COMPARATIVE ECOLOGY OF NORTH ATLANTIC SHORES: DO DIFFERENCES IN PLAYERS MATTER FOR PROCESS?

Environmental factors have stronger effects than biotic processes in patterns of intertidal populations along the southeast coast of Brazil

Rocky shore communities are shaped by complex interactions among environmental drivers and a range of biological processes. Here, we investigated the importance of abiotic and biotic drivers on the population structure of key rocky intertidal species at 62 sites, spanning ∼50% of the Brazilian rocky shoreline (i.e., ∼500 km). Large-scale population patterns were generally explained by differences in ocean temperature and wave exposure. For the gastropod species Lottia subrugosa, differences at smaller scales (i.e., 0.1-1 km) were better explained by other abiotic influences such as freshwater discharge and substrate roughness. Based on the general population patterns of intertidal species identified, three main oceanographic groups were observed: a cold-oligotrophic grouping at northern sites (Lakes sub-region), a eutrophic group associated with large estuaries and urban zones (Santos and Guanabara bays); and a transitional warm-water group found between the two more productive areas. Larger individuals of Stramonita brasiliensis, L. subrugosa and Echinolittorina lineolata were generally found in the cold-oligotrophic system (i.e., upwelling region), while small suspension feeders dominate the warm-eutrophic systems. Evidence of bottom-up regulation was not observed, and top-down regulation effects were only observed between the whelk S. brasiliensis and its mussel prey Pernaperna. Environmental drivers as compared to biotic interactions, therefore, play a key role determining the population structure of multiple intertidal species, across a range of spatial scales along the SW Atlantic shores.

Marine latitudinal diversity gradients are generally absent in intertidal ecosystems

Current latitudinal diversity gradient (LDG) meta-analyses have failed to distinguish one of the most widespread marine habitats, the intertidal zone, as a separate system despite it having unique abiotic challenges and spatially compressed stress gradients that affect the distribution and abundance of resident species. We address this issue by revisiting published literature and datasets on LDGs since 1911 to explore LDG patterns and their strengths in intertidal benthic, subtidal benthic, and pelagic realms and discuss the importance of recognizing intertidal ecosystems as distinct. Rocky shorelines were the most studied intertidal ecosystem encompassing 64.2% of intertidal LDG studies, and 62.9% of studies focused on assemblage composition, while the remaining 37.1% of studies were taxa specific. While our analyses confirmed LDGs in subtidal benthic and pelagic realms, with a decrease in richness toward the poles, we found no consistent intertidal LDGs in any ocean or coastline across hemispheres or biodiversity unit. Analyzing intertidal and subtidal zones as separate systems increased the strength of subtidal benthic LDGs relative to analyses combining these systems. We demonstrate that in intertidal ecosystems across oceans in both hemispheres, a latitudinal decrease in species richness is not readily apparent, which stands in contrast with significant LDG patterns found in the subtidal realm. Intertidal habitat heterogeneity, regional environmental variability and biological interactions can create species-rich hot spots independent of latitude, which may functionally outweigh a typical latitudinal decline in species richness. Although previous work has shown weaker LDGs in benthic than pelagic systems, we demonstrate that this is caused by combining subtidal and intertidal benthic ecosystems into a single benthic category. Thus, we propose that subtidal and intertidal ecosystems cannot be combined into one entity as the physical and biological parameters controlling ecosystem processes are vastly different, even among intertidal ecosystems. Thus, the intertidal zone offers a unique model system in which hypotheses can be further tested to better understand the complex processes underlying LDGs.

Habitat structure shapes temperate reef assemblages across regional environmental gradients

Intertidal artificial habitats are proliferating, but are generally simpler in structure and host lower biodiversity than natural rocky reefs. Eco-engineering aims to enhance the biodiversity of coastal infrastructure, often through physical structural modifications that mimic topographic properties of natural shores. Relationships between biotic assemblages and structural properties of natural and artificial reefs have been extensively studied at sampling scales of up to 1 m2. But evidence that quantified local structural variation has an appreciable influence on biotic assemblages, at a shore-wide scale across regional environmental gradients, is lacking. Here we addressed this knowledge gap with an observational study at 32 natural and artificial intertidal reef sites in Wales, UK. We used multivariate community analysis and permutation tests to examine associations between local physical structure, regional environmental variables and sessile biotic assemblages. A potential influence of local habitat structure on assemblage composition was evident across regional-scale environmental gradients. Compared to natural sites, artificial reefs had lower taxonomic richness, distinct and more variable assemblage composition, and different physical structure. After removing the effect of habitat (natural or artificial), canonical correspondence analysis showed that environmental variables (wave exposure, sea surface temperature and salinity variation), along with two metrics of physical structure (standard deviation in log-transformed detrended roughness and skewness of surface verticality, both at 0.5 m scale), explained 40 % of the variation in assemblage composition among sites. The two structural metrics independently explained 14.5 % of the variation. Associations identified between individual taxa and environmental variables indicated that sites with a higher proportion of horizontal surfaces hosted more canopy macroalgae, which in turn support other algae and invertebrates. Our findings provide evidence to inform scaling-up of structural eco-engineering interventions from experimental contexts to enhance the biodiversity of coastal infrastructure across regional extents.

