Historical Timescales Research Articles

Carbon and environmental footprinting of low carbon UK electricity futures to 2050

Electricity generation contributes a large proportion of the total greenhouse gas emissions in the United Kingdom (UK), due to the predominant use of fossil fuel (coal and natural gas) combustion for this purpose. A range of future UK energy scenarios has been employed to determine their resulting environmental and carbon footprints. Methodologies have been established to calculate these footprints for the UK electricity supply industry on both a historic timescale and in accordance with the three selected scenarios. The latter scenarios, developed by the UK SUPERGEN Consortium on ‘Highly Distributed Power Systems’ (HDPS), were characterised as ‘Business As Usual’ (BAU), ‘Low Carbon’ (LC) and ‘Deep Green’ (DG) futures, and yielded possible electricity demands out to 2050. It was found that the environmental footprint of the current power network is 41 million (M) global hectares (gha). If future trends follow a ‘Business As Usual’ scenario, then this footprint is observed to fall to about 25 Mgha in 2050. The LC scenario implies an extensive penetration of micro-generators in the home to satisfy heat and power demands. However, these energy requirements are minimised by way of improved insulation of the building fabric and other demand reduction measures. In contrast, the DG scenario presupposes a network where centralised renewable energy technologies – mainly large-scale onshore and offshore wind turbines - have an important role in the power generation. However, both the LC and DG scenarios were found to lead to footprints of less than 4 Mgha by 2050. These latter two scenarios were found to give rise to quite similar trajectories over the period 2010–2050. They are therefore more likely to reflect an effective transition pathway in terms of meeting the 2050 UK CO2 reduction targets associated with decarbonisation of its power network. However, this appears unlikely to be achieved by 2030–2040 as advocated by the UK Government's advisory Committee on Climate Change in order to meet overall national carbon reduction targets.

Tectonic controls on the morphodynamics of the Brahmaputra River system in the upper Assam valley, India

The Brahmaputra is one of the largest tropical rivers of the world and is located in an area of high structural instability as evidenced from the presence of a large number of earthquakes in the Himalayan catchment through which it flows. Syntectonic evidence of changes in the morphodynamics is difficult to identify for the large rivers. Nevertheless, we note that the Brahmaputra River has become astonishingly large in planform in a historical timescale. Reconstruction of planform changes over a period of 90years in the upper reaches of the Assam valley shows that the 240-km-long channel belt is widening all along its course in the region. From the average width of 9.74km in 1915, the channel belt has widened to the average width of 14.03km in 2005 (44% widening), and in certain reaches the average widening is as high as 250%. However, the bank line shift is not symmetric along both banks. Further, the planform characteristics of the Brahmaputra River reveal significant spatial and temporal variability from upstream to downstream reaches, and we attribute this variability to tectonogeomorphic zonation of the river based on subsurface configuration and channel slope. Further, the tributaries joining the northern and southern banks of the Brahmaputra differ remarkably in terms of river dynamics, and this is attributed to the differences in tectonic regimes of the Himalaya in the north and the Naga Patkai hills in the south.

Genetically separate populations of the ocean-skater Halobates sericeus (Heteroptera: Gerridae) have been maintained since the late Pleistocene

The oceanic water strider (or ocean‐skater) Halobates sericeus Eschscholtz has a disjunct distribution in the Pacific Ocean, with northern and southern populations widely separated by an equatorial zone. It is sensitive to sea surface conditions and, consequently, its distribution and population structure may provide an insight into environmental changes on the ocean surface on both recent and historical time scales. We assessed the genetic diversity and population structure of H. sericeus in the Pacific Ocean using three gene markers – cytochrome oxidase subunit I (COI), elongation factor 1I± and internal transcribed spacer 1 (ITS‐1). These markers indicate that both populations are evolutionarily distinct with limited gene flow, having separated 20 000–50 000 years ago. This suggests that physical conditions and/or biotic interactions on the surface of the Pacific Ocean have provided significant barriers to gene flow since the late Pleistocene or earlier, creating biotic stability over large geographical and temporal scales in spite of a long history of global climate change. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 797–805.

