The rocky shore environments along the California coast harbor diverse organisms, including fish, invertebrate animals, algae, and plants. Yet, these habitats are among those most impacted by human activities and least protected. Human population in the coastal counties of southern California (Los Angeles, Orange, San Diego, Santa Barbara, and Ventura) has increased dramatically over the last 150 years. In fact, over the past three decades the population of coastal counties in southern California has increased by more than 50% (Murray et al. 1999) and this growth is expected to continue in the foreseeable future.

Such an expanding population can negatively impact the easily accessible rocky habitats at the land-sea interface in a number of different ways. While past studies have focused on negative biological impacts of sewage discharge and industrial pollution (Widdowson, 1971; Thom and Widdowson, 1978; Littler, 1980), there is now considerable evidence that other more episodic disturbances can also be highly damaging to the rocky intertidal biota. These include trampling of the intertidal by foot traffic (Povey and Keough, 1991; Brosnan and Crumrine, 1994; Fletcher and Frid, 1997; Keough and Quinn, 1998; Brown and Taylor, 1999; Murray et al. 1999; Schiel and Taylor 1999), harvesting of intertidal organisms for food, fish bait, aquariums and other needs (Eekhout et al. 1992; Addessi, 1994; Griffiths and Branch, 1997; Kyle et al. 1997) and the removal of rocks and other material such as dead shells that serve as habitats for many invertebrates (Addessi, 1994).

In general, harvesting practices and recreational and other activities of an expanding population are generally considered to be responsible for a serious decline in the biological diversity of southern California rocky shores (Littler, 1980; Littler et al. 1991; Murray and Bray 1994; Lindberg et al. 1998; Murray et al. 1999). However, little quantitative data exist on the nature and extent of such declines, largely due to the lack of baseline information against which to compare present day biological patterns. In other words, we do not know whether the human activities have led to extensive local extinctions of various intertidal species or whether such disturbances change local abundances of species but do not necessarily lead to widespread local extinctions. In terms of environmental quality, the former effect clearly provides a worse case scenario and qualitative evidence suggests that the problem may be widespread (e.g. Murray et al. 1999; Carlton et al. 1999; K. Roy & D. Balch unpublished data on local extinction of Acanthinucella punctulata; Jack Bradbury pers. comm. on nudibranch species; D. Lindberg unpublished data for limpet species) but little effort has been made to quantify the extent of this problem in southern California rocky intertidal habitats. Maintenance of a healthy coastal ecosystem demands that we prevent widespread local extinctions of species and our goal here is to quantify how species compositions in the rocky intertidal habitats of southern California have changed over the past decades.

Understanding the extent of local extinctions affecting populations along California rocky shores is also crucial for estimating or predicting the effects of introduced species in similar habitats. Rocky shores within embayments appear to be relatively immune to introductions. For example, while soft bottom and fouling communities in San Francisco Bay now include hundreds of introduced species, the rocky shores of San Francisco Bay have almost no introduced species with the exception of the Atlantic gastropod Littorina saxatilis which maintains very small and limited populations in the East Bay (Carlton & Cohen 1998). On the outer coast introductions are rarer, but also show a similar pattern with rocky shores seldom experiencing successful invasions. Thus, many rocky shore taxa can serve as "mine canaries" in these habitats and local losses of these key taxa can undoubtedly point to alternative interactions or disruptions of local conditions (e.g. Lindberg et al. 1998). However, to accurately read these signals we need to know the background patterns of local extinctions of these taxa.

While California has a long history of setting up marine protected areas of different kinds, resources for policing these are scant and recent evidence suggests that most of those in southern California "do not seem to be effective in protecting intertidal populations from damaging activities" (Murray et al. 1999, p. 105). The urgency of the situation demands that we determine whether our coastal habitats are seeing significant local extinctions of species and set our future conservation policies accordingly.

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