Transcriptomic response of the intertidal limpet Patella vulgata to temperature extremes

Global warming is challenging wild species in land and water. In the intertidal zone, species are already living at their thermal limits, being vulnerable even to small increases in maximum habitat temperatures. Knowledge of the mechanisms by which many intertidal zone species cope with elevated temperatures is limited. We analysed the molecular thermal stress response of the limpet Patella vulgata under slight and frequent (one-day), and extreme and rare (three-day) warming events. Using RNA-seq to assess differential gene expression among treatments, differing molecular responses were obtained in the two treatments, with more changes in gene expression after the three-day event; with one-third of the differentially expressed transcripts being down-regulated. However, across treatments we observed shifts in gene expression for common aspects of the heat stress response including intra-cellular communication, protein chaperoning, proteolysis and cell cycle arrest. Of the 71,675 transcripts obtained, only 259 were differentially expressed after both heating events. From these, 218 defined the core group (i.e. genes induced by thermal stress with similar expression patterns irrespective of the magnitude of the warming event). The core group was composed of already well-studied genes in heat stress responses in intertidal organisms (e.g. heat shock proteins), but also genes from less explored metabolic pathways, e.g. the ubiquitin system, which were also fundamental regardless of the magnitude of the imposed warming. Moreover, we have also identified 41 signaling genes (i.e. a set of genes responding to both events and with expression patterns specific to the intensity of thermal stress), principally including genes involved in the maintenance of extracellular structure that have previously not been identified as part of the response to thermal stress in intertidal zone organisms. These signaling genes will be useful heat stress molecular biomarkers for monitoring heat stress in natural populations.

Global gradients in intertidal species richness and functional groups

Whether global latitudinal diversity gradients exist in rocky intertidal α-diversity and across functional groups remains unknown. Using literature data from 433 intertidal sites, we investigated α-diversity patterns across 155° of latitude, and whether local-scale or global-scale structuring processes control α-diversity. We, furthermore, investigated how the relative composition of functional groups changes with latitude. α-Diversity differed among hemispheres with a mid-latitudinal peak in the north, and a non-significant unimodal pattern in the south, but there was no support for a tropical-to-polar decrease in α-diversity. Although global-scale drivers had no discernible effect, the local-scale drivers significantly affected α-diversity, and our results reveal that latitudinal diversity gradients are outweighed by local processes. In contrast to α-diversity patterns, species richness of three functional groups (predators, grazers, and suspension feeders) declined with latitude, coinciding with an inverse gradient in algae. Polar and tropical intertidal data were sparse, and more sampling is required to improve knowledge of marine biodiversity.

Cycles of trans-Arctic dispersal and vicariance, and diversification of the amphi-boreal marine fauna

The amphi-boreal faunal element comprises closely related species and conspecific populations with vicarious distributions in the North Atlantic and North Pacific basins. It originated from an initial trans-Arctic dispersal in the Pliocene after the first opening of the Bering Strait, and subsequent inter-oceanic vicariance through the Pleistocene when the passage through the Arctic was severed by glaciations and low sea levels. Opportunities for further trans-Arctic dispersal have risen at times, however, and molecular data now expose more complex patterns of inter-oceanic affinities and dispersal histories. For a general view on the trans-Arctic dynamics and of the roles of potential dispersal-vicariance cycles in generating systematic diversity, we produced new phylogeographic data sets for amphi-boreal taxa in 21 genera of invertebrates and vertebrates, and combined them with similar published data sets of mitochondrial coding gene variation, adding up to 89 inter-oceanic comparisons involving molluscs, crustaceans, echinoderms, polychaetes, fishes and mammals. Only 39% of the cases correspond to a simple history of Pliocene divergence; in most taxonomical groups, the range of divergence estimates implies connections through the entire Pliocene-Pleistocene-Holocene time frame. Repeated inter-oceanic exchange was inferred for 23 taxa, and the latest connection was usually post-glacial. Such repeated invasions have usually led to secondary contacts and occasionally to widespread hybridization between the different invasion waves. Late- or post-glacial exchange was inferred in 46% of the taxa, stressing the importance of the relatively recent invasions to the current diversity in the North Atlantic. Individual taxa also showed complex idiosyncratic patterns and histories, and several instances of cryptic speciation were recognized. In contrast to a simple inter-oceanic vicariance scenario underlying amphi-boreal speciation, the data expose complex patterns of reinvasion and reticulation that complicate the interpretation of taxon boundaries in the region.

