Rocky Intertidal Communities Research Articles

Sampling design for long-term regional trends in marine rocky intertidal communities

Probability-based designs reduce bias and allow inference of results to the pool of sites from which they were chosen. We developed and tested probability-based designs for monitoring marine rocky intertidal assemblages at Glacier Bay National Park and Preserve (GLBA), Alaska. A multilevel design was used that varied in scale and inference. The levels included aerial surveys, extensive sampling of 25 sites, and more intensive sampling of 6 sites. Aerial surveys of a subset of intertidal habitat indicated that the original target habitat of bedrock-dominated sites with slope ≤30° was rare. This unexpected finding illustrated one value of probability-based surveys and led to a shift in the target habitat type to include steeper, more mixed rocky habitat. Subsequently, we evaluated the statistical power of different sampling methods and sampling strategies to detect changes in the abundances of the predominant sessile intertidal taxa: barnacles Balanomorpha, the mussel Mytilus trossulus, and the rockweed Fucus distichus subsp. evanescens. There was greatest power to detect trends in Mytilus and lesser power for barnacles and Fucus. Because of its greater power, the extensive, coarse-grained sampling scheme was adopted in subsequent years over the intensive, fine-grained scheme. The sampling attributes that had the largest effects on power included sampling of "vertical" line transects (vs. horizontal line transects or quadrats) and increasing the number of sites. We also evaluated the power of several management-set parameters. Given equal sampling effort, sampling more sites fewer times had greater power. The information gained through intertidal monitoring is likely to be useful in assessing changes due to climate, including ocean acidification; invasive species; trampling effects; and oil spills.

Spatial and Temporal Variation in Rocky Intertidal Communities Along the Main Hawaiian Islands

Thirteen benthic rocky intertidal communities were quantitatively assessed on Maui, Moloka'i, O'ahu, and Hawai'i Island between the years 2004 and 2007. Our goals were to test for differences in invertebrate and macroalgal abundance and composition to understand how these tropical communities are organized. Percentage cover surveys revealed a diverse intertidal system with 49 macroalgal, 1 cyanobacterial, and 31 invertebrate taxa. Shores were frequently dominated by a few macroalgae and mollusks, and at two sites these organisms were distributed in discrete vertical bands. Common intertidal taxa included the introduced alga Acanthophora spicifera; species in the macroalgal genera Padina, Sargassum, and Laurencia; turf forms of algae; and the mollusks Siphonaria normalis, Nerita picea, and species of littorine snails. Multivariate statistics found community structure to vary among sites and years, but there was lack of evidence for island-specific or substratum-specific assemblages. SIMPROF analysis revealed support for 11 different types of structure. This first description of community-level patterns at multiple intertidal sites along the Main Hawaiian Islands documents substantial spatial variation both among and within shores, as well as substantial temporal variation for select sites. These findings are in contrast to the characterization of a homogeneous tropical system and thus suggest that biotic and abiotic factors in the Main Hawaiian Islands act on a local scale to drive structure.

An Experimental Test of Multi-species Markov Models: Are Barnacles Long-term Facilitators of Mussel Bed Recovery?

Predicting responses of complex ecosystems to environmental impacts is challenging because of the web of species interactions and their potential to generate indirect effects. Multispecies Markov chain models provide one potential predictive framework for complex ecosystems. A previously parameterized Markov model of a rocky intertidal community predicted little effect of removing acorn barnacles from the system on sessile species composition. In contrast, short-term observations of recovery following disturbance suggest that acorn barnacles play a key facilitation role in mussel-bed recovery. In a 6-yr experiment in which balanoid acorn barnacles [Balanus glandula Darwin, 1854 and Semibalanus cariosus (Pallas, 1788)] were chronically and selectively removed, final species composition did not differ significantly from that of model predictions (90.3% of mean composition explained, P > 0.5), and the mussel Mytilus californianus Conrad, 1837 was able to attain its typical dominance. These results demonstrate that multispecies Markov chain models can generate reliable predictions and that acorn-barnacle facilitation does not have a substantial effect on long-term community composition in this system.

