Plant Life of a Desert Archipelago: Flora of the Sonoran Islands in the Gulf of California

Introduction

3 Introduction Like rocks, from large boulders to small pebbles, lying in the middle of a flowing stream, the Midriff Islands of the Gulf of California stand as a partial bridge between the Baja California Peninsula and the Sonora mainland (fig. 1.1A,B). These desert islands, in the central part of the Gulf of California , between latitudes 28°20' and 29°40', display a continuum of adaptation by life to a harshly arid environment in an isolated setting. The high level of endemism found in the mammals and reptiles of the region (Grismer 2002; Lawler et al. 2002) is largely not mirrored in the flora; however, the vegetation and floristic makeup of each island is unique. From the cardón forests of the tiniest island, Cholludo, to the agave-dominated slopes of San Esteban, topographic, climatic, and biological forces have sculpted a set of unparalleled desert worlds. The Comcaac (Seri people) and their ancestors or predecessors established an existence from the eastern Midriff Islands and the Sonora mainland for thousands of years, based on the diverse terrestrial and marine life found there. Remarkably, these islands remain much as they were centuries ago, when the Comcaac were the only human presence in the region. Keep in mind, however, that there have been long and continuing human influences on these islands (Bowen 2009). ThepresentworkfocusesonthemajorityoftheSonoranIslands:Tiburónand its five neighbors, Alcatraz, Cholludo, Dátil, Patos, and San Esteban; San Pedro Mártir, in the middle of the Gulf; and San Pedro Nolasco to the northwest of Guaymas (fig. 1.2). We have built on the local knowledge of the Comcaac and insightsgainedfrommorethanacenturyofscientificfocustobetterunderstand the ecology and botanical diversity of the world’s best-preserved archipelago. GEOLOGIC HISTORy AND ISLAND PHySIOGRAPHy, WITH MICHAEL OSKIN The Gulf of California opened between 15 and 5.5 million years ago (mya), with most evidence indicating two stages of evolution, a proto-Gulf (middle to late Miocene, 15–6 mya) and a modern Gulf (late Miocene and younger, 4 THE ISLANDS AND THEIR VEGETATION Figure 1.1. Midriff Region, Gulf of California. (A) Satellite image of the Gulf of California and Baja California Peninsula, with the Midriff Islands at center. NASA Rapid Fire image, Aqua satellite. (B) Looking from the northwest over the Baja California Peninsula (foreground), the Midriff Islands (middle), and Sonora to the southeast (background). NASA STS-004 shuttle mission, 4 July 1982. (A) (B) INTRODUCTION 5 ca. 6 mya to present; Helenes & Carreño 1999). Outcrops of marine rocks on the margins of the Gulf of California support that marine waters advanced as far as Santa Rosalia, midway up the Baja California Peninsula, by 8 mya, and into southeastern California by 6.5 mya (Oskin & Stock 2003a). Marine sedimentation prior to 11 mya has been suggested from core-hole samples drilled from the northern Gulf of California (Helenes et al. 2009). The Sonoran Islands are a mix of land-bridge islands (part of the mainland during the Pleistocene) and oceanic islands (in deep water, isolated throughout the ice ages). Tiburón and the small islands of Patos to the north, Dátil and Cholludo to the south, and Alcatraz in Bahía Kino are land-bridge islands and were connected to the Sonora mainland as recently as 11,000 years ago. San Esteban and San Pedro Mártir are oceanic islands, and San Pedro Nolasco is a geologically isolated island but was once part of the mainland. Tiburón is part of the transition from the obliquely rifted Gulf of California to the Sonoran Basin and Range Province, marked by mountain ridges with a generally north-south orientation and interspersed with wide, alluvium-filled, low-lying valleys. The Canal del Infiernillo separates Tiburón from the Sonora mainland . This narrow and shallow Canal is a sea-filled valley between two large mountain ranges—the Sierra Seri on the mainland and the Sierra Kunkaak on the island. Farther west on Isla Tiburón, the large Central Valley separates the Sierra Kunkaak from the Sierra Menor. Off the western side of Tiburón there are deep, sedimentfilled ocean basins that plummet hundreds of meters into the Gulf of California. Tiburón is composed of Mesozoic and older (pre-65 mya) intrusive rocks and adjacent metamorphosed sedimentary rocks. These are overlain by volcanic rocks extruded, intruded, or deposited either as part of the magmatic arc that existed from 24 to 16 mya in the area of the present...