Abundance Of Barnacles Research Articles

Predation: a regulating force of intertidal assemblages on the central Portuguese coast?

Predation has long been recognized as an important biological force driving community patterns in intertidal rocky shores throughout the world. Little is known, however, about the role of predation by mobile marine predators in shaping intertidal prey populations in Portuguese rocky shores. The abundance and population structure of crabs were assessed during nocturnal low-tides on two rocky shores to characterize potential predator species. To assess the effect of predation on intertidal species including limpets, barnacles and mussels, predator exclusion experiments using full cage, partial cage and no cage treatments, were set up for two months on two shores on the central Portuguese coast. Pachygrapsus marmoratus (Fabricius) and Eriphia verrucosa (Forsskål) were the most abundant crabs. Results from predator exclusion experiments suggested that predators do not exert a significant control on abundance of limpets, mussels or barnacles on the midshore during the experimental period. Despite the fact that these crabs are known to feed on the analysed prey, several factors may account for the observed absence of impact on prey abundance and these are discussed.

Assessing the consequences of sea level rise: effects of changes in the slope of the substratum on sessile assemblages of rocky seashores

Sea level rise can affect the biodiversity of coastal areas through alterations of features of the substratum, including the geometry of shorelines. Field measurements on the rocky shore of Calafuria (NW Mediterranean Sea) showed that a rise in sea level in the range of 5 to 50 cm would increase the availability of steep substrata (>40°) for assemblages of algae and invertebrates by up to 58% compared to current conditions. Comparisons between assemblages on horizontal and vertical substrata revealed clear differences in the composition and abundance of common taxa. Clearing experiments replicated on vertical and gently sloping surfaces at 2 heights on the shore were per- formed to test hypotheses about the role of recruitment and post-recruitment processes in maintain- ing differences between substrata. Comparisons of recolonizing assemblages with unmanipulated controls allowed us to test the hypothesis that a change in slope of the substratum affected the abil- ity of assemblages to recover after a disturbance. Results indicated that differences in the structure of assemblages between vertical and horizontal substrata were maintained by a combination of recruit- ment and post-recruitment processes, although the relative importance of these processes differed among taxa. Nonetheless, patterns of recovery of assemblages were comparable between the 2 sub- strata. These findings suggested that sea level rise, by increasing the proportion of vertical substrata at the expense of horizontal surfaces, will lead to the expansion of assemblages dominated by encrusting coralline algae and grazing gastropods, and the reduction of abundance of filamentous forms and barnacles. The resilience of assemblages, however, appeared unaffected by the aspect of the substratum.

Effect of predation by the invasive crab Hemigrapsus sanguineus on recruiting barnacles Semibalanus balanoides in western Long Island Sound, USA

Since its introduction fewer than 20 yr ago, the spread of the Asian shore crab Hemigrapsus sanguineus along the east coast of the US has been remarkable. By many accounts it is now the numerically dominant brachyuran in rocky intertidal sites in Long Island Sound. Evidence from several laboratory studies has suggested that H. sanguineus could have significant consequences for barnacle abundance and distribution. In this study we used field caging experiments to determine the effect of this invader on recruitment (sampled at weekly intervals) of the indigenous barnacle Semibalanus balanoides. We found that although H. sanguineus readily consumed settling cypris larvae and early post-settlement juveniles, crab predation did not have a negative impact on barnacle recruitment when predator density was low (15 crabs 0.25 m–2). We also found a measurable decrease in barnacle density at medium (45 crabs 0.25 m–2) and high (90 crabs 0.25 m–2) predator densities after the larval settlement period had ended, but this effect was short-lived. Based on available estimates of Asian shore crab abundance in Long Island Sound, we conclude that crab predation will not have a significant impact on the recruitment success of S. balanoides populations.

