Rocky Shores Research Articles

Variation in metabolic rate during low tide aerial exposure in the Asian shore crab Hemigrapsus sanguineus

Intertidal animals deal with physical gradients daily that create stressful conditions across the shore. These physical gradients influence the physiological performance of organisms, requiring responses that may differ with height on the shore. We examined the respiratory response to aerial exposure in the invasive Asian shore crab Hemigrapsus sanguineus during periods of low tide emersion using two field experiments. The first experiment simultaneously measured respiration of individuals collected from different heights on the shore, which had therefore been emersed for different lengths of time. The second experiment measured respiration of individuals collected at different times from the same tidal height. Respiration rates of crabs in both experiments increased immediately after emersion, nearly doubling by and peaking at ~ 1.5 h of aerial exposure, before decreasing again over the next 1.5 h. These results suggest that the energetic cost of low tide exposure is greatest shortly after emersion during the first half of the typical low tide period, but then decreases thereafter. These respiration patterns facilitate the broad intertidal distribution of this species on rocky shores throughout its range.

Facies models for rocky shorelines and their application to transgressed basement highs in the North Sea

Rocky shorelines form where basement highs are eroded and flooded during marine transgressive events. Despite the Mesozoic North Sea rift generated numerous platform margins and rotated fault blocks which acted as basement highs, rocky shoreline deposits have not been previously reported. In the rock record rocky shoreline deposits are usually represented by thin conglomerates overlying major unconformities, and are typically characterised by their ichnological aspects, rather than their depositional facies. This study uses the sedimentological aspects of modern and Miocene rocky shorelines from Spain and Austria, to create facies models which are then applied to the recognition of rocky shorelines in the Mesozoic of the Central North Sea. Our results demonstrate that structureless, clast-supported, poorly-to-moderately sorted conglomerate-breccia deposits are associated with competent basement lithologies, which produce hard, resistant coastal cliffs around previously overlooked volcanic centres in the subsurface of the North Sea. The basement lithologies in most of the Central North Sea favoured the formation of softer coastal cliffs, with less resistant lithologies that did not generate or preserve gravel size particles, being mostly characterised by low-angle, unconformity-bounded sandstone and fine-grained deposits and precluding the preservation and recognition of Mesozoic rocky shores in much of the North Sea's stratigraphic record.

Relative predation intensity of an intertidal gastropod on artificial coastal defense structures.

Despite seawalls becoming ubiquitous coastal features, and having some physical similarities to natural rocky shores, it remains unclear how these urban habitats influence predator-prey interactions. Predators can affect intertidal mobile prey densities through two pathways: (1) successful predation directly influences prey mortality rates, and (2) direct and indirect effects of predation can scare and induce motile prey to seek safer areas. In this study, we investigated whether intertidal predators affect the density of the marine gastropod, Nerita undata, at four seawall sites in Singapore. Using a tethering method that we developed, we monitored the mortality and other evidence of predation (shell state) of tethered N. undata. Field experiments revealed high (22.5%-82.5%) predation potential of N. undata across the four sites, with significantly higher predation risk at lower shore heights and for snails with mixed shell coloration. Observations and analysis of the shell state after 3 days showed that predation on seawalls was primarily by crushing predators such as fish. Other predators of N. undata include predatory snails, with various feeding methods that left behind different predator signatures. Our results add substantially to the limited knowledge on predator-prey interactions on seawalls, particularly for Nerita undata, and suggest that seawall systems are more dynamic than previously thought. This further highlights the role of these artificial structures as important habitats and feeding grounds in urban coastal ecosystems.

