Intertidal Community Regulation Figure 1. Study sites and percent rock cover of major space occupiers in the mid and low shore levels at R-R and R-S seascapes in New Zealand. Space occupiers include mussels of different size classes, macroalgae, barnacles and free space. CR is located about 1 km west of the nearest extensive rocky shore. TM is located 190 m from the western rocky side of a small sandy bay. MM2 is a small isolated bench located about 40 m from the rocky shore that surrounds the mount. MI2 is a rocky bench 200 m east of the Moturiki Island (see text). Pakiri is a R-S site which is at the beginning of a rocky shore at the end of the sandy Pakiri Beach. At the MI2 low shore, total cover exceeded 100% (because many mussel recruits covered adults) and actual percent cover for those groups is given in numbers on the pie chart. The Kaikoura Peninsula site was sampled in January 2004, Banks Peninsula sites in April 2004, Mt Maunganui sites in June 2004 and the Leigh sites in September 2004. A subset of this data appeared in Rilov and Schiel 2006a. doi:10.1371/journal.pone.0023958.g001 predation experiment (see below), and in the morning (8:00–9:30 AM) of the next day. Numbers of sea stars and crabs (which are mostly nocturnal) were also recorded. The dusk survey included a record of predation activity on mussels on experimental tiles (see below). The predator surveys were intended to provide relative assessments of predator abundance and activity at study sites around the time of the experiments, and were not a thorough investigation of predator densities or temporal changes. Other, qualitative surveys (sometimes during harsher conditions) at R-R and R-S seascapes at different times supported the patterns we report below (Results section). However, we also recognize that within sites there can be great annual and inter-annual variation in densities mainly of recruits of these fishes [e.g., 30]. Predation intensity experiments The experimental units were comprised of 5–15 mm long mussels that were scraped from the rocks and settled on 565 cm carpet- covered plastic tiles that were secured to rocks with stainless steel screws [see, 27,28]. Twenty-five mussels were placed onto each tile, which was then wrapped with soft plastic mesh to secure the mussels until they were firmly attached by byssus threads (3–4 weeks). At the South Island sites, a mixture of Mytilus galloprovincialis and Xenostrobus pulex was used for prey because they were by far the most abundant small mussels in the area and are difficult to distinguish at small sizes. M. galloprovincialis was absent from North Island sites so only X. pulex was used. The experiments included transplanted mussels that were either exposed to the full suite of predators or protected by different types of cages to exclude different guilds of predators (see details in Table 1). It was logistically impossible to do the experiments in all sites at the same time, and the experimental design was somewhat different, depending on logistics and how much prior knowledge we had for each region (Table 1). Prey recruitment rates Mussel recruitment rates were measured at different sites using as substrate plastic-mesh ovoid pot scrubbers (SOS Tuffy Pads, PLoS ONE | www.plosone.org 3 August 2011 | Volume 6 | Issue 8 | e23958