grismer (Author), Harry W. Greene (Foreword)

READ AN EXCERPT

Adobe PDF E-Book
ISBN: 9780520925205
$105.00
Other Formats Available:

Please note: UC Press e-books must be purchased separately from our print books, and require the use of Adobe Digital Editions. If you do not already have Adobe Digital Editions installed on your computer, please download and install the software. To complete your e-book order, please click on the e-book checkout button. A charge will appear on your credit card from Ingram Digital Group.

The Baja California peninsula is home to many forms of life found nowhere else on earth. This, combined with the peninsula's rugged and inaccessible terrain, has made the area one of the last true biological frontiers of North America. L. Lee Grismer is not only the foremost authority on the amphibians and reptiles of Baja California, but also an outstanding photographer. He has produced the most comprehensive work on the herpetofauna of the peninsula and its islands ever published. With its stunning color images, detailed accounts of many little-known species, and descriptions of the region's diverse environment, this is the definitive guide to the amphibians and reptiles of a fascinating and remote region.

The culmination of Grismer's quarter century of fieldwork on the Baja peninsula and his exploration of more than one hundred of its islands in the Pacific Ocean and the Sea of Cortés, this book gives information on the identification, distribution, natural history, and taxonomy of each species of amphibian and reptile found there. Preliminary accounts of the life history of many of the salamanders, frogs, toads, turtles, lizards, and snakes are reported here for the first time, and several species that were almost unknown to science are illustrated in full color. The book also contains new data on species distribution and on the effect of the isolated landscape of the peninsula and its islands on the evolutionary process.

Much of the information gathered here is presented in biogeographical overviews that consider the extremely varied environments of Baja California in both a contemporary and a historical framework. An original and important contribution to science, this book will generate further research for years to come as it becomes a benchmark reference for both professionals and amateurs.
Foreword, by Harry W. Greene
Preface
Acknowledgments
General introduction
1. Salamanders
2. Frogs and Toads
3. Turtles and Tortoises
4. Lizards
5. Worm Lizards
6. Snakes
Appendix A. Insular Species Checklist
Appendix B. Claves taxonómicas
Glossary
Literature Cited
Index
L. Lee Grismer is Professor of Biology at La Sierra University.
“The book is exceptional in quality and design. . . . This book is a necessity for anyone interested in the herpetofauna or general natural history of Baja California and surrounding islands.”—Jerry D. Johnston Journal Of The West
"Grismer brings together an impressive amount of original field and laboratory research, supplemented with a thorough review of existing literature, to present a first-rate accounting of the Baja California herpetofauna. This book sets the stage for further research of the reptiles and amphibians of this remarkable region."—Robert Hansen, Editor, Herpetological Review

"Grismer is without question the foremost authority on the herpetofauna of the Baja Peninsula; no one else could have produced such a detailed, comprehensive treatment of the amphibians and reptiles of the area. The book stands as a monument to Lee's many years of dedicated work on the Baja herpetofauna and will be the standard reference on the subject for many years to come."—Julian C. Lee, author of The Amphibians and Reptiles of the Yucatán Peninsula

GENERAL
INTRODUCTION

  Baja California is the second longest and the most geographically isolated peninsula in the world. Over the last four to five million years, it has undergone a uniquely complex tectonic origin and ecological transformation. What we see today as the Baja California peninsula was originally connected to the west coast of mainland Mexico but was torn away by differential movements of the Pacific and North American plates. Since then, it has been carried approximately three hundred kilometers to the northwest along what has become known as the San Andreas Fault. This separation occurred in various stages of uplift, submergence, and geographical fragmentation. Concurrent with these tectonic upheavals were climatic changes that transformed the peninsula from a generally cool and more mesic region to one of extreme aridity. This change was gradual at first but has rapidly accelerated over the last eight to ten thousand years.

Because of its latitudinal extent, its complex topography, and its location between two very dissimilar bodies of water, Baja California accommodates a wide range of climates. This range in turn supports a striking degree of environmental diversity, with habitats ranging from the extremely hot and arid Lower Colorado Valley Desert in the northeast to the humid, tropical deciduous forests of the south. Additionally, a thick coniferous forest is found in its northern mountains and an endemic pine-oak woodland in the Sierra la Laguna of the Cape Region. Central Baja California is covered with an extensive network of volcanic badlands interrupted by palm-lined oases, and its north-central region is one of only three fog deserts in the world. Yet despite its ecological diversity and unique environmental history, Baja California remains one of the most poorly studied regions in North America.

 

Exploration and Research

Exploration and research in Baja California have not yet enjoyed the same attention or success as investigations in the United States or mainland Mexico. This is not a situation that has developed by chance alone but rather one that has emerged through design. Baja California, on the whole, is extremely arid and rugged. The vast majority of its interior is accessible only by foot or on horseback, and thus a researcher's field time is limited by the amount of fresh water and other supplies that can be carried. The scarcity of permanent sources of fresh water in central Baja California has allowed development only in isolated regions near oases or semipermanent riparian areas in the mountainous regions. Even today, simply traveling from one rancho to the next often requires a moderate degree of isolation and travel through unforgiving terrain.

The logistic difficulties impeding research were made clear in two prominent episodes of Middle American exploration that resulted in a noticeable lack of publications dealing with Baja California's natural history. The first involved the series Biología Centrali-Americana—a 67-volume compilation dedicated to the advancement of knowledge of the flora and fauna of Mexico and Central America. This publication, which spanned the period from 1879 to 1915, expressly excluded Baja California. The second period encompassed the Hobart M. Smith and Edward H. Taylor expeditions in Mexico, during which nearly fifty thousand specimens of amphibians and reptiles were collected between 1932 and 1941. Although this effort culminated in the publication of three major annotated checklists by Smith and Taylor (an unparalleled advancement in our knowledge of Mexico's herpetofauna), not a single specimen was collected from Baja California. And still, in spite of the information presented in this book, a gap remains in our knowledge of the herpetofauna of Baja California and the Gulf of California (the Sea of Cortés). Virtually no detailed ecological or natural history studies have been conducted on any species in Baja California, and there is uncertainty as to the exact distribution of others. The harshness and isolation of the islands in the Gulf of California have dissuaded detailed ecological studies on all but a few species. Additionally, many isolated areas such as the Vizcaíno Peninsula, the Sierra los Cucapás, and the upper elevations of the Sierra la Asamblea, Sierra San Borja, Sierra la Libertad, Sierra Guadalupe, and Sierra la Giganta probably contain species that have not been reported and unknown species that have yet to be described. Baja California and the Sea of Cortés are long overdue for a comprehensive treatment of their herpetofauna.

 

The Environment

A thorough understanding of any faunal element requires a sound knowledge of the historical and contemporary ecology of the region in which it occurs. This is especially true of the herpetofauna of Baja California. Unlike continental regions, Baja California has undergone complex tectonic and ecological transformations as a result of uplift, submergence, isolation, and desertification, and these transformations have contributed to its environmental diversity. Thus an understanding of the dynamic relationships and interactions among these abiotic components, both past and present, is paramount in understanding the relationships, distribution, and geographic variation of its herpetofauna.

 

Paleoenvironmental History

FORMATION OF THE BAJA CALIFORNIA PENINSULA

The most significant factor contributing to the endemism and the biotic diversity seen today in Baja California and the Sea of Cortés is the complex geological origin and evolution of these regions. Between 8 and 13 million years ago (mya), most of Baja California lay submerged beneath the Pacific Ocean and nestled against the northwest coast of mainland Mexico, placing what is today the city of San Felipe against the west coast of Isla Tiburón and placing Cabo San Lucas just north of Puerto Vallarta, Jalisco (Gastil et al. 1983). A shallow epicontinental seaway, the proto-Gulf of California, inundated western Mexico, extending north to at least Isla Tiburón and perhaps into southern California by 6 mya (Stock and Hodges 1989; Winker and Kidwell 1986). With the onset of several complex tectonic events associated with differential movements between the Pacific and North American plates, along what would eventually develop into the San Andreas Fault, Baja California was being torn away from the west coast of Mexico and began to migrate northwest (Lonsdale 1989). At the same time, the peninsular ranges were being uplifted, and depositional filling began forming the Vizcaíno Desert and the Magdalena Plain. During its migration, Baja California underwent many changes in contour and topography. Parts of the peninsula may have been influenced by marine inundations, giving it the appearance of an archipelago, while other portions were giving rise to mountain ranges and were besieged with volcanic activity. For a short period, the Cape Region became separated from the rest of the peninsula and existed as an island, while the Gulf of California remained in the Coachella Valley of southern California at least as far north as Whitewater Canyon in Riverside County (Winker and Kidwell 1986). In fact, it may have been the recent uplift of coastal mountains in southern California that contributed to the Gulf's final regression to its present location.

Overlaid on the geomorphological and physiographic evolution of the Baja California peninsula was a series of climatic changes. These changes were the result of an overall drying trend in North America, which actually began during the Eocene—before the peninsula began to separate from mainland Mexico. The intermittent glacial periods of the Pleistocene and the formidable rainshadows caused by the uplift of the Peninsular Ranges brought severe localized drying trends to Baja California that persist today. In the past, however, the central portion of Baja California was a cool, mesic, volcanic, wooded grassland whose upper elevations supported large stands of oaks and other trees much like those seen today in some portions of the Sierra Madre Occidental of Sonora, Mexico. Horses and giant tortoises ranged through these forests. The Cape Region of Baja California was tropical and supported populations of crocodiles, green iguanas, and boa constrictors, as well as semiaquatic elephants, giant hares, and large cats (Miller 1980). Eventually Baja California became the long, topographically complex, generally arid peninsula that we recognize today. This is not to say that its dynamic past has become dormant. To the contrary, the continued tectonic activity along the San Andreas Fault and its associated fault systems attest to the peninsula's unrest and continued northwestward movement.

It has been this intricate evolution of Baja California's physiography, coupled with its complex historical ecological transformation and currently diverse climatic regimes, that has resulted in organisms becoming isolated in particular regions at various points in time and permitted their invasions and reinvasions into other areas during other periods (Grismer 1994a,b). The overall effect of these events has been the evolution of unique flora and fauna and distinct biotic provinces. We must keep in mind, however, that we are merely observing the evolution of Baja California and its inhabitants at a particular moment (the present). Just as conditions and organisms have changed and evolved in the past, they will continue to do so in the future.

 

FORMATION OF ISLANDS

The islands associated with Baja California vary in age, origin, and geological composition. In the Sea of Cortés, there are three principal types of islands, defined by the manner of their origin: oceanic, continental, and landbridge. Oceanic islands have never been connected to mainland Mexico or to Baja California but originated in the Sea of Cortés. Their origin may be due to uplift, as a result of extensional rotation or compression of the underlying tectonic plates, or to volcanic deposition from a series of underwater eruptions. Both are phenomena associated with the crustal extension and the northwesterly movement of the peninsula as it continues to drift away from mainland Mexico. Isla Tortuga, off the coast of Santa Rosalía, is an example of an oceanic island formed by volcanic deposition.

Continental islands were once connected to mainland Mexico or the Baja California peninsula but became separated as a result of tectonic displacements along coastal fault zones. In the Gulf of California, these are usually islands that broke off the trailing edge of the peninsula as it moved northwest. An example is Isla Ángel de la Guarda.

