Biogeography of Rocky Intertidal Macroinvertebrates of the Southern California Islands

INTRODUCTION The macroinvertebrate fauna of the rocky intertidal zone on the Southern California islands is incompletely known; consequently, it has not been possible to determine zoogeographical relationships among the various islands or between the islands and the mainland. Three studies (Hewatt 1946, Caplan and Boolootian 1967, Littler and Murray 1975) have been published on the rocky intertidal biotas for three of the islands, although only one of these (Hewatt 1946) is a comprehensive faunistic study. Based on extensive collections from rocky and sandy beaches at six stations around Santa Cruz island, Hewatt (1946) recorded a total of 297 invertebrate species. It is noteworthy that, of this total, many were small, inconspicuous forms that dwell among and beneath macrophytes (e.g., articulated corallines) and macroinvertebrates {e.g., mussels). After assembling the latitudinal distribution records for the various species, Hewatt concluded that Santa Cruz Island was located in a transitional area between northern and southern faunas. This conclusion was based, in part, on the determination that nearly equal percentages of the island's rocky intertidal species occurred mainly to the north (30 per cent) or south (27 percent) of Point Conception, while the remaining species (43 percent) were broadly distributed along the mainland. The vertical distribution of the 19 predominant rocky intertidal macroinvertebrates at two sites on the southeastern portion of San Nicolas Island was assessed by Caplan and Boolootian (1967). Their analyses are of little biogeographical value, however, since they were concerned with only those species whose densities exceeded 100/m from any quadrat sampled. Rocky shore species on the leeward, east-facing side of San Clemente Island near Wilson Cove were reported by Littler and Murray (1975). However, since the inception of the intensive Bureau of Land Management intertidal baseline studies (Littler 1977, 1978, 1979, 1980), our knowledge of the fauna there has greatly expanded. Historically, Point Conception has been considered an important marine biogeographical boundary along the Pacific coast of North America (reviewed by Hedgpeth 1957 and Valentine 1966), with the cold-temperate Oregonian Province lying to the north and the warm-temperate Californian Province to the south (Fig. 1). The Oregonian Province and the Aleutian Province are included in the East Pacific Boreal Region (Fig. 1), while the Californian Province describes a warm-temperate region separate from the East Pacific Tropical Region, which includes the Mexican and Panamanian Provinces. The Cortezian Province in the Gulf of California probably should not be considered warm-temperate (as Briggs [1974] has suggested), but rather as having tropical affinities (W. Newman, pers. comm.). The biogeographical boundary at Point Conception does not appear to be sharp. Instead, it can be considered to represent an overlap area, with cold-temperate species ranging two to three degrees in latitude to the south and warm-temperate species ranging two to three degrees to the north. The existence of this overlap zone was clearly described by Newell (1948) using the molluscan range data of Schenck and Keen (1936) and Keen (1937). The area of overlap, termed the Californian Transition Zone, delineates the latitudinal range of a number of short-range endemic species (Newman in press).