Cross-scale variation in top-down and bottom-up control of algal abundance

Previous studies of top-down and bottom-up effects in Oregon rocky intertidal communities suggested that, unlike sea star–mussel dominated food chains, grazer-alga food chains deviated from simple food web predictions. However, this conclusion lacked generality due to limited replication at the site level. We explored how grazing and algal colonization and growth in different zones and multiple sites in intertidal communities varied in relation to processes varying on local to mesoscales, including oceanographic variation. Our approach employed the comparative-experimental method, in which identically-designed and replicated experiments are performed at multiple sites spanning the environmental gradient of interest. In a limpet–algae–barnacle interaction web, we tested the hypothesis that top-down and bottom-up effects were inversely related along the Oregon coast. To distinguish between the alternatives of large-scale oceanographic forcing vs. local control, our experiments spanned scales of meters (between mid and low zones), kilometers (seven sites), 10's to 100's of km (three regions), and ∼ 260 km (central and southern coasts). Identically designed and executed experiments on effects and rates of grazing were conducted in a southern coast region of persistent upwelling and two central coast regions, one with intermittent upwelling in an area of weak offshore currents, and another with intermittent upwelling in an area of strong offshore currents. Grazer effects and grazing rates varied on all scales, indicating a complex interplay of processes operating at zone, site, regional, and coastal scales. Grazing was generally highest in all mid zones and in the low zone at the middle region (Cape Perpetua) on the central coast. Growth of early successional colonists tended to be higher at the northern and southern regions (Capes Foulweather and Blanco). Grazers had no effect on barnacle cover, but their foraging was inhibited by high densities of barnacles. Differences in grazing appear to be a complex consequence of direct and indirect effects of variable oceanographic conditions, limpet recruitment, and barnacle abundance. In contrast to positive associations between predation strength and the magnitude of prey subsidies, higher grazing strength was generally associated with lower magnitude of bottom-up effects and vice versa. Hence in this system predator–prey (sea star–filter feeder) and herbivore–plant (limpet–microalgae) interactions evidently respond differently to ecological subsidies.

Chemical inhibition of barnacle larval settlement by the brown alga Fucus vesiculosus

The possible importance of larval settlement inhibition by Fucus vesiculosus in explain- ing the low abundance of the barnacle Balanus improvisus on algal fronds compared to adjacent rock was investigated by a combination of field and laboratory studies. A field survey showed that the abundance of adult barnacles was significantly lower on F. vesiculosus than on rocks and settlement preference experiments strongly suggested that settlement avoidance for algal fronds during the natural settlement season was chemically mediated. By using a multi-step approach, whereby both exuded and surface associated metabolites were tested on larval settlement, we further investigated the possible existence of chemical antifouling processes in F. vesiculosus. For exuded metabolites, seawater was conditioned for 1 and 3 h with both submerged and desiccated algae. Surface extracts at natural concentrations and seawater conditioned with submerged algae had no significant effect on larval settlement, although there was a tendency for reduced settlement in the 3 h samples. Sea- water conditioned with desiccated algae consistently inhibited settlement and chemical analyses showed that the mean phlorotannin level in the water was 31.5 µg ml -1 , which is ca. 30× higher than concentrations previously shown to inhibit barnacle settlement. Seawater conditioned with sub- merged algae had a mean phlorotannin content of 0.85 µg ml -1 and in samples conditioned for 3 h there was a strong and significant negative relationship between phlorotannin concentration and larval settlement, suggesting that the phlorotannin concentration varied around a level inhibitory to barnacle settlement. Altogether, the results from the laboratory experiments strongly suggested that the low larval preference for algae was caused by exudation of waterborne metabolites, primarily phlorotannins. Finally, field measurements showed that phlorotannins exuded from F. vesiculosus can under natural conditions reach concentrations inhibitory to settlement of barnacle larvae.

A Hawaiian limpet facilitates recruitment of a competitively dominant invasive barnacle

At an intertidal site on the island of Oahu, Hawaii, the abundance of the invasive barnacle Chthamalus proteus was negatively correlated with abundance of the native limpet Siphonaria normalis. As the substratum at this site was a conglomerate of materials, we hypothesized that the observed pattern was the result of differences in substratum preferences, and/or negative interactions between the 2 species, or both. We used manipulative field experiments to test these hypotheses, and then generalized our findings with surveys across several sites that differed in sub- stratum type and abundances of barnacles and limpets. The limpet recruited in highest numbers to light-colored substrata. The barnacle settled in highest numbers on rough substrata, with a trend toward higher settlement on dark-colored substrata. Surprisingly, in experimental plots, recruitment of the barnacle was positively correlated with the density of the limpet. In contrast, limpet densities increased in experimental plots from which barnacles had been removed. Thus the pattern at this site appears to result from a combination of substratum preferences and interactions between the 2 spe- cies. These experiments and surveys of other intertidal sites on Oahu suggest that (1) limpets and bar- nacles have little effect on each other at low densities, (2) at higher densities limpets facilitate settle- ment of barnacles, but barnacles have a negative effect on limpets, and (3) substratum type can mediate the density-dependent interactions between these 2 species by affecting their settlement patterns.