Fingerprinting historical fluvial sediment fluxes

In order to better understand the human impact on fluvial sediment dynamics at various timescales, information on the changing connectivity between the various geomorphic units in a landscape is crucial. Quantitative sediment budgets for various time periods provide a first assessment of changes in coupling between slopes and river systems. However, the application of sediment fingerprinting can yield additional spatial information on the sediment pathways. Furthermore, the fingerprint approach can also be used in cases where detailed sediment budgets are difficult to obtain. Provenance studies have been executed in various sedimentary environments, covering a wide variety of spatial and temporal scales. Here, an overview is provided of the tracer properties that can be used to distinguish between the various sediment sources in a river catchment. From this general compilation, those tracers useful to fingerprint fine-grained floodplain deposits on a historical timescale (decadal to millennial) are identified. Geochemical and mineralogical compositions, mineral magnetic signals and isotope ratios can be considered the most suitable fingerprinting properties. The impact of source area weathering, grain size selectivity of erosion and sedimentary processes and post-depositional alteration on tracer properties from source to sink are considered. Finally, a synopsis is given of the qualitative to fully quantitative approaches that can be used to discriminate the sources of fine-grained alluvial sediments.

Aquatic Insects in Eastern Australia: A Window on Ecology and Evolution of Dispersal in Streams.

Studies of connectivity of natural populations are often conducted at different timescales. Studies that focus on contemporary timescales ask questions about dispersal abilities and dispersal behavior of their study species. In contrast, studies conducted at historical timescales are usually more focused on evolutionary or biogeographic questions. In this paper we present a synthesis of connectivity studies that have addressed both these timescales in Australian Trichoptera and Ephemeroptera. We conclude that: (1) For both groups, the major mechanism of dispersal is by adult flight, with larval drift playing a very minor role and with unusual patterns of genetic structure at fine scales explained by the “patchy recruitment hypothesis”; (2) There is some evidence presented to suggest that at slightly larger spatial scales (∼100 km) caddisflies may be slightly more connected than mayflies; (3) Examinations of three species at historical timescales showed that, in southeast Queensland Australia, despite there being no significant glaciation during the Pleistocene, there are clear impacts of Pleistocene climate changes on their genetic structure; and (4) The use of mitochondrial DNA sequence data has uncovered a number of cryptic species complexes in both trichopterans and ephemeropterans. We conclude with a number of suggestions for further work.

What can DNA tell us about biological invasions?

It is often hoped that population genetics can answer questions about the demographic and geographic dynamics of recent biological invasions. Conversely, invasions with well-known histories are sometimes billed as opportunities to test methods of population genetic inference. In both cases, underappreciated limitations constrain the usefulness of genetic methods. The most significant is that human-caused invasions have occurred on historical timescales that are orders of magnitude smaller than the timescales of mutation and genetic drift for most multicellular organisms. Analyses based on the neutral theory of molecular evolution cannot resolve such rapid dynamics. Invasion histories cannot be reconstructed in the same way as biogeographic changes occurring over millenia. Analyses assuming equilibrium between mutation, drift, gene flow, and selection will rarely be applicable, and even methods designed for explicitly non-equilibrium questions often require longer timescales than the few generations of most invasions of current concern. We identified only a few population genetic questions that are tractable over such short timescales. These include comparison of alternative hypotheses for the geographic origin of an invasion, testing for bottlenecks, and hybridization between native and invasive species. When proposing population genetic analysis of a biological invasion, we recommend that biologists ask (i) whether the questions to be addressed will materially affect management practice or policy, and (ii) whether the proposed analyses can really be expected to address important population genetic questions. Despite our own enthusiasm for population genetic research, we conclude that genetic analysis of biological invasions is justified only under exceptional circumstances.

Detailed chronology of mid-altitude fluvial system response to changing climate and societies at the end of the Little Ice Age (Southwestern Alps and Cévennes, France)

Over a historical timescale, landscapes have been strongly affected by fluctuations in climate and by the impact of human societies. This study examines the historical evolution of mid-altitude fluvial systems in the Western Alps and Cévennes (SE Massif Central) in the context of marked climate and anthropogenic change at the end of the Little Ice Age (late 19th century). This work contributes to the reconstruction of river paleodynamics by investigating the nature and chronology of geomorphological changes in upper river basins. In order to produce a detailed reconstruction of hydromorphological changes, we combined two approaches: the study of historical archives and the use of bioindicators (the dating of terraces using dendrochronology and of silt benches using lichenometry in order to reconstruct the evolution of the river channels). The 19th century is a particularly propitious period for the study of surface dynamics because archives have preserved a wealth of qualitative and quantitative data concerning rivers (economic statistics, meteorological and hydrological observations, illustrative documents, descriptions by contemporary observers). It is also a period for which reliable dating methods are available for detrital deposits in rivers. The period provides new information on how the transition between the Little Ice Age and current climate conditions affected the evolution of drainage basins and stream channels by highlighting a succession of phases in the erosive process (e.g., the preparatory role of the drought of 1830–1838 in the erosion crisis of 1855–1870) and refining the chronology of events (very early onset of riverbed incision). The results highlight the effect of climate (small hydroclimatic fluctuations), amplified by strong anthropization, on the rhythm of landscape change and on the relative stabilization of the landscape at the end of the 19th century. In addition, the synchronization of phenomena on the two sides of the Rhone Valley shows that these changes took place on a regional scale.

