Between the summers of 2008 and 2009, two intertidal zones from Quintay, Chile (33°11′S, 71°1′W) were sampled. The study zones were Playa Chica and Playa Grande, which are respectively a MEABR and an OAA. These study zones represent lower and higher anthropogenic impact scenarios, respectively (Castilla et al. 2007, Gelcich et al. 2008, Molina et al. 2014). Two sites that showed similar wave exposure and rock and boulder presence were selected per zone. The effect of predators on community structure was evaluated by comparing randomly collected rocks at the study site with treatment rocks maintained within cages that excluded predators (Menge 1976).

Specifically, three exclusion cages (20×30×30 cm) were installed at each site and at similar low-intertidal levels. All cages had an aluminium frame covered with wire mesh (10 mm) that was treated with anti-fouling and anti-oxidant paint (see Ojeda and Muñoz 1999). Six rocks (≈15 cm in diameter each) per cage were randomly selected from the same sampling site and placed within each cage. All exclusion cages were sampled monthly for one year, with the exception of some winter months due to adverse climatic conditions. For sampling, the six rocks within each exclusion cage, as well as six rocks outside each exclusion cage (controls), were collected. The collected rocks were placed in individual plastic bags, labelled with the sampling site and date, and transported to the laboratory for subsequent analysis. The rocks taken from inside the exclusion cages were replaced each month after sampling with similar rocks from each sampling site. A control cage was not used as it was impossible to anchor this structure in the boulder habitats.

In the laboratory, the sampled rocks were analysed using 5×5 cm quadrants divided into 25 sub-quadrants (1 cm each) (see Navarrete and Castilla 1990). Invertebrates and algae were classified and counted (number and coverage, respectively) under an Olympus CX31 stereomicroscope. Using the obtained data, the richness and diversity of each site were estimated. Specific richness (S) was established as the total number of species found for each rock sample, and diversity was estimated using the Shannon-Weaver Index.

The representation of trophic webs for the boulder fields of Quintay was supported by predator-prey relationships described in the literature (Paine 1966, Castilla 1981, Angel and Ojeda 2001). Using the collected data, species were grouped according to trophic status. This enabled an approximation of an overall trophic web (frames) that included all of the potential predators and respective prey in intertidal boulder fields, regardless of the sampling zone (see Castilla 1981, Angel and Ojeda 2001).

Spatial variations in richness, density and diversity were assessed using two-way ANOVA (General Linear Models, GLM) to test for differences between study zones (MEABR and OAA) and between treatments (inside and outside exclusion cages). The study zone and treatment were considered fixed factors as interest was focused on the differences inside and outside exclusion cages and between the study zones. Insofar as these zones were inside and outside a MEABR, conclusions were limited to these levels (Bennington and Thayne 1994). Prior to GLM analysis, normal distribution was verified and an a posteriori Tukey analysis was performed to determine differences between factor levels.

Multivariate analysis was based on density data for collected mobile and sessile species. Density data were fourth-root-transformed and standardized (between 0 and 1) to ensure that all species, abundant or rare, contributed similarly to the analysis. The Bray-Curtis index of similarity was used. Nonmetric multidimensional scaling (MDS) was used to display the similarities of mobile and sessile species between study zones (MEABR and OAA) and between treatments (inside and outside exclusion cages). Differences in mobile and sessile community assemblages were tested a priori for significance with the ANOSIM procedure (randomized permutation test; Clarke and Warwick 2001). Similarity analysis (SIMPER) identified those species that accounted for the largest differences between study zones (MEABR and OAA) and between treatments (inside and outside exclusion cages) (Clarke and Warwick 2001).

Significance was established at P<0.05. For analyses concerning descriptors of community structure, the STATISTICA 7.0 (StatSoft.Inc. 2004) and PRIMER 5.0 (PRIMER-E Ltd) statistical software were used.

A total of 67 taxa were recorded, 27 of them corresponding to algae and 40 to invertebrates (Table 1). In the MEABR, 46 were recorded, 17 of them corresponding to algae and 29 to invertebrates (Table 1). In the OAA, 51 species were recorded, 24 of them corresponding to algae and 27 to invertebrates (Table 1). Mollusca and Rhodophyta were the most abundant taxonomic groups in both study zones.

