Predation intensity in a rocky intertidal community

Summary The predation intensity exerted by populations of the gastropod Thais lapillus at different study areas in the rocky intertidal community of New England is unrelated to predator density. Specifically, very similar intensities are exerted by populations differing in density by at least an order of magnitude. Predation intensity is, in part, a joint function of individual rates of prey consumption and various environmental characteristics. Major factors potentially affecting the individual feeding rates of Thais are (1) prey abundance and productivity, (2) other predators, (3) canopy-forming algae, (4) wave shock, (5) desiccation and (6) snail phenotype and/or history. The effects of the first two of these factors seem unimportant. The effects of the latter 4 on prey consumption rates were studied by estimating field feeding rates of snails held in cages with prey in microhabitats which were characterized by one of two alternative states of each factor. For example, microhabitats could be exposed or protected, at higher or lower levels in the mid intertidal, or under a canopy or not. In addition, exposed-phenotype or protected-phenotype snails were used in each experiment.All of factors (3) to (6) had statistically significant effects except wave shock. The latter would probably also have had a significant effect if the experiments had been performed in the stormier part of the year as well as late summer. The results indicate that sparse populations of Thais can exert intense predation pressure on their prey if they are in protected sites covered with a dense canopy (i.e. in cool, moist habitats in calm waters). Areas with sparser canopy (i.e. greater desiccation stress) and more severe wave shock or both apparently reduce average feeding rates of snails. This appears to explain the paradoxical lack of correlation between predation intensity and snail density.An unexpected result with potentially major implications is the nonlinear response of Thais feeding rates to combinations of factors (3) to (6). Four-way analyses of variance on experiments at exposed and protected sites indicate that 7 of 14 1st-order interactions, 2 of 8 2nd-order interactions, and even 1 of 2 3rd-order interactions are statistically significant. These results suggest that individual predators cannot be assumed to be identical, and that socalled higher order interactions cannot be safely ignored in models of interacting multi-species systems. Hence, it appears that to obtain a thorough understanding of the organization of natural communities, both field and theoretical ecologists alike should begin to grapple with such complexities of nature rather than ignore them.     

A test of the Menge-Sutherland model of community organization in a tropical rocky intertidal food web

Summary Menge and Sutherland (1976) predicted that in physically benign habitats: (1) community structure will be most strongly affected be predation, (2) the effect of predation will increase with a decrease in trophic position in the food web, (3) trophically intermediate species will be influenced by both predation and competition, and (4) competition will occur among prey species which successfully escape consumers. These predictions were tested in a tropical rocky intertidal community on the Pacific coast of Panama. The most abundant mobile species included fishes and crabs, which occupied the top trophic level, and predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were encrusting algae, foliose algae, barnacles, and bivalves. Diets were broad and overlapping, and 30.3% of the consumers were omnivorous. Each consumer group had strong effects on prey occurring at lower trophic levels: (1) Fishes and crabs reduced the abundance of predaceous snails, herbivorous molluscs, foliose algae, and sessile invertebrates. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivorous molluscs reduced the abundance of foliose algae and young stages of sessile invertebrates, and altered relative abundances of the encrusting algae. The encrusting algae, although normally the dominant space occupiers, proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. However, the crusts escaped consumers by virtue of superior anti-herbivore defenses and competed for space despite intense grazing. Observations do not support the hypothesis that the trophically intermediate species compete. Hence, with the exception of this last observation, the predictions of the Menge and Sutherland model were supported. Although further work is needed to evaluate other predictions of the model in this community, evidence from this study joins an increasing body of knowledge supporting the model. Contradictory evidence also exists, however, indicating that aspects of the model require revision.     

Microhabitats, thermal heterogeneity, and patterns of physiological stress in the rocky intertidal zone.