Contemporary and historical separation of transequatorial migration between genetically distinct seabird populations

Pelagic seabirds are highly mobile, reducing the likelihood of allopatric speciation where disruption of gene flow between populations is caused by physically insurmountable, extrinsic barriers. Spatial segregation during the non-breeding season appears to provide an intrinsic barrier to gene flow among seabird populations that otherwise occupy nearby or overlapping regions during breeding, but how this is achieved remains unclear. Here we show that the two genetically distinct populations of Cook's petrel (Pterodroma cookii) exhibit transequatorial separation of non-breeding ranges at contemporary (ca. 2-3 yrs) and historical (ca. 100 yrs) time scales. Segregation during the non-breeding season per se appears as an unlikely barrier to gene flow. Instead we provide evidence that habitat specialization during the non-breeding season is associated with breeding asynchrony which, in conjunction with philopatry, restricts gene flow. Habitat specialization during breeding and non-breeding likely promotes evolutionary divergence between these two populations via local adaptation.

Niche as a Determinant of Word Fate in Online Groups

Patterns of word use both reflect and influence a myriad of human activities and interactions. Like other entities that are reproduced and evolve, words rise or decline depending upon a complex interplay between their intrinsic properties and the environments in which they function. Using Internet discussion communities as model systems, we define the concept of a word niche as the relationship between the word and the characteristic features of the environments in which it is used. We develop a method to quantify two important aspects of the size of the word niche: the range of individuals using the word and the range of topics it is used to discuss. Controlling for word frequency, we show that these aspects of the word niche are strong determinants of changes in word frequency. Previous studies have already indicated that word frequency itself is a correlate of word success at historical time scales. Our analysis of changes in word frequencies over time reveals that the relative sizes of word niches are far more important than word frequencies in the dynamics of the entire vocabulary at shorter time scales, as the language adapts to new concepts and social groupings. We also distinguish endogenous versus exogenous factors as additional contributors to the fates of words, and demonstrate the force of this distinction in the rise of novel words. Our results indicate that short-term nonstationarity in word statistics is strongly driven by individual proclivities, including inclinations to provide novel information and to project a distinctive social identity.

A multi‐century record of linked nearshore and coastal change

ABSTRACTUsing geographic information system (GIS) and field measurements the nearshore morphological variability of a headland bay beach at Tenby, West Wales (51·66 N; –4·71 W) was assessed over historical timeframes (1748–2007). Three areas chosen for detailed analysis were the area between mean low water (MLW) and lowest astronomical tide (LAT) contours; LAT and one fathom contours; and one and two fathom contours. Estuary closurec. 1855 has been suggested as the genesis for long‐term beach evolution and did have an initial effect, with northward dune migration and reduced flushing effects. However, this research suggests nearshore bank migration and retrogradation associated with spit collapse took place prior to closure and continued throughout the assessed timeframe. Historical data revealed patterns of shoreward migration demonstrated by changes in orientation, Giltar headland acting as a pivot. Variations in sandbank position correlated with areal reduction of both Giltar spit and White Bank. Temporal offshore areal loss was contrasted against variable gains inshore as offshore banks welded to the beachface. Annual volumetric change analyses represented by profiles that extend 1 km offshore, confirmed Giltar spit and White Bank erosion rates of 91 m3 yr–1and 458 m3 yr–1respectively, and 220 m3 yr–1beachface accretion. Diminution of sediment supply observed over historical timescales was supported by decadal evidence. Here, profile analysis revealed a trend of decreasing volumes both updrift and within the study area, and increasing downdrift volumes. This explained why proximal detachment and sediment redistribution had occurred. Distinct reversal's in shoreline trend (rotation) corresponded to nearshore change; therefore, variations in seabed configuration triggered shoreface dynamic change over century timescales. Five‐year cumulative average changes in North Atlantic Oscillation were further correlated to this reversal. As comparable scenarios are likely to exist at other worldwide coastal locations, similar analyses should be incorporated into shoreline monitoring programmes. Consequently, these assessments would inform shoreline trends and support coastal management decisions. Copyright © 2011 John Wiley & Sons, Ltd.

