Molluscan Grazers Research Articles

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COVER PHOTO: Tide pools at Corona del Mar State Beach, California, USA, are pictured on the cover. Declines in biodiversity in marine systems are occurring at unprecedented rates and it is essential to evaluate the complexities of the resulting transformations on ecosystems. Bracken et al. used tide pools on the southern California shoreline to experimentally manipulate grazer abundance and quantify mechanisms by which molluscan grazers promote algal abundance. Their results (Article e4439; doi: 10.1002/ecy.4439) suggest that mollusks in the pools enhance the abundance of the algae they consume by recycling nutrients and creating sticky slime trails that promote algal recruitment. Photo credit: Matthew E. S. Bracken. image

Immediate Impact of the 2021 Harmful Algal Bloom in Southeast Hokkaido on the Rocky Intertidal Benthic Community and Its Spatial Variation

There has been a limited number of studies on the effects of harmful algal blooms (HABs) on natural rocky intertidal ecosystems. From mid-September to early November 2021, an unprecedented HAB caused by Karenia selliformis hit the Pacific coast of southeast Hokkaido, Japan, for the first time, causing massive mortalities among marine organisms. To clarify the immediate impacts of the HAB on the abundance of 10 rocky intertidal species in four functional groups (macroalgae, sessile invertebrates, molluscan grazers, and molluscan carnivores), we focused on two questions: (1) How did the HAB affect the abundance of each species differently at the regional scale? and (2) How did the impacts of the HAB on the abundance of each functional groups vary spatially, and was the spatial variation of the HAB impacts related to the spatial distribution of the cell density of HAB species? To study these issues, we compared census data for 17 years before the HAB and within one month after it for five shores on the southeast coast of Hokkaido. The results showed that two macroalgae species and all three molluscan grazer species declined significantly after the HAB. Moreover, the decrease in molluscan grazers was significantly correlated with cell density. These results suggest that the impacts of the HAB in southeast Hokkaido on the abundance of rocky intertidal organisms are highly variable depending on species and locality, presumably because of differences in species-specific tolerances to HAB toxins and the spatial variation in the density of the HAB organisms.

Ecological tipping points for an invasive kelp in rocky reef algal communities

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 587:93-104 (2018) - DOI: https://doi.org/10.3354/meps12429 Ecological tipping points for an invasive kelp in rocky reef algal communities David R. Schiel1,*, Stacie A. Lilley1, Paul M. South2 1Marine Ecology Research Group, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand 2Cawthron Institute, Nelson, New Zealand *Corresponding author: david.schiel@canterbury.ac.nz ABSTRACT: Invasive species are affecting coastal ecosystems worldwide and there are many potential mechanisms that allow their spread into native communities. To investigate this phenomenon, we used an experiment in which the canopy of the southern bull kelp Durvillaea antarctica was removed in 2 seasons and community development was followed over 3 yr. The invasive kelp Undaria pinnatifida recruited almost exclusively into plots from which the natural canopy had been removed. We examined, through a series of regression tree models and change point analyses, the gradients of community responses that allowed the successful recruitment of this invasive species. Analyses revealed a range of coinciding conditions, especially a decline in fucoid cover below 20% and an increase in turf cover above 80%, that facilitated Undaria recruitment. The abundance of molluscan grazers and cover of subcanopy algae had lesser effects on Undaria recruitment. In an ecological sense, there was a clear tipping point in the interaction between canopy loss and the subsequent expansion of coralline turf that allowed communities to switch from dominance by native species to seasonal dominance of Undaria. This study illustrates the complex nature of disturbance thresholds and interactions within the native communities that can facilitate the spread and recruitment success of Undaria. KEY WORDS: Tipping point · Threshold · Rocky reef · Native algae · Invasive algae · Regression tree Full text in pdf format PreviousNextCite this article as: Schiel DR, Lilley SA, South PM (2018) Ecological tipping points for an invasive kelp in rocky reef algal communities. Mar Ecol Prog Ser 587:93-104. https://doi.org/10.3354/meps12429 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 587. Online publication date: January 25, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