Analysis of invertebrates revealed greater species richness and density inside than outside exclusion cages (Table 2, Fig. 1A, B). Additionally, inside the exclusion cages, density was higher in the MEABR (posterior Tukey test P<0.05, Fig. 1B). However, invertebrate diversity was higher in the OAA (Table 2, Fig. 1C).

Algae analysis indicated that inside exclusion cages, richness and diversity were greater in the OAA than in the MEABR (Table 2, posterior Tukey test P<0.05; Fig. 2A, C). In terms of algal density, the MEABR showed higher density outside than inside exclusion cages (Table 2, posterior Tukey test P<0.05; Fig. 2B), whereas in the OAA no differences in density were found (posterior Tukey test P>0.05, Fig. 2B).

Regarding species composition, no differences were found between study zones (MEABR and OAA) or inside vs outside exclusion cages for either mobile or sessile species.

The carnivorous predators registered in the MEABR were Actiniaria (Actinia spp., Phymactis clematis and Phymanthea pluvia) and Polycladida (Tythosoceros inca) (Table 1). Herbivorous species included amphipods (Amphipoda spp.), herbivorous decapods (Taliepus dentatus), isopods (Isopoda spp.), gastropods (Austrolittorina peruviana, Austrolittorina araucana, Fissurella spp., Mitrella spp., Prisogaster niger, Scurria ceciliana and Tegula euryomphala), and chitonids (Chiton cumingsi, Chiton spp. and Chiton latus). These herbivores prey on the benthic algae group found in the MEABR (Blidingia spp., Chaetomorpha spp., Codium dimorphum, Ulva spp., Colpomenia spp., Ahnfeltiopsis spp., Centroceras clavulatum, Corallina officinalis chilensis, Erythrotrichia spp., Lithothamnium spp., Mazzaella spp., Mazzaella membranacea, Mesophyllum spp., Polysiphonia mollis, Polysiphonia paniculata, Rhodymenia spp. and Rhodymenia coralline) (Fig. 3A, Table 1). Filter feeders recorded in the MEABR included mobile annelids (Pseudonereis gallapagensis), sessile annelids (Spirorbis spp.), bryozoa (Bryozoa spp.), cirripedes (Jehlius cirratus), copepods (Copepoda spp.), filtering decapods (Allopetrolisthes angulosus, Petrolisthes granulosus, Petrolisthes tuberculosus, Petrolisthes violaceus and Pisoides edwardsii) and Mytilidae (Perumytilus purpuratus and Semimytilus algosus) (Fig. 3A, Table 1).

Three carnivorous predator groups were recorded for the OAA: Asterozoa (Patiria chilensis and Ophiactis kroyeri), Actiniaria (Actinia spp. and Anemonia alicemartinae) and Polycladida (T. inca) (Fig. 3B, Table 1). Among herbivorous species were amphipods (Amphipoda spp.), isopods (Isopoda spp.), Echinoidea (Tetrapygus niger), gastropods (A. peruviana, Mitrella spp., P. niger, S. ceciliana, Tegula euryomphala, T. luctuosa and Turritella cingulata) and chitonids (Chiton cumingsi, Ch. latus and Tonicia disjuncta). The observed benthic algae group for the OAA included Blidingia spp., Chaetomorpha spp., C. dimorphum, Ulva spp., Adenocystis utricularis, Glossophora kunthii, Sphacelaria spp., Ahnfeltiopsis spp., Anisocladella pacifica, C. clavulatum, Chondria spp., C. officinalis chilensis, Erythrotrichia spp., Gelidium spp., Gelidium lingulatum, Lithothamnium spp., Mazzaella spp., M. membranacea, Mesophyllum spp., Nothogenia fastigiata, Porphyra spp., Rhodymenia spp., Rhodymenia corallina and Schottera nicaeensis (Fig. 3B, Table 1). Filter feeder species included Bryozoa (Bryozoa spp.), Veneroida (Protothaca thaca), Mytilidae (P. purpuratus and S. algosus), cirripedes (Balanus flosculus, Balanus laevis and J. cirratus) and sessile annelids (Phragmatopoma spp. and Spirorbis spp.).