Thermal stress has been considered to be among the most important determinants of organismal distribution in the rocky intertidal zone. Yet our understanding of how body temperatures experienced under field conditions vary in space and time, and of how these temperatures translate into physiological performance, is still rudimentary. We continuously monitored temperatures at a site in central California for a period of two years, using loggers designed to mimic the thermal characteristics of mussels, Mytilus californianus. Model mussel temperatures were recorded on both a horizontal and a vertical, north-facing microsite, and in an adjacent tidepool. We periodically measured levels of heat shock proteins (Hsp70), a measure of thermal stress, from mussels at each microsite. Mussel temperatures were consistently higher on the horizontal surface than on the vertical surface, and differences in body temperature between these sites were reflected in the amount of Hsp70. Seasonal peaks in extreme high temperatures ("acute" high temperatures) did not always coincide with peaks in average daily maxima ("chronic" high temperatures), suggesting that the time history of body temperature may be an important factor in determining levels of thermal stress. Temporal patterns in body temperature during low tide were decoupled from patterns in water temperature, suggesting that water temperature is an ineffective metric of thermal stress for intertidal organisms. This study demonstrates that spatial and temporal variability in thermal stress can be highly complex, and "snapshot" sampling of temperature and biochemical indices may not always be a reliable method for defining thermal stress at a site.     

Community stability: effects of limpet removal and reintroduction in a rocky intertidal community

Summary The stability of a high rocky intertidal community was assessed in a controlled field experiment in which the most common consumers, limpets, were temporarily removed. Compared to the unmanipulated plots, the exclusion plots developed greater algal abundance and altered species composition of both algae and barnacles. The community was not perturbed beyond its capacity to recover, since the community structure of the limpet-removal plots converged on the structure of the unmanipulated plots following limpet reintroduction. Different components of the community recovered at different rates, depending on whether or not the species had a size-related escape from the limpets. Algae had no size-related escape from limpets. The difference in algal abundance between manipulated and unmanipulated plots lasted less than six months after limpet reintroduction. Barnacles, however, had a size-related escape from limpets and therefore recovered more slowly. The difference in barnacle species composition between the perturbed and unperturbed plots lasted for 17 months after limpet reintroduction. The length of the limpet removal period (16 or 28 months) did not appear to affect the rate of community recovery.     

Anthropogenic harvesting pressure and changes in life history: insights from a rocky intertidal limpet

The importance of large breeding individuals for maintaining the health of marine fish and invertebrate populations has long been recognized. Unfortunately, decades of human harvesting that preferentially remove larger individuals have led to drastic reductions in body sizes of many of these species. Such size-selective harvesting is particularly worrisome for sequentially hermaphroditic species where the larger size classes are composed primarily of one sex. Whether these species can maintain stable sex ratios under sustained harvesting pressure depends on the level of plasticity of their life-history traits. Here, we show that populations of a marine limpet (Lottia gigantea) can adjust a fundamental aspect of their life history (the timing of sex change) when subjected to size-selective harvesting. As predicted by theoretical models, individuals from harvested populations change sex at smaller sizes and grow at slower rates compared to individuals from protected populations. In addition, the relative size at which the change from male to female occurs remains constant (~0.75; size at sex change/maximum size) across populations, regardless of harvesting pressure. Our results show that population-level demographic and life-history data, in conjunction with existing theory, can be sufficient to predict the responses of sequential hermaphrodites to harvesting pressure. Furthermore, they suggest such species can potentially adapt to size-selective harvesting.     

Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community

Human alteration of nutrient cycling and the densities of important consumers have intensified the importance of understanding how nutrients and consumers influence the structure of ecological systems. We examined the effects of both grazing and nutrient enrichment on algal abundance and diversity in a high-intertidal limpet-macroalgal community on the South Island of New Zealand, a relatively nutrient-poor environment. We used a fully factorial design with three levels each of grazing (manipulations of limpet and snail densities) and nutrients (nutrient-diffusers attached to the rock). Top-down control by grazers appears to be the driving organizing mechanism for algal communities in this system, with strong negative effects of grazing on algal diversity and abundance across all levels of nutrient enrichment. However, in contrast to the conclusions drawn from the analysis of the whole algal community, there was an interactive effect of grazing and enrichment on foliose algae, an important component of the algal system. When herbivory was reduced to very low levels, enrichment generated increases in the abundance and biomass of foliose algae. As expected, top-down control was the primary determinant of algal community structure in this system, controlling abundance and diversity of macrophytes on the upper shore. Contrary to expectations, however, increased nutrients had no community-wide effects, although foliose algal abundance increases were greatest with high nutrients and reduced grazing. It seems likely that most of the corticated algal species have limited capacity to respond to nutrient pulses in this nutrient-poor environment.     