TEMPORAL VARIANCE REVERSES THE IMPACT OF HIGH MEAN INTENSITY OF STRESS IN CLIMATE CHANGE EXPERIMENTS

Extreme climate events produce simultaneous changes to the mean and to the variance of climatic variables over ecological time scales. While several studies have investigated how ecological systems respond to changes in mean values of climate variables, the combined effects of mean and variance are poorly understood. We examined the response of low-shore assemblages of algae and invertebrates of rocky seashores in the northwest Mediterranean to factorial manipulations of mean intensity and temporal variance of aerial exposure, a type of disturbance whose intensity and temporal patterning of occurrence are predicted to change with changing climate conditions. Effects of variance were often in the opposite direction of those elicited by changes in the mean. Increasing aerial exposure at regular intervals had negative effects both on diversity of assemblages and on percent cover of filamentous and coarsely branched algae, but greater temporal variance drastically reduced these effects. The opposite was observed for the abundance of barnacles and encrusting coralline algae, where high temporal variance of aerial exposure either reversed a positive effect of mean intensity (barnacles) or caused a negative effect that did not occur under low temporal variance (encrusting algae). These results provide the first experimental evidence that changes in mean intensity and temporal variance of climatic variables affect natural assemblages of species interactively, suggesting that high temporal variance may mitigate the ecological impacts of ongoing and predicted climate changes.

Using patterns of variability to test for multiple community states on rocky intertidal shores

Predictions based on theory of multiple stable states suggest that larger perturbations should lead to more unpredictable patterns of succession. This prediction was tested in the Gulf of Maine using data from 60 intertidal plots of varying size that were experimentally cleared of the rockweed Ascophyllum nodosum and from 14 benchmark sites from throughout the Gulf. Rockweed was removed from the experimental clearings ranging from 1 to 8 m in diameter in 1996 and data collected in 2004 were used to test effects of clearing size and location on divergence and variability in species composition. Benchmark data were collected in 2005, and the 14 sites were from a dataset on 53 sites throughout the Gulf of Maine. The selected sites were randomly chosen from all sites with > 80% canopy cover by A. nodosum and were expected to be similar to uncleared control plots from the experiment. Experimental removal of A. nodosum resulted in clearings at 12 sites within 4 bays. Abundances of gastropods, barnacles, mussels, and fucoid algae and the percentage cover of barnacles, mussels, fucoid algae, bare space, and other species were sampled. CAP and PERMDISP analyses revealed significant differences in multivariate dispersion and variability with both clearing size and location. Variability generally increased with clearing size and location effects were related to the north–south positioning of the sites. Benchmark sites were similar to the experimental control plots but as variable as the largest clearings. Results suggest that succession in larger clearings has been more unpredictable than in small clearings. The pattern of variability in the experimental clearings is consistent with the predictions of multiple stable states. However, the large amount of variation among the benchmark sites was due to mussels and was unexpected. This unexpected variability underscores the importance of sampling benchmark sites as part of experiments.

Divergent succession and implications for alternative states on rocky intertidal shores

It has been hypothesized that rockweed stands and mussel beds in sheltered bays in the Gulf of Maine, USA, are alternative community states. As a test of this hypothesis, experimental clearings of different sizes were established in stands of the rockweed Ascophyllum nodosum (L.) Le Jolis to determine if successional changes in large clearings developed species assemblages distinctly different from the surrounding A. nodosum stands. Clearings ranging from 1 to 8 m in diameter were created at 12 sites in 4 bays on Swan's Island, Maine, in 1996 to mimic the effects of ice scour, and abundances of gastropods, barnacles, mussels and fucoid algae were monitored until 2002. ANOVAs and MDS showed strong effects of clearing size and divergent successional changes in large clearings. Large clearings were quickly filled in and remained dominated by the alga Fucus vesiculosus L. and the barnacle Semibalanus balanoides (L). There was no evidence for site-specific effects, and Mantel tests showed clearing size was a better predictor of species composition than geographic distances among sites. Results suggest that large pulse disturbances using clearings of 8 m in diameter can initiate divergent successional pathways and have a protracted effect on species composition. Results are also consistent with the hypothesis that mussel beds and rockweed stands in sheltered bays may be alternative community states.

