The rocky intertidal habitats of California harbor a diverse group of plants and animals. In this project we use mollusks and echinoderms as our focal groups. These groups were chosen because: (i) In terms of species numbers, mollusks and echinoderms form the largest component of rocky intertidal biodiversity. (ii) Many mollusk and echinoderm species also play a key role in the organization of intertidal communities and the loss of these species can lead to significant changes in community structures (e.g. Lindberg et al. 1999) (iii) Mollusks and echinoderms are the best sampled marine invertebrates in California. They have been extensively collected by both professionals and amateurs over the last century and hence their past occurrences are much better documented compared to many other groups. (iv) A number of species of mollusks and echinoderms are considered to be important food items, especially in ethnic cuisines. Hence they are often selectively harvested by the coastal population in California (Murray et al. 1999). Other species of mollusks are popular with amateur collectors and are also subject to collecting pressure. (v) Roy and Valentine, in collaboration with D. Jablonski, have already compiled data on the range limits of Californian molluscan species (Valentine, 1966; Roy et al., 1994, 1998, 2000). These data greatly help in ensuring that the occurrence data collected here covers the length of individual species' geographic range in southern California. (vi) The habitats (mussel beds, algal mats) of many intertidal molluscan species are the ones being most impacted by activities such as trampling and poaching (Murray et al. 1999).

At each of our chosen localities we are establishing large permanent plots (around 1000 sq. m) that will be quantitatively sampled once every three months during low tides over two years. The sampling is done using standard ecological methods, noting presence/absence as well as abundance data. However, since our primary goal is to establish presence/absence patterns of species at a locality, transect sampling could miss some rare taxa. So in addition to transects we are exhaustively searching each locality during low tides for any species that are known to occur there from historical records but that do not show up in our transects.

For each locality we will simply ask whether all mollusk and echinoderm species known to be present there in the past are also present today. If we find species that are absent today at a given locality, we will use our compiled timeseries of occurrences to ask when the species was last documented at or near that locality. Similarly for each species we will ask whether it occurs today at all the localities where it is known to occur historically. If it is currently absent from some localities we will compute the proportion of localities at which it is currently extinct. Given that we will be dealing with multiple species at many localities some mismatches are expected simply from random sampling effects. That is, even our intensive searches are expected to miss some rare occurrences; conversely previous sampling would almost certainly have missed some species. We will test whether the observed frequencies of occurrences of species differ from the expectation that each species should be present at all localities where it is known historically using both parametric statistics (Sokal and Rohlf, 1995) as well as randomizations (see Pandolfi 1996; Roy in press). In addition, species ranges are also known to fluctuate in response to changes in oceanographic conditions (e.g. El Nino years) as well as secular changes in temperatures (e.g. Roy et al. 1996) and some of the historical occurrences may reflect such transient range expansions of species into localities where they do not normally live. Comparison of temporal patterns at localities with little human impact versus those with high impacts should let us separate the anthropogenic effects from natural shifts in species distributions; the expectation is that any climate related changes should also be seen at localities protected from human disturbances.

In addition, to quantifying the extent of ongoing local extinctions of mollusks and echinoderms, we will also use our data to explore the underlying causes of the changes. In particular, by comparing sites suffering from different degrees of human impacts we will attempt to separate long term climate-related changes from those that are largely due to immediate anthropogenic causes. Similarly if we detect the presence of significant local extinctions, we will determine which species are most affected and whether their susceptibility results from certain specific ecological or life history traits. Such traits could include modes of reproduction (e.g. brooding or crawl away larvae) that make it harder for a species to recolonize a locality once it has been locally extirpated, or restriction to specific micro-habitats that are being degraded (e.g. many species of small gastropods reside in clumps of intertidal algae that are particularly susceptible to damage from trampling by humans).

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