Scale, environment, and trophic status: the context dependency of community saturation in rocky intertidal communities

Our understanding of the relative influence of different ecological drivers on the number of species in a place remains limited. Assessing the relative influence of local ecological interactions versus regional species pools on local species richness should help bridge this conceptual gap. Plots of local species richness versus regional species pools have been used to address this question, yet after an active quarter-century of research on the relative influence of local interactions versus regional species pools, consensus remains elusive. We propose a conceptual framework that incorporates spatial scale and ecological interaction strength to reconcile current disparities. We then test this framework using a survey of marine rocky intertidal algal and invertebrate communities from the northeast Pacific. We reach two main conclusions. First, these data show that the power of regional species pools to predict local richness disintegrates at small spatial scales coincident with the scale of biological interactions, when studying ecologically interactive groups of species, and in generally more abiotically stressful habitats (e.g., the high intertidal). Second, conclusions of past studies asserting that the regional species pool is the primary driver of local species richness may be artifacts of large spatial scales or ecologically noninteractive groups of species.     

Long term persistence in a rocky intertidal fish assemblage

Environmental Biology of Fishes - A rocky intertidal fish assemblage at Dillon Beach, CA was persistent over 42 months. This time period represents greater than one mean generation time for...     

Effects of avian grazing on the algal community and small invertebrates in the rocky intertidal zone

In this study of a rocky intertidal habitat in northern Japan, feeding by avian consumers had significant effects on algal assemblages and small herbivorous invertebrates. The effects of the birds on algae were different from those of invertebrate grazers such as urchins and gastropods. The abundance of the dominant algal species decreased during the grazing period, increased again after the grazing period, and indirectly affected algal species richness and evenness. Avian grazing also decreased the density of tube-dwelling amphipods on the dominant alga, but did not change the density of mobile and free-living isopods. These results suggest that avian grazers may act as habitat modifiers rather than exploitative competitors for the small herbivorous crustaceans. Avian herbivores consumed only the upper parts of large algal fronds, apparently reducing the amount of suitable microhabitat for the small herbivorous crustaceans, which are subject to a variety of physical or biological stress. Thus, avian herbivores function as ecosystem engineers, regulating community structure in a manner different to invertebrate herbivores in rocky intertidal habitats.     

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.     

Abundance of Petrolisthes armatus (Crustacea: Porcellanidae) on a tropical estuarine intertidal rocky beach, Gulf of Nicoya estuary, Costa Rica

Population of the porcellanid crab Petrolisthes armatus was studied on a rocky intertidal beach located at the Punta Morales peninsula in the mid upper Gulf of Nicoya estuary, Pacific coast of Costa Rica, from December 1997 to November 1998 (14 dates). Horizontal plankton tows (280 micron mesh net) were also made to verify the presence of P. armatus larvae. Crabs were collected every 3 m along three 18 m long transects, at two sites on the beach, by placing a bottomless bucket fringed with canvas to prevent the organisms from escaping under the rim. A total of 15,382 P. armatus were collected. Only 146 (0.95%) crabs had a carapace length longer than 10 mm, and 8995 (58.5%) were in the size range of 2 to 4.5 mm. The remaining crabs 6241 (40.5%) were in the size range of 4.6 to 10 mm. Male and female P. armatus were represented by 2777 and 3518 individuals respectively, with a sex ratio of 1:1.26. Ovigerous females were found at all dates and included 2937 individuals (83% of females). Plankton tows yielded only 73 larvae of P. armatus (Zoea 1), with a density of 1.2 larvae/m3. No statistically significant seasonal trends in the population of this species were detected. P. armatus appears to reproduce continuously the year around in Punta Morales, and some peaks of abundance were present during the dry and rainy seasons. These trends are similar to trends reported for other crustacean species in the Gulf of Nicoya.     