What and when to eat? Investigating the feeding habits of an intertidal herbivorous starfish

Although the ecology of molluscan grazers on intertidal rocky shores is very well documented, there are other abundant grazers, such as herbivorous starfish, that can modify the structure of benthic assemblages yet whose behaviour is poorly understood. The feeding habits of asteroids suggest that they are opportunistic feeders foraging on a broad range of available resources. The reasons for the variation in the food types consumed are not clear. We investigated the feeding habits of the intertidal herbivorous starfish Parvulastra exigua with respect to the temporal and spatial distribution of food and tidal cycles in two populations. We predicted that food choice by P. exigua would be a function of food abundance and that foraging would be independent of tidal cycle. The results showed that this starfish forages on a narrow range of algal food resources, with an apparent selection for microalgae. Foraging during high and low tide was variable across locations; however, most individuals from both populations foraged during any tidal period. These results indicate that P. exigua forages on the same food consumed by many other intertidal grazers, indicating the potential for competition for resources. Grazing on biofilm by P. exigua may play an important role in modifying the structure of algal assemblages on rocky shores by potentially preventing the growth of macroalgae.

Herbivore-Alga Interaction Strength Influences Spatial Heterogeneity in a Kelp-Dominated Intertidal Community.

There is a general consensus that marine herbivores can affect algal species composition and abundance, but little empirical work exists on the role of herbivores as modifiers of the spatial structure of resource assemblages. Here, we test the consumption/bulldozing effects of the molluscan grazer Enoplochiton niger and its influence on the spatial structure of a low intertidal community dominated by the bull kelp Durvillaea antarctica and the kelp Lessonia spicata. Through field experiments conducted at a rocky intertidal shore in north-central Chile (~30°-32°S), the edge of the grazer and algae geographic distributions, we estimated the strength and variability of consumptive effects of the grazer on different functional group of algae. We also used data from abundance field surveys to evaluate spatial co-occurrence patterns of the study species. Exclusion-enclosure experiments showed that E. niger maintained primary space available by preventing algal colonization, even of large brown algae species. The grazing activity of E. niger also reduced spatial heterogeneity of the ephemeral algal species, increasing bare space availability and variability through time in similar ways to those observed for the collective effect with other grazers. Overall, our result suggests that E. niger can be considered an important modifier of the spatial structure of the large brown algae-dominated community. Effects of E. niger on resource variability seem to be directly related to its foraging patterns, large body size, and population densities, which are all relevant factors for management and conservation of the large brown algae community. Our study thus highlights the importance of considering functional roles and identity of generalist consumers on spatial structure of the entire landscape.

Nutrient enrichment alters the consequences of species loss

Summary Loss of biodiversity and nutrient enrichment are two of the most pervasive drivers of change in ecosystems globally. However, little is known about how these disturbances interact to affect ecosystem functioning. We established a field experiment to test for effects of loss of consumer species on algal assemblages (richness and assemblage structure) and ecosystem functioning (ecosystem productivity and algal biomass accumulation) on rocky shores. We manipulated the presence of three common molluscan grazer species in newly created rock pools over 13 months. Our grazer manipulation was based on a subtractive experimental design, and we also manipulated nutrient concentrations to evaluate explicitly how resource availability affected the consequences of species loss. We found that the effects of loss of grazer species on algal assemblages and their functioning depended strongly on both the identity of the grazer species lost and of those remaining in the community. Moreover, the effects of loss of species depended on nutrient availability, making it extremely difficult to predict the effects of species loss generally without detailed knowledge of a system. Our results demonstrate clearly that the effects of grazer species loss on primary producers and ecosystem functioning may not be generalisable based on grazer functional roles or traits. Synthesis. Our findings highlight the need to include key physical drivers, such as nutrient availability, explicitly into biodiversity–ecosystem functioning models in order to move towards a predictive framework that incorporates the effects of both environmental heterogeneity and anthropogenic stressors.

