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Historical and Contemporary Comparisons of the Trans-Atlantic Biota

Vermeij, G. J., G. P. Dietl, and D. G. Reid.
A history of trans-atlantic differences: Body size and diversity of marine invertebrates.

Evolutionarily independent biotas living in geographically separated but physically similar regions often differ strikingly in ecological structure, diversity, and the expression of adaptations. Here we document some differences between the living temperate shallow-water marine biotas of the eastern and western North Atlantic, and examine fossil evidence to illuminate the causes and history of these contrasts. We conducted trans-Atlantic comparisons for three hard-bottom and five sand-bottom guilds of benthic shell-bearing invertebrates. In sand-bottom guilds, large-bodied species attain greater maximum sizes in eastern North America than in Europe. Europe holds the edge in two of the three hard-bottom guilds. These differences are consistent with the large areal extent of American as compared to European sand-bottom habitats, but differences in geographic origin (warm-temperate versus cold-temperate), productivity, and extent of enemy-related selection likely also contribute. Planktonic productivity over sand-bottom environments, as reflected by the maximum sizes of suspension-feeders and their predators, may be higher in those parts of the western Atlantic where suitable source populations reside than in those areas of the eastern Atlantic where European source populations of large-bodied species exist. Most of the differences we document have existed for millions of years, and have been surprisingly little affected by episodes of extinction and species invasion, which were particularly dramatic on the American side of the Atlantic. Maximum body size generally increased from late early Miocene to Pliocene times on both sides of the Atlantic, indicating that the trans-Atlantic differences persisted despite possible changes in the selective and resource regime. Future studies of regional variation should incorporate the dimension of time, and should treat different habitats, life habits, and adaptive syndromes separately.

Jenkins , S., M. Bertness, D. Garbary, S. Hawkins, A. Ingolffson, P. Moore, K. Sebens, P. Snelgrove, D. Wethey, and S. Woodin.
Comparisons of the ecology of shores across the North Atlantic: Do different players matter for process?

The shores of the North Atlantic Ocean provide an excellent system to consider how past events shape present ecological processes and the functioning of ecosystems. There are superficial similarities in littoral soft sediment and hard substratum community composition, but also some striking differences in the distribution of key taxa. These differences are the outcome of large scale events, such as the trans-arctic interchange, which have shaped the species pool, and cycles of local and regional extinction and recolonization, driven at least in part by glaciation. In addition, more recent anthropogenically induced invasions and local extinctions have significantly altered biogeographic distributions. Here we consider how the presence or absence of key taxa, and differences in diversity within functional groups, over large scales influences the outcomes of trophic and non-trophic interactions, and the consequences for community structure and function. Contrasts are considered within the context of the differing environmental regime under which species interactions occur on either side of the north Atlantic. Wherever possible comparisons are made for similar thermal regimes, although the predominantly continental climate of North America versus the oceanic climate of western Europe does create differences in thermal extremes. Experimental studies are used throughout to allow comparison of interaction sign and strength on latitudinal and longitudinal gradients on northern Atlantic shores.
The dominant community structuring processes in both hard substratum and soft sediment shores on either side of the Atlantic are briefly reviewed, and the key taxa in five biogeographic regions which drive these processes are described. For both habitats, experimental studies show similar community structuring processes in North America and Europe. The similarity in community structure and species composition is recognised but there are important differences in the presence, and in some cases the relative abundance, of some key taxa, which have important implications for the way shores are structured.
On hard substratum shores biodiversity of epilithic grazers differs markedly, both qualitatively (in terms of identity) and quantitatively (in terms of number of species), across latitudinal and longitudinal scales. In southern Europe grazer diversity is high with a number of microphagous, patellid limpet and trochid grazers feeding on the epilithic microbial film and associated macroalgal propagules. Diversity declines with increasing latitude and in Iceland these species are totally absent. In the NW Atlantic, patellid limpets and trochids are absent and the dominant grazer is Littorina littorea. Morphological and behavioural differences between the dominant grazers on either side of the Atlantic result in differences in consumer pressure, and a marked contrast in the importance of herbivory versus competitive interactions and predation pressure as community structuring processes.
On soft sediment shores, there are conspicuous differences across the Atlantic in the abundance of both hemichordates, which are important bioturbators, and large digging predators. On western Atlantic shores the number and diversity of digging predators is high; the horseshoe crab, numerous active swimming portunid crabs, several species of large whelks and an abundance of skates and rays exert intense predation pressure and associated biogenic disturbance to sediments. In Europe these taxa are rare or absent. Thus, the importance of biological agents of disturbance is lower on European shores as a consequence both of recent anthropogenic pressure (skates and rays) and natural processes over larger time scales.
Understanding how past events, which contribute to differences in community species composition, impact on community processes is an important element in predicting future change as a result of anthropogenic drivers such as climate change and local and regional extinctions due to habitat loss, pollution and over-exploitation. This and other future research directions are discussed.