Biological interactions and their role in community structure in the rocky intertidal of Helgoland (German Bight,North Sea)

Over 3 successive seasonal cycles (April 1986 to October 1988), field experiments were established within 3 intertidal levels in the sheltered rocky intertidal of Helgoland (North Sea, German Bight). Competitors for space (Mytilus edulis, macroalgae), herbivores (Littorina spp.) and predators (Carcinus maenas) were either excluded from areas (0.25 m²) covered by undisturbed communities or enclosed at natural densities on areas that were cleared before of animals and plants. All the experimental fields (each 0.25 m²) were covered by cages with 4 mm gauze at the sides and a plexiglas top. The results of the experiments in the upper intertidal (occupied byLittorina spp. andEnteromorpha) showed that a natural density of herbivores could not prevent algal settlement and had only little influence on algal growth. Instead abiotic factors (storms, algae washed ashore) decreased the stock of the green algae. Experiments in the mid intertidal, dominated byMytilus (50% cover),Fucus spp. (20%) and grazingL. littorea (100 ind. m^?2) showed that community structure was directly changed both by grazing periwinkles and by competition for space between mussels and macroalgae. WheneverLittorina was excluded, the canopy ofFucus spp. increased continuously and reached total cover within two years. In addition to the increase ofFucus spp., the rock surface and the mussel shells were overgrown byUlva pseudocurvata, which covered the experimental fields during parts of the summer in the absence of herbivores. As soon as perennial species (fucoids) covered most of the experimental areas, the seasonal growth ofUlva decreased drastically. Presence and growth of macroalgae were also controlled by serious competition for space with mussels. EstablishedMytilus prevented the growth of all perennial and ephemeral algae on the rocks. However, the shells of the mussels provided free space for a new settlement ofFucus andUlva. In the lower intertidal (dominated by total algal cover ofF. serratus, herbivores such asL. littorea andL. mariae, and increasing number of predators such asCarcinus), the feeding activity of herbivores can neither prevent the settlement of the fucoid sporelings nor reduce the growth of macroalgae.F. serratus achieved a total canopy on the rock within one year. Doubled density of herbivores prevented the settlement ofFucus and most of the undercover algae. Predation byCarcinus onLittorina spp. had little influence on the herbivore community patterns. However, the crabs supported the establishment of macroalgae by excluding the mussels from the lower intertidal. In summary, the community organization and maintenance in the mid and lower intertidal is influenced to a high degree by biological interactions. Whereas both the relatively important herbivory byL. littorea and competition for space between mussels and macroalgae dominate in the mid intertidal, predation reaches its highest relative degree of importance for community structure in the lower intertidal.     

Molluscan grazing and macroalgal zonation on a rocky intertidal platform at Perth,Western Australia

Abstract Intertidal limestone platforms off Perth show a characteristic pattern of algal zonation, with dense macroalgal beds nearshore bounded by a ‘barren zone’ along the seaward edge. Abalone (Haliotis roei) and several species of limpets and chitons are abundant in the barren zone, which is generally devoid of non-coralline macroalgae. The relative importance of abalone versus limpets and chitons in limiting macroalgal abundance in the barren zone was evaluated by manipulating the presence of each group in a factorial experiment. Percentage algal cover was measured photographically in 0.25m² plots at 1–2 month intervals for 9 months. Mean algal cover (mainly the foliose green alga, Ulva rigida) was highest in plots where all grazers were excluded (77–99%), intermediate where only limpets and chitons were excluded (37–85%), and lowest where only abalone were excluded (4–30%) or where no grazers were excluded (2–19%). The effect of limpets and chitons accounted for 55–89% of the variance in total algal cover, whereas the effect of abalone generally accounted for <10% of the variance. Similar results were obtained in terms of the biomass of Ulva rigida at the end of the experiment. Haliotis roei are relatively large and sedentary herbivores, feeding mainly on drift algae. Their effect on algal abundance was mediated both through pre-emption of space, which might otherwise be colonized by algae, and by grazing around their home scar. Limpets and chitons are smaller than abalone, but were much more abundant. Intensive grazing of the reef surface by limpets and chitons precluded the establishment of non-coralline macroalgae, even where abalone were absent.     