Transition probabilities help identify putative drivers of community change in complex systems

Understanding the role of larger-scale processes in modulating the assembly, structure, and dynamics of communities is critical for forecasting the effects of climate-change and managing ecosystems. Developing this comprehensive perspective is difficult though, because species interactions are complex, interdependent, and dynamic through space and time. Typically, experiments focus on tractable subsets of interactions that will be most critical to investigate and explain shifts in communities, but qualitatively base these choices on experience, natural history, and theory. One quantitative approach to identify the putative forces regulating communities, without reducing system complexity, is estimating transition probabilities among species occupying space (i.e., multispecies Markov chain models). Although not mechanistic, these models estimate the relative frequency and importance of ecological pathways in community assembly and dynamics, and can serve as a framework to identify how pathways change across large scales and which are most important to investigate further. Here, we demonstrate this method in the Gulf of Maine (GOM) intertidal zone, where research has largely focused on the local-scale processes that influence communities, while the mechanisms responsible for more regional shifts in communities are less clear. Transition probabilities of faunal elements were quantified bimonthly for ~2.5 yr in local intertidal communities at three replicate sites in the southern, mid-coast, and northern GOM. Transitions related to mortality, colonization, and replacement by mussels, barnacles, red algae, and encrusting corallines differed regionally, suggesting specific pathways related to consumer pressure and recruitment vary across the GOM with shifting intertidal community structure. Combined with species abundance data and insights from previous research, we develop and evaluate the pathways by which communities likely change in the GOM. Species interactions in local communities can be complex, and this complexity should be incorporated into hypothesis building, experiments, theory, interpretations, and forecasts in ecology. Such a comprehensive approach will be critical to understand how regional shifts in local interactions can drive large-scale community change.

Physiological response of fucoid algae to environmental stress: comparing range centre and southern populations

Climate change has led to alterations in assemblage composition. Species of temperate macroalgae at their southern limits in the Iberian Peninsula have shown shifts in geographical range and a decline in abundance ultimately related to climate, but with the proximate factors largely unknown. We performed manipulative experiments to compare physiological responses of Fucus vesiculosus and Fucus spiralis from Portugal and Wales (UK), representing, respectively, southern and central areas of their distribution, to different intensities of solar radiation and different air temperatures. Following exposure to stressful emerged conditions, Portuguese and Welsh individuals of both fucoid species showed increased frond temperature, high desiccation levels and reduced photophysiological performance that was evident even after a 16 h recovery period, with light and temperature acting in an additive, not an interactive, manner. The level of physiological decline was influenced by geographical origin of populations and species identity, with algae from the south and those living higher on the shore coping better with stressful conditions. The negative effect of summer conditions on photophysiology may contribute to changes in fucoid abundance and distribution in southern Europe. Our results emphasise how physiological performance of geographically distinct populations can differ, which is particularly relevant when predicting responses to climate change.

Climate change, species distribution models, and physiological performance metrics: predicting when biogeographic models are likely to fail

Modeling the biogeographic consequences of climate change requires confidence in model predictions under novel conditions. However, models often fail when extended to new locales, and such instances have been used as evidence of a change in physiological tolerance, that is, a fundamental niche shift. We explore an alternative explanation and propose a method for predicting the likelihood of failure based on physiological performance curves and environmental variance in the original and new environments. We define the transient event margin (TEM) as the gap between energetic performance failure, defined as CT_max, and the upper lethal limit, defined as LT_max. If TEM is large relative to environmental fluctuations, models will likely fail in new locales. If TEM is small relative to environmental fluctuations, models are likely to be robust for new locales, even when mechanism is unknown. Using temperature, we predict when biogeographic models are likely to fail and illustrate this with a case study. We suggest that failure is predictable from an understanding of how climate drives nonlethal physiological responses, but for many species such data have not been collected. Successful biogeographic forecasting thus depends on understanding when the mechanisms limiting distribution of a species will differ among geographic regions, or at different times, resulting in realized niche shifts. TEM allows prediction of the likelihood of such model failure.