Consumer effects on ecosystem functioning in rock pools: roles of species richness and composition

A key challenge in research linking biodiversity and ecosystem functioning is to incorporate the trophic interactions that characterise natural systems. There is a particular shortage of studies investigating consumer species richness and composition (identity) effects in the context of ecosystem development (or succession). We manipulated the richness and composition of an assemblage of molluscan grazers (Patella ulyssiponensis, Gibbula umbilicalis and Littorina littorea) added to rock pools denuded of existing biota. We created monocultures and all possible multispecies combinations in a substitutive design, and ran a field experiment for 13 mo. We used 2 separate nested analyses to isolate the roles of species richness, species composition nested within levels of species richness and the specific effect of the limpet P. ulyssiponensis, a putative key species. We found no evidence that the biomass or productivity of the developing macroalgal assemblage was affected by grazer richness or species composition nested within richness levels. Rather, the presence of P. ulyssiponensis, irrespective of the presence of other grazer species, acted to suppress mean values of these response variables. Biomass and productivity were not strongly related, showing that they provide unique information on ecosystem functioning in this system. Macroalgal species richness was also reduced by P. ulyssiponensis, and correlated positively with macroalgal biomass, indicating a link between these response variables. Macroalgal species composition was largely insensitive to either species richness or the presence of P. ulyssiponensis, but responded to particular combinations of species within levels of these factors. The key role of P. ulyssiponensis in determining ecosystem functioning is apparent from our results, but we note that consumer species richness may play an important role under more heterogeneous conditions.

Combined effects of nutrient enrichment, sedimentation and grazer loss on rock pool assemblages

Coastal areas around the world are experiencing increasing disturbance from multiple stressors caused by anthropogenic activities. Although there is good knowledge about the impacts of individual stressors, there is less understanding of the consequences of several stressors acting simultaneously. Eutrophication and sediment deposition are widely recognized as major problems for the functioning of coastal systems, and they are expected to increase during the next decades. In a field experiment, using rock pools as a model system, different levels of nutrients and sedimentation were applied in a factorial experimental design that also accounted for the influence of molluscan grazers. Sedimentation significantly changed assemblage structure, mainly owing to an increase in turfing and filamentous algae and a decrease in crustose algae. Nutrients also caused an increase in the cover of green filamentous algae, which in turn was synergistically magnified by the removal of grazers. Here we showed that these stressors can individually alter the structure of rock pools assemblages; and that in this system top-down control (by grazers) is more important than bottom-up factors (nutrients) in controlling macroalgal assemblage structure. The combined effect of grazers loss and nutrients was larger than the sum of their individual effects. This study enhanced mechanistic understanding of the impacts of multiple stressors on coastal ecosystems, which will help to develop management strategies and conservation of the marine environment.

Spatial heterogeneity increases the importance of species richness for an ecosystem process

The role of biodiversity in mediating ecosystem processes has been the subject of focused theoretical and empirical attention since the mid‐1990s. Theory predicts that the balance between species richness and identity effects will critically depend on the degree of environmental heterogeneity, which dictates the extent to which differences between species in patterns of resource use can be expressed. We conducted a mesocosm experiment to explicitly test this hypothesis. We manipulated the richness and identity of intertidal molluscan grazers, as well as the spatial heterogeneity of the substrate upon which they grazed. The magnitude of algal consumption was used as our focal ecosystem process. The grazer treatments consisted of three monocultures and a single polyculture including all three species; heterogeneity was represented as the proportion of topographically complex and flat substrate. Species identity had strong effects on homogeneous substrates, with the identity of the best‐performing species dependent on the substrate. On the heterogeneous substrate, suitable conditions for all three species were represented, allowing the expression of spatial complementarity of resource use and the enhancement of total algal consumption. Our findings provide the first explicit experimental evidence that spatial heterogeneity of the physical environment can play a key role in mediating effects of species diversity.

