Barnacle Density Research Articles

The role of intraspecific trait variation in driving post-metamorphic survival: Implications for recruitment in open populations.

Most ecological studies attempting to understand causes of population dynamics and community structure disregard intraspecific trait variation. We quantified the importance of natural intra-cohort variation in body size and density of juveniles for recruitment of a sessile marine organism, the barnacle Semibalanus balanoides. Barnacles are representative of species organised in metapopulations, that is, as open local populations connected by larval dispersal. We tracked the individual growth and survival of a cohort of juvenile barnacles from two shores of North Wales. Barnacles settled as larvae in spring of 2002 on previously cleared rock. The density of these new recruits was experimentally manipulated in June and randomly selected individuals were monitored from June to October to evaluate the role of barnacle size and density in predicting survival. In doing so we characterised density at three spatial scales (quadrat: 25 cm2, cells within quadrats: 25 mm2 and neighbourhood: number of neighbours in physical contact with the target barnacle). At all scales, variations in juvenile body size exacerbated the effect of density-dependent mortality on population size. While density-dependent mortality was very intense in the small-sized individuals, large-sized individuals experienced very weak density-dependent mortality and showed high survival rates. Using the concept of 'Jensen inequality', we show that important biases in estimations of survival, based on population size only, occur at high barnacle densities, where survival is low. Our study highlights the role of body size variation in understanding dynamics of open populations.

First-Year Performance of the Pervious Oyster Shell Habitat (POSH) along Two Energetic Shorelines in Northeast Florida

Novel living shoreline methods are being developed to minimize negative environmental impact while maintaining strength and effectiveness in high-energy systems. The “Pervious Oyster Shell Habitat” (POSH) is a novel structure composed of oyster shells bound by a thin layer of Portland cement into the shape of a dome. The structure’s makeup greatly reduces its environmental impact while providing optimal substrate for the provision of oyster reef habitat. Previous laboratory testing has demonstrated that the structure is robust, and this follow-up study assesses the structure’s performance in the estuarine environment. Oyster and barnacle densities were compared between POSH modules and the industry standard “Oyster Ball” model Reef BallTM along two energetic shorelines in northeast Florida. Oyster densities on the POSH were high and significantly greater than on the Oyster Ball at both sites. Barnacle densities did not differ between structures and did not appear to affect oyster recruitment. The size distribution of oysters on POSH and Oyster Ball modules was measured to assess the demographics and growth of oysters over time. Overall, demographics were similar among the two structures. Differences in oyster densities and demographics were greater at our more energetic site. Results show that the POSH can be an optimal structure for early oyster recruitment and reef development in energetic systems and should be considered by restoration stakeholders.

Subtle ecosystem effects of microplastic exposure in marine mesocosms including fish

For two months, communities in 5.8 m3 outdoor marine mesocosms were exposed to 700 μm sphere-shaped polystyrene (PS) beads in dosages between 0.08 and 80 g/m2. Barnacle (Semibalanus balanoides) densities were reduced at dosages of 0.8 g/m2 onwards without following a standard dose response curve. Lugworms and fish (Solea solea) ingested PS-beads without accumulating them. Lugworms (Arenicola marina) ingested the beads nonselective with the sediment without negative effects. The fish seemed to ingest the plastics only occasionally and at the final sampling day even in the highest dosed mesocosms (>30 beads/cm2) only 20% contained plastic. The condition index of the fish was slightly reduced in mesocosms with dosages of 0.8 g/m2 onwards. No difference in condition was found between fish with and without ingested plastic across mesocosms, illustrating the difficulty to relate plastic ingestion with condition from field data. The fish also ingested mollusks with shells exceeding the size of the PS-beads. Bivalves rejected the PS-beads as pseudofeces, without obvious impact on their condition. Mussel's (Mytilus edulis) pseudofeces present an effective matrix to monitor microplastic presence in the water column. Species richness and diversity of the pelagic and benthic community were not affected although, a trend was found that the lower microplastic dosages had a positive effect on the total abundance of benthic invertebrates. In general, the observed effects at even the highest exposure concentrations were that subtle that they will be obscured by natural variation in the field. This underlines the importance of experiments under semi-field conditions for meaningful assessment of the ecological impact of microplastics. This study was performed with the real life, non-toxic, sphere-shaped polystyrene beads as were lost during an actual spill near the Dutch Wadden sea in January 2019. We recommend future mesocosm studies with other types of microplastics, including microfibers, weathered microplastics from sea, and smaller sized particles down to nanoplastics.