By far the most common type of island in the Sea of Cortés, and along all of Baja California's Pacific coast (with the exception of the Islas San Benito), is the landbridge island. Landbridge islands are relatively young islands (no more than fifteen thousand years old) that were once connected to the mainland or the peninsula. Many of them are simply the emergent peaks of nearby coastal ranges. For the most part, these peaks became isolated because of a rise in sea level. Occasionally landbridge islands are formed from coastal submergence where the earth's crust is thinned as it is stretched. This thinning causes the ground to sink and water to enter from a nearby sea. Such an event may have been the first step in the formation of the proto-Gulf of California. Some landbridge islands have been formed by erosion, which can cut off prominent points of coastal areas and leave them isolated. Isla Espíritu Santo, off the coast of La Paz, may have been created by coastal erosion. There are a few other landbridge islands—such as Isla Willard, which forms part of the northern end of Bahía San Luis Gonzaga, and Isla Requesón, in Bahía Concepción—that are connected to the peninsula by a sandy isthmus at low tide but separated at high tide.

 

Physical Characteristics

An in-depth knowledge of the physiography (shapes and contours) of Baja California is a prerequisite for understanding the geographical orientation of its various phytogeographic regions. Because the distribution of many species is correlated with phytogeography, understanding the physiographic nature of Baja California is essential to comprehending why much of the herpetofauna occurs where it does. Additionally, this knowledge provides insight into the various climatic regimes of Baja California, which reciprocally dictate the distribution of the phytogeographic regions. Thus, physiographic insight is fundamental to understanding the geographical interactions between the herpetofauna and the environment. Major geographic features and towns mentioned below are shown in map 1.

Today Baja California is a thin northwest to southeast-tending peninsula nearly 1,300 km long. It is situated between 32° 30{pr} N latitude and 117° W longitude at its northwestern corner and 23° N and 110° W at its southern tip. Its width ranges from approximately 240 km along the U.S.-Mexican border to less than 30 km at the Isthmus of La Paz. It is separated from the state of Sonora by the Río Colorado in the north and from the rest of Sonora and mainland Mexico by the Gulf of California, approximately 160 km wide. The area of Baja California is approximately 143,400 km2, and its coastline is approximately 3,300 km long. Associated with the coastline are forty-five major islands, each at least 1.3 km2 in area. Several smaller islands are also associated with Baja California, and an additional ten or so major islands are principally associated with the Mexican states of Sonora and Sinaloa (maps 2 and 3).

The most distinctive feature of Baja California's dramatically sculpted topography is a series of mountain ranges, known collectively as the Peninsular Ranges, that run nearly uninterrupted from its northern border to the Isthmus of La Paz. This massive fault-block system is tilted westward toward the Pacific Ocean, and its crest lies slightly to the east of the peninsula's central axis. On the west side the mountains slope gradually toward the coast, whereas the eastern face rises abruptly out of the desert floor, often presenting a precipitous escarpment. Many passes and minor depressions have been cut into the main crest of the Peninsular Ranges, and other, minor fault systems have given rise to additional ranges that radiate outward from the main massif at various angles. The result is a rugged and complex topography that allows the Peninsular Ranges to be conveniently divided into a series of less extensive ranges.

Dominating northern Baja California are two major mountain ranges known as the Sierra Juárez and the Sierra San Pedro Mártir. The Sierra Juárez, which reaches an elevation of nearly 1,410 m, is part of a mountain range that extends from southern California. At its upper elevations, the terrain is flat and supports an ephemeral body of water known as Laguna Hanson. Its eastern face is rocky and drops sharply into a dry-lake basin known as Laguna Salada. The Sierra San Pedro Mártir is a much more dominant feature. Its highest peak, Picacho del Diablo, reaches 3,096 m, and from this elevation one can see both coastlines of Baja California as well as that of Sonora. Situated just west of the crest of the Sierra San Pedro Mártir are three large meadows: La Grulla, La Encantada, and Vallecitos. These meadows, which lie between 1,800 and 2,400 m elevation, drain a large segment of the western face of the Sierra San Pedro Mártir, in a stair-step fashion, toward the Pacific Ocean. Delimiting the southern end of the Sierra Juárez and the northern end of the Sierra San Pedro Mártir is Paso de San Matías. The most significant mountain pass in the Peninsular Ranges, it is situated at the eastern end of Valle la Trinidad, which extends 100 km from southeast to northwest. It lies between 800 m at its northwestern end just east of Ensenada and 975 m at its southeastern terminus at Paso de San Matías. The valley is constricted at the town of Valle la Trinidad by a small, northeast to southwest-tending range known as the Sierra Warner. Northwest of Valle la Trinidad and north of Ensenada is the prominent Valle Guadalupe.

East of the Sierra Juárez lies a series of smaller mountains. The Sierra los Cucapás, Sierra el Mayor, and most of the Sierra las Pintas are separated from the Sierra Juárez by the Laguna Salada basin. The Sierra de los Cucapás and Sierra el Mayor, the more northerly of these ranges, are a contiguous, isolated mass extending south approximately 80 km below the U.S.-Mexican border. These ranges support only scant vegetation and a depauperate herpetofauna. Their highest peak reaches 1,000 m, and the entire range is surrounded by the ancient Río Colorado flood plain. The Sierra los Cucapás is a relatively narrow range, tending southeast and composed primarily of granitic rock. Its southern end, which is marked by a narrow, low pass known as Cañón David, supports broad expanses of basaltic desert pavement. South of this pass lies the Sierra el Mayor, which is more volcanic in composition and somewhat more spread out, angling off in a more southerly direction. It is estimated that as little as ten thousand years ago these mountains existed as an island or islands, and this isolation prompted the evolution of some of their endemic flora and fauna. Currently, the Laguna Salada basin borders their western foothills. Not long ago it was little more than an ephemeral dry lake that filled only during seasons of above-average rainfall. Lately, water from the Río Colorado has been channeled in through the "no-man's-land" separating the Sierra el Mayor from the more southerly Sierra las Pintas. In the early 1980s the level of this lake rose so much that it stretched the entire length of the mountain ranges and wrapped eastward around the southern terminus of the Sierra el Mayor. Mexican Highway 5 had to be elevated to avoid flooding, and culverts were constructed to allow water to pass beneath the highway.

Sixty kilometers south of the Sierra el Mayor and across the southwestern margin of the Laguna Salada basin lies the northern end of the Sierra las Pintas, a contorted, jagged, colorful volcanic range whose peaks protrude through the southern end of the dry lakebed. The Sierra las Pintas and Sierra Juárez are linked at their southern bases by a wide set of rocky, volcanic hills that come together in the north to form the Sierra las Tinajas. This range continues north into the Laguna Salada basin as an isolated finger of land between the Sierra Juárez and Sierra las Pintas. The southern connection of the Sierra Juárez, Sierra las Pintas, and Sierra las Tinajas may account, in part, for the greater floral and faunal diversity of the latter two as compared to that of the more isolated Sierra los Cucapás and Sierra el Mayor.

Immediately south of the Sierra las Pintas lies the Sierra San Felipe, which reaches 1,332 m in elevation and is separated from the Sierra San Pedro Mártir to the west by the Laguna Diablo basin in Valle San Felipe. This is an ephemeral dry lake filled primarily by runoff from the eastern slopes of the Sierra San Pedro Mártir during bouts of heavy summer precipitation. Currently, the Mexican government is sponsoring agricultural development of Valle San Felipe, with the primary crop being Jojoba (Simmondsia chinensis). The Sierra San Felipe angles away from the Sierra San Pedro Mártir to the southeast and generally runs uninterrupted past Bahía San Felipe. At this point it grades into a small series of extremely rugged and broken volcanic mesas and canyons known as the Sierra Santa Isabel and Sierra Santa Rosa. These ranges reach 1,200 m in elevation and slope gradually toward the Gulf of California.

The cismontane areas west of the Sierra Juárez and Sierra San Pedro Mártir, from the U.S.-Mexican border to just north of Cabo Colonet, approximately 200 km south of Tijuana, consist of foothills, plains, and mesas of varying sizes. These features are occasionally interrupted by wide canyons and deep arroyos. This region descends gently toward the Pacific Coast from the crest of the Peninsular Ranges and in some places terminates in precipitous coastal bluffs. The dominant geographic feature of this region lies inland from Punta Santo Tomás, approximately 110 km south of the U.S.-Mexican border. This is a small range of three narrowly connected mountains known from north to south as the Sierra Peralta, Sierra Warner, and Sierra San Miguel. The foothills of these ranges stop short of the Pacific and give way to a series of narrow, coastal plains known collectively as the San Quintín Plain. This plain extends south to Rancho Socorro, where it gives rise to an even narrower plain extending nearly to El Rosario. The Sierra Warner and the Sierra Peralta form a portion of the northwestern border of Valle la Trinidad. Between the Sierra San Miguel in the west and Sierra San Pedro Mártir in the east lie a series of narrow, arid valleys known as Valle San José. These valleys are buffered from cool Pacific breezes by these western ranges and consequently heat up sharply during the summer. As the Sierra San Miguel extend southward beyond El Rosario, they become a jumbled mass of low, often volcanic foothills that blend imperceptibly into the southern end of the Sierra San Pedro Mártir and the Sierra Santa Isabel.

Cismontane Baja California south of El Rosario is a rugged, complex mass of canyons, mountains, low hills, volcanic mesas, ephemeral dry lakes, and deep, sinuous arroyos that collectively make up the Sierra Columbia. The southern sides of the Sierra Columbia abruptly merge into the Vizcaíno Desert just north of the 28th parallel. The dominant features of this area are Mesa San Carlos and Mesa la Sepultura. Mesa San Carlos is a wide, flat, volcanic structure roughly 500 m high and 15 km long on the west coast, approximately 60 km south of El Rosario. Mesa la Sepultura is situated approximately 27 km southeast of El Rosario inland from Mesa San Carlos.

From Mesa San Carlos south to Laguna Manuela, just north of Guerrero Negro, the Sierra Columbia comes within a few kilometers of the coast. Here it constitutes the lower western slopes of the elevated central portion of north-central Baja California, leaving a flat, low-lying, wind-blown, fog-drenched, narrow coastal strip between them and the Pacific Ocean. Inland from the Sierra Columbia and west of the Sierra Santa Isabel, Sierra Calamajué, and Sierra la Asamblea of the Gulf coast, the peninsula opens up into a long, narrow, relatively flat region crisscrossed by a series of low hills and valleys. At the center of this area is a rugged and extensive region composed of granitic uplifted blocks and boulders, through which course palm-lined arroyos supporting sources of semipermanent freshwater such as those at Cataviña and Misión Santa María. The most intriguing feature of this region, El Pedregoso, is a gigantic, isolated pile of granitic boulders stacked some 200 m above the floor of Valle Gato. This entire rocky region is drained primarily by the picturesque arroyos La Bocana and Santa María, which intricately negotiate the southern end of the Sierra Santa Isabel down to the narrow plain along the Gulf coast. Along their courses they support various sources of semipermanent surface water. South of the boulder region the peninsula gives way to a series of large, ephemeral lakes. The first of these, Laguna Chapala Seca, is located approximately 60 km south of Cataviña, just west of the Sierra Calamajué. Two other much larger dry lakes, Valle Agua Marga and Valle Laguna Seca, occur northwest of Bahía de los ángeles.