Epibiotic barnacles and crabs as indicators of Caretta caretta distribution and movements in the Mediterranean Sea

The occurrence of barnacles and crabs on Caretta caretta specimens found in the waters around Italy was analysed. In the seas surrounding Italy balanomorph barnacles occur mainly on turtles frequenting shallow waters and crabs display a strong west–east gradient. Patterns of turtle distribution and movements are proposed. The abundance of turtle-specific barnacles may be dependent on, and so an indicator of, turtle density.

Distribution of barnacle epizoites of the crab Portunus pelagicus in the Moreton Bay region, eastern Australia

The parasites of some decapod crustaceans are known to cause sterilisation of their hosts, and can thus have an important impact on the population dynamics of infested species. Blue swimmer crabs (Portunus pelagicus) collected in three areas around Moreton Bay, Australia were examined for the presence of epizoic barnacles in their branchial chambers and on their carapace. Of the 952 crabs inspected 92% were infested with Octolasmis spp. The mean number of barnacles (predominantly Octolasmis warwickii) per carapace and gill chamber (mainly O. angulata) were 2.35 and 71.1, respectively. Barnacle infestation of gills was found to differ significantly by area, season and sex with the deeper offshore areas exhibiting the highest number of barnacles. The distribution within the hosts showed barnacles were more likely to be distributed in areas closer to the inhalant aperture. Highest abundances were found on the proximal surface of the hypobranchial side of gills 3, 4 and 5. Host moult stage and parasitism by Sacculina granifera were also found to affect the abundance of epizoic barnacles in some areas.

TRAIT-MEDIATED EFFECTS IN ROCKY INTERTIDAL FOOD CHAINS: PREDATOR RISK CUES ALTER PREY FEEDING RATES

The influence of predation on rocky intertidal community structure has long emphasized the importance of indirect interactions. Most efforts in this area have focused on the density-mediated, or lethal effects, of predators on prey density. Recently, there has been growing interest in trait-mediated indirect interactions (TMIIs): the presence of a predator in the environment influences the interaction between two other species (prey and their resource) by altering a trait of the prey species. For example, waterborne cues released by predators can cause changes in prey species behavior, such as feeding rates, thereby altering the impact of the prey species on their resources. Thus, TMIIs represent the nonlethal effects of predators that contrast with the more traditional emphasis on lethal indirect effects. Marine ecologists are just beginning to explore the role of TMIIs in their systems. We examined whether risk cues released by a ubiquitous crab predator (Carcinus maenas) influence the abundance of two dominant species in the rocky intertidal zone (barnacles [Semibalanus balanoides] and fucoid algae [Ascophyllum nodosum]) by altering the behavior of two of its snail prey (Nucella lapillus and Littorina littorea). We found that the presence of green crab risk cues can have strong cascading indirect effects on the abundance of barnacles and fucoid algae. N. lapillus exposed to risk cues consumed up to 29% fewer barnacles compared to conspecifics feeding in the absence of risk cues, whereas L. littorea exposed to risk cues consumed 459% fewer fucoids compared to conspecifics feeding in the absence of risk cues. These cascading interactions appear to reflect suppression of snail feeding by predator risk cues. In both food chains, snails exhibited more refuge-seeking behavior and grew less in the presence of risk cues. Our experiments suggest that TMIIs may have an important and underappreciated influence on species interactions that shape community dynamics on rocky intertidal shores. Corresponding Editor: P. T. Raimondi

Do Locals Rule? Interactions between Native Intertidal Animals and a Caribbean Barnacle in Hawai'i