Interaction of top down and bottom up factors in intertidal rockpools: Effects on early successional macroalgal community composition, abundance and productivity

Increasing levels of eutrophication in coastal waters and recognition of natural variation in supply of nutrients, through for example variations in upwelling intensity, have led to an increasing demand to understand the role of bottom up forcing in coastal communities. Rocky intertidal habitats are highly disturbed and are composed of a patchy mosaic of assemblages in different successional stages. The role of consumer control of algae in such habitats is well recognised but the way consumer control interacts with variation in resource supply is still debated. We examined, using a factorial manipulative experiment, the way top down control by molluscan grazers interacts with nutrient supply to determine the community composition and productivity of early successional communities in intertidal rockpools. The experiment was run over a six-month period, through the summer months, when nutrients were potentially limiting. Rockpools were cleared of biota and nutrient supply enhanced using slow release fertiliser, while the level of molluscan grazing was reduced through regular removals. Macroalgal colonisation was rapid. Positive effects of nutrient enrichment on macroalgal cover were generally lower than the negative effects of grazer control, although at the peak of algal cover, nutrient effects were apparent irrespective of grazer treatment. There were distinct differences in the identity of dominant species among nutrient treatments with green Ulva spp. dominating under enhanced nutrient conditions and red Ceramium spp. dominating under ambient conditions. These differences contributed to a mismatch between effect estimates based on cover and those based on direct measures of productivity. The use of percentage cover as a surrogate of biomass in intertidal studies is discussed. The results indicate that in a disturbed environment enhanced nutrient supply can stimulate macroalgal blooms despite strong top down control by intertidal grazers.

Effects of Chiton granosus (Frembly, 1827) and other molluscan grazers on algal succession in wave exposed mid-intertidal rocky shores of central Chile

Molluscan grazers can have important effects on the abundance, colonization rates, and successional pathways of algal assemblages and the entire intertidal community. In general, early successional algae are more readily consumed than corticated algae and kelps, which usually get established later in the community succession. To generalize, however, the effect of different grazers on algal assemblages must be examined on different coasts and under different scenarios. This information could help us understand the mechanisms of ecosystem processes and situations in which general models do not apply. Along the coast of Chile, humans harvest large keyhole limpets, which seem to be the only invertebrate grazers capable of controlling the dominant corticated alga Mazzaella laminarioides, a canopy-forming species that can cover extensive areas of the mid intertidal zone. In this scenario, where large limpets are harvested, the overall effects of the diverse molluscan assemblage of limpets, chitons and snails on algal succession and on corticated algae in particular are not clear. We conducted a 26-month-long experiment to evaluate the effects of molluscan grazers on mid-intertidal algal succession and to isolate the effects of Chiton granosus, the most conspicuous member of the assemblage at these tidal elevations. At sites heavily impacted by humans the molluscan grazer assemblage had strong negative effects on colonization and abundance of green algae such as ulvoids and Blidingia minima. In doing so, the grazer assemblage had a strong negative indirect effect on the establishments of chironomid fly larvae, which were only observed on green algal mats and rarely on bare rock. No significant effects were detected on epilithic microalgae, and effects on sessile invertebrates were highly variable over space and time. C. granosus also had significant negative effects on green algae but did not account for the total grazing pressure exerted by the guild. Limited foraging excursions (ca. 35 cm) from refuges and moderate site (crevice) fidelity in this species may contribute to the patchiness in green algal distribution observed in the field. Nearly 13 months after rock surface were experimentally cleared, M. laminarioides appeared in all experimental plots, but increased over three times faster in enclosures containing C. granosus than in exclosures plots or controls, suggesting that moderate levels of herbivory could actually facilitate the establishment of this alga in the succession and that the green algal cover found in the absence of grazers may delay its establishment.

