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.

The genetic consequences of living on the edge of distributional ranges have been the subject of a largely unresolved debate. Populations occurring along persistent low latitude ranges (rear-edge) are expected to retain high and unique genetic diversity. In contrast, currently less favourable environmental conditions limiting population size at such range-edges may have caused genetic erosion that prevails over past historical effects, with potential consequences on reducing future adaptive capacity. The present study provides an empirical test of whether population declines towards a peripheral range might be reflected on decreasing diversity and increasing population isolation and differentiation. We compare population genetic differentiation and diversity with trends in abundance along a latitudinal gradient towards the peripheral distribution range of Saccorhiza polyschides , a large brown seaweed that is the main structural species of kelp forests in SW Europe. Signatures of recent bottleneck events were also evaluated to determine whether the recently recorded distributional shifts had a negative influence on effective population size. Our findings show decreasing population density and increasing spatial fragmentation and local extinctions towards the southern edge. Genetic data revealed two well supported groups with a central contact zone. As predicted, higher differentiation and signs of bottlenecks were found at the southern edge region. However, a decrease in genetic diversity associated with this pattern was not verified. Surprisingly, genetic diversity increased towards the edge despite bottlenecks and much lower densities, suggesting that extinctions and recolonizations have not strongly reduced diversity or that diversity might have been even higher there in the past, a process of shifting genetic baselines.

Wind-driven upwelling variability and local topography cause an upwelling shadow in the northern region of Monterey Bay, California, to persist seasonally. The present study applied partial least squares regression to a 7-yr time series collected within this retentive feature for the purpose of evaluating the environmental controls on total autotrophic phytoplankton (as chlorophyll a) and picoplankton (Synechococcus spp., picoeukaryotes, and heterotrophic bacteria) abundance. A bloom threshold was defined and applied to all biological groups to evaluate seasonal and inter-annual abundance patterns. Microbial and phytoplankton abundances in the upwelling shadow were positively associated with warmer, nutrient-depleted water. Consistent with these results, two-thirds of phytoplankton blooms occurred in October−November, when surface temperatures were warm and ammonium concentrations were greatest. These blooms were predominantly composed of dinoflagellates, 64% of which were known toxin-producing species. Although the overall relationship of phytoplankton to river discharge rates was negative, phytoplankton blooms in 2006, 2007, 2010, and 2012 followed early rainfall events, which flush nitrogen from the surrounding farms into the bay. Despite the fact that the regional measure of upwelling, the Bakun upwelling index, is seasonally low in the autumn, pulses of cold, nutrientreplete water were advected into the upwelling shadow, additionally supporting late-year blooms. Physical and chemical processes occurring over multiple time scales controlled bloom dynamics in the upwelling shadow of Monterey Bay.

Humans were considered external drivers in much foundational ecological research. A recognition that humans are embedded in the complex interaction networks we study can provide new insight into our ecological paradigms. Here, we use time-series data spanning three decades to explore the effects of human harvesting on otter–urchin–kelp trophic cascades in southeast Alaska. These effects were inferred from variation in sea urchin and kelp abundance following the post fur trade repatriation of otters and a subsequent localized reduction of otters by human harvest in one location. In an example of a classic trophic cascade, otter repatriation was followed by a 99% reduction in urchin biomass density and a greater than 99% increase in kelp density region wide. Recent spatially concentrated harvesting of otters was associated with a localized 70% decline in otter abundance in one location, with urchins increasing and kelps declining in accordance with the spatial pattern of otter occupancy within that region. While the otter–urchin–kelp trophic cascade has been associated with alternative community states at the regional scale, this research highlights how small-scale variability in otter occupancy, ostensibly due to spatial variability in harvesting or the risk landscape for otters, can result in within-region patchiness in these community states.

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