Expert variability provides perspective on the strengths and weaknesses of citizen‐driven intertidal monitoring program

Citizen scientist programs are a means to efficiently conduct large-scale surveys of ecosystems or managed species, provided that concerns over the quality and use of data generated by nonexperts can be addressed. This study presents actions taken in a citizen science program to assure data quality and demonstrates the validity of citizen-generated data. In this case the accuracy of data collected by secondary school students as citizens in a program that quantitatively sampled benthic rocky intertidal communities at 13 sites on Maui, Molokai, Oahu, and Hawai'i island during the years 2004-2007 was evaluated. In 2007, two independent research teams collected data simultaneously with students at five sites on eight sampling dates. Comparisons of Shannon diversity and Bray-Curtis similarity values computed and simulated from student and researcher collected data revealed that nonexpert students accurately collect community-level data within the range of the variation that occurs between researchers. Students were, however, likely to misidentify cryptic and rare species. These findings have direct implications for the conservation goals of the monitoring program as the assessment reveals that students are likely to misidentify early alien introductions but are able to monitor the abundances of native and introduced species once they become established. The validity assessment designed for this investigation is unique in that it directly compares consistent errors made by citizens in data collection to expert variability to identify usage limitations and can be a guide for future studies that involve the efforts of trained volunteers.

Seaweed richness and herbivory increase rate of community recovery from disturbance.

The importance of herbivores and of plant diversity for community succession and recovery from disturbance is well documented. However, few studies have assessed the relative magnitude of, or potential interactions between, these factors. To determine the combined effect of herbivory and surrounding algal species richness on the recovery of a rocky intertidal community, we conducted a 27-month field experiment assessing algal recruitment and succession in cleared patches that mimic naturally forming gaps in the ambient community. We crossed two herbivore treatments, ambient and reduced abundance, with monocultures and polycultures of the four most common algal species in a mid-high rocky intertidal zone of northern California. We found that both the presence of herbivores and high surrounding algal richness increased recovery rates, and the effect of algal richness was twice the magnitude of that of herbivores. The increased recovery rate of patches containing herbivores was due to the consumption of fast-growing, early colonist species that preempt space from perennial, late-successional species. Mechanisms linking algal richness and recovery are more numerous. In polycultures, herbivore abundance and species composition is altered, desiccation rates are lower, and propagule recruitment, survival, and growth are higher compared to monocultures, all of which could contribute the observed effect of surrounding species richness. Herbivory and species richness should jointly accelerate recovery wherever palatable species inhibit late-successional, herbivore-resistant species and recruitment and survival of new colonists is promoted by local species richness. These appear to be common features of rocky-shore seaweed, and perhaps other, communities.

PRESERVATION POTENTIAL AND DIVERSITY OF TROPICAL ROCKY SHORE GASTROPOD COMMUNITIES, SAN SALVADOR ISLAND, BAHAMAS

This study explores spatial distribution, species diversity, and preservation potential of modern rocky intertidal gastropod communities from San Salvador Island, Bahamas. Using a hierarchical sampling approach, 17,703 intertidal gastropod specimens representing 9 genera and 15 species were recorded from 480 quadrats along 120 transects at 40 sites sampled at ten localities around the island. All localities are dominated by several species of littorinids and neritids. The rank-abundance structure was comparable across all localities, with the same species dominating at all but one locality. The hierarchical sampling scheme revealed a gradual increase in diversity across sampling levels, with most notable increase observed between site-level and locality-level diversity estimates. Habitat diversity estimates did not vary from bare and exposed surfaces to sheltered pits, crevices, and tide pools. Across energy regimes, there was no meaningful shift in habitat occurrences to more sheltered, protected habitats. Species occurrences in sheltered habitats should augment fossilization potential of those assemblages because, as shown in some paleontological studies, there is evidence of rocky intertidal environments from such settings as tide pools, pits, and crevices. Indeed, when data are restricted to more preservable, sheltered settings only, a good representation of total biodiversity and overall community structure of sampled faunal associations is still retained. Moreover, because the sampled communities appear remarkably homogeneous across and within localities, even a highly fragmentary fossil record (e.g., small area in one locality) would likely collect a substantial fraction of biodiversity and community structure of rocky intertidal communities of San Salvador Island.