Historical and Contemporary Drivers of Species Diversity and Range Shifts

Yasuhara, M., and T. M. Cronin.
Climatic influences on deep-sea ostracode (Crustacea) species diversity for the last three million years.

In this paper, we review patterns of deep-sea fossil ostracode (small bivalved Crustacea) species diversity in the context of climatic and oceanographic changes reconstructed from sediment cores and proxy records over the past three million years, mainly in the North Atlantic Ocean. We also discuss potential causal factors, especially temperature and productivity, influencing large-scale diversity patterns. Available evidence from fossil ostracodes suggest that climatic changes occurring over timescales ranging from thousands to hundreds of thousands of years significantly affected deep-sea species diversity at local and probably regional spatial scales. Some fossil ostracode records support the suggestion, based on meta analysis of Quaternary deep-sea foraminiferal data, that temperature is a significant predictor of deep-sea species diversity, although the role of productivity may also be significant at certain times and regions of the ocean.

Witman, J. A., Pershing, M. Cusson, P. Archambault, N. Mieskowska, and B. Helmuth.
The relation between productivity and species diversity in temperate- arctic marine ecosystems

Energy variables, such as evapotranspiration, temperature and productivity explain significant variation in the diversity of many groups of terrestrial plants and animals at regional to global scales. Although the ocean represents the largest continuous habitat on earth, with a broad spectrum of primary productivity and species richness, little is known about how productivity influences species diversity in marine systems. To search for general relationships between productivity and species richness in the ocean, we analyzed data from one pelagic and three benthic marine ecosystems across local – global spatial scales using a standardized proxy for productivity, satellite-derived chlorophyll a. Theoretically, the form of the function between productivity and species diversity is either monotonically increasing or decreasing, or parabolic (hump shaped). Our analysis found widespread support for the parabolic model, as there were significant quadratic relationships between the species richness of local communities and mean chlorophyll a for zooplankton communities in the Gulf of Maine, epifaunal invertebrate communities on subtidal rock walls and on navigation buoys in the Gulf of St. Lawrence, and soft bottom macrobenthos of the Canadian Arctic. The parabolic model held on regional (> 200 km) and global (> 4000 km) spatial scales. No linear relationships between productivity and species richness were found. Although the parabolic model was generally supported in temperate- arctic marine ecosystems, competitive dominance at high productivity, the classic mechanistic explanation for declining of diversity at high productivity, was only supported for epifaunal communities on rock walls. In two of the data sets, (epifaunal communities on navigation buoys, Arctic macrobenthos), declining diversity past the maximum at intermediate levels of chlorophyll a covaried with the effects of salinity, suggesting that environmental stress as well as productivity, influences diversity in these systems. In the zooplankton communities, the declining limb of the parabola was driven by the effects of the spring phytoplankton bloom, when diversity was low diversity and productivity high, indicating the importance of temporal variation in driving the relation. These co-varying effects of salinity and time may commonly arise in broad scale studies of productivity and diversity in marine ecosystems when attempting to sample the largest range of productivity, often encompassing a coastal – oceanic gradient, and when using time averaged data, suggesting caution in invoking traditional mechanistic explanations for the parabolic model in marine systems.

Greene, C. H., and A. J. Pershing.
Arctic Climate Change and its Impacts on the Ecology of the North Atlantic.