Soft-bottom tidepools within mixed reefs of native mussels and introduced oysters – refuge for associated species and parasites?

AbstractThe introduction of Pacific oysters to the sedimentary south-eastern North Sea coast and their establishment on intertidal native blue mussel beds has caused the development of mixed reefs of mussels and oysters with extensive tidepools. Tidepools have been intensively studied at rocky shores where they show community structures, which usually differ from that of the surrounding emerging substrates. Tidepools at sedimentary coasts, however, have received less attention. We compared the community structure and species interactions inside and outside tidepools in oyster reefs by determining densities of snails, barnacles and amphipods. Snail densities were similar in and outside tidepools. Barnacle coverage on bivalve shells, however, was lower inside tidepools, which may be caused by higher predation pressure and increased snail grazing under permanently submerged conditions, as was revealed by field and laboratory experiments. Additionally, we studied the occurrence of copepod and trematode parasites in blue mussels inside and outside tidepools. Prevalence and intensity of parasitic copepods was higher in mussels inside tidepools. Trematode parasites, by contrast, showed a lower intensity in mussels inside tidepools. This can be explained by high amphipod densities found inside tidepools because trematode larvae represent a food source of amphipods. Our study suggests that the community structure of oyster reefs within tidepools is not a submerged equivalent to that of intertidal reefs. As their counterparts at rocky shores, they show their own species distribution patterns with particular species interactions and only provide refuge for specific species such as parasitic copepods.

Salinity gradients and interspecific competition determine the distribution of chthamalid barnacles in a subtropical estuary

Distribution patterns in the upper intertidal zone, where diversity is low, are often explained by the variation in physical factors. However, certain environmental conditions may hinder resource monopolization by a dominant species, allowing for the coexistence of species. Under these conditions, interspecific interactions may become relevant. In southeastern Brazil, the upper intertidal zone of exposed rocky shores is typically colonized by the barnacle Chthamalus bisinuatus. In estuaries, two other species, Chthamalus proteus and Microeuraphia rhizophorae, may occur on hard substrates, although patterns of coexistence at relevant spatial scales are presently unknown, which prevents a better understanding of the ecology of rocky shore systems in the region. This study aimed to (i) describe the distribution patterns of chthamalid barnacles along the Paranaguá Estuarine Complex (PEC) and (ii) evaluate the vertical distribution of species in places where substantial coexistence was observed to preliminarily test whether their distribution is compatible with a lottery model of resource acquisition or compatible with a process of interspecific interference competition. A well-defined barnacle community was observed along the upper intertidal zone, from the euhaline to the oligohaline sector, with the cover area being larger in the euhaline sector. The effects of hydrodynamic exposure were limited to the euhaline sector, where barnacles covered larger extensions of the rocky intertidal habitat at more exposed sites. Barnacle density did not vary consistently among sectors or levels of hydrodynamic exposure but changed according to certain combinations of these factors. As a result, the realized habitat tended to decrease from the mouth of the estuary to the oligohaline sector. Three species were recorded in this study: Chthamalus bisinuatus, practically limited to the euhaline sector; Microeuraphia rhizophorae, limited to the polyhaline and mesohaline sectors; and Chthamalus proteus, limited to the polyhaline, mesohaline and oligohaline sectors. Habitat sharing was substantial only in 3 of the 16 sampled sites and only between Microeuraphia rhizophorae and Chthamalus proteus. At low barnacle densities, up to 40% substrate cover, there was no apparent vertical segregation between these two species. Above 40–50% cover, Microeuraphia rhizophorae occupied a higher position on the shore than Chthamalus proteus. These results suggest a lottery model when vacant space is still abundant and interspecific interference competition when barnacle abundance surpasses a critical abundance threshold. In this latter case, Chthamalus proteus displaces Microeuraphia rhizophorae to upper zone levels, where conditions for growth and reproduction are probably less favorable.