Notes on the egg capsule and variable embryonic development of Nerita melanotragus (Gastropoda: Neritidae)

The black nerite Nerita melanotragus (previously known as N. atramentosa) occurs along the rocky intertidal shores of New Zealand and southeastern Australia. Although commonly used in ecological studies, recent observations suggest that the spe-cies may not be strictly planktonic as previously recorded. The current study investigates whether only planktonic larvae emerge from N. melanotragus egg capsules in the field by analyzing encapsulation period, developmental stage, temperature and salinity through the digital monitoring of 386 individual egg capsules over 65 days. The persistence of egg capsules in rock pools was highly variable (1–54 days). Encapsulation period was strongly related to developmental stage (Adjusted R2 = 0.8293, p < 0.0001), and competent larvae and post-metamorphic juveniles were recorded from some capsules after an encap-sulation period of 35 or more days. Encapsulation period was significantly and parabolically related to both temperature and salinity, with encapsulation period decreasing during extreme conditions. Results indicate that egg capsules can persist for much longer than would be expected with a strict planktonic developmental mode, thereby challenging the status of N. melan-otragus as a consistent planktonic developer. The potential of this species for poecilogony or hatching plasticity is discussed in light of recent studies examining the role of larval dispersal and biogeographic barriers in structuring rocky intertidal commu-nities.

Spatial distribution pattern analysis of subtidal macroalgae assemblages by a non-destructive rapid assessment method

The extensive field work carried out over the last century has allowed the worldwide description of general distribution patterns and specific composition of rocky intertidal communities. However, the information concerning subtidal communities on hard substrates is more recent and scarce due to the difficulties associated with working in such environments. In this work, a non-destructive method is applied to the study and mapping of subtidal rocky bottom macroalgae assemblages on the coast of Cantabria (N Spain) which is quick, easy and economical. Gelidium corneum and Cystoseira baccata were the dominant species, however, the composition and coverage of macroalgae assemblages varied significantly at different locations and depth ranges. The high presence of Laminaria ochroleuca and Saccorhiza polyschides, characteristic of colder waters, shows the transitional character of this coastal area. The results obtained throughout this study have been very useful to the application of the European Water Framework Directive (WFD 2000/60/EC) and could be of great interest for the future conservation and management of these ecosystems (e.g. Habitats Directive 92/43/EEC).

Gradient effects on structuring of soft-bottom benthic infauna: Macoma balthica and predation, recruitment, and food availability

Infaunal benthic community structure can be affected by factors that operate locally (e.g., predation, competition, and disturbance) and regionally (e.g., recruitment, nutrient delivery). To determine the mechanisms driving distribution of a dominant infaunal clam, Macoma balthica, along a salinity (i.e., stress) gradient in the York River, Chesapeake Bay, I experimentally examined the effects of predation, recruitment, and food availability in shallow, subtidal, soft-bottom habitats. Specifically, in upriver and downriver zones I quantified: (1) the abundance of M. balthica; (2) the effect of predation on survival of M. balthica; (3) the abundance and foraging efficiency of major epibenthic predators; (4) the recruitment of M. balthica; and (5) the food availability for clams (sedimentary carbon). Consumer stress models predict that predators are more stressed in zones where salinity is lower and more variable (i.e., upriver); thus, predator-induced mortality should be lower upriver, potentially elevating the density of infaunal prey. In contrast to these models, in the York River, (1) natural abundance of M. balthica was higher upriver, (2) both predation-induced mortality in manipulative field experiments as well as field measurements of natural mortality of clams over time were higher upriver, though per capita foraging efficiency was lower, (3) predator abundance but not size was greater upriver where predation intensity was higher, (4) recruitment was lower upriver, and (5) food availability (sedimentary carbon) was greater upriver. Clam density, environmental stress, and per capita foraging efficiency were in accord with a consumer stress model of community regulation, whereas higher predator abundance and predation intensity in zones of higher environmental stress were contrary to the model's predictions. Recruitment did not dictate overall patterns of clam abundance, but habitat quality and nutrients appeared to drive prey and predator abundances. The patterns in this soft-bottom community are explained best by a productivity model, as reflected in a community governed principally by resource availability (i.e., bottom–up control), at a broad scale. This research suggests that structuring forces are similar for rocky intertidal, salt marsh, and soft-bottom communities at broad spatial scales.

