The California Islands

Abstract

The California Islands off the Pacific coast of southern and Baja California display almost diagrammatically the insular phenomena that characterize fringing archipelagos. To a lesser extent they also illustrate some of the insular features associated with oceanic islands. Several of these islands have been much studied and are relatively accessible. The location, size, elevation, and geological history of the islands are described. To determine the degree of disharmony of the island floras, they are analyzed in comparison with selected California mainland floras. The degree of richness of the island floras is discussed in relation to size of the islands, proximity to the mainland, and other variables. Such phenomena of island life as the immigrant pattern of vertebrate distribution, relict versus autochthonous endemism of plants and animals, insular gigantism, continuous flowering, high degree of hybridization, vulnerability, and loss of dispersibility in island species are considered in reference to the California Islands. Some suggestions as to available facilities on the islands and persons or institutions to contact are given in an epilogue. Off the Pacific coast of southern California and Baja California, Mexico, lie the islands variously called the Channel Islands or California Offshore Islands and the Baja California Islands or, collectively, the California Islands. Although they are in no sense tropical, they are worthy of mention in this symposium because they are a good example of a fringing archipelago and because they illustrate many of the insular phenomena that one might otherwise have to travel far to tropical areas to study. Some of them are quite accessible from the heavily populated areas of southern California, and some of them have been rather thoroughly studied by biologists. Although they are, thus, rather well known to California biologists, they are surprisingly little known to non-Californians. It is hoped that this short discussion of them and of some of their insular characteristics will create more interest in them among those biologists concerned with evolutionary and environmental phenomena. The Southern California and Baja California Islands consist of 16 major islands or island groups spread over some 500 miles between Point Conception, California, and Punta Eugenia, Baja California, Mexico (Fig. 1). From east to west the Northern Channel Islands are Anacapa, Santa Cruz, Santa Rosa, and San Miguel. The Southern Channel Islands are San Nicolas, Santa Barbara, Santa Catalina, and San Clemente Islands. The Baja California Islands include Los Coronados just south of San Diego, Todos Santos off Ensenada, San Martin off San Quintin, San Geronimo south of El Rosario, the San Benitos, Cedros, and Natividad off Punta Eugenia, and Guadalupe 157 miles off the Baja California peninsula. In order of size, with area in square miles, the islands are: Cedros (134), Guadalupe (98), Santa Cruz (96), Santa Rosa (84), Santa Catalina (75), San 11 am grateful to Dr. Reid Moran for critically reviewing the manuscript, for making many valuable suggestions, and for supplying useful unpublished information. 2Rancho Santa Ana Botanic Garden, Claremont, California 91711. ANN. MISSOURI BOT. GARD. 56: 391-408. 1969. This content downloaded from 157.55.39.247 on Wed, 27 Apr 2016 05:48:03 UTC All use subject to http://about.jstor.org/terms [VOL. 5 6 392 ANNALS OF THE MISSOURI BOTANICAL GARDEN Clemente (56), San Nicolas (22), San Miguel (14), Natividad (2.8), San Benito (2.5), Anacapa (1.1), Santa Barbara (1.0), Los Coronados (1.0), San Martin (0.9), Todos Santos (0.5), and San Geronimo (0.2). Their distances from the mainland, in statute miles, are: San Martin (3), Todos Santos (4), Natividad (5), San Geronimo (6), Los Coronados (8), Anacapa (13), Cedros (14), Santa Cruz (19), Santa Catalina (20), San Miguel (26), Santa Rosa (27), Santa Barbara (38), San Benito (41), San Clemente (49), San Nicolas (61), and Guadalupe (157). All these figures are from a summary by Philbrick (1967). The highest elevations on the largest islands, in feet, are: Guadalupe (4,257), Cedros (3,950), Santa Cruz (2,165), Santa Catalina (2,069), San Clemente (1,965), Santa Rosa (1,574), and San Nicolas (907).