Effects of intensity and seasonal timing of disturbances on a rocky intertidal benthic community on the southern coast of Korea

The effects of intensity and timing of disturbances on recovery of marine benthic organisms were investigated on a rocky intertidal shore in Gwangyang Bay, Korea. We hypothesized that the recovery pattern of the benthic community structure would be affected by disturbance intensity and season. Twenty-eight permanent plots were set up, with disturbance intensity (cleared plots and sterile plots) and seasonal disturbance (fall 1999 and spring 2000) incorporated into the experimental design. To monitor natural seasonal variation in benthic community abundances, we established seven permanent unmanipulated plots. Turf-forming algae were observed in the unmanipulated plots throughout the experimental period, whereas green algae and invertebrate presence varied with season. In the disturbance-intensity experiment, turf-forming and green algae were dominant in cleared plots. The highest coverage of sessile organisms was observed in sterile plots, which exhibited the highest species richness because of their relatively low macroalgal coverage. Seasonal effects of disturbance were an important factor in the recovery pattern of benthic organisms under high disturbance intensity. Coverage of green algae was higher in sterile spring plots than in sterile fall plots; this result was attributed to low spatial competition, as the disturbances occurred just before green algal blooms. On the other hand, the abundances of barnacles and bivalves were highest on sterile fall plots, as these organisms were suppressed by green algal blooms in other periods. These results indicate that the effects of disturbance intensity on benthic community recovery patterns can be influenced by season of disturbance.     

Physiological community ecology: variation in metabolic activity of ecologically important rocky intertidal invertebrates along environmental gradients

Rocky intertidal invertebrates live in heterogeneous habitatscharacterized by steep gradients in wave activity, tidal flux,temperature, food quality and food availability. These environmentalfactors impact metabolic activity via changes in energy inputand stress-induced alteration of energetic demands. For keystonespecies, small environmentally induced shifts in metabolic activitymay lead to disproportionately large impacts on community structurevia changes in growth or survival of these key species. Herewe use biochemical indicators to assess how natural differencesin wave exposure, temperature and food availability may affectmetabolic activity of mussels, barnacles, whelks and sea starsliving at rocky intertidal sites with different physical andoceanographic characteristics. We show that oxygen consumptionrate is correlated with the activity of key metabolic enzymes(e.g., citrate synthase and malate dehydrogenase) for some intertidalspecies, and concentrations of these enzymes in certain tissuesare lower for starved individuals than for those that are wellfed. We also show that the ratio of RNA to DNA (an index ofprotein synthetic capacity) is highly variable in nature andcorrelates with short-term changes in food availability. Wealso observed striking patterns in enzyme activity and RNA/DNAin nature, which are related to differences in rocky intertidalcommunity structure. Differences among species and habitatsare most pronounced in summer and are linked to high nearshoreproductivity at sites favored by suspension feeders and to exposureto stressful low-tide air temperatures in areas of low wavesplash. These studies illustrate the great promise of usingbiochemical indicators to test ecological models, which predictchanges in community structure along environmental gradients.Our results also suggest that biochemical indices must be carefullyvalidated with laboratory studies, so that the indicator selectedis likely to respond to the environmental variables of interest.     

Feeding guild structure of a rocky intertidal fish assemblage in central Chile

The identification and analysis of the guild structure of vertebrate assemblages has played a fundamental role in the understanding of the underlying mechanisms responsible for their community organization and structuring. This approach generally has not been undertaken for temperate water intertidal fish assemblages. In central Chile, fishes are important components of the intertidal community, but no studies attempting to understand their organization and structuring have been done. In the present study, the diets of 13 of the most abundant species which inhabit tidepools in the rocky intertidal zone of central Chile were determined. A total of 660 fishes was collected at 5 sites: Los Molles, Con-Cón, Quintay, El Tabo, and Las Cruces. Dietary overlap between all species pairs was calculated and a phenogram of dietary similarity was constructed and analyzed using a bootstrapping technique to objectively determine guild membership. The results showed that the intertidal fish assemblage of central Chile can be divided into three feeding guilds: two guilds consisting of carnivorous species and one guild of omnivorous and herbivorous species. The possible causes and implications of the resulting guild structure and the potential effects of predation by these fishes on other components of the intertidal community are discussed.