Development of a tool for assessing the ecological quality status of intertidal coastal rocky assemblages, within Atlantic Iberian coasts

The aim of this paper is to develop a new methodology for assessing the quality of coastal waters along the Atlantic Iberian coasts, based upon Basque coast rocky intertidal assemblages, compliant with the European Water Framework Directive (WFD, 2000/60/EC). Biological data collected over a 20-year period, during the gradual introduction of a sewerage plan, are compared to several reference stations in order to differentiate various degrees of community alteration. A quality index (RICQI: Rocky Intertidal Community Quality Index) is drawn up, on the basis of: indicator species abundance; morphologically complex algae cover; species richness; and faunal cover (herbivore and suspensivore cover, proportion of fauna with respect to the whole assemblage). An independent dataset collected in Plentzia Bay (Basque coast, N. Spain), before and after the set-up of a wastewater treatment plant, is used in order to validate RICQI. A conceptual model based on our results is proposed, which describes successional stages of assemblages along a gradient of increasing environmental disturbance and associated values of the metrics included in the index. The performance of this new approach is compared with that of the quality of rocky bottoms index, used presently as the official method for assessing the ecological status of rocky assemblages in the Atlantic coastal waters of Spain. Both indices respond to changes in community structure, associated with pollution removal. However, the RICQI index shows a more accurate response, identifying different degrees of disturbance.

Green Crab (Carcinus maenas) Foraging Efficiency Reduced by Fast Flows

Predators can strongly influence prey populations and the structure and function of ecosystems, but these effects can be modified by environmental stress. For example, fluid velocity and turbulence can alter the impact of predators by limiting their environmental range and altering their foraging ability. We investigated how hydrodynamics affected the foraging behavior of the green crab (Carcinus maenas), which is invading marine habitats throughout the world. High flow velocities are known to reduce green crab predation rates and our study sought to identify the mechanisms by which flow affects green crabs. We performed a series of experiments with green crabs to determine: 1) if their ability to find prey was altered by flow in the field, 2) how flow velocity influenced their foraging efficiency, and 3) how flow velocity affected their handling time of prey. In a field study, we caught significantly fewer crabs in baited traps at sites with fast versus slow flows even though crabs were more abundant in high flow areas. This finding suggests that higher velocity flows impair the ability of green crabs to locate prey. In laboratory flume assays, green crabs foraged less efficiently when flow velocity was increased. Moreover, green crabs required significantly more time to consume prey in high velocity flows. Our data indicate that flow can impose significant chemosensory and physical constraints on green crabs. Hence, hydrodynamics may strongly influence the role that green crabs and other predators play in rocky intertidal communities.

Plants

Plants

Sampling errors create bias in Markov models for community dynamics: the problem and a method for its solution

Repeated, spatially explicit sampling is widely used to characterize the dynamics of sessile communities in both terrestrial and aquatic systems, yet our understanding of the consequences of errors made in such sampling is limited. In particular, when Markov transition probabilities are calculated by tracking individual points over time, misidentification of the same spatial locations will result in biased estimates of transition probabilities, successional rates, and community trajectories. Nonetheless, to date, all published studies that use such data have implicitly assumed that resampling occurs without error when making estimates of transition rates. Here, we develop and test a straightforward maximum likelihood approach, based on simple field estimates of resampling errors, to arrive at corrected estimates of transition rates between species in a rocky intertidal community. We compare community Markov models based on raw and corrected transition estimates using data from Endocladia muricata-dominated plots in a California intertidal assemblage, finding that uncorrected predictions of succession consistently overestimate recovery time. We tested the precision and accuracy of the approach using simulated datasets and found good performance of our estimation method over a range of realistic sample sizes and error rates.