The species composition of the Quintay boulder fields showed patterns similar to those from other reported intertidal zones, such as platforms (Alveal 1971, Santelices et al. 1977, Castilla 1981, see Table 1). The study zones (MEABR and OAA) displayed taxonomic groups typical of the rocky intertidal zone (Castilla 1981). However, important differences in community structure were recorded. Both boulder fields lacked top predators described for rocky platforms. Additionally, taxonomic groups that strongly attach to primary substrates were found, including benthic algae (e.g. Lithothamnium spp.), Mytilidae (e.g. P. purpuratus), Cirripedia (e.g. J. cirratus) and sessile annelids (e.g. Spirorbis spp.). Interestingly, intertidal boulder fields were a nursery habitat for all species, as evidenced by the presence of juveniles for all registered taxonomic groups (see Table 1). This finding highlights the importance of boulder fields for studying species development and the relationship of boulder fields with intertidal and subtidal diversity.

The differences in community structure between study sites are likely associated with coastal morphology and the environmental protection status of MEABRs. Moreover, the studied OAA is a well-described retention zone that, due to seawater circulation and coastal morphology, has increased phytoplankton abundance (Mace and Morgan 2006, Henríquez et al. 2007, Palma et al. 2009), which might help to explain the greater diversity of invertebrates in comparison with the MEABR (Fig. 1C). On the other hand, the Quintay MEABR may have had a differential impact on study zones.

General analyses of MEABRs have found greater species diversity and density than in OAAs, as well as different species composition (Durán and Castilla 1989, Gelcich et al. 2008, Molina et al. 2014). Similarly, greater benthic resource abundances in MEABRs than in OAAs could impact various species, even those not considered within management plans, through the direct and indirect effects of interspecific interactions such as competition and predation (for details see Castilla and Gelcich 2008, Gelcich et al. 2008). For example, some taxonomic groups, such as Arthropoda (e.g. crabs from the Petrolisthes genus) and Actiniaria (e.g. P. clematis and P. pluvia), may be favoured on the boulders of Quintay MEABR, (Fig. 3A, Table 1).

Studies evaluating the impacts of MEABRs on abundance and diversity have focussed on the subtidal system, a more stable habitat than boulder fields. Previous studies indicate significantly greater abundance and diversity in MEABRs than in OAAs (Gelcich et al. 2008, Molina et al. 2014). Additionally, the regular extraction of top predators (e.g. C. concholepas) (Molina et al. 2014) may promote increased diversity by preventing the monopolization of major environmental resources by species with higher competitive capabilities (Paine 1966). This finding was supported by observations at the Quintay OAA, thus revealing a different functional dynamic related to the combined impact of top predator extraction and boulder field variability.

The greater richness and density of invertebrates within exclusion cages (Fig. 1A, B) suggests that the cage might have provided shelter against predators, thereby acting as an additional protective measure within the MEABR, which would explain the high density at this site (Fig. 1B). Echinodermata and Mollusca showed higher species diversity and density in the Quintay MEABR than in the OAA (see Table 1), an observation consistent with results obtained by Molina et al. (2014). Most molluscs and sea urchins are generalist grazers that scrape the substratum, thus removing spores, macroalgae plantlets, epiphytes and microalgae. The diet of these species is considerably similar (Aguilera 2011), with important impacts on benthic algae populations (Contreras and Castilla 1987). Therefore, the MEABR and exclusion cages promoted a protected area for mostly herbivorous invertebrate recruits (Fig. 3A). This protective status (i.e. MEABR regulations and exclusion cages) would differentially impact algae and invertebrates (Figs 1 and 2).

Indeed, the protection given to herbivorous invertebrates inside exclusion cages in the MEABR would increase foraging pressure, explaining the decreased algae density inside exclusion cages (Fig. 2B). This would additionally explain the lower richness and diversity of algae within the exclusion cages in the MEABR than in the OAA (Fig. 2A, C). Although there are no significant differences between zones (MEABR vs OAA), there was a tendency outside exclusion cages towards lower algae richness and diversity in the OAA, a zone where the Tetrapygus niger sea urchin was also recorded (see Table 1). The grazing activities of this species have been described as intense, generating large halos in the bed of intertidal benthic algae (Contreras and Castilla 1987).