Influence of environmental variables over multiple spatial scales on the population structure of a key marine invertebrate

Quantifying scale-dependent patterns and linking ecological to environmental variation is required to understand mechanisms regulating biodiversity. We conducted a large-scale survey in rocky shores along the SE Brazilian coast to examine spatial variability in body size and density of an intertidal barnacle (Chthamalus bisinuatus) and its relationships with benthic and oceanographic predictors. Both the size and density of barnacles showed most variation at the smallest spatial scales. On average, barnacle body size was larger on shores located in areas characterised by higher chlorophyll levels, colder waters, low wave action and low influence of freshwater. Barnacles were more abundant at wave-exposed shores. We identified critical scales of spatial variation of an important species and linked population patterns to essential environmental predictors. Our results show that populations of this barnacle are coupled to scale-dependent oceanographic variation. This study offers insights into the mechanisms regulating coastal populations along a little studied coastline.

Barnacle colonization on Spartina alterniflora in Georgia salt marshes

The distribution of barnacles within a habitat is influenced by numerous biotic and abiotic factors that impact barnacle recruitment and/or settlement. In Georgia salt marshes, barnacles (specifically Chthamalus fragilis) have been observed colonizing leaves and stems of S. alterniflora, which is the dominant plant species in marshes along the Gulf and East Coast. The aims of this study were to: 1) identify spatial and short-term monthly patterns of barnacle colonization within individual S. alterniflora as well as across sites of varying marine influence during peak barnacle recruitment and plant growth and 2) determine the effect of barnacle settlement on S. alterniflora growth and productivity. Patterns of barnacle settlement and measurements of plant growth and productivity were assessed through a field survey and field experiment in three salt marshes of varying proximity to oceanic inputs. Two marsh sites were dominated by tall-form S. alterniflora and one was composed of short-form plants. Results suggested spatial and short-term monthly patterns in barnacle settlement both across sites and within individual plants. Specifically, there was significantly greater barnacle density on stems and leaves of plants in salt marshes with greater marine influence. Furthermore, there was a decrease in barnacle settlement on S. alterniflora leaves and increase in settlement on stems from June to August, suggesting short-term monthly patterns in the portion of the plant colonized. Within individual plants, there was a significantly greater density of large barnacles on the middle segments of the stem and greater barnacle colonization in the non-axils of the stems compared to leaf axils. A potential negative impact of barnacle settlement on S. alterniflora was observed at the site with the greatest barnacle density and marine influence, which was also the site dominated by short-form S. alterniflora. These plants showed significantly lower photosynthetic efficiency in August, but it is unclear whether this was related to barnacle settlement, edaphic conditions, or both. Information on the patterns of barnacle settlement and its impact on S. alterniflora will be vital to coastal managers in salt marsh conservation strategies.

Production and vertical distribution of invertebrates on riprap shorelines in Chesapeake Bay: A novel rocky intertidal habitat

