California's Islands and Offshore Rocks

The following materials are exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.

The islands, rocks, and sea stacks offshore of California's coast are unique habitats that provide protected breeding sites for thousands of migrating seabirds and marine mammals. Though their isolation from the mainland and relative inaccessibility make islands and offshore rocks prime sanctuaries for birds and marine mammals, they have historically been sites of plunder and slaughter of wildlife for profit. The 19th century trade in seal skins and seabird eggs resulted in the virtual elimination of northern elephant seals, northern fur seals, common murres, Cassisn's auklets, and tufted puffins from the California coast. California's islands, in particular the Channel Islands, harbor many rare, endemic species that have evolved or survived as a consequence of their isolation from the mainland, and the characteristics of their specialized island habitats. Today, Ano Nuevo Island is a state reserve, and national marine sanctuaries protect the Farallon Islands and five of the eight Channel Islands.

More than half the California population of resident and migratory seabirds nests on the Farallon Islands, a group of seven sparsely vegetated granite outcroppings located offshore of the Golden Gate channel. The rest breed mostly on the small islands and offshore rocks between the Oregon border and Cape Mendocino in Humboldt county. Only seven acres in area, Castle Rock, offshore of the Del Norte County Coast, is California's second largest seabird rookery after 65-acre Southeast Farallon Island; Castle Rock is also a Steller sea lion haul-out and a harbor seal nursery.

The breeding season of many seabird species that nest on California's islands and offshore rocks coincides with annual up- welling and the resulting increase in biological productivity of ocean waters along the California coast. Beginning in spring, a layer of deep, cold, nutrient-rich water rises, initiating surface blooms of phytoplankton; these microscopic marine plants are then grazed by zooplankton such as krill, a shrimp-like animal that is important in the diet of breeding seabirds such as Cassin's auklets and common murres.

San Miguel Island, one of the eight Channel Islands, is the only place in the world where five pinniped species--California sea lions, Steller sea lions, harbor seals, northern fur seals, and northern elephant seals--congregate and breed. Ano Nuevo Island, in San Mateo County, has the largest Steller sea lion rookery in California, and also supports a large population of breeding northern elephant seals.

The Channel Islands and the Farallon Islands were created as a result of geologic activity millions of years ago. The Farallons are composed of 89-million-year-old granite that emerged as molten rock from below the sea floor; uplifting of the Pacific Plate during the mid-Pleistocene completed the island-making process. The four northern Channel Islands are actually the western terminus of the Santa Monica Mountains, separated from the mainland by the waters of the Santa Barbara Channel. Twenty thousand years ago, when worldwide sea level was 300 feet lower than at present, the four islands formed one large island. The four southern Channel Islands are thought to have once been connected to the mountains of the Peninsular Ranges, but faulting in the San Pedro Channel 30 million years ago cut them off from the mainland.

Offshore rocks and sea stacks are of more recent origin than California's islands. North of Point Conception, where most offshore rocks are found, storm waves generated in the North Pacific buffet the shore and whittle away the coastal cliffs, leaving isolated stands of the most resistant rock. Fewer rocks lie offshore of the Southern California coast, where the buffering effect of the Channel Islands and the Southern California Blight-- an indentation and southeasterly shift in the coastline south of Point Conception--reduces the impact of storm waves on coastal cliffs.

Island endemics, species that are found only on islands, are the result of divergent genetic evolution from mainland species. The island fox, found on six of the eight Channel Islands, has no predators and is considerably smaller than its mainland ancestor, the gray fox. The Santa Cruz Island scrub jay, another Channel Island endemic, is noticeably larger than its mainland relatives. Endemic island plant species may be climatic relicts, like the island ironwood tree that grows on four of the Channel Islands. The wet, cool island climate is similar to the climate of the Pleistocene, when these trees were widespread in western North America. The island populations were able to survive the post- Pleistocene climatic changes that resulted in the demise of ironwood forests on the mainland.

| Search | Comment | Natural Resources | Coastal Resources | Coastal Geography |