The Cartographers of Life: Two Centuries of Mapping the Natural History of Baja California

The Cartographers of Life: Two Centuries of Mapping the Natural History of Baja California PEDRO P. GARCILLÁN, CHARLOTTE E. GONZÁLEZ ABRAHAM, AND EXEQUIEL EZCURRA In 1964, the outstanding biogeographer Leon Croizat published a book entitled Space, Time, Form: The Biological Synthesis. He never visited Baja California, but few places in the world can condense more deeply the powerful metaphor of Croizat’s title. Indeed, biogeography has been for centuries a powerful way of looking at the world from a holistic perspective in which the large-scale processes that mold and maintain life on Earth can be seen, identified, and understood. THE BIOGEOGRAPHICAL SINGULARITY OF BAJA CALIFORNIA The peninsula of Baja California, in northwestern Mexico (fig. 1a), is the Mexican part of “Peninsular California” (Gastil, Phillips, and RodríguezTorres 1972), a biogeographic region stretching from the southern base of the Transverse Ranges in California to the Cape Region in the tip of Baja California Sur. The peninsula rides on the Pacific tectonic plate, separated from the rest of North America by the San Andreas Fault and the deep spreading centers of the Gulf of California. A series of factors have contributed to make it one of the most uniquely diverse regions in the world. Geological formation. Six million of years ago, a sliver of continental crust started to drift away from the Mexican mainland (Riddle et al. PEDRO P. GARCILLÁN holds a Ph.D. in biological sciences from the Universidad Nacional Autónoma de México and is a researcher at EcoDesertica, in Baja California. CHARLOTTE E. GONZÁLEZ-ABRAHAM is a researcher at EcoDesertica. EXEQUIEL EZCURRA is director of the University of California Institute for Mexico and the United States (UC Mexus), and professor of plant ecology at the University of California, Riverside. Journal of the Southwest 52, 1 (Spring 2010) : 1–40 Figure 1. Physical geography of Baja California: (a) Topography and places cited in the text; (b) annual mean precipitation, (c) percentage of winter precipitation , and (d) annual median temperature. Cartographers ✜ 3 2000). The Sea of Cortés, a result of the tectonic spreading centers in the Earth’s deep mantle that push and drive the peninsula’s drift, has kept Baja California biologically separated from the mainland, and the peninsula, in turn, has kept the Sea of Cortés engulfed, sequestering it from the Pacific Ocean. In this region, where land and sea mutually embrace each other, superimposed patches of insularity exist at even smaller scales, resulting in an amazingly diverse array of ecosystems, species, and unique life-forms. Holocene climatic cycles. The successive expansion and contraction of ecosystems and biomes induced by glacial climatic cycles during the Quaternary created a series of “sky islands” in the highest ranges of Baja California, where remnants of the ancestral Madro-Tertiary temperate flora that covered the region during the Pliocene still survive in isolation (Axelrod 1958, 1979; Van Devender 1990; Riddle and Honeycutt 1990). Similarly, the deep, moist canyons that dissect the mountain slopes harbor palm oases with relicts of moister tropical vegetation (Grismer and McGuire 1993; Arriaga and Rodríguez-Estrella 1997). Isolated and distinct within the general matrix of sparse desert vegetation, these sky islands and oases create a complex mosaic of contrasting ecosystems. Latitudinal span. The long latitudinal span of the peninsula, ranging from south of the Tropic of Cancer, at 22°50’N, to 32°45’N, determines the first great axis of ecological variation by creating along this elongated splinter of land a long ecological transition between the northern temperate region showered by winter rains and the southern dry tropical forest soaked by tropical storms, with an extensive desert area bridging both realms (figs. 1b, c). Oceanic influence. The narrow peninsula also harbors a dramatic eastwest transition: While the Pacific coast is strongly influenced by the cold California Current and has a cool, foggy, oceanic climate, the eastern coast is washed by the warm, enclosed waters of the Gulf of California, and the climate is continental—extremely hot in summer and cold in winter (figs. 1b, c, d). Topography. Finally, a mountainous backbone runs along the length of the peninsula, introducing a third environmental gradient (fig. 1a): On the one hand...