Along the Gulf side of the peninsula, between Bahía San Luis Gonzaga in the north and Bahía de los ángeles in the south, lie the Sierra Calamajué and, to the south, the much more extensive Sierra la Asamblea. The latter is a prominent granitic uplift reaching nearly 1,700 m in elevation and maintaining two boulder-lined meadows fringed with Mexican Blue Fan Palms (Brahea armata) and Piñon Pine trees (Pinus monophylla) in its upper elevations. The Sierra la Asamblea blends into the Sierra San Borja to the south through a series of small, rugged, volcanic hills in the vicinity of Bahía de los ángeles. The Sierra San Borja and the more southerly Sierra la Libertad coalesce as a mixture of granitic and volcanic ranges and mesas reaching 1,940 m in elevation. They extend south of Bahía de los ángeles and grade into the volcanic foothills of the Sierra San Francisco at the northern edge of the Magdalena Region and eastern edge of the Vizcaíno Desert. Consequently, the Sierra San Borja and Sierra la Libertad lie inland from the Gulf coast and form the western edge of Valle San Rafael, which extends from Bahía de los ángeles to below Bahía San Francisquito.

Just north of Guerrero Negro on the Pacific coast the Vizcaíno Desert begins. This is a low-lying, flat, wedge-shaped area extending 150 km between the volcanic foothills and badlands of the Sierra San Francisco and the Sierra Guadalupe to the east and the Vizcaíno Peninsula to the west. The southern tip of this triangular desert reaches as far as Bahía San Juanico, where it grades into the northern end of the Magdalena Plain. Together the Magdalena Plain and Vizcaíno Desert make up a distinctive cool coastal desert. The dune region of the Vizcaíno Desert is unique in North America: its dunes appear as long, parallel furrows that have formed under the influence of continuous light, steady breezes (Wiggins 1980). The southern portion of the Vizcaíno Desert, south of Laguna San Ignacio, is a broad, flat, sandy plain precipitously edged to the east by the volcanic foothills of the Sierra Guadalupe.

The Vizcaíno Peninsula is a prominent point of land jutting northwest from the center of Baja California. It is composed of the Sierra Vizcaíno in the north and the Sierra Santa Clara at its base. The Sierra Vizcaíno thrusts northwest into the Pacific Ocean, terminating at Punta Eugenia and emerging offshore to form Isla Natividad and Isla Cedros. The southwestern slopes of the Sierra Vizcaíno give rise to a narrow coastal plain that frequently terminates in precipitous ocean bluffs. Punta Eugenia and, in part, Islas Natividad and Cedros cradle the southern portion of the extensive Bahía Sebastián Vizcaíno along the west coast of central Baja California. Where the head of this bay inundates the shallow plain of the northern Vizcaíno Desert, one of the largest lagoon systems in North America has developed. Originally one system, it has now been transformed into three separate lagoons: Laguna Manuela in the north, Laguna Guerrero Negro in the center, and the extensive Laguna Ojo de Liebre (Scammon's Lagoon) in the south. On the southwestern margin of the Vizcaíno Desert is the Sierra Santa Clara, which lies immediately north of a large Pacific inundation known as Laguna San Ignacio. The Sierra Santa Clara forms a series of seven closely proximate, heavily eroded, ancient volcanic peaks and remains one of the least explored regions in Baja California.

Northeast of the Vizcaíno Desert lies the Sierra San Francisco. This range is a dominant feature of central Baja California, with a majestic expanse of volcanic mesas and peaks reaching over 2,100 m. This system is embellished with a complex network of deep, sheer, palm-lined canyons containing permanent fresh water. Within these canyons are found some of the most elaborate cave paintings in Baja California. The Sierra San Francisco gives way in the southeast to a low-lying, broad expanse of volcanic fields and cinder cones through which courses the Río San Ignacio before it empties into Laguna San Ignacio. Just east of the town of San Ignacio, the Peninsular Ranges continue as the Sierra Guadalupe in the north and Sierra de la Giganta in the south. The northernmost portion of the Sierra Guadalupe begins majestically with Volcán las Tres Vírgenes. These are three volcanic peaks that arise from a common base approximately 30 km north of Santa Rosalía. The dominant peak attains an elevation of nearly 2,000 m, and in some years snow can be seen on its north-facing slopes. These are active volcanoes that occasionally spew vapors from their lower slopes. The area surrounding their base is crisscrossed with lava flows that may be as recent as 1746.

The Sierra Guadalupe extends south to near the southern end of Bahía Concepción and has peaks reaching as high as 1,800 m just west of Mulegé. In the north, these mountains begin as two ranges separated by a wide valley wherein Misión Guadalupe is situated. These ranges, characterized by precipitous cliffs, provide some of the most spectacular and dramatic landscapes in all of Baja California. They come together southwest of Mulegé and eventually give way to the Sierra la Giganta, which reaches 1,767 m in elevation at Cerro la Giganta northeast of San José Comondú. From here the range continues irregularly south to the vicinity of the Gulf coast village of San Evaristo. From San Evaristo south, the Sierra la Giganta gradually decreases in elevation, blending smoothly and nearly imperceptibly into the Magdalena Plain at the Isthmus of La Paz. Viewed from the Gulf of California, the eastern face of the Sierra la Giganta appears to rise out of the sea, presenting a mural of strange geologic formations and sinuous bands of color. This face is an abrupt clifflike escarpment dissected by numerous arroyos and sculpted with jagged foothills and volcanic flows that wind their way to the water's edge.

The volcanic badlands immediately west of the Sierra Guadalupe and Sierra la Giganta consist of mesas and deep, palm-lined arroyos created by thousands of years of erosion from streams draining the western slopes of the mountains. Many of these arroyos have large supplies of permanent fresh water that have supplied villages such as La Purísima, San Ysidoro, San José Comondú, and San Miguel Comondú.

The western foothills and badlands of the Sierra la Giganta slope toward the Pacific Ocean and grade into the Magdalena Plain. This is a 55-km wide, flat, sandy area east of Bahía Magdalena, extending south some 300 km, where it tapers to a point near the town of Todos Santos in the Cape Region. The Magdalena Plain was formed from thousands of years of fluvial deposits from the western slopes of the Sierra la Giganta. Currently, its fertile soil supports extensive agriculture. Unfortunately, however, as groundwater for irrigation is being pumped out faster than it is being replaced, seawater incursions from the adjacent coastline are contaminating groundwater supplies. At the southern terminus of the Sierra la Giganta, the Magdalena Plain angles east and fans out across the Isthmus of La Paz, contacting the Gulf of California on the northwestern shore of Bahía de la Paz. Along its central western edge, the Magdalena Plain is bordered by Bahía Magdalena. This bay runs parallel to the west coast for approximately 50 km, where it joins Laguna Santo Domingo in the north. Bahía Magdalena is bordered in the west by Isla Magdalena (shaped like a crooked finger), the mountainous Isla Santa Margarita, and the sandbar Isla Creciente. Laguna Santo Domingo consists of a narrow body of water flowing out of Bahía Magdalena and running parallel to the coast for approximately 145 km. It is flanked to the west by the thin, sandy, northern portion of Isla Magdalena. This western border is interrupted in three locations that form inlets into the bay.

The southernmost portion of Baja California, south of the Isthmus of La Paz, is known as the Cape Region. It is likely that this region had an origin related to but separate from that of the remainder of the peninsula, and throughout its evolution it has made intermittent land-positive connections with the northern areas. The Cape Region is dominated by a range of mountains whose axis begins at Punta Coyote in the north, just east of La Paz, and runs nearly due south, diagonally across the Cape Region, to Cabo San Lucas. Collectively, these mountains are known as the Sierra la Laguna, but the southwestern portion occasionally goes by the name of Sierra la Victoria. Jutting northward from the central portion of the Sierra la Laguna near the town of San Bartolo and reaching the Gulf coast at Ensenada de los Muertos is the Sierra del Álamo. This range forms the southern border of a large, alluvial plain known as San Juan los Planes. Its northern border is formed by the northern section of the Sierra la Laguna, often referred to as the Sierra la Pintada and Sierra la Palmillosa. The highest peak of the Sierra la Laguna, El Picacho, is a pinnacle of rock lying east of Miraflores that rises to nearly 2,200 m. The northern portion of the Sierra la Laguna, south of San Bartolo, supports a large meadow called La Laguna, lying at roughly 1,600 m. This meadow once supported a shallow lake; hence the name. The Sierra la Laguna is a single fault-block, but, unlike the mountains to the north, it tilts to the east, so that the most precipitous slopes face the Pacific Ocean. There are a few large arroyos that cut through the crest of the Sierra la Laguna, the most prominent of which are Arroyo Santiago and Cañón San Bernardo.

A smaller, disjunct series of mountains, the Sierra la Trinidad, fringe the southeastern margin of the Cape Region and border the Gulf coast. The highest peak, Cerro del Venado, reaches about 1,000 m. Approximately ten thousand years ago the Sierra la Laguna and Sierra la Trinidad were separated by a shallow seaway extending through Valle San José, which separates them today.

The islands off the Pacific coast of Baja California are all landbridge in origin, except for the Islas San Benito, which are oceanic (see map 2). The largest and most environmentally diverse Pacific island is Isla Cedros, which reaches nearly 1,200 m in elevation. The remaining islands are generally low, small, and rocky, with the notable exception of Isla Creciente, the long, narrow sandbar enclosing the southern end of Bahía Magdalena. The islands in the Gulf of California are extremely variable in size, habitat, physiography, and geological origin (see map 3). Generally speaking, all are arid and rocky with ridges running from north to south, which on Ángel de la Guarda form an extensive system of mountains. These ranges are granitic or volcanic. Some islands, such as Isla Coronados, Isla Tortuga, and Isla San Luis, are merely the emergent tops of oceanic volcanoes.

 

Climate

With the exception of the northwestern and southeastern sections of the peninsula, most of Baja California and the Sea of Cortés form the southwestern portion of the Sonoran Desert (Axelrod 1979). The climate is characterized by relatively high annual mean temperatures with low precipitation. Baja California's length, complex topography, and location place it weakly under the influence of moisture from four different sources (Hastings and Turner 1965). It lies at the southern edge of winter cyclonic systems from the westerlies that affect weather patterns in the north; at the western edge of summer monsoons originating in the Gulf of Mexico that affect weather patterns of eastern Baja California and the Gulf of California; at the eastern edge of tropical storms and hurricanes that range across the eastern North Pacific in autumn and affect weather patterns of southern Baja California; and at the western limit of fall activity in the easterlies. However, the region is dominated by two major climatic regimes: winter cyclonic storms from the north and fall hurricanes from the south (Turner and Brown 1982).

Northern Baja California receives the majority of its precipitation from winter storms, which usually originate in the western Pacific and sweep southeast over the peninsula (Hastings and Turner 1965). These storm fronts weaken as they stretch farther south (Humphrey 1974), and usually they do not extend past Laguna San Ignacio. Occasionally, high-pressure cells over the western United States will push storms south into central Baja California and the Cape Region, where they are known as equipatas and are usually accompanied by cold temperatures. The gradual rise of the western slopes of the Sierra Juárez and the Sierra San Pedro Mártir induces a great deal of precipitation in the form of either rain or snow and relieves these passing storm fronts of the majority of their moisture. Consequently, little rain falls east of these mountains (Markham 1972; Hastings and Humphrey 1969), although their associated low-pressure systems are responsible for strong winds. Because of this rainshadow effect, the adjacent desert regions are the hottest (Meigs 1953) and driest areas in North America, receiving only about half as much annual rainfall as Death Valley, California (Markham 1972).