Seabird-vectored pelagic nutrients integrated into temperate intertidal rocky shores

Seabirds provide inter-habitat connectivity by transporting vast quantities of nutrients from their pelagic feeding grounds to terrestrial and marine ecosystems via their nitrogen and phosphorous rich guano. However, it remains unclear whether seabird nutrients are cycling through many nearshore, temperate systems, such as United Kingdom (UK) rocky shores. Furthermore, it also remains unknown how seabird nutrient subsidies impact biodiversity, productivity, and recolonisation rates in UK rocky intertidal systems. Here, the impact of seabird nutrient subsidies to the Farne Islands, Northumberland, areas of high seabird densities during their breeding seasons, was compared to nearby low seabird density areas on the Northumberland coast. Estimated seabird derived nitrogen inputs and seabird densities were 420-1,025 and 949-2,279 times higher, respectively, on seabird islands than on the mainland. Seabird derived nutrient inputs led to substantially enriched nitrogen stable isotope (δ15N) values and total nitrogen content in terrestrial soil, plants, and lichen, and intertidal barnacles, and limpets, and higher δ15N values in macroalgae and turf algae. The seabird islands contained marginally greater algal species richness and higher densities of intertidal predators. However, other drivers could not be ruled out, such as nutrient inputs from mainland sources and marginally higher wave exposure at the mainland sites. No difference in limpet size, biomass, or abundance existed between the seabird islands and the mainland. On plots artificially cleared to simulate a disturbance event, barnacles recolonised faster on mainland sites. These findings provide evidence that seabird vectored pelagic nutrients are incorporated into terrestrial and intertidal ecosystems of UK islands, and therefore play an important role in facilitating ecosystem connectivity.

Predation effects on benthic communities are maximized by greater taxonomic richness and diversity in Subtropical SW Atlantic

Predation effects on benthic communities are locally influenced by biological and ecological processes, such as recruitment, competition, and habitat features. We studied three Southwestern Atlantic (SWA) subtropical reef habitats (a rhodolith bed, a coral reef, and a rocky shore) to test if the predation effects on benthic communities overwhelm the habitat-context differences. We deployed panels protected and unprotected from predators on the three habitats, retrieved the panels after 45 days and quantified the recruitment of benthic sessile organisms. Despite differences in the composition of benthic assemblages, the effect of predation was mainly driven by the reduction of the ubiquitous polychaeta Spirorbidae in the three habitats. For the rhodolith bed, which supported a higher taxonomic richness and diversity than the other habitats, predation effects were higher and also driven by the reduction of other taxa. Although we recorded a greater predation effect in more diverse reefs, we attribute this result to small scale traits intrinsic to each habitat instead large-scale oceanographic patterns. Our results add pieces into the debate about how predation affects marine habitats and the importance of small-scale understanding for direct management actions in marine conservation.

Local and global stressors as major drivers of the drastic regression of brown macroalgae forests in an oceanic island

Similar to other coastal regions worldwide, forests created by brown macroalgae have severely declined in recent decades across the Macaronesian oceanic archipelagos (northeastern Atlantic), eroding the provision of ecosystem services. However, the putative effects of natural and anthropogenic stressors (both local and global) on such declines across spatial and temporal scales remain unresolved. Our research endeavored to investigate the connection between local and global stressors and the distribution and extent of the brown macroalgae Gongolaria abies-marina in the rocky intertidal and adjacent subtidal zones of Gran Canaria over the past four decades. We also quantified the presence of populations at small scales, according to local micro-habitat topography (“open rock” versus “refuge”). Through herbarium records, we additionally analyzed the historical variation in the thallus size of the species. Finally, we experimentally assessed the thermotolerance of embryonic stages to warming. The main environmental drivers explaining the regression of G.abies-marina were the increasing number of marine heatwaves, while the number of local human impacts (quantified through the HAPI index) also accounted for further regression in the extent of marine forests. Warming experimentally reduced the survival and size of macroalgal embryos. A progressive miniaturization of the species, currently restricted to micro-habitat refuges as a survival strategy, seems likely to be the final stage in the progressive disappearance of this macroalgae from the island’s rocky shores.

