We introduce a demographic model for a local population of sessile marine invertebrates that have a pelagic larval phase. The processes in the model are the settling of larvae onto empty space, and the growth and mortality of the settled organisms. The rate of settlement per unit of unoccupied space is assumed to be determined by factors outside of the local system. The model predicts the number of animals of each age in the local system through time. The model is offered in both discrete and continuous—time versions. The principal result is that the growth of the settled organisms is destabilizing. In the model, there is always a state where recruitment balances mortality. However, growth can interfere with recruitment and can destabilize this steady state, provided also that the settlement rate is sufficiently high. The model suggests that two qualitatively distinct pictures of population structure result, depending on the settlement rate. In the high settlement limit, the intertidal landscape is a mosaic of cohorts, punctuated with occasional gaps of vacant substrate. In the low settlement limit, the intertidal landscape has vacant space and organisms of all ages mixed together and spatial variation in abundance is caused by microgeographic variation in settlement and mortality rates.

Changes in the invertebrate fauna of a California rocky intertidal community between the period 1931 to 1933 and the period 1993 to 1994 indicate that species' ranges shifted northward, consistent with predictions of change associated with climate warming. Of 45 invertebrate species, the abundances of eight of nine southern species increased and the abundances of five of eight northern species decreased. No trend was evident for cosmopolitan species. Annual mean shoreline ocean temperatures at the site increased by 0.75 degrees C during the past 60 years, and mean summer maximum temperatures from 1983 to 1993 were 2.2 degrees C warmer than for the period 1921 to 1931.

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Changes in the abundance of macroinvertebrate species documented in a rocky intertidal community between surveys in 1931–1933 and 1993–1996 are consistent with the predicted effects of recent climate warming. We resampled 57 0.84‐m2 plots of an intertidal transect first surveyed by W. G. Hewatt at Hopkins Marine Station (HMS), Pacific Grove, California, between 1931 and 1933. Replicating precisely the location of the plots and methodology used by Hewatt, we documented changes in the abundances of 46 invertebrate species, indicating that this intertidal community changed significantly during the 60 yr between surveys. Changes in abundance were related to geographic ranges of species. Most southern species (10 of 11) increased in abundance, whereas most northern species (5 of 7) decreased. Cosmopolitan species showed no clear trend, with 12 increasing and 16 decreasing. Although Hewatt did not record algal species as thoroughly as invertebrates, we were able to document a massive decline in cover of Pelvetia compressa, a cosmopolitan fucoid alga that is typically more common in the southern part of its range. Shoreline ocean temperature, taken daily at HMS, warmed by 0.79°C during this 60‐yr period, with average summer temperatures up to 1.94°C warmer in the 13 yr preceding our study than in the 13 yr preceding Hewatt's. The hypothesis that climatic warming drove the observed range‐related community shifts is supported further by historical records and data from other investigators. Several alternative hypotheses to explain changes in the invertebrate community at HMS, including habitat changes, anthropogenic effects, indirect biological interactions, El Niño–Southern Oscillation (ENSO) events, and upwelling are considered to be less important than climate change. Changes in species' abundances over a short period (3 yr) were relatively small compared to large species shifts over 60 yr and were unrelated to geographic range of the species, indicating that short‐term population fluctuations play a relatively minor role in the long‐term community changes that we observed.

Analyses of sulfide, methane, oxygen, and CO, in pore-water sampdes from three cold seep sites in Monterey Bay indicate that fluid chemistry is a strong determinant of the distribution of chemolithoautotrophic vesicomyid clams. The distribution of Culyptogena pacijca and Culyptogenu kilmeri were aligned closely with sulfide concentrations at all cold seeps and reflected species-specific capabilities for sulfide binding. Live clams occurred only in sediment where sulfide was detectable. Sulfide was not detected in the absence of vesicomyid clams. The relative abundances of five vesicomyid species varied greatly among seeps. C. kilmeri accounted for 85-99% of all vesicomyids at seeps with high sulfide content, and C. puciJicu dominated (73%) seeps with low sulfide levels. These species were also partially segregated along sulfide gradients from the center to the margin of seeps, analogous to zonation of rocky intertidal communities. We hypothesize that the absence of thiotrophic or methanotrophic mytilid mussels from Monterey Bay cold seeps is related to the lack of physiological specializations for concentrating reduced sulfur compounds or methane and the absence of hypersaline brines that could extend the persistence of methane or sulfide-rich fluids very near the sea floor.

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One of the most storied biological expeditions is the 1940 trip to the Sea of Cortez (Gulf of California) by author John Steinbeck and his close friend Edward F Ricketts, a professional biologist. Steinbeck and Ricketts visited intertidal sites around the Gulf and made extensive collections, taking notes on fauna and natural history. In 2004, we retraced the Steinbeck and Ricketts voyage, visiting the same intertidal sites during the same season and using the authors' extensive natural history notes as a baseline for comparison. Although we found many of the same species as they did, populations were, in many cases, not as geographically widespread, and individuals were fewer in number and smaller in size. In particular, echinoderms and large gastropods showed declines, as did most large pelagic vertebrate species. One of the most remarkable changes in the pelagic community is the present abundance of jumbo squid, Dosidicus gigas, a species not reported in 1940. Although Steinbeck and Ricketts, by their own admission, “could not yet relate the microcosm of the Gulf with the macrocosm of the sea”, the changes we observed with historical perspective are in agreement with documented changes in ocean and coastal ecosystems around the world.