Effects of physical ecosystem engineering and herbivory on intertidal community structure

Physical ecosystem engineers play dominant roles in a wide variety of communities. While many of the direct, positive effects of ecosystem engineers are readily apparent, the roles of engineers are often mediated by indirect interactions stemming from the facilitation of one or a few key species. Although direct and indirect effects are both critical drivers of community dynamics, they are rarely considered together with regards to ecosystem engineering. In the present study barnacle and herbivorous gastropod densities are experimentally manipulated to investigate the direct positive effects of habitat provision by barnacles as well as indirect effects mediated by mollus- can grazers. Molluscan grazers (Littorina spp.) and herbivorous arthropods were positively influ- enced by the presence of barnacles. Arthropod abundance and species richness were lower when Littorina spp. were present. This pattern was not influenced by barnacle cover, suggesting that competition among herbivore functional groups was strong but independent of biogenic habitat complexity. In addition, Littorina spp. had strong negative effects on the filamentous alga Urospora penicilliformis, but this effect was only seen in the absence of barnacles. Finally, Littorina spp. reduced the recruitment of the principal habitat-forming barnacle Balanus glandula, suggesting that Littorina spp. may mediate a negative feedback loop in B. glandula population dynamics. Given the ubiquity of ecosystem engineers, similar combinations of direct and indirect influences may have far-reaching consequences for community dynamics and species richness in a wide range of systems.

Monitoring spatio-temporal variation in molluscan grazing pressure in seasonal, tropical rock pools

In Hong Kong, low-shore rock pools support a patchy mosaic of different successional stages of crustose coralline algae. Echinoid and molluscan grazers are abundant in these low-shore rock pools and spatial and temporal variation in grazing pressure is thought to play an important role in structuring these assemblages. Direct records of graze marks using a modified wax-disc method were used to monitor spatial (different pools) and temporal (periods, 3 or 4 days each; spring vs. neap tides; summer vs. winter) variations in grazing pressure. Radula marks of Chlorostoma argyrostoma, Lunella coronata and Nerita albicilla were common in all pools. Grazing pressure was highly variable with tidal state and season but also within these two factors. Lunella and Nerita exerted highest grazing pressure at neap tides during summer, whilst Chlorostoma exerted highest grazing pressure in winter. Although overall patterns of temporal variation were similar and consistent in all pools, there was between pool variation in grazing pressure and individual species showed different patterns. Within the seasonal and tidal variations, large period-to-period differences were found for all species, illustrating small-scale temporal variation. Fine-scale spatial and temporal patterns of variation in grazing pressure of different species, therefore, contribute to the highly dynamic nature of these low-shore rock pools.

Microspatial variation in marine biofilm abundance on intertidal rock surfaces

The effect of substrate surface roughness on small-scale patchiness and the ability of molluscan grazers to feed on intertidal biofilms was examined in a factorial experiment. Granite slabs were treated to create 4 different levels of surface roughness, and biofilm and macroalgae were allowed to recruit. Biofilm cover varied greatly with slab roughness, and was spatially patchy at a scale of millimetres. Diatoms dominated the biofilm, but were less abundant on surfaces with the smallest pits. Cover of diatoms and cyanobacteria was affected by surface roughness, with increased abundance around surface features. Different species of grazer varied in their success at removing certain diatoms and cyanobacteria from slabs of varying roughness, due to either the morphology of the different food types or grazer radula structure. Cover of macroalgal species on the slabs of different roughness also varied, and one species, Hypnea sp., did not recruit on smooth slabs. Rock roughness, therefore, affects both the biofilm and algal species that recruit and their abundance. Grazers were able to remove algae from slabs of all roughness with no apparent species-specific differences in their ability. However, grazer species appear to be more or less efficient at feeding according to the level of roughness, and this combination of variation in rock roughness and grazer efficiency may explain the observed small-scale patchiness on rocky shores in Hong Kong.

