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

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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 cau