Geographic variation in diversity of wave exposed rocky intertidal communities along central Chile

A lo largo de la costa de Chile central, los patrones geograficos de diversidad han sido inferidos a partir de revisiones literarias y colecciones de museos para algunos grupos taxonomicos. Sin embargo, aun no contamos con una evaluacion integral, y en terreno, de la riqueza de especies macrobentonicas intermareales. En un estudio de largo plazo conducido entre 1998 y 2005 caracterizamos los patrones latitudinales en la biodiversidad del intermareal rocoso en 18 sitios a lo largo de la costa de Chile central (29-36° S). En cada sitio, el numero de especies sesiles y moviles fue cuantificado en cuadrantes de 0.25 m2. Usamos dos estimadores de la riqueza de especies: la riqueza local observada, correspondiente a la asintota de la curva de acumulacion de especies, y el indice Chao2, el cual considera el efecto de especies raras en la estimacion de la diversidad local. Identificamos 71 especies pertenecientes a 66 generos y a un total de 86 taxa. Los grupos mas diversos fueron moluscos (27 taxa) y macroalgas (43 taxa). La riqueza de especies mostro un patron complejo en el cual areas de alta riqueza especifica aparecieron intercaladas con areas de baja riqueza. Ademas, observamos una alta variabilidad espacial en el numero de especies herbivoras y un patron latitudinal inverso en el numero de especies algales. Nuestros resultados apuntan a la necesidad de incorporar una variedad de grupos taxonomicos y un rango de escalas espaciales en estudios de biodiversidad.

Non-linear density-dependent effects of an intertidal ecosystem engineer

Ecosystem engineering is an important process in a variety of ecosystems. However, the relationship between engineer density and engineering impact remains poorly understood. We used experiments and a mathematical model to examine the role of engineer density in a rocky intertidal community in northern California. In this system, the whelk Nucella ostrina preys on barnacles (Balanus glandula and Chthamalus dalli), leaving empty barnacle tests as a resource (favorable microhabitat) for other species. Field experiments demonstrated that N. ostrina predation increased the availability of empty tests of both barnacle species, reduced the density of the competitively dominant B. glandula, and indirectly increased the density of the competitively inferior C. dalli. Empty barnacle tests altered microhabitat humidity, but not temperature, and presumably provided a refuge from wave action. The herbivorous snail Littorina plena was positively associated with empty test availability in both observational comparisons and experimental manipulations of empty test availability, and L. plena density was elevated in areas with foraging N. ostrina. To explore the effects of variation in N. ostrina predation, we constructed a demographic matrix model for barnacles in which we varied predation intensity. The model predicted that number of available empty tests increases with predation intensity to a point, but declines when predation pressure was strong enough to severely reduce adult barnacle densities. The modeled number of available empty tests therefore peaked at an intermediate level of N. ostrina predation. Non-linear relationships between engineer density and engineer impact may be a generally important attribute of systems in which engineers influence the population dynamics of the species that they manipulate.

Reestablishment of the Southern California Rocky Intertidal Brown Alga,Silvetia compressa: An Experimental Investigation of Techniques and Abiotic and Biotic Factors That Affect Restoration Success