Freezing temperatures are rare in the western coastal areas but quite common in the northern mountains. Snow begins to fall on these ranges in late November but usually does not persist for more than a few days, except at the higher elevations in the Sierra San Pedro Mártir, where it may last considerably longer and fall as late as March. Further south, snow is rare but has been observed on Cerro Matomí, Cerro San Borja, and Volcán las Tres Vírgenes; during the unusually cold winter of 1987-1988, snow fell west of Bahía de los Ángeles and was observed on Isla Ángel de la Guarda. From March to November, the northwest portion of the peninsula is characterized by windy days and cloudy nights. Although little rain falls (Humphrey 1974), early-morning low cloud cover usually extends inland to blanket the foothills of the Peninsular Ranges and adds a significant amount to this region's precipitation (Markham 1972). Much of this cloud cover comes in the form of radiational fog, which develops as the ground ceases to radiate heat during the night. This causes air temperatures to drop, and if humidity is high enough the water vapor condenses to form fog (Logan 1968). Such fogs, which are common in the northwest during the spring and fall, usually burn off by mid-morning.

The northern winter storms generally lose their effectiveness somewhere between Bahía de los Ángeles in the northeast and Laguna San Ignacio in the southwest (Aschmann 1959). South of here, the peninsula, and more specifically the Gulf coast, receives the majority of its precipitation from two kinds of summer storms. The more common are convectional storm systems, which result from the orographic lifting and cooling of humid tropical air over the mountains (Humphrey 1974). As the northern desert regions of North America heat up during the summer, the rising air creates a low-pressure cell that draws warm, moist air north from the tropical Pacific areas. As this air mass gains momentum and surges across the Gulf of California, it picks up additional moisture. When it moves onto land, it slams into the precipitous eastern face of the Peninsular Ranges, forcing this moisture-laden air to rise as it crosses the mountains. As it rises, it quickly cools and releases its moisture in the form of heavy and often violent rainshowers with spectacular displays of thunder and lightning. When tropical moisture is present, such storms (aguaceros) occur almost daily in the higher elevations of the Sierra San Pedro Mártir, Sierra la Asamblea, Sierra San Borja, Sierra la Libertad, Sierra Guadalupe, Sierra la Giganta, and Sierra la Laguna; this is the case from July to September (Humphrey 1974) and occasionally into early October. Often the tops of these storms flatten and spread out, bringing rain to the surrounding lowland areas as well, but they may be extremely localized, affecting only the upper elevations of the mountains.

The region is also affected by hurricanes. These are powerful anticyclonic systems that develop over the warm waters off the west coast of southern and central Mexico from July through November (Markham 1972). Instead of turning out to sea, a hurricane may sweep up the Gulf of California, hitting the east coast of the peninsula, or cross the Pacific Ocean and hit the southwestern coastline. Hurricanes that do not come onshore, but continue up the Gulf of California, feed on the warm waters and increase in intensity as they move north, generating winds of over 200 kph. On September 28, 1976, Hurricane Kathleen hit La Paz with 208-kph winds and torrential rains; these caused a dam above the town to burst, burying two to three thousand people in its debris. On rare occasions, severe hurricanes have reached as far north as southern California and have even caused deaths at San Felipe on the east coast and El Rosario in the west. After moving off the warm Gulf waters onto land, hurricanes soon dissipate.

Chubascos or toritos are smaller anticyclonic systems that usually develop locally, within the Gulf of California. They too are characterized by strong winds and locally heavy rains and can cause severe localized damage when they move onto land. Although they generally last only a few hours, they are some of the most dangerous storms in the Gulf of California. They build very rapidly and have caused numerous deaths among boaters and fishermen who find themselves suddenly trapped in strong winds and rough seas.

These southern summer storms affect the southeastern portion of the peninsula much more than they do the northern and western areas. In fact, they often extend no farther north than Bahía de los Ángeles in the east and Laguna San Ignacio in the west. When both winter and summer storms of the same season falter, the central portion of Baja California may receive no rainfall at all. Occasionally, southern cismontane areas are unaffected by such storms because they lie in the rainshadow of the Sierra la Giganta (Markham 1972).

The strongest influence on the temperature of Baja California is the dissimilarity of its surrounding bodies of water. The Pacific Ocean dominates the temperature regime of western Baja California with its south-flowing, cold California Current, which is responsible for the coastal advection fog (Shreve 1951) and cloud cover that, with steady onshore breezes, keep the coastal temperatures relatively low. In consequence, the west coast of Baja California, from approximately El Rosario south to Todos Santos, is a cool coastal desert that Humphrey (1974) referred to as the Pacific Coastal Desert. In fact, the northern portion of this area, south to Punta Eugenia and inland 6 to 10 km, is one of only three fog deserts in the world (Meigs 1966). Its fogginess results from warm, moist belts of air cooling as they descend and contact the cold surface waters of the California Current. As this air cools, its moisture forms fog near the surface of the water and is carried inland by continuous onshore breezes. The California Current is submerged beneath the north-flowing, slightly warmer Davidson Current in the vicinity of Bahía Magdalena during the fall. Here, along the Magdalena Plain, the Pacific Ocean also influences the climate of the peninsula, although cloud cover is less and annual mean temperatures are slightly higher (Hastings and Turner 1969).

The temperature of the southern and eastern portions of Baja California is controlled primarily by the Sea of Cortés. A much warmer body of water than the Pacific (Robinson 1973), the Sea of Cortés offers little amelioration to the peninsula in the form of precipitation or cooling. The areas east of the Peninsular Ranges receive no cooling from onshore Pacific breezes, and consequently the Gulf coast becomes extremely hot during the summer (Markham 1972).

 

Phytogeographic Provinces

Baja California's wide range of climatic variables and well-sculpted topography support clearly defined phytogeographic regions (Shreve and Wiggins 1964; Wiggins 1980; Grismer 1994b). Phytogeographic regions are good indicators of natural biotic provinces because they are ecological reflections of the interactions of climate, topography, and soil. They are distinguished from one another with the notion that many of the plants therein share the same ecological and environmental history. A comparison of the distribution and geographic variation of the herpetofauna with the phytogeographic regions of Baja California (map 4) reveals that the two coincide very closely (Grismer 1994b). Therefore, it is important to understand not only the floristic composition of these regions, which sets them apart from one another, but also the environmental reasons why they exist where they do. The following characterization of the various phytogeographic regions follows Grismer 1994b and is adapted from Shreve and Wiggins 1964, Wiggins 1980, Turner and Brown 1982, and Cody et al. 1983.

 

CALIFORNIA REGION

The California Region is a relatively cool, mesic area occupying the cismontane areas of northern Baja California from the U.S.-Mexican border south to the vicinity of El Socorro. Its eastern border lies on the lower western slopes of the Sierra Juárez and Sierra San Pedro Mártir at the edge of the Jeffrey Pine Belt. This region, which is basically a southern extension of the coastal scrub and chaparral communities of southern California, occupies the Pacific coastal scrub and foothill chaparral areas of northwestern Baja California. The Pacific coastal scrub areas (fig. 1) occur in the sporadic, flat, low-lying western portions of this region from sea level to the edge of the chaparral belt at approximately 300 to 600 m elevation. The dominant plant species here are drought-deciduous forms such as California Sagebrush (Artemisia californica), White and Black Sage (Salvia apiana and S. mellifera), California Buckwheat (Eriogonum fasciculatum), and Coastal Agave (Agave shawii).

The chaparral portion of the California Region occurs primarily in canyons in the western part of the region and at the higher elevations in the foothills of the Peninsular Ranges to the east. This chaparral assemblage is characterized by larger (1-3 m) evergreen shrubs (fig. 2) such as Chamise (Adenostoma fasciculatum), Hoary Leaf-Lilac (Ceanothus crassifolia), Chaparral Ash (Fraxinus trifoliata), Toyon (Heteromeles arbutifolia), Scrub Oak (Quercus berberidifolia), Laurel Sumac (Malosma laurina), Lemonadeberry (Rhus integrifolia), and Red Shank (Adenostoma sparsifolium). South and inland from Cabo Colonet, this area begins a gradual transition into the more arid southerly Vizcaíno Region.

 

BAJA CALIFORNIA CONIFEROUS FOREST REGION

The Baja California Coniferous Forest Region is the southernmost disjunct and depauperate section of the broader and more inclusive Sierran Montane Conifer Forest (Pase 1982), which, in Baja California, is composed of two disjunct sections. This cool mesic area occupies the upper elevations of the northern Sierra Juárez and southern Sierra San Pedro Mártir above the chaparral belt of the California Region in the west and the creosote bush scrub of the Lower Colorado Valley Region in the east. It extends south from the U.S.-Mexican border approximately 300 km to near Cerro Matomí and is a southern extension of the coniferous forests of southern California. The Baja California Coniferous Forest Region receives more precipitation than any other area in northern Baja California. The majority of this comes from cold northern winter storms, but a significant portion also results from convectional southern summer storms. Consequently, the floristic composition is relatively diverse, composed of many large shrubs and trees but with a conspicuous lack of understory vegetation. The dominant species of this region are Piñon Pine (Pinus quadrifolia) and Jeffrey Pine (P. jeffreyi), the former being more prevalent in the Sierra Juárez (fig. 3) and the latter more prevalent in the Sierra San Pedro Mártir in the higher elevations. Other conifers, such as Lodgepole Pine (P. contorta subsp. murayana), can be found among the Jeffrey Pine, Sugar Pine (P. lambertiana), White Fir (Abies concolor), Quaking Aspen (Populus tremuloides), and Incense Cedar (Calocedrus decurrens) (fig. 4).

 

LOWER COLORADO VALLEY REGION

The Lower Colorado Valley Region is the largest subdivision of the Sonoran Desert, but only a thin southern extension of its western portion enters Baja California. It occupies the areas between the western coastline (or the Río Colorado in the extreme northeast) of the Gulf of California and the eastern foothills of the Peninsular Ranges, extending from the U.S.-Mexican border south to just below Bahía de los Ángeles. In the northern portion of eastern Baja California, the region is a low-lying, flat, sandy, hot, and arid area lying in the rainshadow of the Sierra Juárez and Sierra San Pedro Mártir. South of here, from Puertecitos to Bahía San Luis Gonzaga, and again south to Bahía de los Ángeles, the region is rugged and characterized by small, eroded volcanic terraces and mountain ranges. Owing to its temperature and aridity, the Lower Colorado Valley Region of Baja California is dominated by various small-leafed, drought-resistant plants (fig. 5). The most common of these in the flat, sandy areas to the north are Creosote Bush (Larrea tridentata), White Bursage (Ambrosia dumosa), Ocotillo (Fouquieria splendens), Brittlebush (Encelia farinosa), and Desert Agave (Agave deserti). In the arroyos and rugged foothill areas, Mesquite (Prosopis glandulosa), Smoke Tree (Psorothamnus spinosa), Little-Leaf Palo Verde (Cercidium microphyllum), Ironwood (Olneya tesota), and chollas (Opuntia spp.) are also common.

