Predator impacts on intertidal community structure have been studied for rocky platforms, but intertidal boulder fields, a habitat with a greater extension and heterogeneity, have not yet been considered. Keeping in mind that disturbances are considered an important force in determining intertidal habitat diversity, the aims of this work were to describe and quantify boulder field community structure and to assess boulder field community dynamics by proposing possible food webs, taking into consideration predatory and anthropogenic impacts. These aims were achieved by installing predator-exclusion cages outfitted with rocks that were monitored monthly over one year in two study zones, a Management and Exploitation Area for Benthic Resources (MEABR, Playa Chica) and open-access area (OAA, Playa Grande). For both study zones, juveniles were the dominant observed ontogenetic state and invertebrate richness and density were higher inside exclusion cages. Furthermore, the MEABR had a differentiated impact on community structure and dynamics in comparison with the OAA. In conclusion, the roles played by boulder fields in intertidal diversity, especially in recruitment and as a nursery zone, are important to consider in management plans.
El impacto depredador sobre la estructura comunitaria intermareal ha sido estudiado para plataformas rocosas, pero no ha sido abordado en campos de bolones intermareales, un hábitat de mayor extensión y heterogeneidad. Considerando que las perturbaciones son una fuerza importante en determinar la diversidad del hábitat intermareal, los objetivos de este trabajo fueron describir y cuantificar la estructura comunitaria de campos de bolones, y evaluar la dinámica comunitaria de este hábitat proponiendo posibles tramas tróficas; considerando el impacto de depredadores y antropogénico. Estos objetivos se lograron mediante la instalación de jaulas de exclusión de depredadores equipadas con rocas, que fueron monitoreadas mensualmente durante un año en dos zonas de estudio, un Área de Manejo y Exclusión de Recursos Bentónicos (AMERB, Playa Chica) y un área de acceso abierto (AAA, Playa Grande). Para ambas zonas de estudio, los juveniles fueron el estado ontogenético dominante, y la riqueza y densidad de invertebrados fueron mayores dentro de las jaulas de exclusión. Además, el AMERB tuvo un impacto diferenciado sobre la estructura y dinámica comunitaria comparado al AAA. En conclusión, el rol que cumplen los campos de bolones en la diversidad intermareal, especialmente en el reclutamiento y área de crianza, son importantes y debiesen ser considerados en los planes de manejo.
Factors that promote variations in biodiversity are highly important considering the impact that diversity has on ecosystem functioning (
The conservation and sustainable-use mechanisms for marine resources include Marine Protected Areas and no-take zones (
Due to dense, diverse organism assemblages, the rocky intertidal zone is an ideal "natural laboratory” for understanding the factors that govern intertidal community organization, a topic of numerous studies. Steep environmental gradients, rapid organism turnover, and abundant sessile and slow-moving organisms grant this zone experimental tractability (
Understanding community dynamics requires a basic knowledge of community interactions between member species, which define community structure and determine how effects are transmitted between species. For instance, predators can directly (e.g. by consumption) and indirectly (e.g. through the trophic cascade) affect community structure (
Predator impacts on intertidal community structure have been studied for rocky platforms, but intertidal boulder fields—a habitat with a greater extension and heterogeneity—have not yet been considered (
Rocky platforms have decreased diversity due to space monopolization by highly competitive species (
Therefore, the aims of this study were (1) to describe and quantify boulder field community structure; and (2) to assess boulder field community dynamics by proposing possible boulder field food webs, taking into consideration predatory and anthropogenic impacts. In the two zones were assessed, a MEABR and an open-access area (OAA), we expected to find notable differences in community structure and trophic web between the evaluated boulder fields.