Previous research has indicated that many rocky intertidal macrophyte communities in southern California, and other locations around the world, have shifted from larger, highly productive, fleshy seaweeds toward a smaller, less productive, disturbance‐tolerant flora. In widespread decline are ecologically important, canopy‐forming, brown seaweeds, such as the southern California rockweed speciesSilvetia compressa. Restoration efforts are common for depleted biogenic species in other habitats, but restoration within rocky intertidal zones, particularly on wave‐exposed coasts, has been largely unexplored. In two phases, we attempted to restoreSilvetiapopulations on a southern California shore by transplanting live plants and experimentally investigating factors that affect their survival. In Phase I, we implemented a three‐way factorial design where juvenileSilvetiathalli were transplanted at four sites with a combination of simulated canopy and herbivore exclusion treatments. Transplant survival was low, although enhanced by the presence of a canopy; site and herbivore presence did not affect survival. In Phase II, we used a two‐way factorial design, transplanting two size classes of rockweeds (juveniles and reproductive adults) on horizontal and partially shaded, north‐facing vertical surfaces at a target location where this rockweed has been missing since at least the 1970s. Transplant survival was moderate but lower than natural survival rates. Larger thalli exhibited significantly higher survival rates than smaller thalli in both the transplanted and naturally occurring populations, particularly on vertical surfaces. Higher mortality on horizontal surfaces may have been due to differences in desiccation stress and human trampling. Transplanting reproductive adults resulted in the subsequent recruitment of new individuals.

Temperature and community consequences of the loss of foundation species: Surfgrass ( Phyllospadix spp., Hooker) in tidepools

By ameliorating environmental conditions, mediating species interactions and regulating resource availability in ecosystems, habitat-forming, foundation species underlie the structure and function of many ecological communities. Current and predicted human-induced changes to ecosystems require a mechanistic understanding of how foundation species enable the persistence of diverse, functioning ecological communities. By manipulating the abundance of a putative foundation species of rocky intertidal tidepool communities, the seagrass Phyllospadix spp. (surfgrass), this study assesses how surfgrass affects the thermal environment and structure of tidepool communities over a period of two years. Surfgrass removal pools experienced temperatures up to 10 °C higher than control pools during low tide. Changes in community composition of sessile organism accompanied this change in thermal regime. Over two years, removal pools saw a significant decline in the abundance of coralline crusts and bare space and an increase in the abundance of foliose red algae while control pool communities remained stable. At the level of functional groups, community similarity metrics showed pool communities became more variable in space in response to surfgrass removal, demonstrating that pool communities diverge dramatically in the absence of surfgrass. In removal pools, recovery of surfgrass was limited with less than 25% of pre-removal abundance after two years. However, removal pool communities shifted toward species compositions that favor surfgrass recruitment and suggest a mechanism by which surfgrass recovery may be enhanced over the long-term. Surfgrass clearly play a foundational role in tidepools by reducing pool temperatures and act to stabilize tidepool communities.

Supply-side ecology, barnacle recruitment, and rocky intertidal community dynamics: Do settlement surface and limpet disturbance matter?

The supply of new recruits can be a critical determinant of community structure, but estimating recruitment density can be challenging due to variation in larval supply, settlement, substratum, species interactions and physical stresses. We evaluated the effect of surface type and limpet “bulldozing” on settlement and recruitment of barnacles, using replicated field experiments sampled monthly at each of four sites along the central Oregon coast in 2001, 2003, and 2008. A particular goal was to evaluate the efficacy of Saf-T-Walk, a widely used textured surface, in quantifying actual patterns and levels of recruitment. In 2001 and 2003 experiments, surfaces were bare rock (ROCK) and Saf-T-Walk (STW). In 2008 experiments, to further evaluate the influence of surface rugosity, we added two additional artificial substratum types — smooth PVC plates (no texture) and travertine plates (TRAV, low texture). Limpet activity was manipulated using barriers of anti-fouling paint. Results indicated the overwhelming importance of surface texture. Recruitment density was highest in 2001, lower in 2003, and lowest in 2008. Recruitment on STW, the most textured surface, ranged up to 87 times greater than on ROCK, and the rank order of surfaces for recruitment density was STW > ROCK > TRAV > PVC which matches that for texture. Settlement differences were even greater, with up to 1180-fold higher settlement on STW than ROCK. Limpets usually had a negative effect on recruitment of Balanus glandula on ROCK, likely due to bulldozing and direct consumption, but had a positive effect on STW. These impacts were reduced or negated when recruitment density was higher (e.g., 2001) or lower than average (e.g., 2008). Limpets had no consistent effect on Chthamalus dalli. Abundances of settlers and recruits on the different surfaces were positively correlated (explaining up to 71% of the variance), and correlations between STW and other surfaces generally were stronger in the absence of limpets. Although compared to bare rock, recruitment to STW overestimates absolute rates of recruitment, use of STW settlement plates provides an effective and relatively efficient means of determining spatial and temporal patterns of recruitment at local to geographic scales.

