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The structure of ecological communities reflects a tension among forces that alter populations. Marine ecologists previously emphasized control by locally operating forces (predation, competition, and disturbance), but newer studies suggest that inputs from large-scale oceanographically modulated subsidies (nutrients, particulates, and propagules) can strongly influence community structure and dynamics. On New Zealand rocky shores, the magnitude of such subsidies differs profoundly between contrasting oceanographic regimes. Community structure, and particularly the pace of community dynamics, differ dramatically between intermittent upwelling regimes compared with relatively persistent downwelling regimes. We suggest that subsidy rates are a key determinant of the intensity of species interactions, and thus of structure in marine systems, and perhaps also nonmarine communities. Many ecological processes determine the structure and dynamics of communities and ecosystems. Theoretical and experimental advances have led to a growing awareness that different processes operate at different spatial and temporal scales (1, 2). Capturing the full richness of ecosystem dynamics thus requires studies ranging across a wide range of scales and allowing effective evaluation of the relative impact of the relevant factors. Doing so can be challenging. The logistics are daunting, especially in using the power of experimentation over large spatial scales. One solution, the ''comparative-experimental'' approach (3), involves replicated local-scale experimentation at multiple sites spanning larger scales, coupled with local-scale repeated sampling of factors that vary at characteristically larger scales.In its early development, marine community ecology focused on the dynamic consequences of primarily local-scale processes such as species interactions and physical disturbance (4). More recently, marine and nonmarine ecologists alike have documented the influence on communities of larger-scale phenomena including subsidies of materials and propagules transferred between adjacent ecosystems (5-8). Although evidence for the importance of subsidies is growing, questions remain about their impact, generality, magnitude, and interdependence and the physical and biotic mechanisms that underlie them.Here we use the comparative-experimental approach to address the role of large-scale oceanographic phenomena in structuring communities on New Zealand rocky shores. Based on earlier results (6, 9), we predicted that intertidal community structure and dynamics would reflect the coastal oceanographic regime. We hypothesized that the influences of oceanographically modulated subsidies (propagules, as a cause of increases in population density of benthic species, and the concentration of phytoplankton and detritus, as food for filter feeders) would be high with upwelling and low with downwelling. Study SystemPrevious research on the west and east coasts of the South Island of New Zealand (pairs of sites 100-500 m apart on each coast) revealed strik...

A recent model predicts that species interactions in benthic marine communities vary predictably with upwelling regimes. To test this model, we studied the Pisaster-Mytilus 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 intermittentupwelling 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 by transplanting mussels into the sea star's low-zone foraging range and comparing the rate of mussel loss in ϩPisaster plots to those in ϪPisaster plots. To evaluate the relation between predation rates and key ecological processes and conditions, we quantified phytoplankton concentration and rates of mussel recruitment, mussel growth, mussel abundance, and sea star abundance.Predictions of the model are expressed as responses of predator and prey abundance, and species interaction strength (per capita and per population or total impact at the population level). As predicted by theory, per capita predation rates were independent of upwelling regime, with no variation with region. Contrary to expectation however, perpopulation predation rates were similar between intermittent-and persistent-upwelling regions but were greater under strong upwelling than under weak upwelling conditions. The greatest variation in per-population predation rates was at the level of site within region. Also contrary to theory, average abundances of prey (mussel cover) and predators (sea stars) were similar among oceanographic regions and varied mostly at the site level.As expected from theory, predation rate was high where sea star density was high, a condition that often coincided with a high food supply (phytoplankton) for filter feeders, including larvae, and high recruitment. With the exception of two sites having dense sea star populations and thus high predation, low values of either or both were associated with low predation, suggesting that the supply of prey often depended on conditions that favored subsidies of both phytoplankton and new larvae to prey populations. The occurrence of high predator density and high predation at sites of low inputs of particulate food and propagules suggests that understanding sea star life history is a key to a fuller understanding of variation in predation on a coastal scale. Evidence suggests that often sporadic recruitment of sea stars along the coast is balanced by great longevity, which tends to even out predation impact on coastal intertidal communities.

Patterns of recruitment in marine ecosystems can reflect the distribution of adults, dispersal by ocean currents, or patterns of mortality after settlement. In turn, patterns of recruitment can play an important role in determining patterns of adult abundance and community dynamics. Here we examine the biogeographic structure of recruitment variability along the U.S. West Coast and examine its association with temperature variability. From 1997 to 2004 we monitored monthly recruitment rates of dominant intertidal invertebrates, mussels and barnacles, at 26 rocky shore sites on the West Coast of the United States, from northern Oregon to southern California, a span of 1750 km of coastline. We examined spatial variation in the dynamics of recruitment rates and their relationship to coastal oceanography using satellite‐derived time series of monthly sea surface temperature (SST). Recruitment rates showed a biogeographic structure with large regions under similar dynamics delimited by abrupt transitions. The seasonal peak in recruitment rates for both mussels and barnacles changed from a late summer–early fall peak in Oregon to winter or early spring in northern California, and then back toward summer in southern California. Recruitment rates varied greatly in magnitude across the latitudinal range. The barnacle Balanus glandula and mussels (Mytilus spp.) showed a decline of two orders of magnitude south of Oregon. In contrast, recruitment rates of barnacles of the genus Chthamalus showed a variable pattern across the region examined. The spatial distribution of associations between raw SST and recruitment rates for all species showed positive associations, indicating recruitment during warm months, for all species in Oregon, northern California, and several sites in south‐central California. By considerably extending the spatial and temporal scales beyond that of previous studies on larval recruitment rates in this system, our study has identified major biogeographic breaks around Cape Blanco and Point Conception despite considerable spatial and temporal variation within each region and among species. These large differences in recruitment rates across biogeographic scales highlight the need for a better understanding of larval responses to ocean circulation patterns in the conservation and management of coastal ecosystems.