Intertidal Community Regulation Table 1. Design of predation experiments. Island South South North Area Banks Peninsula Kaikoura Peninsula Mt Maunganui Seascape 26 R-R sites 26 R-S sites 1 R-R site 26 R-R sites 26 R-S sites SL Low Low+ Mid Low Treatment No Cage No Cage Partial cage Full Cage No Cage Full Cage Species on tiles Experiment duration MG & XP MG & XP XP November 22, 2003–January 6, 2004 November 24, 2003–February 10, 2004 August 30, 2004–September 4, 2004 R-R = reef-to-reef, R-S = reef-to-sand. MG = Mytilus galloprovincialis, XP = Xenostrobus pulex. SL = Shore level. (At the South Island Banks Peninsula sites we used only tiles exposed to all predators (no-cage treatments) because we showed in a previous study that survival was always high for months inside full cages in this region at all seascape types (Rilov and Schiel 2006b). At Kaikoura we used no-cage, a partial-cage (3 cm gap at the bottom; excludes fish but not large invertebrate predators such as crabs, sea stars or whelks) and a full-cage (excludes all large predators) in both the low and mid shore levels. At the North Island Mt Maunganui sites we used the no-cage treatment and a full-cage control. Cage size was 10610610 cm, mesh gauge was 16616 mm (for full details on cage design see Rilov & Schiel 2006a,b)). doi:10.1371/journal.pone.0023958.t001 The Clorox Company, CA, USA) that were fastened to rocks with stainless steel screws and washers (n = 5). Mussels attach to the filaments of the mesh balls that mimic the filamentous algal and mussel byssal thread surfaces that constitute common settlement sites in nature [e.g., 31] and it provides moisture and protection from most macro-predators. Many other studies that tested benthic-pelagic coupling models have used mussel recruitment to tuffies as a proxy for prey larval supply [e.g., 20,24,32]. At Banks Peninsula (South Island), settlement was monitored weekly or biweekly between January–December 2004, and at Kaikoura Peninsula (South Island), biweekly between December 2003 and May 2004. At the distant North Island sites, monitoring was done monthly between August–December 2004 (Mt Maunganui) and September–December 2004 (Leigh). Samples were stored in 220uC freezers prior to sorting and enumeration. In the lab, mussels were extracted from collectors using fast-flowing seawater, sieved and counted. Recruitment rate was expressed as mussels per collector per day. Predation-recruitment-mussel cover and predator abundance relationships To test these relationships, we used all data from this study complemented by some data from earlier studies (Rilov and Schiel 2006a, 2006b), and by mussel recruitment data collected by B. Menge using the same methods over the same time period at three sites (Woodpecker Bay, Nine Mile Bluff on the west coast, and Raramai on the east coast of the South Island) for which we conducted predation but no recruitment measurements (B. Menge, personal communication). All data types and sources are summarized in Table S1. Average daily recruitment rates were calculated for the main recruitment season for which data were available. To standardize predation intensity to natural mortality rates, we used an index of interaction strength. There are several different ways interaction strength has been calculated in the past [see review, 33]. We chose a modified version of an index used by Connolly and Roughgarden [13] for which we had the available (NE {NC ) data: I ~ NE where I is the interaction strength index, NE is the percent mortality (the inverse of survival) of small mussels on tiles exposed to all predators (experimental plot) and NC is mortality on tiles inside a cage (control plot). The index reflects the mussel mortality in plots exposed to all predators relative to protected plots and the values in this modified index would normally move between 0 (no interaction) and 1 (very strong interaction) but can also reach negative infinity if there is higher mortality for some reason inside than outside cages (this was never seen in our experiments). In this measure of predation intensity, the mean percent mortality for days 3 or 5 (depending on the experiment monitoring regime that was determined by logistics and sea conditions) for all predation experiments was used because most unprotected mussels were gone in R-R sites within the first few days of the experiment. We also looked for population-level effects by testing a variety of potential predictors for percent mussel cover (only mussels.15 mm, i.e., cover excluding recruits). These were 1) predation rate, 2) mussel recruitment, and 3) the combined effect of predator abundance and mussel recruitment. Data for whelks, fish and sea stars were pooled by functional group (i.e., whelks = all predatory whelk species, fish = the two labrid species (Banded wrasse, Notolabrus fucicola and Spotty, Notolabrus celidotus), sea stars = mostly Stichaster australis but also cushion stars if they were present). We have no fish data for the west coast sites (too rough and murky to sample) but labrids are known to be rare on the central parts of this coast where our study sites were located (Don Neale, New Zealand Department of Conservation, personal communication). We also have no data of fish abundances at KP or Raramai (on the east coast of the South Island) but labrids are known to be abundant on subtidal reefs in this area [34]. All the sites for which we had no fish data were not included in the correlations of mussels with fish abundance. Data Analysis Abundance data from the community surveys were analyzed using 3-way mixed-model ANOVA (after arcsin square-root transformation) for each island (North, South) separately with seascape (R-S, R-R), and shore height (Mid, Low) as fixed factors, and site as a random factor nested within Seascape. Seascape, shore height and site were tested as main effects. Using data from the current study and a previous one ([28], plus unpublished data from R-S sites) we compared fish visitation rates (both labrid species combined) between islands (fixed factor), seascapes (fixed) and sites (random, nested in seascape) in a 3-way mixed-model ANOVA (after square-root transformation). Mussel percent survival in the predation experiment was tested using repeated-measures ANOVA after arcsin-square-root transformation. In the Banks Peninsula (South Island) and the Mt. Maunganui (North Island) experiments, seascape and site (nested in seascape) were tested with time as the within-subjects factor. In the Kaikoura Peninsula (South Island) experiment, shore-level and treatment were fixed factors and time PLoS ONE | www.plosone.org 4 August 2011 | Volume 6 | Issue 8 | e23958