Chthamalus proteus, a high intertidal barnacle from the Gulf of Mexico and the Caribbean, is one of the most conspicuous recent introductions to Hawaiian waters. It arrived sometime between the early 1970s, when the last thorough survey of intertidal barnacles on O‘ahu was done (Matsuda 1973), and the late 1990s, when it was photographed by an amateur biologist who was putting together a book on Hawai‘i’s sea creatures (Southward et al. 1998). On O‘ahu, the barnacle attains high densities in protected bays and lagoons, particularly where there are few other intertidal organisms. Chthamalus proteus is somewhat less abundant in semiprotected waters where it co-occurs with a suite of native organisms. We examined interactions between C. proteus and two native Hawaiian species, the barnacle Nesochthamalus intertextus and the pulmonate limpet Siphonaria normalis. The native barnacle is highly abundant on seawalls in Waikiki and frequently overgrows or undercuts the invader. Nesochthamalus intertextus grows twice as fast as C. proteus, which may allow the native to compete more effectively for space. To examine whether the native barnacle is limiting the distribution of the invader, we established 20 permanent quadrats along a seawall. In half of the quadrats native barnacles within 5 mm of invasive barnacles were removed; the remaining quadrats served as controls. Growth and survivorship of invasive barnacles in the quadrats have been tracked for 1 yr. An additional 12 quadrats were cleared of all native intertidal organisms. Recruitment of C. proteus to these quadrats and to 12 control quadrats was recorded for 10 months. Invasive barnacles in the treatment quadrats grew faster than those in control plots. Neither recruitment nor mortality was significantly different in treatment versus control plots, however, suggesting that factors other than competition (such as larval supply) also play a role in the distribution of the invader. It is more usual to ask whether an invader is impacting a native. There were too few of the invasive barnacles at Waikiki to test this with any statistical power. At Kualoa Beach, on O‘ahu’s windward side, the situation is reversed: the invader is abundant and the native is relatively rare. An experiment similar to the one in Waikiki was established in March 2000 to test the effect of C. proteus on the growth, mortality, and recruitment of N. intertextus. After 1 yr, there appeared to be no effect of the invader on the native in any of these categories. Interactions in Kāne‘ohe Bay between C. proteus and the pulmonate limpet S. normalis were also examined. The pulmonates are highly abundant (mean 30/100 cm) at Coconut Island on a seawall that is composed of several rock types. A survey of the wall showed that the invasive barnacles were less abundant on light-colored rock and that there was an additional negative effect of pulmonate density on barnacle abundance. The pulmonates were more abundant on light-colored rock; their densities were not additionally affected by barnacle numbers. Four types of cement settlement panels (dark smooth, dark rough, light smooth, light rough) were placed in the intertidal to test for settlement preferences of C. proteus and S. normalis. Barnacles did not show a preference for color, but limpets recruited preferentially to light-colored panels. Further experiments examining the effect of the pulmonates on recruitment of C.

An investigation into Rock Surface Wetness as a Parameter Contributing to the Distribution of the Intertidal BarnaclesChthamalus stellatus and Chthamalus montagui

Areas which were under the influence of runoff seeping slowly from upshore rockpools were identified at two shores in Co. Cork, Ireland. Surveys of adult and 0+Chthamalus stellatus and C. montagui were carried out in these areas and compared with adjacent dry areas. A second survey on cleared quadrats examined barnacle abundance in wet and in dry areas at 1 and 5 months after the initiation of settlement. Chthamalus stellatus adults and 0+ individuals were significantly more abundant in wet areas than in dry areas, which in the context of previous laboratory findings suggest that desiccation or some other parameter associated with wetness may be an important factor in limiting the vertical distribution of this species. Chthamalus montagui adults and 0+ individuals were significantly more abundant in dry areas than in wet areas. The lower vertical limit of this species may therefore also be set by some wetness parameter. In 1 month and 5 month old stages, the differences in abundance between wet and dry conditions were apparent in both species but were only significant in both age classes of C. montagui at one of the study shores. The results are discussed in the context of vertical and geographical distribution patterns exhibited by adults of each species on European shores.

Development of sessile marine assemblages on fixed versus moving substrata

Epibiotic assemblages have been shown previously to differ between pontoons and rocky reefs. This may occur for a variety of reasons, one of which is that pontoons move, whereas reefs do not. Effects of movement of the substratum are particularly pertinent to studies involving settlement panels because these experimental units are often suspended from pontoons such that they can move up and down, or attached to ropes such that they can rotate. The development of epibiotic assemblages on panels that were fixed, moved up and down, or rotated, was studied to test hypotheses about the effects of movement on sessile organisms. The covers of barnacles, sponges and ascidians increased greatly with increasing movement and/or rotation of the substratum, whereas species of red and brown foliose algae and tubiculous polychaetes generally decreased with increasing movement and/or rotation of the substratum. In general, assemblages on panels that moved up and down were most similar to those on rotating panels. One of the most obvious differences was the 2 to 3-fold greater biomass on rotating panels compared to fixed panels, which was attributed to the greater abundance of barnacles. It was concluded that, although movement of a surface can influence the composition of assemblages, this factor on its own may explain only some of the differences reported between pontoons and reefs. Furthermore, the method of deployment of settlement panels was shown to have great effects on the types of assemblages that develop. Differences in water flow were proposed to explain some of the observed patterns, but this idea needs to be investigated further.