 

VIZCAÍNO REGION

The Vizcaíno Region is an arid region occupying the central third of western Baja California. The marine influence of the Pacific Ocean leaves this region cooler than the other desert areas, but because of its southerly and cismontane location it receives little in the way of winter or summer rainfall. Therefore it is composed mostly of desert species. The northern margin of the Vizcaíno Region begins near El Rosario and extends east to the crest of Sierra San Miguel and south to the vicinity of San Ignacio. Here it is bordered by the volcanic foothills of the more southerly Magdalena Region. From this point, it arcs southwest along the foothills of the Magdalena Region to the Pacific Coast near Bahía San Juanico. In the north, maritime influences extend inland approximately 6 to 10 km, and vegetation along this narrow coastal strip is scant. This area, extending from El Rosario to Laguna San Ignacio, forms a unique subdivision within the Vizcaíno Region that Meigs (1966) considered to be a "fog type" temperate desert. Vegetation in open areas along this strip is stunted, widely spaced, and depauperate (fig. 6) because of continuous onshore winds, which release little moisture in their passage. In areas protected from the winds but available to precipitation, floral diversity increases sharply. The dominant plants of this coastal subregion are bursages (Ambrosia chenopodifolia and A. camphorata), Coastal Agave (Agave shawii subsp. goldmaniana), Bush Sunflower (Encelia farinosa), Palo Adán (Fouquieria diguetii), and Jojoba (Simmondsia chinensis). Inland from this coastal subregion and above 500 m elevation, the conspicuous species are Blue Agave (Agave cerulata), bursages (Ambrosia dumosa and A. camphorata), Palo Adán, Cirio (Fouquieria columnaris), Cardón (Pachycereus pringlei), Elephant Tree (Pachycormus discolor), Pitahaya Agria (Stenocereus gummosus), Little-Leaf Palo Verde (Cercidium microphyllum), and Mesquite (Prosopis glandulosa) .

The southern portion of the Vizcaíno Region, which is much flatter, comprises the Vizcaíno Desert. The Vizcaíno Desert is a triangularly shaped area beginning just south of Punta Rosarito and extending inland to the low, rocky foothills of the Peninsular Ranges. From here it continues south along the southern and eastern margins of the foothills to near Bahía San Juanico. The western margin of the Vizcaíno Desert north of Guerrero Negro is bordered by the Pacific Ocean. South of Guerrero Negro, it is bordered by the mountainous Vizcaíno Peninsula, from Punta Eugenia south to Laguna San Ignacio. South of Laguna San Ignacio, the Vizcaíno Desert is bordered by the Pacific Ocean once again. The Vizcaíno Desert is characterized by cool, cloudy, and breezy days most of the year. As a result, the vegetation is relatively depauperate and prostrate. Much of the Vizcaíno Desert (fig. 8) is characterized by Datilillo (Yucca valida), Palo Adán (Fouquieria diguetii), Lomboy (Jatropha cinerea), Yerba Reuma (Frankenia palmeri), and salt bushes (Atriplex julacea and A. polycarpa). In the west-central portion surrounding the town of Guerrero Negro, the vegetation becomes prostrate and uniform (fig. 9). Here the dominant plants are A. julacea and F. palmeri.

 

CENTRAL GULF COAST REGION

The Central Gulf Coast Region is a narrow zone beginning near Bahía de los Ángeles and continuing south along the eastern foothills and escarpments of the Peninsular Ranges to the Isthmus of La Paz. At the isthmus, it is interrupted by the eastern expanse of the Magdalena Region but begins again on the coastline northeast of La Paz and continues south to Ensenada de los Muertos in the Cape Region. South of this ensenada, it is interrupted a second time by the Arid Tropical Region but persists once more on the coastline east of Sierra la Trinidad. This region is very hot and arid and receives virtually no winter rainfall. The majority of its precipitation comes in the form of runoff from southern convectional storms in the bordering Peninsular Ranges during the summer. Occasionally it receives rainfall from chubascos and large convectional storms that flatten and spread out at high altitudes. As a result, the Central Gulf Coast Region is composed of desert vegetation and is characterized by Leatherplant (Jatropha cuneata), Lomboy (Jatropha cinerea ), Palo Adán (Fouquieria diguetii), Copál (Bursera hindsiana), Elephant Tree (B. microphylla), Cardón (Pachycereus pringlei), and Little-Leaf Palo Verde (Cercidium microphyllum) (fig. 10). In the Cape Region, the flora of the Central Gulf Coast Region consists of additional dominants such as Pitahaya Agria (Stenocereus gummosus).

 

MAGDALENA REGION

The Magdalena Region occupies the western desert slopes and Pacific drainages of the Sierra Guadalupe and Sierra la Giganta. The Magdalena Region begins at San Ignacio in the north and arcs southwest toward the coast, contacting the Pacific near Bahía San Juanico. Here it continues south to just north of Todos Santos. The eastern border extends south from San Ignacio along the base of the Peninsular Ranges to the Isthmus of La Paz, where it arcs east and contacts the Gulf coast. Its relatively warmer climate is influenced by the warmer Davidson Current during the fall and the colder California Current at other times of the year. However, morning fogs are common, and annual precipitation is low and unpredictable, owing to the rainshadow effect of the Sierra la Giganta and its southerly location, which keeps it outside most winter storm tracks. In fact, this area has gone as long as five years without rain.

The Magdalena Region is topographically divisible into two sections. An eastern portion extends between San Ignacio and the Isthmus of la Paz. This area consists of a "badlands" composed of volcanic hills, fields, and mesas extending along the base of the Sierra la Giganta (fig. 11). It is edged to the west by a small southern section of the Vizcaíno Desert and the Magdalena Plain. Here the dominant species are Creosote Bush (Larrea tridentata), Leatherplant (Jatropha cuneata), Lomboy (Jatropha cinerea), chollas (Opuntia spp.), Palo Blanco (Lysiloma candida), Pitahaya Agria (Stenocereus gummosus), Organ Pipe Cactus (Stenocereus thurberi), Palo Adán (Fouquieria diguetii), Little-Leaf Palo Verde (Cercidium microphyllum), and Cardón (Pachycereus pringlei). Palm-lined oases (fig. 12) are characteristic of some of the deeper canyons and arroyos in these volcanic badlands, and there the dominant species are the Mexican Fan Palm (Washingtonia robusta) and Palma Palmía (Brahea brandegeei).

West of the Comondú area, the Magdalena Region opens up into the Magdalena Plain, which constitutes the region's western section. This section occupies nearly half the width of the southern third of Baja California. Here the volcanic fields become less complex along the fringe of the western foothills of the Sierra la Giganta and eventually give way to a low, flat, open and extensive sandy plain that borders the Pacific Ocean. The Magdalena Plain is composed primarily of fluvial deposits from the western slopes of the Sierra la Giganta and in unusually wet years has served as a large, shallow, freshwater basin. The climate of the Magdalena Plain is influenced strongly by the cool Pacific Ocean. This region is often shrouded in morning advectional fogs whose moisture supports the growth of Spanish Moss (Ramalina reticulata) on the vegetation (fig. 13). In fact, the fogs produce enough moisture to support the growth of bromeliads (Tillandsia recurvata) on the high-tension wires in the towns of Ciudad Constitución and Ciudad Insurgentes, and there are even road signs warning of a zona de neblina (fog zone) along the main highway. The dominant floral components of the Magdalena Plain (fig. 14) are Mesquite (Prosopis glandulosa), Coast Desert Thorn (Lycium californicum), chollas (Opuntia spp.), Palo Adán (Fouquieria diguetii), Pitahaya Agria (Stenocereus gummosus), Organ Pipe Cactus (Stenocereus thurberi), and Cardón (Pachycereus pringlei).

 

ARID TROPICAL REGION

The Arid Tropical Region is composed of two disjunct sections. The first occurs on the mountainous upper elevations of the Sierra Guadalupe and Sierra la Giganta, beginning approximately due east of Mulegé and terminating just north of the Isthmus of La Paz. This region varies in elevation from 504 to 2,088 m, and the majority of its precipitation comes between June and October, from southern convectional storms that leave between 200 and 300 mm of rain each year. Winter fogs coming off the Gulf of California sometimes reach this area as well. Because of its elevation, temperatures are cooler than along the Gulf coast, and, coupled with the relatively high amount of summer rainfall, they enable the region to support many large tropical deciduous shrubs and trees. The dominant floristic components are Palo Blanco (Lysiloma candida), Creosote Bush (Larrea tridentata), Palo Fierro (Pithecellobium confine), Vinorama (Acacia brandegeana), Palo Chino (Acacia peninsularis), Mesquite (Prosopis glandulosa), Palo Zorillo (Cassia emarginata), Pitahaya Dulce (Stenocereus thurberi), and Little-Leaf Palo Verde (Cercidium microphyllum) (fig. 15). Also very common in the Sierra la Giganta are several species of pincushion cactus (Mammillaria spp.) and chollas (Opuntia spp.). In the bottoms of many canyons that have water or underground streams, it is common to find thick stands of Palma Palmía (Brahea brandegeei) and the fig Zalate (Ficus palmeri).

The southern section of the Arid Tropical Region encompasses the low-lying, flat areas of the Cape Region, beginning at the Isthmus of La Paz and continuing south to Cabo San Lucas. The Arid Tropical Region of the Cape surrounds the Sierra la Laguna. As a result, weather patterns and floristic makeup can be significantly different on opposite sides of the mountains. On the west side of the Sierra la Laguna, temperatures are slightly cooler because of the influence of the Pacific Ocean, and precipitation is generally lower because this area lies in a rainshadow and is not greatly affected by the summer storms. The western area is basically a mesquite and thorn-scrub woodland characterized by many of the same species that occur in the lower elevations of the Sierra la Giganta. The Arid Tropical Region on the east side of the Sierra la Laguna receives much more precipitation and has been considered by some to be an impoverished jungle (fig. 16). Species present on the west side of the Sierra la Laguna also occur here but are not usually the dominant forms. More characteristic of the eastern flora are species such as Coralvine (Antigonon leptopus), Palo Fierro (Pithecellobium confine), Vinorama (Acacia brandegeana), Cardón-Barbón (Pachycereus pecten-aboriginum), Coral Tree (Erythrina flabelliformis), Ciraelo (Cyrtocarpa edulis), Trumpet Bush (Tecoma stans), Little-Leaf Palo Verde (Cercidium microphyllum), Palo Blanco (Lysiloma candida), Plumeria (Plumeria acutifolia), Copál (Bursera hindsiana), Elephant Tree (Bursera microphylla), and Leatherplant (Jatropha cuneata).

 

SIERRA LA LAGUNA REGION

The Sierra la Laguna Region occupies the upper elevations of the Sierra la Laguna, which runs diagonally across the Cape Region from near La Paz to Cabo San Lucas. Owing to its geographic position and altitude, this region has a variety of climates, ranging from the hot, relatively dry foothills of the north to the wet, tropical climate of its upper elevations in the south. These high elevations induce summer storms, moving north out of the southern Gulf and Pacific, to deposit much of their moisture. This area receives as much as 700 mm of rainfall each year, more than any other area in southern Baja California. The vegetation of this region corresponds to its climates. In areas below 800 m, the dominant plants are Copál (Bursera hindsiana), Elephant Tree (Bursera microphylla), Morning Glory (Ipomoea spp.), Palo Blanco (Lysiloma candida), and Mauto (Lysiloma divaricata). In areas lying between 800 and 1,300 m there is a transition to oak and pine-oak communities (fig. 17) dominated by Encino Negro (Quercus devia), Encino Roble (Quercus tuberculata), and a pine (Pinus cembroides).

 

INSULAR REGIONS

Because of climatic similarities, insular flora of the Gulf of California and along the Pacific coast are generally extensions of those of the adjacent continental phytogeographic regions. Any differences are usually due to the island's relatively small size and low elevation and its being surrounded by water: for example, more moderate mean temperatures and slightly less diverse vegetational structure and composition. Generally, on islands greater than 3 km2, however, the vegetation is essentially the same as on the adjacent mainland (Cody et al. 1983). Therefore, all islands within the Gulf of California, with the exception of the Isla Encantada Archipelago, lie within the Central Gulf Coast Region (fig. 18). The Isla Encantada Archipelago is included in the Lower Colorado Valley Region. On the Pacific coast, Islas Coronado and Todos Santos are part of the California Region. Islas San Martín and San Gerónimo lie within the transition of the California and Vizcaíno regions and support species from both phytogeographic regions. Islas San Benito (three islands), Cedros, Natividad, San Roque, and Asunción are dominated by Vizcaíno Region flora, although Isla Cedros supports relict chaparral and pine communities at its upper elevations (Oberbauer 1993). The large coastal islands of Magdalena and Santa Margarita lie within the Magdalena Region. Some small Gulf islands, such as Encantada, Raza, and San Pedro Mártir, serve as seabird rookeries, and the accumulation of guano on these islands has become so thick that it has nearly choked out most forms of vegetation (fig. 19).