Sediment on rocky intertidal reefs: Effects on early post-settlement stages of habitat-forming seaweeds

Modification of the coastal environment by human activities often leads to an increase in sedimentation of nearshore waters, with potential impacts on benthic marine assemblages. Here we assess the relationships between the levels of sedimentation, wave exposure and benthic organisms on rocky intertidal platforms around the Kaikoura Peninsula in southern New Zealand. We designed and tested five sediment traps to provide a tool for measuring the relative abundance of sediment across sites. Using field- and laboratory-based experiments, we tested hypotheses concerning whether different levels of sedimentation affected algal germling survival and algal zygote attachment, and whether the interactions of grazers and sediments affected germling survival. Levels of sediment and exposure were inversely related across seven sites. The fucoid alga Hormosira banksii characterized the more sedimented wave-sheltered and intermediately wave-exposed sites, with up to 80% cover in the lower mid-tidal zone, while the bull kelp Durvillaea antarctica characterized the three most wave-exposed sites. Grazing molluscs were found across all sites but species abundances varied by sediment and exposure levels. We did two 11-day trials testing the effects of different levels of sediment and different species of molluscan grazers on the survival of 1-week-old Hormosira germlings. Generally, there was no significant treatment effect of grazers, but mortality varied considerably among sites. In particular, one site had very high levels of sediment, which resulted in 100% mortality of germlings across all grazer treatments. Removing sediment at 1-week, 2-week and 4-week intervals made no difference to the survival of Hormosira germlings. In laboratory-based experiments, a light dusting of sediment reduced the percentage of zygotes of Hormosira by 34% and Durvillaea by 71% that attached to primary substratum, and a complete cover of sediment prevented attachment altogether. Overall, the effects of sediments and its interaction with molluscan grazing were highly variable but often large, particularly on the attachment of zygotes to primary substratum.

Cirripedios en la dieta del molusco herbívoro Chiton granosus Frembly, 1827 (Mollusca, Placophora) presente en el intermareal rocoso de Iquique, norte de Chile

Preliminary quantitative information is given on the presence of barnacle cyprids and post-metamorphic stages in the diet of the molluscan grazer Chiton granosus Frembly 1827 (Mollusca: Placophora) sampled at Iquique, Chile. In December 2000, we observed the presence of barnacle cyprids and post-metamorphic stages at a high relative percentage of abundance (N = 30, 54,1% and 39,9%, respectively) and they were frequent in all of the individuals analyzed. Moreover, a high frequency of ocurrence of cyprid larvae (F% = 67) was recorded in the distal portion of the intestine; of these larvae, 15 were observed with signs of vital activity (appendage movement) in 45% of all the stomachs analyzed. Ingestion of barnacle cyprids and post-metamorphic stages was observed at a high frequency throughout the five months of the study (N = 10, 96% ± 6,5 and 78% ± 23,6).

A novel interaction between nutrients and grazers alters relative dominance of marine habitats

Strong top-down control by grazers is considered a moderating influence on the nega- tive effects of elevated nutrients on marine algae. Little experimental work has been done in a sys- tem that has weak grazing pressure (weak top-down control), which is also subjected to elevated nutrients. We experimentally elevated nutrient concentration to test (1) the effects of nutrient enrich- ment on algal assemblages in the presence and absence of canopies (Ecklonia radiata) and (2) the interactive effects of nutrients and molluscan grazers (meso-grazers) on algal assemblages in the absence of canopies. We established that the loss of canopy-forming algae is likely to be a key pre- cursor to nutrient-driven changes of assemblages of benthic algae, because nutrients had no effects on algal assemblages in the presence of canopy-forming algae. In the absence of canopy-forming algae, space was monopolised by filamentous, turf-forming algae, and it was only in the presence of grazers that nutrients caused a change to the relative covers of algal habitat that monopolise canopy- free space. When grazers were present at natural densities, elevated nutrients reduced the monopoly of turf-forming algae in favour of foliose algae. These results demonstrate a novel interaction between nutrients (bottom-up control) and grazing pressure (top-down control), which are funda- mental to predictions about management of human activities that continue to reduce densities of herbivores and increase nutrient availability on temperate coasts.

