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RESEARCH ecological interactions Motivation MotivationExperimental investigations of species interactions on rocky shores have had a powerful influence on advances in ecology. Until recently, however, most of this research was constrained in scope and scale with respect to space and time. As a consequence, general principles were sometimes difficult to perceive. Divergent results among studies could result from different dynamics or simply from different investigators focusing on a narrow portion of a much larger issue. Research Questions
ApproachPISCO scientists have thus embarked upon an intensive, multi-site and multi-factor investigation of the effects of species interactions on community structure. Studies of predation rate in the mid- and low-intertidal zones involve translocation of mussels to mid-zone gaps in mussel beds and to low-intertidal zones dominated by algae. Mid-zone experiments examine predation by whelks (Nucella spp.) on blue mussels (Mytilus galloprovincialis in California, M. trossulus in Oregon). Experiments include + predator and - predator treatments, with predator deletion achieved by either manual removals or exclosures. In the low zone, we have examined the separate and combined effects of two predator species: whelks (Nucella spp.) and sea stars (Pisaster ochraceus). Studies of growth of sessile invertebrates and algae are conducted annually at multiple sites in each region. Mussel growth is quantified as the shell increment added per year. Barnacle growth is quantified using plexiglass settlement plates treated to cause widely-spaced settlement of juveniles. This method limits the problem of crowding, which can impede growth (basal diameter), and allows for measurements of growth. Growth of the alga Egregia menziesii, the only abundant macroalga that occurs across the entire PISCO range, also is quantified by frequent measurement of individually marked plants. Pilot studies are underway in one or more regions of the PISCO study area to examine other interactions, including grazer rates, grazer impacts, rates of competition, and preemption. Once effective approaches are developed for these other interactions, each will be considered for inclusion as PISCO-wide studies. 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. Research Findings Predation intensity, however, is different from the overall effect of predators. We initiated field experiments in 2001 that measure the predation effect to determine how the predation rate and effect interact. These experiments involve controlled exclusion of Pisaster ochraceus from areas of the shore at multiple sites. Based on classic experiments in Washington State, we expect that in the absence of sea stars, mussels will cascade down the shore, displacing algae and invertebrates. We also expect the predation effect of sea stars to vary in relation to mussel recruitment and growth, which PISCO scientists have shown vary dramatically among sites and years. Because changes in mussel bed distribution can take several years to occur, this experiment will become part of PISCO's long-term monitoring program. Combined, these studies will reveal how extensively both post-recruitment predation and oceanographic conditions can modify rocky intertidal communities. By Bruce Menge, Principal Investigator, and Renee Davis-Born, and Lydia Bergen, Policy Coordinators |
A long-held assumption in coastal ecological research based on these spatially and temporally limited results was that the dynamics of rocky intertidal and subtidal communities were largely independent of oceanic factors such as nutrients, phytoplankton, zooplankton, temperature, and currents. Recent work--much of which has been conducted by PISCO scientists--makes this assumption problematic. Studies along the coasts of California and Oregon demonstrated that benthic and nearshore pelagic environments were often strongly linked via recruitment, larval transport, the flux of food for particle-feeding invertebrates, and nutrients. Hence, in order to fully understand the dynamics of these communities, and thereby derive the knowledge needed to conserve and protect them, spatial and temporal scales need to be dramatically expanded to match the scales upon which both benthic and pelagic components of the system vary. Such increases in scale also demand expansions of research scope to include a much more integrative and interdisciplinary approach to understanding of nearshore community and ecosystem dynamics. 
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. In 1999, our studies of the effects of whelks and the ochre sea star on mussels found that whelks have weak effects on these prey species throughout the PISCO study area. Thus, experiments in subsequent years examine sea star-mussel interactions at multiple sites in Oregon, central California, and southern California. We performed experiments to test the strength of predation of the ochre sea star, a keystone predator, on its primary prey, the California mussel (M. californianus), at 14 sites from Central Oregon to Southern California. PISCO conducted similar experiments to measure the intensity of competition, the intensity of grazing, and the growth rates of several key species. 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 should correlate with both local productivity and recruitment. Experiments in 1999 and 2000 partially confirmed these predictions. Predation was relatively high north of Point Conception where recruitment and productivity vary among sites, and predation was relatively low south of Point Conception where recruitment and productivity were low. 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.