Anthropogenic Impacts on Rocky Intertidal Mollusks in Southern California: Compiling Historical Baseline and Quantifying the Extent of the Problem

Anthropogenic activities such as illegal harvesting and trampling is generally considered to be responsible for a decline in the biological diversity of rocky intertidal habitats along the southern California coast (Littler, 1980; Littler et al. 1991; Murray and Bray 1994; Lindberg et al. 1998; Murray et al. 1999; Roy et al. 2003). Yet relatively little information exists about the nature and extent of such declines, largely due to the lack of baseline data against which one can compare the exploited or disturbed populations. This is a difficult problem because at present very few rocky intertidal assemblages in southern California are undisturbed enough to serve as an ecological baseline(s) against which other sites can be compared. Similarly, reliable ecological census data from the past are very rare and we will never be able to quantify temporal changes in abundances of species except at a few localities (such as that by Barry et al. 1995). However, data on past species occurrences (i.e. presence/absence data) can be compiled much more easily and across much larger spatial and temporal scales. Museum collections and the literature are a rich source of such data and once compiled they have the potential to provide us with a picture of how species occurrence patterns have changed over the last century or so. The primary goal of this project was to use historical data collected over the last century to construct a long-term perspective that can be used to quantify the ecological consequences of anthropogenic impacts on rocky intertidal molluscan species and populations. In particular, we refined and expanded a database of historical occurrences and body sizes of rocky intertidal molluscan species at multiple localities in southern California. These data, for the first time, provide a comprehensive picture of our knowledge of past occurrences of rocky intertidal molluscan species in southern California. We then used analyses of site occupancy patterns over time to identify sites with the best historical data on species occurrences and undertook ecological surveys at a selected subset of these localities. We are using comparative analyses of the historical information and the survey data to quantify how patterns of site occupancy as well as body sizes of individual species have changed over time.