Along the North American Mid-Atlantic coast, soft-sediment shorelines are armored with hard substrata, such as riprap (rock revetments), for erosion protection. These structures are valuable for determining anthropogenic effects on marine ecosystems, and riprap potentially represents a habitat similar to an “emerging ecosystem” with productive species assemblages. The goals of this project were to (i) quantify species diversity, secondary production, and vertical distribution associated with armored (i.e., riprap) habitats in various locations in Chesapeake Bay and (ii) compare benthic epifaunal production on riprap with infaunal production on natural soft-sediment habitats in a system where natural rocky intertidal habitats are absent. Density, diversity, and secondary production of benthic epifauna on riprap and benthic infauna were compared in adjacent habitats in lower (Lynnhaven River) and upper (Patuxent River) Chesapeake Bay. To compare riprap habitat between tributaries that differed in physical characteristics and with natural rocky intertidal habitats from other systems, oyster, mussel, and barnacle vertical distribution on riprap in southern (Lynnhaven River) and northern (Piankatank River) locations was quantified. The hypothesis was that riprap assemblages and secondary production would differ between rivers and among regions due to differences in physical variables such as temperature (thermal stress in northern locations), or osmotic stress (greater upriver), and patterns would be comparable to natural rocky habitats. We predicted that productivity would be relatively high and relate positively with diversity on riprap, suggesting that it is a habitat within an “emerging ecosystem” where new combinations of species and habitat appear. Benthic epifaunal production on riprap was sevenfold greater than natural infaunal production, and both tended to be highest in the Lynnhaven. Vertical distribution on riprap was similar to that on natural rocky intertidal habitats: oyster and mussel densities tended to be highest in the mid- and low-intertidal zones, whereas barnacle density was greatest in the high intertidal at the southern location and the low intertidal in the northern location. Secondary productivity on riprap was high relative to the natural, infaunal, soft-bottom habitat in this area. Species richness and epifaunal non-oyster production were positively related to oyster production. We demonstrate that intertidal riprap-armored shorelines are a productive novel habitat in an emerging ecosystem, and they enhance species diversity, abundance, and production. Thus, riprap shorelines mimic the ecological characteristics of natural hard-bottom habitats, and they may contribute substantially to ecosystem function throughout the Atlantic and Gulf of Mexico coasts due to riprap's widespread use and distribution.

Density of intertidal barnacles along their full elevational range of distribution conforms to the abundant-centre hypothesis.

The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at its distribution centre and decrease similarly towards distribution margins. The ACH has been deduced from a theory that postulates that environmental conditions should be most favourable for a species at the centre of its distribution. This idealised density pattern, however, has been supported by limited field studies, as natural patterns are often more complex. It is thus of interest to examine under what conditions compliance with the ACH could be favoured. Such conditions could be smooth environmental gradients with limited habitat patchiness throughout the distribution range of a species. Thus, we tested the ACH by measuring the density of an intertidal barnacle (Semibalanus balanoides) across its full vertical distribution range (from low to high elevations) on a rocky shore with similar substrate properties across elevations. To do a reliable test, we surveyed eight elevation zones applying an equal sampling effort in each zone. Average barnacle density conformed to the ACH, as it peaked at the middle of the vertical distribution range of this species. The same underlying theory predicts a similar unimodal pattern for maximum body size, but this trait was decoupled from density, as maximum barnacle size increased from low to high elevations. Overall, although the ACH is not a universal predictive tool as once envisioned, it may predict some cases well, as shown by this study. Therefore, the ACH should not be discarded completely, and its domain of application should be further evaluated.

Distribution patterns of epibiotic barnacles on the Hawksbill turtle, Eretmochelys imbricata, nesting in Iran

The hawksbill turtle (Eretmochelys imbricata,) is a critically endangered species of marine turtle that occurs in the Persian Gulf. The present study analyzed the frequency and distribution of epibiont barnacles residing on 122 the carapace and plastron of nesting hawksbill turtles (mean CCL = 70.22 ± 4.3 cm; mean CCW = 64.77 ± 3.2 cm) from the northern Persian Gulf in 2017. Barnacles frequency on the carapace, plastron, flippers, and head were studied in 29 turtles of the 122 turtles. 99.2 % of the nesting hawksbill turtles hosted barnacles and 72% carried between 1 and 20 barnacles (n= 122 turtles). The plastron, carapace, flippers, and head areas of the 29 turtles hosted 2804 836, 585 and 454 barnacles, respectively. The number of barnacles smaller than 0.5 cm on carapace represented 38.8% of the total of 836 barnacles. Based on the combined totals on the plastron, flippers, and head, barnacles smaller than 0.5 cm represented 69.2% of the barnacles counted on the turtles. There was a significant difference among the barnacle frequency on different scales of carapace, plastron, and four flippers (P<0.05). The presence of numerous barnacles may influence the hydrodynamic surface and water flow around the turtle’s body. Moreover, the high density of barnacles may influence the health of the turtles. The number of barnacles residing on a hawksbill turtle may result from the foraging behavior and habitat of the turtles.