Sampling design begets conclusions: the statistical basis for detection of injury to and recovery of shoreline communities after the 'Exxon Valdez' oil spill

The joint effect of multiple initial decisions made about sampling design in evaluation of environmental impacts using observational field assessments influences the ability to detect and ac- curately estimate responses. The design can dictate in advance whether the study can identify even large impacts that truly exist. Following the 'Exxon Valdez' oil spill in 1989, 4 separate studies of ef- fects of the spill on the intertidal biota were conducted. Studies overlapped sufficiently in geographic area, shoreline habitat, and biological response variables to permit contrasts showing how the aggre- gate of multiple design decisions led to differences in conclusions. The SEP (Shoreline Ecology Pro- gram) supported by Exxon and the CHIA (Coastal Habitat Injury Assessment) funded by the Exxon Valdez Oil Spill Trustee Council shared a core approach of establishing a stratified random design of site selection. The Exxon-supported GOA (Gulf of Alaska) study and the NOAA (National Oceano- graphic and Atmospheric Administration) Hazmat (Hazardous Materials) study both chose to employ subjective choices of fixed sites. Despite many common goals, these 4 studies differed greatly in: (1) sampling effort (area covered per sample quadrat, sample replication within sites, numbers of study sites per category, numbers of samplings, and total areas sampled) and sampling design (philo- sophy of targeting sampling effort, complete randomization versus matched pair designs, sampling frame, treatment of habitat heterogeneity within sites, interspersion of sites, and control of shoreline treatment and oiling intensity); (2) analytical methodology (analysis of covariance versus paired designs, treatment of subsamples as replicates in F-ratios, logic of inferring recovery, and power cal- culations); and (3) choice of biological response variables (taxonomic level of analysis, aggregating versus splitting separate communities, and scope of communities and habitats examined). The CHIA and NOAA Hazmat studies of epibiotic responses in sheltered rocky shores of Prince William Sound made several decisions to enhance detection power and produced similar conclusions about large reductions in total biotic cover of intertidal space, Fucus cover, mussel abundance, abundance of the limpet Tectura persona and a balanoid barnacle, and increases in open space and abundance of an opportunistic barnacle, Chthamalus dalli. In contrast, the SEP study of this same habitat and geo- graphic region adopted design choices resulting in lower power of detection in 12 (vs CHIA and vs NOAA Hazmat) of 15 separate decisions (with one tie in each contrast). Accordingly, the SEP study was able to detect declines only in Fucus cover and occasionally in total limpet abundance but not in total epifaunal or mussel or balanoid barnacle abundance and, unlike the results of the other 2 stud- ies, most of the taxa analyzed showed apparent increases rather than decreases from oiling and shoreline treatment. The more powerful GOA and CHIA studies of impacts of oiling in the Gulf of Alaska, where oil grounded 1 to 8 wk later and in more weathered condition than in Prince William Sound, showed more consistent and larger reductions in intertidal biota in the sheltered rocky habitat than did the SEP study of Prince William Sound. Thus, the combined effects of many design decisions that reduced power to detect impacts in the SEP study led to failure to demonstrate large impacts of the spill documented by other studies of the same habitat in the same and the more remote region.