Hydrodynamic sensory stressors produce nonlinear predation patterns

Predators often have large effects on community structure, but these effects can be minimized in habitats subjected to intense physical stress. For example, predators exert large effects on rocky intertidal communities on wave-protected shores but are usually absent from wave-swept shores where hydrodynamic forces prevent them from foraging effectively. The physical environment also can affect predation levels when stressors are not severe enough to be physically risky. In these situations, environmental conditions may constrain a predator's ability to locate prey and alleviate predation pressure. Yet, stress models of community structure have rarely considered the implications of such sensory or behavioral stressors, particularly when the sensory abilities of both predators and prey are affected by the same types of environmental conditions. Ecologists may classify certain environmental conditions as refuges if they impede predator foraging, but these conditions may not actually decrease predation levels if they simultaneously increase prey vulnerability to consumers. Using blue crabs (Callinectes sapidus) and hard clams (Mercenaria mercenaria) as a model system, we investigated the relationship between predation intensity and environmental stress in the form of hydrodynamics (i.e., flow velocity and turbulence). Blue crabs and hard clams are less responsive to each other in faster, more turbulent flows, but studies exploring how flow modulates the outcomes of crab-clam interactions in the field are lacking. We manipulated turbulence within field sites and compared predation levels within and between sites that differed in flow velocity and turbulence. Our results suggest that blue crabs are most effective foragers in flows with intermediate velocities and turbulence levels. Although these conditions are not ideal for blue crabs, lab studies indicate that they also compromise the ability of clams to detect and react to approaching crabs and, thereby, increase clam vulnerability to predators. Our results suggest that environmental stresses on perception (sensory stressors) may not cause a steady decay in predation rates when they simultaneously affect the behaviors of both predators and prey. Moreover, the relative contribution of lethal vs. nonlethal predator effects in communities also may be influenced by environmental forces that enhance the predator-avoidance abilities of prey or the foraging efficiency of predators.

Genetic structure among 50 species of the northeastern Pacific rocky intertidal community.

Comparing many species' population genetic patterns across the same seascape can identify species with different levels of structure, and suggest hypotheses about the processes that cause such variation for species in the same ecosystem. This comparative approach helps focus on geographic barriers and selective or demographic processes that define genetic connectivity on an ecosystem scale, the understanding of which is particularly important for large-scale management efforts. Moreover, a multispecies dataset has great statistical advantages over single-species studies, lending explanatory power in an effort to uncover the mechanisms driving population structure. Here, we analyze a 50-species dataset of Pacific nearshore invertebrates with the aim of discovering the most influential structuring factors along the Pacific coast of North America. We collected cytochrome c oxidase I (COI) mtDNA data from populations of 34 species of marine invertebrates sampled coarsely at four coastal locations in California, Oregon, and Alaska, and added published data from 16 additional species. All nine species with non-pelagic development have strong genetic structure. For the 41 species with pelagic development, 13 show significant genetic differentiation, nine of which show striking FST levels of 0.1–0.6. Finer scale geographic investigations show unexpected regional patterns of genetic change near Cape Mendocino in northern California for five of the six species tested. The region between Oregon and Alaska is a second focus of intraspecific genetic change, showing differentiation in half the species tested. Across regions, strong genetic subdivision occurs more often than expected in mid-to-high intertidal species, a result that may reflect reduced gene flow due to natural selection along coastal environmental gradients. Finally, the results highlight the importance of making primary research accessible to policymakers, as unexpected barriers to marine dispersal break the coast into separate demographic zones that may require their own management plans.