The influence of the rock mineralogy on population density of Chthamalus (Crustacea: Cirripedia) in the Ligurian Sea (NW Mediterranean Sea)

Settlement, recruitment and survival of sessile marine species are driven by many biotic and abiotic factors. Among them, substrate mineral composition is generally a neglected topic, despite it proved to be a relevant contributing variable in driving the structure of benthic communities. Thanks to their ecology, Chthamalus species are a good proxy to test the role of substrate in affecting settlement and final population density on exposed rocky shores. Differences in the number of individuals were analysed in eight localities along the Eastern Ligurian Riviera (north-western Mediterranean Sea), from Portovenere to Manara Cape. In this sector of the Ligurian littoral, the coast is constituted by different rocks characterised by variable concentrations of calcite, silicates and quartz. This situation constitutes an ideal setting to evaluate the influence of mineral composition of the rocks in structuring the epilithic macrobiota communities in energetic splash zones and under similar physical pressures. Rocks rich in calcites turned out to be more suitable for Chthamalus species than substrates containing high amounts of silicates, and of quartz in particular. Additionally, also the grain size of the main mineral component of the rocks, determining in turn the surface roughness seemed to influence barnacle densities, with a significant preference for finer-grained substrates.

Numerical simulations of biofouling effects on the tidal turbine hydrodynamic

Biofouling by benthic organisms must be considered for tidal turbine operation and maintenance because it modifies hydrodynamics (drag and resistance) and could be detrimental to the turbine performance. We investigate vortices modification downstream a tidal turbine due to biofouling using numerical modeling. Firstly, 2D flow downstream a clean Darrieus vertical axis tidal turbine is simulated using a dynamic mesh for different tip speed ratio. Results agree the former studies. Simulations are very sensitive to turbulence modeling. To ensure an acceptable computing time, only LES and RANS are used. Secondly, an airfoil with barnacles is modeled in two dimensions for various fouling height and spacing with different flow incidences. Barnacle height has more influence on flow than the barnacle density. Then barnacles and mussels with various characteristics are fixed on blades. Vorticity fields are strongly changed by organism shapes. Mussels size has little impact on vorticity patterns. A few mussels could have stronger impact than a fully colonization. A 3D simulation is performed with a shape of barnacles from new in-situ measurements. Finally a colonized tidal turbine is simulated is modelled.

Depth and coral cover drive the distribution of a coral macroborer across two reef systems.

Bioerosion, the removal of calcium carbonate from coral frameworks by living organisms, influences a variety of reef features, from their topographic complexity to the net balance of carbonate budgets. Little is known, however, about how macroborers, which bore into reef substrates leaving traces greater than 0.1 mm diameter, are distributed across coral reefs, particularly reef systems with high (>50%) stony coral cover or at mesophotic depths (≥30 m). Here, we present an accurate and efficient method for quantifying macroborer densities from stony coral hosts via image analysis, using the bioeroding barnacle, Lithotrya dorsalis, and its host coral, Orbicella franksi, as a case study. We found that in 2014, L. dorsalis densities varied consistently with depth and host percent cover in two Atlantic reef systems: the Flower Garden Banks (FGB, northwest Gulf of Mexico) and the U.S. Virgin Islands (USVI). Although average barnacle density was nearly 4.5 times greater overall in the FGB than in the USVI, barnacle density decreased with depth in both reef regions. Barnacle density also scaled negatively with increasing coral cover in the study areas, suggesting that barnacle populations are not strictly space-limited in their distribution and settlement opportunities. Our findings suggest that depth and host coral cover, and potentially, local factors may strongly influence the abundance of macroborers, and thus the rate of CaCO3 loss, in a given reef system. Our image analysis method for quantifying macroborers can be standardized across historical and modern reef records to better understand how borers impact host growth and reef health.