Influence of habitat features on the colonisation of native and non-indigenous species

Marine artificial structures provide substrates on which organisms can settle and grow. These structures facilitate establishment and spread of non-indigenous species, in part due to their distinct physical features (substrate material, movement, orientation) compared to natural habitat analogues such as rocky shores, and because following construction, they have abundant resources (space) for species to colonise. Despite the perceived importance of these habitat features, few studies have directly compared distributions of native and non-indigenous species or considered how functional identity and associated environmental preferences drive associations. We undertook a meta-analysis to investigate whether colonisation of native and non-indigenous species varies between artificial structures with features most closely resembling natural habitats (natural substrates, fixed structures, surfaces oriented upwards) and those least resembling natural habitats (artificial materials, floating structures, downfacing or vertical surfaces), or whether functional identity is the primary driver of differences. Analyses were done at global and more local (SE Australia) scales to investigate if patterns held regardless of scale. Our results suggest that functional group (i.e., algae, ascidians. barnacles, bryozoans, polychaetes) rather than species classification (i.e., native or non-indigenous) are the main drivers of differences in communities between different types of artificial structures. Specifically, there were differences in the abundance of ascidians, barnacles, and polychaetes between (1) upfacing and downfacing/vertical surfaces, and (2) floating and fixed substrates. When differences were detected, taxa were most abundant on features least resembling natural habitats. Results varied between global and SE Australian analyses, potentially due to reduced variability across studies in the SE Australian dataset. Thus, the functional group and associated preferences of the highest threat NIS in the area should be considered in design strategies (e.g., ecological engineering) to limit their establishment on newly built infrastructure.

Species contributions to ecosystem stability change with disturbance type

Simultaneous exposure to multiple stressors complicates the challenge of predicting ecological responses to global environmental change. Here, we show that the contributions of individual species and functional groups to the overall stability of ecosystems can be modified by the presence of different stressors, both individually and in combination. By disturbing natural rocky shore communities with nutrients and sediments and simulating extinction of predatory whelks and grazers, we also found that consumers can simultaneously stabilise and destabilise communities along different stability dimensions, irrespective of their trophic position. Our results suggest that our experimental disturbances influenced consumer contributions to stability indirectly by modifying the interactions between consumers and macroalgae in different ways. These findings merit further exploration in different systems exposed to a range of different stressors to better understand how perturbations of different kinds can modify the multifaceted contributions of species to the overall stability of ecosystems.

Does the incorporation of shell waste from aquaculture in the construction of marine facilities affect the structure of the marine sessile community?

The growth of the human population causes significant harm to ecosystems, directly affecting the biological diversity of coastal areas by replacing natural habitats with artificial structures such as breakwaters, ports, and marinas. The hard substrate from those marine facilities lacks the topographic complexity of natural habitats. Because of that, artificial habitats usually do not support a diverse community to the same extent as rocky shores in the surroundings. To address this issue and bring a strategic solution to the improper disposal of shell waste from aquaculture farms, we evaluated how increasing the environmental heterogeneity of walls by incorporating mussel and oyster shells on artificial concrete affected the diversity of sessile organisms from the subtidal zone. Adding shells to concrete positively affected ascidians' richness. Substrates with added shells supported more species than flat substrates in total. They promoted species that did not occur on flat substrates that simulated the traditional walls of marinas and harbors. However, it did not affect the number of bryozoans and the average species richness. Consequently, incorporating shells resulted in communities with completely distinct structures from those on flat substrates. Adding shells affected the community structure, reducing the dominance by the exotic bryozoan Schizoporella errata, and promoting the occurrence of other groups, such as ascidians. Using shell residues from aquaculture on marina walls adds substrate for colonization. Still, it is also likely to provide refuges for fragile and vulnerable organisms, like crevices and pits in natural habitats. Because of that, the increment in diversity was mostly group-specific and restricted to ascidians. This research reinforces the importance of creating complex artificial coastal structures, inspired by the blue economy, for a more heterogeneous coverage of sessile communities and reduced presence and dominance of exotic species. Thus, the strategy tested here, besides the effects on the sessile community, also supports efforts to reduce inappropriate waste disposal in the environment.