 

The Herpetofauna

History of Herpetological Research

The first written report concerning amphibians and reptiles to emerge from Baja California came from the Jesuit missionary Francisco Javier Clavigero in 1789. His account of the amphibians and reptiles comes primarily from an unpublished manuscript written by the Jesuit Miguel del Barco during his tenure and travels in Baja California from 1738 until the expulsion of the Jesuit order from Mexico in 1768. Clavigero writes:

In California there are few kinds of reptiles, to wit, large and small lizards, frogs, toads, turtles, and snakes. Among the species of large and small lizards, we do not know that there are any poisonous ones. Frogs are very scarce. Toads are abundant when it rains, but they disappear entirely when the land is dry again. Among the turtles, besides the common land variety and the fresh-water ones, there are two other species of large sea turtles. One of these is that from whose shell tortoise shell is obtained to use in prized and curious works.. ;nbsp;. ;nbsp;.There are two kinds of snakes, the rattlesnakes or crotali, as Linnaeus calls them, and those which are not. The latter are smaller than the former; but their poison is more active.
The second report came from the German naturalist F. Deppe as a result of his overland trek from La Paz to San Diego during the spring of 1837. Deppe stated, "The peninsula is poor beyond description in living forms of organic life. Neither reptiles nor insects were seen" (quoted in Lichtenstein 1839). These reports remained the most current sources of published information concerning the herpetology of Baja California for nearly half a century.

Biological exploration in Baja California and the Sea of Cortés did not really get under way until the early to mid-nineteenth century. The first expeditions were primarily concerned with the collection of plants and birds, and very few amphibians and reptiles were taken. The first significant herpetological collections to come out of Baja California were made by L. ;nbsp;J. Xantus de Vésey in the Cape Region between 1859 and 1861. During the next twenty-five years or so, great naturalists such as Spencer F. Baird, Lyman Belding, M. ;nbsp;P. ;nbsp;E. Botta, Townsend S. Brandegee, Walter E. Bryant, Gustav Eisen, William M. Gabb, and others added so significantly to these collections that by 1895 John Van Denburgh was able to publish the first review of the herpetofauna of Baja California and its associated islands. We now know that Van Denburgh's 1895 review of Baja California herpetology was very incomplete, but it served as a starting point on which others could build.

During the years 1888-1892, the first major collection from central Baja California was made by Léon Diguet, a French chemical engineer employed by the Boleo Mining Company at Santa Rosalía. Based on this material, in 1899 François Mocquard published a descriptive work that significantly augmented that of Van Denburgh. In 1911, the United States Bureau of Fisheries and the American Museum of Natural History sponsored the Albatross Expedition, whose mission was a biological reconnaissance of the coast and bordering islands of Baja California. The expedition was in the charge of Charles H. Townsend, who devoted special attention to the collection of reptiles and fishes. As a result of Townsend's efforts, Mary C. Dickerson (1919) described one new genus and twenty-three new species of lizards. Shortly afterward, Charles Nelson presented a faunal analysis of the Baja California herpetofauna, and Karl P. Schmidt produced a comprehensive, descriptive work. Both were among the first to entertain hypotheses concerning the evolutionary origin of the herpetofauna as a faunal unit. John Van Denburgh's 1922 Reptiles of Western North America considered species from Baja California and some of the islands in the Sea of Cortés but excluded the amphibian fauna. Schmidt 1922 and Van Denburgh 1922 have remained the best descriptive accounts of the Baja California and Gulf of California herpetofauna to date.

Other works concerning large segments of Baja California's herpetofauna were mostly island checklists (Van Denburgh 1905; Van Denburgh and Slevin 1914, 1921a; Cliff 1954a,b; Soulé and Sloan 1966; Savage 1967; Bostic 1975; Wilcox 1980; Murphy and Ottley 1984, Grismer 1993, 1999b, 2001a), none of which treated simultaneously the Pacific and Gulf islands. Savage (1960) presented a dispersal-based hypothesis to account for the origin of the herpetofauna of Baja California, and later Murphy (1983a,b), Welsh (1988), and Grismer (1994a,b, 2000) discussed vicariant models.

 

Biogeography

FAUNAL RELATIONSHIPS

As currently understood, the native, nonmarine herpetofauna of Baja California, its associated Pacific islands, and the islands in the Sea of Cortés is composed of 161 species. Of these, 4 are salamanders, 13 are frogs, 4 are turtles, 84 are lizards, 55 are snakes, and one is an amphisbaenian. Readily apparent is the depauperate nature of the amphibians compared to the modest diversity of lizards and snakes. This is easily accounted for by the generally hot and dry climate of Baja California. In fact, of the species of amphibians present, most are restricted to the northwestern portion of the peninsula in cismontane or montane mesic areas or just marginally enter Baja California in the extreme northeast, in association with the Río Colorado and its delta. Only three species of amphibians naturally range into Baja California Sur. Three others, the Bullfrog (Rana catesbeiana), Río Grande Leopard Frog (R. berlandieri), and Forrer's Grass Frog (R. forreri), have been introduced.

Much of the herpetofauna of Baja California and the Sea of Cortés occurs in the United States and mainland Mexico, but a large percentage is endemic to Baja California. In this work, the term endemic pertains to species restricted to the region of study as a geographical and biological unit (Grismer 1994a) rather than a political one. Therefore, some species, such as the Peninsular Leaf-Toed Gecko (Phyllodactylus xanti) and the Baja California Rat Snake (Bogertophis rosaliae), which range continuously throughout the peninsula and enter southern California, are considered endemic to Baja California. In this context, the northern limit of Baja California is the northern limit of the Peninsular Ranges at Mount San Jacinto in Riverside County, California, in the east and the Los Angeles Basin in Los Angeles County, California, in the west. These areas are all part of the same massive fault-block system from which the Baja California peninsula is derived. Some taxa, however, such as the Sandstone Night Lizard (Xantusia gracilis), which are endemic to the Baja California peninsula as defined here, occur outside the geopolitical boundary of Baja California and are not covered in this work.

Of the 161 nonmarine species in the region of study, 23 percent are shared only with the United States, 29 percent are shared with the United States and Mexico, less than 1 percent are shared only with Mexico, and 48 percent occur only in Baja California. Of these, approximately 64 percent are believed to be more closely related to and in some cases derived from mainland Mexican lineages, 6 percent are more closely related to species in the United States, and 30 percent are related to species occurring in the United States and Mexico (Grismer 1994a,b). Thus, it is clear that the origin of this herpetofauna is more intimately associated with mainland Mexico than with the United States (table 1). These numbers do not reflect the fact that 12 percent of the species shared exclusively by Baja California and the United States only marginally enter the northern areas. At least five additional species have been introduced into the region of study.

 

Historical Biogeography

Grismer (1994a) divided the peninsular herpetofauna into various biogeographical complexes based on hypotheses concerning their evolutionary origin. Since that time, the phylogenetic relationships of some of these species have been reevaluated, and some of them have been placed in different groups (see table 2). One group of species that is endemic to Baja California appears to have evolved in southern Baja California as the peninsula was separating from mainland Mexico during the Miocene and is referred to as the Southern Miocene Vicariant Complex. All species (or monophyletic groups of endemic species) of this complex have close relatives in southwestern Mexico. Additionally, because of the Cape Region's intermittent isolation from the more northerly portions of the peninsula, many of these forms have distinctive pattern classes, or in some cases sister species, in the southern portion of the peninsula, whose contact zones are generally situated around the Isthmus of La Paz. Among the fossil species known from the Cape Region are the large land tortoise (Geochelone sp.), boa constrictor (Boa sp.), and crocodile (Crocodylus sp.).

Many species were widespread in northern Baja California, southwestern United States, and northwestern Mexico when northern Baja California existed as a western extension of mainland Mexico. Eventually the distribution of these forms was bisected by the northern extension of the newly forming proto-Gulf of California in the late Miocene to early Pliocene, when it extended at least as far north as Banning Pass in Riverside County, California. This separation prompted the formation of closely related forms on either side of the northern portion of the Gulf of California. When the Gulf finally receded in the Pleistocene, these sister taxa came into contact with one another and are now parapatric near the head of the Gulf of California or occur on opposite sides of the Coachella Valley. This group is referred to as the Northern Pliocene Vicariant Complex.

Another group, composed of species that are currently distributed continuously around the head of the Gulf of California in close association with the Sonoran, Mojave, and Great Basin deserts, is referred to as the Western Desert Complex. Presumably these species evolved outside Baja California in association with the formation of these deserts and entered the region from the northeast after the regression of the Sea of Cortés and the union of the southern peninsular regions. As a result, their geographic variation on the peninsula is generally weak and clinal, and their closest relatives occur in northern mainland Mexico. A portion of this group includes species that are situated at the head of the Gulf of California and only marginally enter Baja California. These species are closely associated with the low-lying, sandy regions left behind after the regression of the Gulf or the flora thereon and have not dispersed further south into Baja California. A few species with diverse origins outside Baja California are restricted to its northeast portion. These forms probably entered Baja California after the regression of the Gulf. These are strictly associated with the Río Colorado, its drainages, or its flood-plain vegetational associates and are referred to as the Río Colorado Group.

Similarly, a group of mesophilic forms occurring in northwestern Baja California appears to have entered from North America. These are referred to as the Northwestern Complex. The dispersal of these forms into Baja California may have been facilitated by the uplift of coastal mountains in southern California during the Pleistocene. This uplift may have forced the Gulf to recede to its present position and provided a mesic corridor through which these species could disperse southward. The fact that they generally dispersed no further south than El Rosario was likely the result of the simultaneous formation of the arid Vizcaíno Region, which would have served as an effective xeric barrier to the dispersal of these mesophilic forms, just as it does today. The closest relatives of these forms are found in the Pacific Northwest and central United States.

Another group of mesophilic species is restricted to cismontane and montane woodland mesic environments of northern Baja California. These species were presumably widespread throughout North America during cooler and wetter periods, but with the onset of Pleistocene drying trends became fragmented and restricted to higher elevations where cool and moist climates still prevail. The closest relatives of these species occur in the northern portions of the Sierra Madre Occidental and the mountains of central Arizona. This group is referred to as the Chaparral-Madrean Woodland Complex.

One group of species is ubiquitous throughout Baja California, the United States, and much of Mexico, along with its closest living relatives. Because these species are so widespread, it is exceedingly difficult to form viable hypotheses concerning their historical origin in Baja California. Finally, there is one other group of species whose relationships to other species remain obscure and of whose distribution our knowledge is incomplete. Before any reasonable hypotheses can be made regarding its evolutionary origin in Baja California, some of these issues must be more fully addressed.