Cascading effects of a top predator on intraspecific competition at intermediate and basal trophic levels

Abstract Predators can impact competition among prey by altering prey density via consumption or by causing prey to modify their traits or foraging behavior. Yet, differences between these two mechanisms may lead to different cascading impacts on lower trophic levels. Using a crab‐snail‐barnacle rocky intertidal food chain, we tested the effects of predation risk from crabs (top predators) on intraspecific competition among snails (intermediate consumers) and emergent indirect effects on the density of and competition among barnacles (basal resources). The per capita foraging and growth rates of snails declined with high conspecific density. Predation risk from crabs, which caused even larger reductions in snail foraging and growth, weakened competition among snails, whereas a 45% increase in barnacle density had no detectable effect on snail competition. Intraspecific competition between barnacles, however, depended on the interactive effects of barnacle density, snail density, and crab predation risk. Barnacles developed hummocking morphologies as they grew and competed for space. Hummock formation (a proxy for competition) increased as a result of either greater initial barnacle density or reduced snail foraging pressure, but these effects depended on predation risk. The effects of crab predation risk on snail foraging behavior weakened an otherwise strong relationship between barnacle density and hummock development: hummocking increased with barnacle density in the absence of crabs but remained relatively high when crabs were present. In communities with similar final barnacle densities, hummocking was more common in those with crabs than those without crabs. The extent to which predators can drive trophic cascades by suppressing the foraging rates of their prey is highly context‐dependent: the positive trait‐mediated indirect effect of predators on basal resource abundance is stronger when many prey respond simultaneously to the threat of predation. However, our results demonstrate that top predators can also enhance competition among basal resources even when their indirect effect on resource abundance is relatively weak. Hence, the cascading effects of predators on competition within lower trophic levels may play an important but under‐appreciated role in the dynamics of basal resource populations and the communities they support. A plain language summary is available for this article.

Unimodal relationship between small-scale barnacle recruitment and the density of pre-existing barnacle adults.

Recruitment is a key demographic process for population persistence. This paper focuses on barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), ice scour is common during the winter. At the onset of intertidal barnacle recruitment in early May (after sea ice has fully melted), mostly only adult barnacles and bare substrate are visible at high elevations in wave-exposed habitats. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm−2), the relationship between adult density and recruit density (scaled to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm−2), the relationship between adult and recruit density was positive and resembled the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40% of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the previously documented fact that settlement-seeking larvae are chemically and visually attracted to adults, which might be important for local population persistence. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may limit recruit growth and survival, the negative nature of the relationship suggests that adult barnacles at increasingly high densities stimulate larvae to settle elsewhere. The unimodal pattern may be especially common on shores with moderate rates of larval supply to the shore, because high rates of larval supply may swamp the coast with settlers, decoupling recruit density from local adult abundance.

KEPADATAN DAN SEBARAN TERITIP (Amphibalanus spp.) DI PELABUHAN KOTA DUMAI

Barnacle (Amphibalanus spp.) density and distribution implemented in September-November 2013 in the port of Dumai, Riau, which aims to explore the causes of inequality in the spatial distribution of density Barnacles (Amphibalanus spp). in different media attachment were influenced by marine environmental conditions in the port of Dumai. Barnacle density observation method in this study is the stratified random sampling method using wood, fiber and iron as a media attachment. The results showed the existence of inequality Barnacle density (Amphibalanus spp). on the media attachment with different types and colors. The highest average Barnacle density was found in colorless timber, which ranges between 265-506 ind/m2. While the lowest average Barnacle density was found in white fiber and it ranged between 25-68 ind/m2.

Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabsCarcinus maenascaused juvenile and sub‐adult snailsNucella lapillusto increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnaclesSemibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs.Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.

Combining Traits and Density to Model Recruitment of Sessile Organisms

We propose an integrative approach that explains patterns of recruitment to adult populations in sessile organisms by considering the numbers of individuals and their body size. A recruitment model, based on a small number of parameters, was developed for sessile organisms and tested using the barnacle Semibalanus balanoides, a marine invertebrate inhabiting North Atlantic intertidal shores. Incorporating barnacle body size improved model fit beyond that based on density alone, showing that growth played an important role in how resource limitation affected survival. Our approach uncovered the following: First, changes in the shape of the recruitment curve resulted from the balance between individual growth and mortality. Second, recruitment was limited by the least plastic trait used to characterise body size, operculum area. Basal area, a trait that responded to increases in barnacle density, did not contribute significantly to explain patterns of recruitment. Third, some temporal variation is explained by changes in the amount of space occupied by shells of dead barnacles: at high cover barnacles are densely packed and these shells remain long after death. Fourth, seasonal variation and spatial variation in survival can be separated from that resulting from resource limitation; survival was predicted for two different shores and four sampling times using a single recruitment model. We conclude that applying this integrative approach to recruitment will lead to a considerable advance in understanding patterns of mortality of early stages of sessile organisms.

Multidecadal signals within co-occurring intertidal barnacles Semibalanus balanoides and Chthamalus spp. linked to the Atlantic Multidecadal Oscillation

Few links have been established between the Atlantic Multidecadal Oscillation and long-term dynamics of marine systems due to the scarcity of sustained biological time-series with sufficient multi-decadal coverage. The abundances of co-occurring boreal and Lusitanian species of barnacle have been recorded annually at a rocky shore in Devon, southwest England since 1953. Multidecadal cycles in relative abundances of the cold-water Semibalanus balanoides and warm-water Chthamalus spp. are strongly correlated with both local sea surface temperatures, and a ‘Warm Index’ of barnacle abundance shows strong links to the basin-scale Atlantic Multidecadal Oscillation. In contrast there are weak or no observed relationships with the North Atlantic Oscillation for either species. The shorter lifecycle of S. balanoides compared to the chthamalids and the increase in spring and summer temperatures to which newly settled S. balanoides recruits have been exposed during the last decade are likely mechanisms by which barnacle densities are responding to low-frequency temperature variability expressed in the Atlantic Multidecadal Oscillation.

Physiological tolerance predicts species composition at different scales in a barnacle guild

This study examined how the species composition of an intertidal barnacle guild varied according to physical gradients in the environment at small scales governed by microclimates, medium scales of wave exposure and large scales of latitude. Barnacle distributions at small and medium scales were sampled in Ireland between 51°29′ and 52°44′N and 6°50′ and 10°08′W. Sampling on European shores spanned ~18° latitude from 37°05′ to 55°16′N. Barnacle surveys mainly took place in 2003–2004. An index of wave fetch was calculated along the wave exposure gradient using a digital coastline-based model that was supported by a biological exposure scale. A ‘dryness’ index was defined according to mean monthly wind speed, fetch along the average wind direction and mean monthly air or sea surface temperatures for 2 years (January 2001–December 2002) which is the period when the most recent adults in the barnacle community would have settled and grown to adulthood. The proportion of the dry-loving barnacle Chthamalus montagui Southward increased within the barnacle guild at all scales as the habitat became warmer and drier. Barnacle densities were high in all habitats, mean densities ranged from a minimum of 4.16 cm−2 on moderately exposed shores to a maximum of 6.27 cm−2 in sunlit or south-facing microclimates. Percentage cover of barnacles across the gradient of latitudes was usually >70%. The results suggest that the distribution and abundance of interacting barnacle species on European coasts is strongly controlled by abiotic factors, most likely temperature and desiccation.