The role of food value on host use by the herbivorous amphipod Sunamphitoe pelagica

Mesograzers often use macrophytes as both food and habitat and may have strong effects on primary producers. In this context, understanding the factors mediating their interaction with macrophytes, such as host food value, is an important step to predict the impacts of these herbivores on aquatic ecosystems. The amphipod Sunamphitoe pelagica is a mesograzer with distribution restricted to a few brown macroalgal hosts, however it is unclear if food value drives host use by this mesograzer. Herein, we investigated the distribution of S. pelagica and the food value of its hosts. For that, the abundance of S. pelagica on the macroalgae Sargassum filipendula, Padina gymnospora, and Dichotomaria marginata was evaluated seasonally in a subtidal rocky shore at Fortaleza Beach (23°32′S, 45°10′W), state of São Paulo, Brazil. Furthermore, we conducted laboratory experiments to test feeding preference and rate, as well as the performance (e.g. survival, growth, and reproductive potential) of S. pelagica using the same macroalgal hosts used in the field abundance investigation. Overall, the mesograzer was more abundant on the brown macroalgae Sargassum and Padina than on the red macroalga Dichotomaria. In both feeding preference and rate experiments, S. pelagica consumed more Sargassum and Padina than Dichotomaria. Moreover, the consumption of Padina and Sargassum resulted in high performance of this mesograzer, while juveniles raised on Dichotomaria had a low survival, perishing within few days of experiment. These results suggest the host use by S. pelagica is strongly driven by the food value of macroalgal hosts and are in accordance with the narrow host breadth of this mesograzer. Therefore, considering the vulnerability of large macroalgae to climate change and habitat alterations, the restricted host use by S. pelagica to few brown macroalgae may increase its risk of local extinction.

Microplastic concentrations and risk assessment in water, sediment and invertebrates from Simon's Town, South Africa

Plastic pollution is an ever-increasing threat globally and poor waste management in South Africa has caused an increase in plastic leakage into the environment. Plastic waste in the environment are categorized according to size and plastic particles smaller than 5 mm in size are regarded as microplastics (MPs), and little to no research has been done on MPs pollution within the marine coastal environment and rocky shores in South Africa. Sampling was done in February 2020 at a rocky shore within Simon's Town Marina, Cape Town. MPs were extracted from collected water (n = 5), sediment (n = 5) and biota (n ≤ 30) samples. The extracted MPs were further classified based on shape, colour, size and an attenuated total reflectance Fourier-transform infrared (ATR-FTIR) instrument was utilized for polymer type identification The risks posed by MPs because of concentration at which they occurred and chemical composition were assessed in all the sample types. As expected, MPs were higher in sediment (38 ± 2 MP/kg) than in water (0.37 ± 0.06 MP/L) as the area has low water energy, allowing MP particles to settle within the sediment. Filter-feeding organisms had the lowest average MP particle concentrations (0.28 ± 0.04 MP/g) but displayed the highest variation of MP particle colours due to the non-selective feeding strategy, where other feeding strategies ingested mostly black/grey particles. The dominant MP size was between 100 μm and 500 μm in size for all samples combined, with the most abundant MP polymer type being nylon (27.27 %), polyethylene terephthalate (PET) (18.18 %) and natural MP particles such as cotton (18.18 %). The risk assessment indicated that polymer type poses a greater risk of MP pollution than MP concentrations.