 

ECOLOGICAL BIOGEOGRAPHY

Although some species of the Baja California herpetofauna are ubiquitous in distribution, many others can be placed into loosely defined ecological groups in accordance with their general natural history and distribution (Grismer 1994b; table 3). Thus, unlike historical biogeography, ecological biogeography (or ecogeography) groups species according to contemporary environmental factors and is not concerned with their geographic or phylogenetic history. Looking at the herpetofauna in terms of its ecological biogeography is useful because it serves to underscore the diversity of this region and the wide array of adaptive types that inhabit it.

There is a Northwestern Mesophilic Group containing cismontane species whose distributions closely follow or are contained within the California Region. These species avoid the more arid portions of the peninsula and thus generally range no further south than the vicinity of El Rosario. A distinctive Northern Montane Group occurs in the Baja California Coniferous Forest Region of the Sierra Juárez and Sierra San Pedro Mártir. Some of the members of this group are endemic to these mountains, and all represent the fragmented southern distribution of more widely ranging northerly species or closely related forms. A Northeastern Xerophilic Group of species occupies northeastern Baja California in the Lower Colorado Valley Region, east of the Peninsular Ranges, and generally extends no further south than Bahía San Luis Gonzaga. Most of the members of this group have various specializations for living in the open, sandy, and arid extremes of this area. Another group, whose distribution in Baja California is contained entirely within the Lower Colorado Valley Region, is the Colorado River Riparian Group. Species of this group enter Baja California along the Río Colorado and its tributaries. Some are strictly aquatic; others find shelter and food in the vegetation along the watercourses or irrigation canals. None of the members of this group would occur in Baja California if it were not for these water sources. A Northern Transpeninsular Mesophilic Group contains species that are widespread in the mesic California and Baja California Coniferous Forest regions of the north; but as they extend south into the more arid regions of the peninsula, their distributions become highly fragmented and restricted to mesic refugia such as springs, oases, and mountaintops (Grismer 1988a, 1990a; Grismer and McGuire 1993; Grismer and Mellink 1994; Grismer et al. 1994; McGuire and Grismer 1993). Those that extend into the Cape Region become more widespread in distribution once again in the more mesic Arid Tropical and Sierra la Laguna regions. A similar but more widely distributed Transpeninsular Xerophilic Group ranges throughout the arid Lower Colorado Valley, Vizcaíno, Central Gulf Coast, and Magdalena regions of the peninsula. Members of this group are conspicuously absent from the mesic California and Baja California Coniferous Forest regions of the northwest and, to a large extent, the Sierra la Laguna Region of the Cape. There is also a Saxicolous Group that is generally restricted to the more arid, rocky areas of the Peninsular Ranges. Members of this group usually range throughout many phytogeographic regions and show little geographic variation. The limiting factor of their distribution seems to be the presence of rock, and the species of this group are usually endemic to Baja California. There is a Southern Xerophilic Group that is also composed entirely of endemic species. Its members are widespread in the more southern, xeric regions of the peninsula but do not enter the Lower Colorado Valley Region of the northeast or the northern mesic areas. These species generally do not range further north than El Rosario in the west and Bahía de los Ángeles in the east. A Southern Mesophilic Group, which is similar in ecology to the Northern Transpeninsular Mesophilic Group, is composed of endemic species that are widespread in the mesic Arid Tropical and Sierra la Laguna regions of the Cape Region. As these species extend north into the more arid central regions of the peninsula, their distributions become fragmented and restricted to springs, oases, and mountaintops. Last, there is a Cape Region Group composed of endemic species that are confined to the mesic Arid Tropical and the Sierra la Laguna regions south of the Isthmus of La Paz.

 

DISPERSAL CORRIDORS, DISPERSAL BARRIERS, AND HABITAT ISLANDS

The distribution limits of any reproductively cohesive group of organisms or any population result from the interaction between the population's genetic limitations and the ecology of the region within which it occurs. For example, organisms that are not genetically adapted for living in deserts usually do not do so, and the distribution limits of the population to which they belong often track the fringes of adjacent desert environments. Such relationships between populations and their environments are most obvious in areas where geographically proximate environmental extremes result from interactions between major physiographic features and climate. This principle is eloquently demonstrated in the general ecogeography of the herpetofauna of Baja California and even more so in general trends involving dispersal barriers, dispersal corridors, and habitat islands.

Dispersal corridors are generally localized regions whose physiography and ecology allow certain groups of species access into regions where they would not occur without such corridors. Five such corridors occur in northern Baja California (see map 1). The first is an east to west-tending depression in the Peninsular Ranges known as Paso de San Matías. This is a narrow, low-elevation pass, approximately 0.5 km wide, 980 m in elevation, and 6.5 km long. This pass separates the Sierra Juárez in the north, which reaches an elevation of 1,800 m, from the Sierra San Pedro Mártir in the south, which reaches 3,007 m. Paso de San Matías provides a corridor through which some desert species extend west into the coastal California Region (Grismer 1994a, 1997; Grismer and McGuire 1996; Welsh and Bury 1984), whereas, in other parts of their distribution, they are unable to cross the Peninsular Ranges. Species that are generally restricted to arid regions but are filtering through Paso de San Matías into the cooler, more mesic coastal areas of Valle la Trinidad, immediately to the west of Paso de San Matías, include the Desert Iguana (Dipsosaurus dorsalis), Zebra-Tailed Lizard (Callisaurus draconoides), Long-Nosed Leopard Lizard (Gambelia wislizenii), Banded Rock Lizard (Petrosaurus mearnsi), Desert Spiny Lizard (Sceloporus magister), Desert Night Lizard (Xantusia vigilis), Shovel-Nosed Snake (Chionactis occipitalis), and Ground Snake (Sonora semiannulata). Interestingly, however, there appear to be no coastal species extending east onto the desert floor. Apparently desert environments exceed the physiological capabilities of most cismontane species, although coastal environments do not exceed those of some desert species.

Another dispersal corridor, known as Valle San José, lies between the Sierra San Miguel to the west and the Sierra San Pedro Mártir to the east. This is a northwest to southeast-tending valley approximately 47 km long. Its northern end is confluent with Valle la Trinidad, and its southern end opens onto the Vizcaíno Region near the southern terminus of the Sierra San Pedro Mártir. The westerly Sierra San Miguel blocks the cooling maritime influence of the Pacific Ocean, leaving the valley hot and arid. This allows a northerly invasion of the arid-adapted Vizcaíno Region flora (Shreve 1936). It also enables arid-adapted species of reptiles common to the Vizcaíno Region to extend northward, west of the Peninsular Ranges into the California Region (Welsh 1988; Grismer 1997). Such species include the Baja California Leopard Lizard (Gambelia copeii), Baja California Collared Lizard (Crotaphytus vestigium), Zebra-Tailed Lizard (Callisaurus draconoides), Desert Night Lizard (Xantusia vigilis), Sand Snake (Chilomeniscus stramineus), and Baja California Gopher Snake (Pituophis vertebralis). Some of these species extend north all the way into Valle la Trinidad (e.g., C. draconoides, G. copeii, and P. vertebralis), and one continues into the United States through similar habitat (G. copeii).

Three other major dispersal corridors in northern Baja California are particularly significant to saxicolous (rock-dwelling) species. The first is the Sierra San Felipe. This large, rocky mountain range, extending some 70 km from the northern end of Laguna Diablo in the north to the Sierra Santa Isabel to the south, reaches over 1,200 m elevation. The Sierra San Felipe lies 18 km to the east of the Sierra San Pedro Mártir, the main block of the Peninsular Ranges, across the dry lake known as Laguna Diablo. A connection with the southern end of the Sierra San Pedro Mártir, by way of a complex network of rocky canyons and low mountain ranges generally referred to as the Sierra Santa Isabel, provides a dispersal corridor through which saxicolous Peninsular Ranges species can extend their distribution eastward into the Lower Colorado Valley Region. Species utilizing this corridor are the Baja California Collared Lizard (Crotaphytus vestigium), Northern Chuckwalla (Sauromalus obesus), Banded Rock Lizard (Petrosaurus mearnsi), Granite Spiny Lizard (Sceloporus orcutti), Peninsular Leaf-Toed Gecko (Phyllodactylus xanti), Rosy Boa (Lichanura trivirgata), Lyre Snake (Trimorphodon biscutatus), and Speckled Rattlesnake (Crotalus mitchellii).

The remaining two dispersal corridors in northern Baja California are the Sierra las Tinajas and the Sierra las Pintas. Both ranges extend from a common base on the southern end of the Sierra Juárez, just north of Paso de San Matías, and fan out to the northeast like two large fingers. The Sierra las Pintas is the larger and more easterly of the two. Like the Sierra San Felipe, these rocky, volcanic, mountain ranges extend east into the Lower Colorado Valley Region and allow certain saxicolous species to extend their distributions. Saxicolous species known from these ranges are the Baja California Collared Lizard (Crotaphytus vestigium), Northern Chuckwalla (Sauromalus obesus), Banded Rock Lizard (Petrosaurus mearnsi), Barefoot Banded Gecko (Coleonyx switaki), Peninsular Leaf-Toed Gecko (Phyllodactylus xanti), Rosy Boa (Lichanura trivirgata), and Speckled Rattlesnake (Crotalus mitchellii). One can see that these ranges are depauperate relative to the Sierra San Felipe, and potential colonizers still occur in the Sierra Juárez. Noticeably absent are the Granite Spiny Lizard (Sceloporus orcutti) and Lyre Snake (Trimorphodon biscutatus).

Dispersal barriers prevent populations from extending their distributions from one locale to an adjacent one. They are usually the result of major physiographic features such as mountain ranges, but they may also be zones of ecological transition from one environmental region into another as a result of climate. An example of the former would be the northern Peninsular Ranges, which prevent species from dispersing from the Lower Colorado Valley Region into the California Region simply because they are unable to cross the mountains. An example of an environmental barrier created by an ecological transition occurs in the cismontane region between San Quintín and El Rosario. Here, although there are no large physiographic features that would prevent the southern dispersal of many of the more northerly-occurring species of northwestern Baja California, a number of these species range no further south than El Rosario. This is because the degree of aridity in this region begins to exceed their physiological capabilities and has altered the ecology of the environment. There are also some less obvious physiographic dispersal barriers within climatically homogeneous regions that play a significant role in the local distribution of many species. One such barrier is a large volcanic flow in the Lower Colorado Valley Region between Puertecitos and El Huerfanito. This flow, which is approximately 28 km wide and of relatively recent origin, bisects what would otherwise be a generally flat, narrow, sandy coastal desert plain extending south from San Felipe to Bahía San Luis Gonzaga. Its presence limits the southern distribution of some widely distributed species of the Lower Colorado Valley Region to the vicinity of Puertecitos. These include the Long-Nosed Leopard Lizard (Gambelia wislizenii), Desert Spiny Lizard (Sceloporus magister), Glossy Snake (Arizona elegans), and Western Shovel-Nosed Snake (Chionactis occipitalis). It also limits the northern distribution of southern species along the east coast of the peninsula to the vicinity of El Huerfanito (e.g., Gambelia copeii).