Large-scale variation in combined impacts of canopy loss and disturbance on community structure and ecosystem functioning

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.

Biogeographical boundaries, functional group structure and diversity of Rocky Shore communities along the Argentinean coast

We investigate the extent to which functional structure and spatial variability of intertidal communities coincide with major biogeographical boundaries, areas where extensive compositional changes in the biota are observed over a limited geographic extension. We then investigate whether spatial variation in the biomass of functional groups, over geographic (10's km) and local (10's m) scales, could be associated to species diversity within and among these groups. Functional community structure expressed as abundance (density, cover and biomass) and composition of major functional groups was quantified through field surveys at 20 rocky intertidal shores spanning six degrees of latitude along the southwest Atlantic coast of Argentina and extending across the boundaries between the Argentinean and Magellanic Provinces. Patterns of abundance of individual functional groups were not uniformly matched with biogeographical regions. Only ephemeral algae showed an abrupt geographical discontinuity coincident with changes in biogeographic boundaries, and this was limited to the mid intertidal zone. We identified 3-4 main 'groups' of sites in terms of the total and relative abundance of the major functional groups, but these did not coincide with biogeographical boundaries, nor did they follow latitudinal arrangement. Thus, processes that determine the functional structure of these intertidal communities are insensitive to biogeographical boundaries. Over both geographical and local spatial scales, and for most functional groups and tidal levels, increases in species richness within the functional group was significantly associated to increased total biomass and reduced spatial variability of the group. These results suggest that species belonging to the same functional group are sufficiently uncorrelated over space (i.e. metres and site-to-site ) to stabilize patterns of biomass variability and, in this manner, provide a buffer, or "insurance", against spatial variability in environmental conditions.

Local adaptation in adult feeding preference and juvenile performance in the generalist herbivore Idotea balthica

Populations can respond to environmental heterogeneity by genetic adaptation to local conditions. Evidence for local adaptation in herbivores with relatively broad host breadth is scarce, either because generalists rarely locally adapt or because fewer studies have tested for local adaptation. The marine isopod Idotea balthica, a small (<3 cm) generalist herbivore common to estuaries of the northwestern Atlantic, is found on multiple macroalgae and sea grasses north of 42°N, while more southerly populations utilize sea grass-dominated and macroalgal-poor habitats. Feeding preference assays revealed a latitudinal shift in preference hierarchy that mirrors this geographic variation in host availability. Northern populations have higher feeding preference for fresh and freeze-dried tissue of the brown macroalga Fucus vesiculosus and consumed more of its water-soluble and lipophilic extracts relative to southern populations. In contrast, southern populations have a relatively higher preference for the green macroalga Ulva linza and sea grass Zostera marina. The rank of hosts in feeding assays exhibited by northern adults (Fucus = Ulva > Zostera) and southern adults (Ulva > Fucus > Zostera) closely mirrored ranking of juvenile growth rates, suggesting that preference and performance are strongly correlated across these macrophytes. Several of our assays included isopods that had parents reared under uniform laboratory conditions, indicating that geographic differences are genetically mediated and unlikely to reflect phenotypic plasticity or maternal effects. Local adaptation in host use traits may be common in broadly distributed, generalist herbivores in marine and terrestrial systems, and will manifest itself as local shifts in the preference ranking of hosts.

Mulinia lateralis (Mollusca: Bivalvia) die-off in South Carolina: discovery of a vector for two elasmobranch cestode species

Unexplained and episodic die-offs of the dwarf surf clam, Mulinia lateralis , have been reported on the West Atlantic coast, with such an occurrence in South Carolina in June 2010. A sample of live clams from the 2010 South Carolina event was collected, and 200 clams were measured and necropsied. Two species of tapeworm larvae were observed. Plerocercoids (Duplicibothrium sp.) occupied the digestive gland ducts, and merocercoids (Rhodobothrium sp.) were found beneath the mantle. Specimens of both species were sequenced to obtain partial 28S rRNA gene sequences, and they were identified as the tetraphyllidean D. minutum and the rhinebothriidean R. paucitesticulare, based on an NCBI Standard Nucleotide BLAST search. Of the 200 clams, 2.1% were infected with merocercoids (mean intensity 1.3 ± 0.2) and 75% with plerocercoids (mean intensity 4.3 ± 3.7). Intensity of infection by plerocercoids increased significantly with individual shell length. The presence of plerocercoids was associated with enlargement of the digestive gland ducts, but no other pathology was observed. Because uninfected clams were abundant among the stranded molluscs, these parasites are not considered to be the causative agent of the die-off. This is a new host record for both elasmobranch tapeworms.