The Multiphasic Teeth of Chiton Articulatus, an Abrasion‐Resistant and Self‐Sharpening Tool for Hard Algae Collection

AbstractChiton articulatus is a species of mollusk living in the tropical Pacific intertidal rocky shores of Mexico. This species feeds on solid waste organic sources, including hard crustose algae that grow on rocky substrates, by grazing on them with its radula, a flexible chitinous membrane lined with mineralized major lateral teeth. In this study, the composition, morphology, and resulting mechanics of the mature teeth of this species, which have yet to be examined, are revealed. The results show the presence of multiphasic mature teeth, each consisting of aligned hard magnetite nanoparticles on the leading edge of the tooth underneath which are magnetite lamellae, followed by goethite, lepidocrocite, and eventually hydroxyapatite near the trailing edge. This multiregional structure demonstrates a gradation in hardness as well as different microstructural features integrated with tough interfaces. The combination of these microstructural and phase arrangements results in an abrasion‐resistant tough structure with a self‐sharpening ability. The results of this work will help contribute to developing new bioinspired designs while also helping to understand the evolution and feeding habits of these intriguing invertebrates.

Important mesopredator restricts feeding in response to risk cues from a new invasive predator

Invasive predators can alter community structure and dynamics in their invaded communities through direct consumption of prey and by inducing prey to alter traits, such as behavior or morphology, to avoid predation. These changes can modify the strength or even the direction of species interactions which can have cascading effects throughout a community. On rocky shores in the Gulf of Maine, an important mesopredator, the whelk Nucella lapillus, alters its feeding behavior when it detects waterborne chemical cues of a long-established invasive crab, Carcinus maenas, potentially impacting the abundance and distribution patterns of sessile species and energy flow through the food web. A more recent invasive crab, Hemigrapsus sanguineus, has become a dominant predator in the southern Gulf of Maine rocky intertidal, but our understanding of its impacts on community structure and dynamics remains incomplete. I examined whether chemical cues from Hemigrapsus alter the feeding behavior of Nucella to better understand how native fauna are responding to this relatively new invasive predator and its potential impacts on the community. Nucella exhibited a risk-induced change in feeding behavior by significantly reducing its feeding rate on blue mussels (Mytilus edulis) when Hemigrapsus cues were detected. These findings demonstrate the capacity for rapid behavioral responses to new invasive species and highlight the potential for such interactions to alter community structure and dynamics.

Physico-chemical alteration of pyroxene in coastal environments: Empirical constraints from New Zealand and the Azores

Developing approaches to determine the modes of transport and weathering of mafic detrital minerals in natural sedimentary environments is critical to our understanding of sediment production, dispersal and provenance, as well as carbon capture under natural and enhanced weathering regimes. We integrated the characterisation of morphological and surface textures with the surface composition of recent detrital clinopyroxene grains concentrated in a sandy coastal area from the western North Island of New Zealand and a rocky shore area of Santa Maria Island in the Azores Archipelago. Using the compactness shape descriptor, 341 grains were subdivided into elongated, elongated angular, euhedral, angular and subangular groups, with each group further characterised using morphological and microtextural indicators of abrasion, breakage and dissolution/chemical weathering. In both studied environments, the clinopyroxenes are dominated by elongated to angular shapes with flat cleavage surfaces and conchoidal fractures. This is consistent with dominant subaqueous transport by rivers and longshore currents for New Zealand, and dominant wave action with limited sediment supply by rivers along the rocky shore of Santa Maria. More abundant subabraded and abraded shapes with bulbous and elongated depression microtextures are observed in New Zealand, which indicates additional effects by aeolian transport that are not seen in Santa Maria. Consistent with semi-quantitative EDS spot analyses that document fresh clinopyroxenes surfaces, chemical weathering textures are very rare to absent. In Santa Maria, sampling of recent beach sand was complemented by ∼125 kyr-old palaeobeach clinopyroxenes, but these yielded similar textural results without evidence for chemical weathering. However, the surface of these older clinopyroxenes includes small adhering smectite (typically <50 μm in width and a few μm in thickness) interpreted to reflect incipient cementation with buffering of acidic fluids by more reactive lithic fragments in the deposits. The lack of chemical weathering but pervasive evidence for mechanical breakage of clinopyroxenes indicate that, at the sand size, these minerals break faster than they dissolve due to frequent subaqueous and/or aeolian reworking in the studied high-energy environments and temperate climatic conditions.