In this paper, we review the record of Arctic climate change in the past to provide insights for interpreting the ecological responses to changes in climate recently observed in the North Atlantic. From these insights, we also attempt to predict ecological responses to future changes in climate.
A recurring theme in earth’s history is the importance of the Arctic atmosphere, ocean, and cryosphere in regulating global climate on a variety of spatial and temporal scales. When the extent of sea ice and continental ice sheets begins to expand, positive ice-albedo feedback mechanisms serve to accelerate an expansion of the cryosphere. When melting begins to reduce the extent of sea ice and continental ice sheets, the same ice-albedo feedback mechanisms run in reverse and accelerate a contraction of the cryosphere. Processes that trigger these ice-albedo feedback mechanisms can operate on time scales of millions of years, such as the transition from greenhouse to icehouse conditions over much of the Cenozoic era, to time scales of decades to millennia, such as the transitions from interstadial to stadial conditions during the last ice age.
A second recurring theme in earth’s history is the importance of freshwater export from the Arctic in regulating global- to basin-scale ocean circulation patterns and climate. On a global scale, large discharges of low-salinity water into the Nordic Seas can lead to intense stratification of the upper water column and a disruption of North Atlantic Deep Water (NADW) formation. A reduction in the formation of NADW can lead to a slowing down of the global ocean’s meridional overturning circulation. This can have climatic ramifications for the entire Earth system. On a basin scale, smaller discharges of low-salinity from the Arctic, such as the Great Salinity Anomalies (GSAs) of recent decades, also can impact ocean circulation patterns and climate.
During recent decades, historically unprecedented changes have been observed in the Arctic. The emergence of a strongly cyclonic atmospheric circulation pattern has resulted in an extensive reorganization of upper-ocean circulation patterns in the Arctic Ocean. This has led to an increase in the proportion of low-salinity Pacific water passing through the Arctic Ocean and entering the North Atlantic through the Canadian Archipelago. In addition, climate warming has increased river inflow and sea ice melting in the Arctic. This has led to greater freshwater export from the Arctic, with recent GSAs freshening shelf waters in the Northwest Atlantic from the Labrador Sea to the Middle Atlantic Bight.
The changes in Arctic Ocean circulation patterns and freshwater export have been associated with two major types of ecological responses in the North Atlantic. The first of these responses has been a series of biogeographic range expansions by boreal plankton, including renewal of the trans-Arctic exchanges of Pacific fauna and flora with the Atlantic. The second response has been a dramatic regime shift in the shelf ecosystems of the Northwest Atlantic. This regime shift resulted from freshening and stratification of the shelf waters, which in turn has been linked to changes in the abundances and seasonal cycles of phytoplankton, zooplankton, and higher trophic-level consumer populations.
It is reasonable to predict that the recently observed ecological responses to Arctic climate change in the North Atlantic will continue into the near future. It is more difficult to predict ecological responses to climate change in the more distant future as the earth’s climate system is changing so rapidly, and scientists are only beginning to understand its inherent nonlinearities. If tipping points in the climate system are exceeded, then future changes in Arctic Ocean circulation patterns and freshwater export will be more difficult to predict. Under such circumstances, it will also be much more difficult to predict whether biogeographic and ecosystem responses in the Northwest Atlantic will continue along the same trajectories as at present or begin to shift in new directions.

Broitman, B. R., N. Mieszkowska, B. Helmuth, and C. A. Blanchette.
Large-scale climate effects on recruitment variability of rocky shore intertidal invertebrates of the Eastern North Atlantic.

