can vary geographically, and mussels are not dominant intertidal organisms in all regions. At R-S seascapes located in biogeo- graphical regions where intertidal predators (sea stars, whelks) are influential (i.e., can exert strong predation pressure) we expect patchy/ephemeral to very low mussel cover in the low shore depending on recruitment rates (scenarios A, B in Fig. 6; e.g., the Oregon coast at Cape Perpetua). Indeed, in Oregon R-S sites such as SH (see site name codes in Fig. 6 legend), or ones on New Zealand’s central-northern west coast, temporary mussel beds can be formed in the low shore as the predators are first overwhelmed by the prey recruit numbers but over time eventually eat them [G. Rilov, personal observations, B. Menge personal communication, and see, 15]. Where intertidal predators are rare or not influential (e.g., whelks in some places such as New Zealand), mussel cover is expected to be high even when recruitment is low or medium because of low predation rates (scenarios C, D; e.g., R-S sites on the New Zealand east coast). In R-R seascapes, we expect mussel cover to be low or spatially patchy due to low recruitment rates. Where both intertidal (sea stars) and subtidal (fish, crabs) predators are present, mussel beds are restricted to mid-shore levels (scenarios E, F, probably the situation in JH on New Zealand southern west coast). Where only intertidal predators are influential we still expect low mussel cover because they usually feed on all mussel size classes (scenarios G, H, e.g., sites in Oregon at Cape Foulweather). Where intertidal predators are rare (or not influential) and subtidal ones present, low shore mussel beds are expected to be spatially patchy in medium recruitment sites (because most fish and crabs prey only on small mussels, mussels in microhabitats escape in size to form patches), and there is scarcity of mussel recruits between patches, or absence where recruitment is very low (scenarios I, J, e.g., most New Zealand east coast sites, [27]). Although this proposed model describes processes on a local scale, these scenarios can also be scaled-up to coastlines, based on difference in geomorphologies among coasts. For example, in New Zealand, most rocky seascapes along the west coast of the North Island and the northern and central west coast of the South Island have subtidal benches with nearby subtidal sandy or pebble bottoms. Predatory reef fish such as labrids are rare in these habitats, probably due to the rarity of kelp beds there [34]. Most R- S benches are indeed covered by extensive mussel beds in the higher parts of the low shore and in midshore levels, with very high recruitment levels and occasional high densities of sea stars [29,41]. In contrast, many parts of eastern New Zealand (both Islands) can be categorized as R-R seascapes with abundant predatory fish [34,42] and very low mussel recruitment (Rilov unpublished data). At many of these sites, low shore mussel beds are patchy, scarce, or altogether absent. Dotted along the coast, often very close to R-R sites, R-S rocky benches occur, and mussel cover on them is frequently high, probably due to enhanced recruitment (see suggested mechanism above) and reduced predation. The apparent non-linear ‘‘threshold’’ increase in sea star abundance beyond ca 100 mussel recruits per collector per day or ca 90% mussel cover (see Fig. 5) may be more a product of biogeography than of a threshold numerical response. The two sites with the highest sea star abundance were on the west coast where mussel recruitment and cover are indeed also high. Menge et al. [15] suggested that the different coastal oceanography on the two coasts (intermittent upwelling vs. downwelling) determines the pace of processes on the shore, and that ‘‘high abundances of prey may influence predator abundance by increasing survival of predator recruits, increasing predator growth rates…’’. However, we found small isolated sites on the east coast with very high mussel recruitment rates but very low sea star abundances. Unless the numerical Intertidal Community Regulation response, or more precisely the demographic response, is operating on a coastal scale the prey-predator abundance relationship alone does not hold, and other explanations for the significant difference in sea star abundances between coasts are required. The lack of a liner relationship between prey (mussel) recruitment/abundance and predator density fits well with the conclusion of Wieters et al. [23] who showed that such relationships are expected only for predators with non-pelagic larvae, and sea stars do have pelagic larvae. Conclusions. Four major concepts have been stressed in this study: (1) predation can be more important than recruitment for the abundance of a dominant space occupier (mussels). (2) Local recruitment of prey and predation pressure on it (species interaction) can be negatively correlated because (3) seascape can greatly influence both larval recruitment rates of community dominants and predation on these community dominants, and therefore community structure in coastal systems – a good example of context-dependency. (4) The inclusion of interactions (e.g., predation) with the early life stages of benthic community dominants in experimental studies is crucial for the understanding of patterns and processes on the shore [see also, 43]. Furthermore, while mesoscale coastal processes (e.g., upwelling, eddies) may greatly contribute to regional variability in community structure [13,15,20], we demonstrate here that very local processes can result in just as much variability among sites with similar wave exposure within the same area. Including these concepts when designing future studies will increase the power of predictive models describing coastal community function and therefore help in achieving greater understanding of underlying processes. Finally, the importance of landscape configuration to species interactions and how they affect communities have certainly become a focus of study in current ecological research. Recent examples come for instance from the African savanna where herbivore hotspots were related to landscape features that affect predation and nutrient input [44], and where movement and foraging activity of lions was related to landscape features and prey distribution [45,46]. Our study contributes to this line of research by supplying evidence from a marine system on the importance of the local predator guild, the way it is affected by the local landscape, and the subsequent effects on community structure and processes. One can also view the intertidal-subtidal food-web linkage described in our study as a good example of an edge or refuge effect. Within this framework, forest edge effects are well known, and nest and seed predation rates in the forest have been linked to their proximity to an open area where mammalian nest predators come from [47,48,49]. Similarly, edge or refuge effects are known for marine environments. Lobsters living on New Zealand subtidal reefs, have been shown to fiercely forage on nearby sandy bottom prey [50], and in New England lobsters have been shown to move up to the intertidal at night to prey [51]. The next step is to find unifying or cross-ecosystem models that describe and explain these patterns and processes. Supporting Information Table S1 Site location, name, type of seascape, code name and type of information used in this study. Mussel rock cover, recruitment and predation intensity was used for the multivariate analysis. The maximum tidal range on the east coast is ca. 2.4 meters. We define the low shore there as extending from 0–0.8 m (Lowest Astronomical Tide) and the mid shore from 0.8– 1.6 m. The maximum tidal range on the west coast is ca. 3.7 meters. The low and mid shore zones on this coast extend from 0–1.2 m and 1.2–2.4 m, respectively. (DOC) PLoS ONE | www.plosone.org 10 August 2011 | Volume 6 | Issue 8 | e23958