In both study zones, the high abundance of red algae could be associated with the trophic morphology of the herbivores detected on boulders (Chiton spp. and Fissurella spp.) (Santelices and Correa 1985, Santelices et al. 1986, Camus et al. 2008) and of species that tolerate high habitat variability. Studies on intertidal rocky platforms have primarily addressed adult individuals with completely developed mouthparts, which prefer Calcarea algae species and other species resistant to grazing (Steneck and Dethier 1994). In boulder fields, only juveniles and invertebrate recruits were found (e.g. T. niger, C. cumingsi, C. latus; Fig. 1B), which may lead to the dominance of the Rhodophyta algae group in this habitat (Santelices 1990, Muñoz and Ojeda 2000, Aguilera 2011). On the other hand, species such as red crustose algae (e.g. Lithothamnium spp.) showed traits that would promote success in the highly variable boulder field habitat, which, in turn, would result in greater abundance.

The OAA boulder field food web revealed the presence of Asterozoa predators such as the sea stars P. chilensis and O. kroyeri. Similar predator species were found for intertidal platforms (Castilla 1981). Asterozoa species have a significant impact on community structure and dynamics since they prey on almost all of the sampled species (Castilla 1981, Navarrete et al. 2000, Navarrete and Manzur 2008, Fig. 3B). Sea stars are a determinant factor in species coexistence, with the predation of dominant community species by sea stars decreasing competitive impacts by preventing competitive exclusion, thereby increasing local diversity (Paine 1966). Moreover, the evidence obtained revealed the presence of other carnivorous groups such as anemones (Actinia spp. and A. alicemartinae) and planaria (T. inca), species with a broad trophic spectrum whose effects on boulder field communities are as yet unknown but should be considered (Sebens and Paine 1978, Zamponi 1979, Acuña and Zamponi 1996).

The MEABR boulder field trophic web revealed the presence of two carnivorous groups (Actiniaria and Polycladida) (Fig. 3A). In turn, the OAA showed three predator groups (Asterozoa, Actiniaria and Polycladida). The Asterozoa group was absent from the MEABR, which may be the result of an increased abundance of commercial predator species that would force Asterozoa to other intertidal or subtidal habitats; however, interactions between predators were not evaluated in this study. Interestingly, both boulder fields lacked the top predators described for rocky platforms, including C. concholepas and Sicyases sanguineus (Castilla 1981). Moreover, species of the intertidal boulder fields that used primary substrates included benthic algae (e.g. C. dimorphum, G. lingulatum, Lithothamnium spp. and Ulva spp), Mytilidae (e.g. P. purpuratus and S. algosus), Cirripedia (e.g. B. flosculus, B. laevis and J. cirratus) and sessile annelids (e.g. Spirorbis spp.). In turn, the rocky platforms of central Chile only contain benthic algae and invertebrates such as Mytilidae and Cirripedia (Castilla 1981).

In conclusion, rocks sampled from both Quintay zones showed similar invertebrate compositions and algae patterns to those of other intertidal habitats (e.g. Alveal 1971, Santelices et al. 1977, Castilla 1981). The most abundant taxonomic group was Mollusca, as was found for rocky platforms, and a comparable result was obtained for algae species (Otaíza and Santelices 1985, Santelices et al. 1986, Aguilera 2011). The analysed boulders of the MEABR and OAA showed taxonomic groups characteristic of intertidal zones (Castilla 1981). Evidence of diversity and food web structure variability likely associated with MEABR protection was detected. Moreover, the high diversity observed in the OAA revealed a different functional dynamic that is likely associated with the combined impact of top predator extraction and boulder field variability. Finally, boulder fields are a necessary habitat for the recruitment of, and as a nursery for, all intertidal species, as is supported by the present findings of only juvenile invertebrate individuals. Therefore, the functional impact of boulder fields on the structural dynamics of intertidal communities should be considered in any management plans.