Age-dependent zonation of the periwinkle Littorina littorea (L.) in the Wadden Sea

On sedimentary tidal flats near the island of Sylt (German Bight, North Sea) abundance and size distribution of periwinkles, Littorina littorea L., were studied in low intertidal and in shallow and deep subtidal mussel beds (Mytilus edulis L.). In low intertidal mussel beds, surveys revealed that high densities (1,369±571 m–2) of juvenile snails (≤13 mm) were positively correlated with strong barnacle epigrowth (Semibalanus balanoides L. and Balanus crenatus Bruguiere) on mussels. A subsequent field experiment showed that recruitment of L. littorea was restricted to the intertidal zone. Abundances of periwinkles (213±114 m–2) and barnacles abruptly decreased in the adjacent shallow subtidal zone, which served as a habitat for older snails (>13 mm). L. littorea was completely absent from disjunct deep (5 m) subtidal mussel beds. Snail abundance varied seasonally with maxima of >4,000 m–2 in low intertidal mussel beds in October and minima in July, just before the onset of new recruitment. I suggest that the presence of cracks and crevices among the dense barnacle overgrowth in intertidal mussel beds favoured recruitment and survival of juvenile snails. Larger (older) specimens are assumed to actively migrate to the less favourable adjacent subtidal. Therefore, intertidal mussel beds are considered as nurseries for the population of L. littorea in the Wadden Sea.

RECRUITMENT VS. POSTRECRUITMENT PROCESSES AS DETERMINANTS OF BARNACLE POPULATION ABUNDANCE

Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations...

Recruitment vs. Postrecruitment Processes as Determinants of Barnacle Population Abundance

Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations of site, zone, and exposure, the effects of physical and biotic factors ranged from strong to weak. For Chthamalus, recruitment generally had a stronger impact on density of adults than did postrecruitment processes, while for Balanus, postrecruitment factors tended to have stronger effects than did recruitment. These differences were an apparent consequence of differential susceptibilities to postrecruitment processes. Chthamalus was more tolerant of both biotic and abiotic forces than was Balanus and usually had high survival during periods of high Balanus mortality. Heat and desiccation were identified as the primary postrecruitment mortality factors in the high zone, whereas biotic interactions (competition and predation) were the most likely postrecruitment mortality processes in the mid-zone. Mortality was generally density dependent at the site with the strongest effects of postrecruitment processes, and generally density independent at the site with the strongest effects of recruitment. To determine whether trends were more consistent with the recruit–adult hypothesis at larger scales, data for each species were pooled across exposures (zone scale) and across exposures and zones (site scale) and also were compared to literature data from sites around the world (global scale). Each analysis led to the conclusion that recruitment can be a strong determinant of density of adults, but that the magnitude of this relationship depends on context, varies with species, and can be strongly modified by postrecruitment processes. Even when density of recruits is low, postrecruitment factors can be important in determining density of adults. Thus recruitment is a necessary but sometimes insufficient determinant of adult population density. These results support earlier suggestions that predictive models must incorporate both recruitment and postrecruitment factors and will thus depend on understanding the coupling between benthic and pelagic processes.

Population ecology of the barnacle Chthamalus stellatus in the northwest Mediterranean

This study examined patterns in the distribution and demography of the barnacle Chthamalus stellatus (Poli) at different spatial scales in the northwest Mediterranean. Preliminary data indicated that the abundance and size of barnacles decreased from high-shore to low-shore habitats. The generality of these patterns was investigated at several locations (10s to 100s of km apart), at several sites within locations (100s to 1000s of m apart) and at different times. Patterns were consistent with the preliminary observations, despite considerable spatial and temporal variability at small and large spatial scales. The following models were proposed to explain the observed patterns: (1) recruitment was intrinsically greater high on the shore, (2) limitation of recruitment due to pre-emption of the substratum was greater low than high on the shore, (3) environmental conditions reduce growth low on the shore, and (4) mortality was greater low on the shore. The predictions of these models were tested by examining patterns of recruitment, growth and mortality of barnacles and availability of free space in relation to height on the shore, at several spatial scales and through time. Successful recruitment of barnacles was observed at different heights on the shore where resident organisms were removed, despite a trend toward a larger number of recruits high on the shore at 1 location (Livorno). Availability of bare rock for recruitment was greater high on the shore, implying that pre-emption of the substratum was more intense low on the shore. There was no evidence to suggest that barnacles grew faster on the high shore than on the low shore. The opposite pattern was observed for young barnacles in several cases. Mortality rates were generally greater in low-shore than high-shore habitats for young and for adult barnacles. Patterns emerged over a background of considerable spatial and temporal variation. These results emphasized the importance of pre-emption of space and mortality of juveniles in generating patterns in the distribution and structure of populations of barnacles on rocky shores in the northwest Mediterranean.