Island biogeography in the Sea of Cortéz

The birth of this volume was a symposium on the same subject at the University of California, Los Angeles, in 1977. That this title has come along so many years later is a blessing, presumably to be attributed to the editors. For once the papers from a symposium have been rewritten, reviewed and assembled with the idea that a collaboration can be coherent and cohesive. One would not expect 15 writers to agree on everything; they certainly do not. But at least they are discussing the same region and addressing some of the same issues. Individual writers were given the choice between using "Sea of Cortez" and "Gulf of California" (so the reviewer gets his choice, too). Whichever name is used, the body of water is the 1070 kin-long, 100 to 200 kin-wide division between the peninsula of Baja California and mainland Mexico. The specific focus was the islands, of which there are about 25 major ones, including 67 kin-long Angel de la Guarda (elevation 1315 m) and TiburOn with an area of 1,000 km 2, and about 10 very small islets. These desert islands are, on relative terms, some of the most pristine in the world. The species of plants and vertebrates present on the various islands are now reasonably well known, partly through the efforts of the authors. Many gaps remain (and are freely admitted), and with basic inventories only just completed, one can imagine the potential for the study of the biology of individual species and of community ecology. Discussions of invertebrates are not included. Two chapters deal with birds. Chapter 8 (pp. 210-245) by Martin L. Cody is on the land birds. Cody begins the chapter with a discussion of the potential sources of the island avifaunas, especially the Sonoran desert which borders the Gulf of California. The Sonoran desert as a whole does not have a particularly distinct avifauna; most species occur widely in other deserts or other habitats. The "Gilded" Flicker is the closest o being typical of the region and endemic. Because the islands are low and dry, montane colonists need not apply, and the nearby montane regions come in for only limited discussions. Also included is a brief but adequate discussion of patterns of (primarily) subspecific differentiation along Baja, as well as other oddities of the peninsular avifauna. To provide comparative data for his investigations of the islands, Cody did some basic census work on the mainland (the species lists are in the appendices). Seven sites are in the typical desert habitats around the northern and central Gulf, and four in the thorn scrub of the summer rain region on both sides of the mouth of the Gulf. Not surprisingly, the birds of the islands are drawn from the surrounding deserts. No species are endemic to the islands, and only a few weakly differentiated races are unique to the islands. It is always interesting to see which species are absent: Greater Roadrunner is not a surprise, but the absence of Cactus Wren from all but one of the cactus-rich islands is startling. Perusal of the species lists reveals other interesting occurrences -for instance, Great Horned Owl, which has not colonized the Channel Islands of California, is present on 10 of the large islands. Much of the purpose of this volume is to discuss island biogeography, and Cody devotes most of his chapter to a discussion of the observed patterns. While I am not facile with the mathematical models of this discipline, most of Cody's conclusions seemed reasonable. A latitudinal change in the species composition is related to the brushier thorn scrub habitats of the southern islands and adjacent source areas. The richest island, TiburOn, is the largest, lies only 2 km from the mainland, and was formerly connected by a landbridge (current channel depth: 3 fathoms). Cody found that 79% of the variability in bird species could be attributed to the area of the island. Distance from shore (never too far in the Gulf anyway) and the former