Recruitment is a key life history stage that influences the persistence and success of populations throughout a species’ geographic range. Climate has been shown to exert a pervasive influence on ecological processes including recruitment within the marine environment, but accurately predicting the responses of species to future alterations in the global climate requires that we understand the spatial and temporal scales over which these effects occur. Through a literature survey, we examine the relationship between annual recruitment and the NAO index (NAOI) in a suite of intertidal invertebrate species from the eastern North Atlantic over a period spanning the 1970s to the 2000s. Strong correlations are evident between recruitment strength and NAOI in all species studied. However, differences between the northern and southern sectors of the North Atlantic are apparent, suggesting that different climatic drivers relating to the NAOI may be affecting recruitment.
Atmospheric forcing appears to be the dominant driver of recruitment success at higher latitudes in Scotland, Wales and Ireland, with increased recruitment success in years with a strong positive NAO index where sea surface temperatures are higher. A six month time lag, reflecting the effects of winter and early spring, is evident in all significant correlations for this region. In contrast, oceanographic forcing appears to predominate along the coastline of southern Europe, with enhanced recruitment success being linked to both increases in sea surface temperature and reduced upwelling that occur during positive NAO index years. There is no temporal lag in correlations between NAOI and recruitment in this region, indicating an immediate response of the target species. These results suggest that the effects of climate on larval recruitment are related to large-scale environmental processes influencing both pre- and post-settlement processes. We highlight the role of climate as a basin-wide synchronizer of ecological processes on rocky shore ecosystems, but demonstrate that large-scale processes do not account for all the observed variance in recruitment. Our results show that the use of basin-scale climate indices such as the NAO is a good first step to identify responses to changes in climate and generate more focused, testable hypotheses regarding the mechanisms underlying these trends.

Schmidt, P., E. Serrão, G. Pearson, C. Riginos, P. Rawson, J. Hilbish, S. Brawley, G. Trussell, E. Carrington, D. Wethey, J. W. Grahame, F. Bonhomme, and D. A. Rand.
Ecological genetics in the North Atlantic intertidal: Environmental gradients, replicated clines, 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 rigorous population genetic analyses of parallel adaptation to environmental stress and heterogeneity. 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. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and add rigor to the process of candidate gene identification. 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 versus external environmental factors. We argue that the North Atlantic intertidal community provides a superb model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and the extensive experimental literature supporting a deep knowledge of 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 organisms 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.

Historical and Contemporary Drivers Shaping the Genetics of the North Atlantic Biota

Christine A. Maggs, Rita Castilho, David Foltz, Christy Hensler, Taimour Jolly, Jeanine Olsen, Kathryn E Perez, Wytze Stam, Risto Vainola, Frederique Viard and John Wares.
Are Phylogeographic Models of Glacial Refugia Adequate for Marine Systems?

A goal of phylogeography is to relate patterns of genetic differentiation to potential historical geographical isolating events.  Quaternary glaciations, particularly the Last Glacial Maximum (25-18 ka), greatly affected the distributions and population sizes of temperate marine species as their ranges retreated southwards to escape ice sheets.  Current genetic models of glacial refugia and routes of recolonization include these predictions: low genetic diversity at high latitudes, with a small number of alleles/haplotypes dominating disproportionately large areas; high diversity in the south, with populations in or near different southern refugia that were geographically isolated during the LGM being most divergent.  These models are based on terrestrial biota, however, and it is not clear to what extent they can be extended to the marine environment.To test this, we used coalescent simulations to explore the possible outcomes, in terms of haplotype networks, of isolating a hypothetical formerly panmictic population into two geographical regions.  We also generated models of genetic signatures for different scenarios of glacial isolation and secondary contact. Ten genetic data sets with appropriate geographical sampling and molecular markers were selected for North Atlantic marine organisms (algae, invertebrates, fish).  Special attention was paid to potential cryptic species and anthropogenic introductions.  Data sets were grouped according to phylogeographic pattern. In addition to haplotype distributions derived from the original papers, we reanalyzed haplotype richness for all datasets to create a uniform set of diversity plots. We hypothesized that periglacial refugia supported populations with lower effective population sizes and greater chance of local extinction /colonization events than southern areas with larger ancestral populations; that these southern areas experienced southward range shifts, but not appreciable range contractions or population bottlenecks; and that northern isolates would show a stronger genetic signature of recent population expansion.  Our analysis suggests that for marine organisms the genetic signatures of periglacial and southern refugia can be distinguished, and that the terrestrial model can broadly be applied to them. Our analyses further illustrate that there were several periglacial refugia in more northern latitudes giving credence to recent climatic reconstructions that infer more open waters than had previously been considered likely.