Variation in community structure across vertical intertidal stress gradients: how does it compare with horizontal variation at different scales?

In rocky intertidal habitats, the pronounced increase in environmental stress from low to high elevations greatly affects community structure, that is, the combined measure of species identity and their relative abundance. Recent studies have shown that ecological variation also occurs along the coastline at a variety of spatial scales. Little is known, however, on how vertical variation compares with horizontal variation measured at increasing spatial scales (in terms of sampling interval). Because broad-scale processes can generate geographical patterns in community structure, we tested the hypothesis that vertical ecological variation is higher than fine-scale horizontal variation but lower than broad-scale horizontal variation. To test this prediction, we compared the variation in community structure across intertidal elevations on rocky shores of Helgoland Island with independent estimates of horizontal variation measured at the scale of patches (quadrats separated by 10s of cm), sites (quadrats separated by a few m), and shores (quadrats separated by 100s to 1000s of m). The multivariate analyses done on community structure supported our prediction. Specifically, vertical variation was significantly higher than patch- and site-scale horizontal variation but lower than shore-scale horizontal variation. Similar patterns were found for the variation in abundance of foundation taxa such as Fucus spp. and Mastocarpus stellatus, suggesting that the effects of these canopy-forming algae, known to function as ecosystem engineers, may explain part of the observed variability in community structure. Our findings suggest that broad-scale processes affecting species performance increase ecological variability relative to the pervasive fine-scale patchiness already described for marine coasts and the well known variation caused by vertical stress gradients. Our results also indicate that experimental research aiming to understand community structure on marine shores should benefit from applying a multi-scale approach.

Influence of a breakwater on nearby rocky intertidal community structure

It is widely recognised that coastal-defence structures generally affect the structure of the assemblages they support, yet their impact on adjacent systems has been largely ignored. Breakwaters modify the nearby physical environment (e.g. wave action) suggesting a local impact on biological parameters. In the present study, an ACI (After-Control-Impact) design was used to test the general hypothesis that the artificial sheltering of an exposed coast has a strong effect on the structure and functioning of adjacent systems. The effects of a reduction in hydrodynamics were clear for a number of taxa and included the replacement of barnacles, limpets and frondose algae by an increasing cover of ephemeral algae. These effects were evident both at early and late successional stages. Results suggest that the artificial sheltering of naturally exposed coasts can have a strong impact promoting a shift from consumer- to producer-dominated communities, which has important ecological and energetic consequences for the ecosystem.

Ecological genetics in the North Atlantic: environmental gradients and adaptation at specific loci

The North Atlantic intertidal community provides a rich set of organismal and environmental material for the study of ecological genetics. Clearly defined environmental gradients exist at multiple spatial scales: there are broad latitudinal trends in temperature, meso-scale changes in salinity along estuaries, and smaller scale gradients in desiccation and temperature spanning the intertidal range. The geology and geography of the American and European coasts provide natural replication of these gradients, allowing for population genetic analyses of parallel adaptation to environmental stress and heterogeneity. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and aid in the process of candidate gene identification. In this paper, we review studies of marine organisms that illustrate associations between an environmental gradient and specific genetic markers. Such highly differentiated markers become candidate genes for adaptation to the environmental factors in question, but the functional significance of genetic variants must be comprehensively evaluated. We present a set of predictions about locus-specific selection across latitudinal, estuarine, and intertidal gradients that are likely to exist in the North Atlantic. We further present new data and analyses that support and contradict these simple selection models. Some taxa show pronounced clinal variation at certain loci against a background of mild clinal variation at many loci. These cases illustrate the procedures necessary for distinguishing selection driven by internal genomic vs. external environmental factors. We suggest that the North Atlantic intertidal community provides a model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and an extensive experimental literature on ecological function. While these organisms are typically poor genetic and genomic models, advances in comparative genomics have provided access to molecular tools that can now be applied to taxa with well-defined ecologies. As many of the organisms we discuss have tight physiological limits driven by climatic factors, this synthesis of molecular population genetics with marine ecology could provide a sensitive means of assessing evolutionary responses to climate change.