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 (
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
In the laboratory, the sampled rocks were analysed using 5×5 cm quadrants divided into 25 sub-quadrants (1 cm each) (see
The representation of trophic webs for the boulder fields of Quintay was supported by predator-prey relationships described in the literature (
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 (
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;
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 (
Taxa | Species | MEABRs | OAA | ||
---|---|---|---|---|---|
In | Out | In | Out | ||
Annelida | × | × | |||
Annelida | × | ||||
Annelida | × | × | × | × | |
Arthropoda | × | ||||
Arthropoda | Amphipoda spp. Latreille, 1816 | × | × | × | × |
Arthropoda | × | × | |||
Arthropoda | × | × | |||
Arthropoda | Copepoda spp. | × | |||
Arthropoda | Isopoda spp. Latreille, 1817 | × | × | × | × |
Arthropoda | × | × | |||
Arthropoda | × | ||||
Arthropoda | × | ||||
Arthropoda | × | ||||
Arthropoda | × | ||||
Arthropoda | × | ||||
Bryozoa | Bryozoa spp. | × | × | × | × |
Chlorophyta | × | × | × | ||
Chlorophyta | × | × | × | × | |
Chlorophyta | × | × | × | × | |
Chlorophyta | × | × | × | × | |
Cnidaria | × | × | × | ||
Cnidaria | × | ||||
Cnidaria | × | ||||
Cnidaria | × | ||||
Echinodermata | × | ||||
Echinodermata | × | ||||
Echinodermata | × | ||||
Mollusca | × | × | |||
Mollusca | × | ||||
Mollusca | × | × | × | × | |
Mollusca | × | × | × | ||
Mollusca | × | ||||
Mollusca | × | ||||
Mollusca | × | × | |||
Mollusca | × | × | × | × | |
Mollusca | × | × | × | × | |
Mollusca | × | ||||
Mollusca | × | × | × | × | |
Mollusca | × | × | × | ||
Mollusca | × | × | × | × | |
Mollusca | × | ||||
Mollusca | × | ||||
Mollusca | × | ||||
Ochrophyta | × | ||||
Ochrophyta | × | ||||
Ochrophyta | × | × | |||
Ochrophyta | × | ||||
Platyhelminthes | × | × | |||
Rhodophyta | × | × | × | ||
Rhodophyta | × | ||||
Rhodophyta | × | × | |||
Rhodophyta | × | ||||
Rhodophyta | × | × | × | ||
Rhodophyta | × | × | × | × | |
Rhodophyta | × | ||||
Rhodophyta | × | × | |||
Rhodophyta | × | × | × | × | |
Rhodophyta | × | × | × | × | |
Rhodophyta | × | × | × | ||
Rhodophyta | × | × | × | × | |
Rhodophyta | × | ||||
Rhodophyta | × | ||||
Rhodophyta | × | ||||
Rhodophyta | × | ||||
Rhodophyta | × | × | × | × | |
Rhodophyta | × | × | × | ||
Rhodophyta | × | × |
Analysis of invertebrates revealed greater species richness and density inside than outside exclusion cages (
Variable | Effect | Invertebrate | Algae |
---|---|---|---|
Richness | Study zone (S) | F(1, 169)=3.31, P=0.070 | F(1, 169)=4.07, P=0.045 |
Treatment (T) | F(1, 169)=11.45, P<0.001 | F(1, 169)=0.67, P=0.412 | |
(S) * (T) | F(1, 169)=1.16, P=0.282 | F(1, 169)=7.77, P=0.005 | |
Density | Study zone (S) | F(1, 169)=14.66, P<0.001 | F(1, 169)=0.29, P=0.589 |
Treatment (T) | F(1, 169)=85.05, P<0.001 | F(1, 169)=37.9, P=0.053 | |
(S) * (T) | F(1, 169)=16.27, P<0.001 | F(1, 169)=74.58, P=0.006 | |
Diversity | Study zone (S) | F(1, 169)=13.21, P<0.001 | F(1, 169)=3.49, P=0.063 |
Treatment (T) | F(1, 169)=0.06, P=0.803 | F(1, 169)=1.14, P=0.287 | |
(S) * (T) | F(1, 169)=0.34, P=0.557 | F(1, 169)=6.70, P=0.01 |
Algae analysis indicated that inside exclusion cages, richness and diversity were greater in the OAA than in the MEABR (
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 (
Three carnivorous predator groups were recorded for the OAA: Asterozoa (
The species composition of the Quintay boulder fields showed patterns similar to those from other reported intertidal zones, such as platforms (
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 (
General analyses of MEABRs have found greater species diversity and density than in OAAs, as well as different species composition (
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 (
The greater richness and density of invertebrates within exclusion cages (
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 (
In both study zones, the high abundance of red algae could be associated with the trophic morphology of the herbivores detected on boulders (
The OAA boulder field food web revealed the presence of Asterozoa predators such as the sea stars
The MEABR boulder field trophic web revealed the presence of two carnivorous groups (Actiniaria and Polycladida) (
In conclusion, rocks sampled from both Quintay zones showed similar invertebrate compositions and algae patterns to those of other intertidal habitats (e.g.
This study was funded by Universidad Andres Bello grants DI 0508/R, DI 17-10R, DI 16-12/R, and DI-495-14/R awarded to JP, and Universidad Andres Bello grant DI-02-11/I awarded to MRG-H.