We have embarked upon an intensive, multi-site and multi-factor investigation of the effects of species interactions on community structure (see Findings for results of these studies). Experiments enable us to tease apart the relative importance of relationships among species, the variation in those relationships across the PISCO region and the influence of nearshore waters on benthic species. Intertidal systems have been the main arena for experimental examination of these interactions.
PISCO's main research questions focus on understanding how rates of predation and growth of several key species vary along the coast, and how these interactions are influenced by nearshore ocean conditions. Predation studies have largely focused on two of the most important intertidal predators: the seastar (Pisaster ochraceus) and the whelk, (Nucella spp.) and their main prey, mussels (Mytilus spp.) and barnacles (Chthamalus spp. and Balanus glandula). Growth studies have also targeted mussels and barnacles, as well as the kelp Egregia menziesii. Links to nearshore oceanic conditions and processes, and to recruitment and related processes are examined by performing all studies at the same core set of study sites. Mechanistic investigation of the factors and processes that underlie micro-, meso- and macro-scale variation are pursued through a variety of additional experiments and measurements.
Experimental studies examining predation intensity have found generally weak effects of whelks on mussels and strong effects of seastars on mussels. To further examine the influence of coastal conditions on seastar predation, we studied the seastar-mussel interaction at 14 rocky intertidal sites distributed among three oceanographic regions along a 1300-km stretch of the U.S. West Coast. Regions included an intermittent upwelling region (northern), a persistent-upwelling region (central), and a region of weak and infrequent upwelling (southern). We quantified predation rates by the sea star Pisaster ochraceus on its main prey Mytilus californianus as well as phytoplankton concentration and rates of mussel recruitment, mussel growth, mussel abundance, and sea star abundance. By combining the results of these experiments with our increasing understanding of the nearshore conditions--such as currents and food supply--at each of the areas, we have started to uncover some predominant factors that drive variation in how species interact and grow.
We have learned that much of the spatial and temporal variation in recruitment and top-down processes, such as predation and herbivory, can be explained by nearshore oceanographic processes such as upwelling and relaxation events, and by the interactions between water masses and the shoreline topography. In particular, we predicted that predation rate wouldcorrelate with both local productivity and recruitment. Our experiments have partially confirmed these predictions. Predation was relatively high in areas of strong and intermittent upwelling, where recruitment and productivity vary among sites, and predation was relatively low in the weak upwelling region to the south of Point Conception where recruitment and productivity were low. Additionally growth rates of the prey (mussels) were generally similar across upwelling regions, suggesting that factors other than food availability (i.e. temperature, wave exposure) may strongly influence growth. Thus, while recruitment and productivity may affect predation intensity, the effect is not simple and needs further resolution. Our studies suggest that predation intensity relates most closely to the abundance of the sea star. We are thus focusing on the factors that underlie sea star abundance.