Community structure and zonation patterns of rocky shores at high latitudes: an interocean comparison

AbstractAim To compare the structure of communities of sheltered rocky shores at high latitudes in widely separated regions in order to search for similarities and dissimilarities.Locations Three regions with similar physical environments were investigated: south‐western Iceland, southern Alaska, and the Magellanic region.Methods Cover of algae and sessile animals was estimated and animals were counted in quadrats laid along a number of transects in each of the three regions. Transects were divided into 10 height intervals with reference to tidal levels, and comparisons made on the basis of average distributions of species within each of 11 defined guilds.Results Icelandic and Alaskan shores had a number of species in common, although these were in the minority. No species was common to southern and northern regions, with the possible exception of Mytilus edulis. Most defined guilds were present in all regions, but their taxonomic composition was often radically different. The molluscan filtering guild was the guild showing greatest similarity in the three regions, while canopy‐forming algae were absent from the Magellanic region and balanid filterers were virtually absent from Iceland. Crustacean grazers were only prominent in Iceland, and molluscan grazers were more common there than in the other regions. Zonation of species was recognizable and species diversity increased seawards in all regions.Main conclusions Few, if any, assembly rules could be detected when comparing regions, except for the universal increase in species diversity seawards. The absence of ‘key’ species in one region may affect the distribution of other species. All major opportunities for resource exploitation are not utilized in all regions. In some instances it is possible to recognize ecological equivalents in different regions. No recurrent distinct zones are apparent when regions are compared. A universal descriptive system of zonation, such as developed by Stephenson & Stephenson (1949) and Lewis (1964), was found to be not applicable.

Extending the generality of ecological models to artificial floating habitats

Marine assemblages on natural hard substrata are generally different from those on artificial habitats. There is, however, the potential for certain ecological processes to operate on both types of structures. On the sides of floating pontoons in Sydney Harbour, there were strong patterns of vertical distribution of sessile epibiotic organisms and molluscan grazers across relatively small spatial scales (in three defined zones, namely splash, shallow and deep). Patterns of vertical distribution of the tubeworms Hydroides spp. were reversed depending on the cover of mussels. A manipulative experiment was done to test if patterns of vertical distribution of Hydroides spp. were due to (1) the functioning of mussels or (2) the structure provided by mussels. Neither the functioning nor structure of mussels accounted for the patterns of distribution of Hydroides spp. Mussels increased recruitment of Hydroides spp., in the shallow and deep zones, and this was not due to increased surface area of the mussel shells. Manipulation of numbers of grazers and covers of sessile epibiota showed that the observed negative relationship between grazers and epibiota was due to grazers reducing recruitment of epibiota and epibiota decreasing survival of grazers. Most importantly, processes that accounted for patterns of distribution of mobile and sessile organisms on artificial floating structures were similar to those repeatedly shown to create such patterns on natural rocky shores.

An assessment of variation in molluscan grazing pressure on Hong Kong rocky shores

Several methods of assessment have been used to document variation in grazing pressure on temperate rocky shores, although often these methods are applied without consideration of local conditions or species. In this study, a comparison was made between abundance counts of inactive molluscan grazers at low tide, direct observations of grazer activity and distribution throughout day and night tidal cycles, and records of grazing marks on wax discs, for the mid-shore of Hong Kong. The abundance of grazers found during low-tide counts varied among dates, sites and species. This method, however, did not record all grazer species that day/night observations showed to migrate from the low shore with the rising tide. Low-tide counts, therefore, underestimate grazing pressure (number of active grazers per unit area) and grazer guild (number of species). Grazing marks on wax discs also recorded a greater number of species than the low-tide counts of inactive grazers, and included grazers that were seen to migrate up shore during day/night observations. Certain limpet species, however, avoided the wax and did not leave grazing marks, showing this method to also underestimate grazing pressure. All methods showed grazing pressure to be variable at spatial scales of tens of metres or less and also temporally variable between sampling dates. The sole use of either low-tide counts or wax discs is likely to underestimate grazing pressure, due to variation in shore topography and grazer foraging behaviour, especially on shores with a narrow tidal range such as in Hong Kong. To gain a more accurate assessment of total grazing pressure, it is suggested that recording of grazing marks on wax discs should be used in conjunction with direct day/night observations.