Intertidal mussels as ecosystem engineers: maintenance of invertebrate assemblages amid intertidal stress gradients

Species that form complex biogenic structures may increase niche space for many others and are assumed to be paramount for whole‐assemblage stability and ecosystem functioning. Because they often ameliorate physical conditions, ecosystem engineering species are also predicted to become more important with environmental stress. Testing this hypothesis, however, has proven difficult because isolating facilitation effects along the entire stress gradient is challenging. Here we addressed motile invertebrate assemblages associated to the mid‐intertidal chthamalid barnacle cover at four rocky shores, either with (‘facilitated') or without (‘control') a secondary natural cover of small mussels (Mytilaster solisianus and Brachidontes darwinianus). Following a factorial balanced design, replicate samples were taken at three tidal heights in all sites, encompassing a clear gradient of thermal stress and desiccation potential along the vertical range of the mussel‐enriched zone. While observations on general abundance were mostly consistent with stress‐independent facilitation, results for richness, diversity and assemblage structure indicated overarching stress‐dependent responses. All these later three response variables steadily declined from the low to the high level in the control barnacle habitat but remained unaltered in the mussel‐facilitated habitat. Increased facilitation higher on the shore is mostly due to retention of stress‐vulnerable groups such as polychaetes, flatworms and large nematodes which virtually collapse in the higher control habitat, further indicating that mechanisms of mussel facilitation involve relief of environmental stress rather than protection from higher‐order consumers. Our results suggest that mussel ecosystem engineering was fully preserved during the summer season when sampling took place. Ongoing research simulating heat waves compatible to climate‐change scenarios will test whether mussel facilitation would hold over the coming decades.

Investigating the Upper Holocene palaeoenvironment and human subsistence strategy in the Khor Rori coastal area by studying mollusc remains from the Inqitat plateau (Dhofar, Sultanate of Oman)

ABSTRACTArchaeological shells have a great potential for reconstructing past environments and human–environment interactions. The Inqitat plateau (Khor Rori Archaeological Park, Oman) shows rich mollusc assemblages, present in the HAS1 settlement and in a shell midden nearby (Iron Age). By analysing the environmental requirements of the species identified in the assemblages, we reconstruct the past coastal environment and investigate its exploitation by the local population and human activities during the settlement occupation. This is the first multidisciplinary investigation of Iron Age molluscs from Dhofar. Our results indicate that the environment surrounding the site was characterised by rocky shores and shallow‐water sandy to sandy‐muddy substrates, where locals collected shells. Mangrove species were recovered in few numbers, indicating that this environment was restricted or not exploited. Furthermore, a different use of shells between circular houses within the settlement and the midden was observed; in the former, species used as ornaments are more abundant, whereas, in the latter, species used for food purposes are found, confirming the primary use of the midden as a dump. The presence of easily accessible settings for mollusc collection in the site's vicinity and the richness in food resources due to a coastal upwelling makes this region very attractive for human populations.

Contributions from citizen science to climate change education: monitoring species distribution on rocky shores involving elementary students

ABSTRACT Citizen science is a promising field for research in climate change. It has also been recognised with great potential in the field of climate change education, especially in the increase of scientific knowledge and science process skills. The study aims to examine the contributions of citizen science to learning outcomes regarding climate change and the development of science skills. A mixed-methods approach involving a pretest, posttest, records inserted in the iNaturalist platform, semi-structured interviews with the researchers, and a questionnaire to iNaturalist users that validated the observations inserted by students, were applied to access students’ knowledge and development of science skills. Findings showed an improvement in scientific knowledge related to consequences of climate change, species identification and students’ development of science skills, such as observation, data collection and teamwork. Indeed, this study supports that students can be involved in all stages of scientific research and the data can be used in scientific investigations, improving the size and quality of citizen science databases. This investigation reinforces the importance of citizen science projects in a school context to improve the student’s skills and scientific knowledge regarding climate change.