Similarly, the continuity and volcanic nature of the southern extent of the Sierra Columbia, the Sierra la Libertad, the Sierra Guadalupe, and the Sierra la Giganta in central Baja California may limit many lowland species to areas west of the Peninsular Ranges despite the existence of suitable habitat along the east coast of the peninsula. The Sierra Columbia, which contacts the west coast of the peninsula at approximately 17 km north of Jesús María, marks the northern boundary of the Vizcaíno Desert. From here, this range arcs southwest toward the interior of the peninsula, contacting the Sierra la Libertad and forming the eastern boundary of the Vizcaíno Desert. The Sierra la Libertad continues south as a generally unbroken, rocky barrier that merges into the Sierra Guadalupe just southeast of San Ignacio. The Sierra Guadalupe, in turn, grades into the Sierra la Giganta, which continues south to the northern edge of the Isthmus of La Paz. This mountainous barrier delimits the southern edge of the Vizcaíno Desert and the northern and eastern boundaries of the Magdalena Plain. At the Isthmus of La Paz, the Sierra la Giganta gently fades into the Magdalena Plain, which then extends east across the peninsula, contacting the Gulf coast at Bahía de la Paz. These mountains may act as a dispersal barrier to certain species, denying them access to the low-lying Central Gulf Coast Region to the east. Species whose distributions are generally restricted to the Vizcaíno Desert and Magdalena Plain and only reach the Gulf coast through the Isthmus of La Paz are the Five-Toed Worm Lizard (Bipes biporus), the Baja California Leopard Lizard (Gambelia copeii), and the Baja California Glossy Snake (Arizona pacata).

Ever since the legendary works of Alfred Russel Wallace and Charles Darwin, islands have held a special place of reverence for evolutionary biologists. Their isolation and general inaccessibility often preserves them as natural laboratories for evolutionary studies. When we think of islands we generally envision some landmass surrounded by water, comprising a fortuitous array of species that happened to disperse over water from an adjacent continental source. This is a limited conception, however, because there are other types of islands that are not surrounded by water and may be far more isolated than those that are. These are known as habitat islands. A habitat island is a localized region of habitat within, and surrounded by, a much broader area of a differing habitat. Examples include permanent water holes in the desert that maintain a streamside flora and an array of aquatic fauna. Unlike the case of an island surrounded by water, where colonization through overwater dispersal is a passive, ongoing (albeit slow) phenomenon, in habitat islands many species lack dispersal opportunities and are truly isolated. The reason is simple. For organisms to reach a habitat island they would have to actively disperse across uninhabitable terrain, something of which they are not biologically capable. It is difficult to envision a small frog hopping across a wide expanse of desert to reach an isolated water source. However, being washed from the side of a riverbank during a storm and floating out to an island at sea on vegetation is more plausible. Therefore, habitat islands differ from other islands in the way they maintain their species diversity. Islands surrounded by water have been shown repeatedly to be dynamic and evolving communities regulated by the colonization and extinction of species (MacArthur and Wilson 1967). Habitat islands, in contrast, are more like museums whose exhibits (species) are relics of bygone environments. In other words, many of the species occupying habitat islands were always there, but the habitat has shrunk around them. Habitat islands are extremely important to evolutionary biologists and paleoecologists because they serve as living windows through which we can look into the past and glimpse the organisms that lived in an area during a previous ecological episode.

There are three major types of habitat island in Baja California. The first is formed by rocky mountain ranges surrounded by broad, flat expanses of low-lying, sandy desert. These ranges offer refuge for saxicolous species, and the surrounding desert prevents immigration and colonization from adjacent rocky areas. There are three such mountain ranges in Baja California. In the northeastern portion of the peninsula, just south of the U.S.-Mexican border, lie the Sierra los Cucapás and Sierra el Mayor. These form an extensive range of mountains, approximately 80 km long, bordered by the Río Colorado flood plain on the east and Laguna Salada on the west. This poorly explored mountain range harbors a depauperate, saxicolous herpetofauna containing the endemic Sierra los Cucapás Collared Lizard (Crotaphytus grismeri), Northern Chuckwalla (Sauromalus obesus), Speckled Rattlesnake (Crotalus mitchellii), and perhaps others (McGuire 1994). In west-central Baja California, two closely situated mountain ranges, the Sierra Vizcaíno and the Sierra Santa Clara, define the Vizcaíno Peninsula. The Sierra Vizcaíno lies northwest of the Sierra Santa Clara and extends southeast from near Punta Eugenia approximately 115 km into the Vizcaíno Desert. This mountain range is very poorly explored, and the Northern Chuckwalla (Sauromalus obesus) is the only saxicolous species currently known to occur there (Grismer et al. 1994). The Sierra Santa Clara, located approximately 50 km to the southeast, consists of seven closely spaced and eroding volcanic peaks. This system is known to harbor the Northern Chuckwalla (Sauromalus obesus), Night Snake (Hypsiglena torquata), and Lyre Snake (Trimorphodon biscutatus). Conspicuously absent from all three of these mountain ranges are the common and widespread Central Baja Banded Rock Lizard (Petrosaurus repens), Granite Spiny Lizard (Sceloporus orcutti), and Peninsular Leaf-Toed Gecko (Phyllodactylus xanti). Their absence may be explained by the fact that these ranges never had a rocky connection with the adjacent Peninsular Ranges. It is likely that the area was colonized by saxicolous species through chance overwater dispersals during a period when they were surrounded by water. The former three species just happened not to disperse (Grismer et al. 1994; McGuire 1994).

The second form of habitat island harbors isolated populations of species on high mountaintops. In northern Baja California, examples include the upper elevations of the Sierra Juárez and Sierra San Pedro Mártir. These mountains extend approximately 160 km south of the U.S.-Mexican border, and the Sierra San Pedro Mártir reaches over 3,000 m elevation. Both are dominated by Baja California Coniferous Forest Region flora and offer refuge to some interesting montane species at the southernmost extent of their distributions. Both the Sierra Juárez and Sierra San Pedro Mártir maintain allopatric (isolated) populations of the California Mountain Kingsnake (Lampropeltis zonata). The Sierra San Pedro Mártir also maintains populations of the Large-Blotched Ensatina Salamander (Ensatina klauberi), Foothill Yellow-Legged Frog (Rana boylii; possibly extinct), Southern Sagebrush Lizard (Sceloporus vandenburgianus), and Western Terrestrial Garter Snake (Thamnophis elegans) (Mahrdt et al. 1999; Welsh 1988). The Sierra la Asamblea is another isolated mountaintop habitat island. This mountain range rises precipitously out of the arid Vizcaíno Region just north of Bahía de los Ángeles and reaches over 1,400 m elevation. Its upper elevations support a relict chaparral, manzanita, and piñon pine woodland in which a population of the Southern Alligator Lizard (Elgaria multicarinata) was discovered (Grismer and Mellink 1994). This mountaintop is accessible only by climbing and has been visited only twice by herpetologists. It is likely that additional relict cismontane species will be found during future visits. Last, the upper elevations of the Sierra la Laguna in the Cape Region of Baja California serve as a habitat island. The Sierra la Laguna extends generally from La Paz to Cabo San Lucas, reaches over 2,100 m, and maintains an endemic pine-oak woodland at its upper elevations. Endemic to this region is a population of the small, secretive, and peculiar Desert Night Lizard (Xantusia vigilis).

The last major type of habitat island in Baja California consists of widely scattered oases that extend fragmentedly for approximately 700 km from Misión San Fernando Velicatá in the north to at least the La Presa region in the south (Grismer and McGuire 1993). Up to fifteen thousand years before the present, Baja California was much more mesic than it is today, and many mesophilic species had transpeninsular or nearly transpeninsular distributions (Grismer 1994b; Welsh 1988), ranging widely throughout central Baja California. With the rapid onset of extreme aridification between fifteen and eight thousand years before the present, Baja California was transformed into an arid peninsula, and the only mesic habitat that remained was that near permanent sources of water. This reduced the transpeninsular distribution of many mesophilic species to being widespread and continuous only in northwestern Baja California and the Cape Region, where aridity is not a factor. In central Baja California, their distributions became fragmented and restricted to the mesic refugia of these relict oases. Today the oases range in size from extensive bodies of water, such as those at La Purísima, to small, deep pools no more than two meters across, such as that found at Las Palmas (Grismer and McGuire 1993). Nonetheless, all maintain relict populations of various combinations of species (Grismer and McGuire 1993). Species commonly found in the oases are the California Treefrog (Hyla cadaverina), Pacific Treefrog (Hyla regilla), Baja California Slider (Trachemys nebulosa), and Two-Striped Garter Snake (Thamnophis hammondii).

 

Conservation and Commercialization

Conservation issues are a rapidly growing concern for government agencies in Mexico. Several biosphere reserves have been created in key peninsular areas, and all the islands within the Gulf of California and along Baja California's Pacific coast are now protected. Because of the high degree of both peninsular and insular endemism and the monetary value that unfortunately accompanies such phenomena, the commercial market for this region's herpetofauna has grown at a staggering rate over the last few years. Even though this commercialization was started by just a few notorious reptile collectors and dealers trafficking in illegal wildlife, it has now become a serious threat to many species in the region of study.

Mellink (1995) notes some of the negative environmental effects that illegal reptile collecting has had in Baja California. For example, certain areas of the Sierra Juárez and Sierra San Pedro Mártir have been heavily degraded by collectors breaking apart rock piles in search of California Mountain Kingsnakes (Lampropeltis zonata). Similar devastation can be seen along Mexican Highway 1 as it passes through rocky areas in northern Baja California (such as Cataviña and Jaraguay), where collectors are looking primarily for Rosy Boas (Lichanura trivirgata) and banded rock lizards (Petrosaurus). The sheer numbers of specimens that have been removed from some areas are also alarming. I was told by two Mexican nationals from La Paz that they helped one American reptile dealer collect over one thousand San Lucan Banded Rock Lizards (Petrosaurus thalassinus) in the Cape Region for over two years. Many of these were sold, and others provided the parental stock for illegal captive propagation.

Commercialization of this herpetofauna threatens insular endemics most of all. For example, the Isla Todos Santos Mountain Kingsnake (Lampropeltis herrerae) is confined to a single, tiny island off the coast of Ensenada. Mellink (1995) reported finding snake traps on the island baited with live mice, and Anglos with pillowcases turning over rocks. I know of one collector who took a gravid female off the island. The long-term potential problems of removing gravid individuals from populations of species with extremely limited gene pools is obvious. The same problems affect islands in the Gulf of California. For example, when I saw several species of endemic rattlesnakes appearing on reptile price lists from Florida, I became very concerned. I talked to a Mexican national who participated in the collection efforts of the American who shipped those reptiles to Florida, and he told me that nearly one hundred snakes were collected. I have found abandoned pitfall traps on several islands filled with dead animals because the person who set them never came back to close them up. Mellink provides many more examples.

Despite the sanctimonious claims of many dealers, breeders, and collectors, unregulated captive propagation of illegally obtained animals has done nothing, and will do nothing, to protect against the potential loss or local extirpation of Baja California or Gulf of California amphibians and reptiles. Additionally, as Mellink notes, many individuals from different populations are crossed to produce more commercially valuable color phases, thus creating individuals with genetic constitutions and color patterns that do not even occur in the wild. Furthermore, the commercialization of this herpetofauna not only has created its own market but also has resulted in the environmental degradation of many areas (Mellink 1995). When people see the outrageous prices for which some species sell and realize that they are only a few hours or days away from areas where these species can be collected, some develop an urge to be adventurous and go catch their own.

Besides the fact that overcollecting, destroying microhabitats, and smuggling reptiles across the border into the United States is considered an illegal act in both countries, it is environmentally and ethically unconscionable, and it constitutes an act of aggression against our ecosystems. So the next time you see a price list with protected species on it or attend a reptile expo where Baja California and Gulf of California herps are being paraded for sale in hundreds of little plastic boxes, remember that the original populations from which these specimens came were most likely illegally taken (Mellink 1995). Then ask yourself if you want to be a part of, or cater to, that level of our society that abuses wildlife for profit.

Join UC Press


Members receive 20-40% discounts on book purchases. Find out more