Microplastics in rocky shore mollusks of different feeding habits: An assessment of sentinel performance

Microplastics (MPs) accumulation in rocky shore organisms has limited knowledge. This study investigated MPs accumulation in filter-feeding oysters, herbivorous limpets and carnivorous snails to assess their performance as sentinel species in the MPs trophic transfer. The samples were obtained along a contamination gradient in the Santos Estuarine System, Brazil. All three studied species showed MPs concentrations related to the contamination gradient, being the oysters the species that showed the highest levels, followed by limpets and snails (average of less and most contaminated sites of 1.06–8.90, 2.28–5.69 and 0.44–2.10 MP g−1, respectively), suggesting that MPs ingestion rates are linked to feeding habits. MPs were mainly polystyrene and polyacetal. The polymer types did not vary among sites nor species. Despite minor differences in percentages and diversity of size, shape, and color classes, the analyzed species were equally able to demonstrate dominance of small, fiber, transparent, black and blue MPs. Thus, oysters, limpets, and snails are proposed as sentinels of MPs in monitoring assessments.

Spatial distribution of lower intertidal decapods on the northern Patagonian coast (Pelluhuín beach, Puerto Montt, 41°29′S, Chile)

Abstract The decapod fauna of Chile’s intertidal shores in inner seas south of 40°S has relatively low diversity because of the presence of low-salinity waters due to river inputs and glacial smelts; nevertheless it is possible that the same decapods species are found as on the northern and central Chilean coast. The aim of the present study was to determine the spatial distribution patterns of lower intertidal decapods on Pelluhuín beach, a small beach south of Puerto Montt, northern Patagonia. Data were obtained by counting individuals from random quadrants in intertidal zones; to the obtained data the variance/mean ratio was applied to determine if the specimens have a random, aggregate or uniform distribution, which are associated with Poisson, negative binomial or positive binomial distributions respectively. Among four of the species observed, a uniform distribution (positive binomial) was reported, and one had an aggregated pattern (negative binomial). The sites correspond to rocky shores in semi-urban zones, and in a protected zone. Our results on the interpretative probabilistic models of aggregated distribution patterns agree with previously reported observations of decapods on the rocky shores of Northern and Central Chile, specifically in interpretative probabilistic models.

Effects of harvesting and an invasive mussel on intertidal rocky shore communities based on historical and spatial comparisons.

Intertidal rocky shores are the most accessible marine habitats and therefore heavily impacted by harvesting. In recent years, they have also been increasingly invaded by alien species, which compounds the effects of harvesting on rocky shore community composition and functioning. Recent survey data, combined with historical data from 1970, were used to assess temporal changes over the intervening period in rocky shore communities at two sites (Wireless Point and Wireless Island). Three kinds of changes emerged: (1) the appearance of alien species; (2) the effects of increased harvesting pressure; and (3) the direct and indirect effects of these changes on other species. A striking result was transformation of mid-shore zones on exposed shores by the appearance of the invasive Mediterranean mussel Mytilus galloprovincialis, and the indirect effects of this on the demography and vertical zonation patterns of the granular limpet Scutellastra granularis. Adult limpets have become excluded by the mussel, whereas juveniles find a secondary home on the shells of the mussel and their abundance has increased. To further disentangle the effects of harvesting from those of alien invasions, a spatial comparison was made between two currently unharvested no-take sites (Scarborough South and Scarborough North) and two regularly harvested sites (Kommetjie and Wireless Point). Harvesting has substantially depleted the granite limpet Cymbula granatina and Argenville's limpet Scutellastra argenvillei. This has led to the proliferation of opportunistic seaweeds, such as Ulva spp. The dual effects of alien invasive species and over-harvesting have major ecosystem effects but do not necessarily diminish biodiversity because the alternative habitats that have developed provide opportunities for colonisation by additional species.