Modelling the effect of directional spatial ecological processes at different scales

During the last 20 years, ecologists discovered the importance of including spatial relationships in models of species distributions. Among the latest developments in modelling how species are spatially structured are eigenfunction-based spatial filtering methods such as Moran's eigenvector maps (MEM) and principal coordinates of neighbour matrices (PCNM). Although these methods are very powerful and flexible, they are only suited to study distributions resulting from non-directional spatial processes. The asymmetric eigenvector map (AEM) framework, a new eigenfunction-based spatial filtering method, fills this theoretical gap. AEM was specifically designed to model spatial structures hypothesized to be produced by directional spatial processes. Water currents, prevailing wind on mountainsides, river networks, and glaciations at historical time scales are some of the situations where AEM can be used. This paper presents three applications of the method illustrating different combinations of: sampling schemes (regular and irregular), data types (univariate and multivariate), and spatial scales (metres, kilometres, and hundreds of kilometres). The applications include the distribution of a crustacean (Atya) in a river, bacterial production in a lake, and the distribution of the copepodite stages of a crustacean on the Atlantic oceanic shelf. In each application, a comparison is made between AEM, MEM, and PCNM. No environmental components were included in the comparisons. AEM was a strong predictor in all cases, explaining 59.8% for Atya distribution, 51.4% of the bacterial production variation, and 38.4% for the copepodite distributions. AEM outperformed MEM and PCNM in these applications, offering a powerful and more appropriate tool for spatial modelling of species distributions under directional forcing and leading to a better understanding of the processes at work in these systems.

Does reduced MHC diversity decrease viability of vertebrate populations?

Loss of genetic variation may render populations more vulnerable to pathogens due to inbreeding depression and depletion of variation in genes responsible for immunity against parasites. Here we review the evidence for the significance of variation in genes of the Major Histocompatibility Complex (MHC) for conservation efforts. MHC molecules present pathogen-derived antigens to the effector cells of the immune system and thus trigger the adaptive immune response. Some MHC genes are the most variable functional genes in the vertebrate genome. Their variation is clearly of adaptive significance and there is considerable evidence that its maintenance is mainly due to balancing selection imposed by pathogens. However, while the evidence for selection shaping MHC variation on the historical timescale is compelling, a correlation between levels of MHC variation and variation at neutral loci is often observed, indicating that on a shorter timescale drift also substantially affects MHC, leading to depletion of MHC diversity. The evidence that the loss of MHC variation negatively affects population survival is so far equivocal and difficult to separate from effects of general inbreeding. Some species with depleted MHC variation seem to be particularly susceptible to infection, but other species thrive and expand following severe bottlenecks that have drastically limited their MHC variation. However, while the latter demonstrate that MHC variation is not always critical for population survival, these species may in fact represent rare examples of survival despite of the loss of MHC variation. There is clearly a compelling need for data that would disclose the possible consequences of MHC diversity for population viability. In particular, we need more data on the impact of MHC allelic richness on the abundance of parasites or prevalence of disease in populations, while controlling for the role of general inbreeding. Before such evidence accumulates, captive breeding programs and other conservation measures aimed at inbreeding avoidance should be favoured over those protecting only MHC variation, especially since inbreeding avoidance programs would usually conserve both types of genetic diversity simultaneously.

Invasive species can't cover their tracks: using microsatellites to assist management of starling (Sturnus vulgaris) populations in Western Australia

Invasive species are known to cause environmental and economic damage, requiring management by control agencies worldwide. These species often become well established in new environments long before their detection, resulting in a lack of knowledge regarding their history and dynamics. When new invasions are discovered, information regarding the source and pathway of the invasion, and the degree of connectivity with other populations can greatly benefit management strategies. Here we use invasive common starling (Sturnus vulgaris) populations from Australia to demonstrate that genetic techniques can provide this information to aid management, even when applied to highly vagile species over continental scales. Analysis of data from 11 microsatellites in 662 individuals sampled at 17 localities across their introduced range in Australia revealed four populations. One population consisted of all sampling sites from the expansion front in Western Australia, where control efforts are focused. Despite evidence of genetic exchange over both contemporary and historical timescales, gene flow is low between this population and all three more easterly populations. This suggests that localized control of starlings on the expansion front may be an achievable goal and the long-standing practice of targeting select proximal eastern source populations may be ineffective on its own. However, even with low levels of gene flow, successful control of starlings on the expansion front will require vigilance, and genetic monitoring of this population can provide essential information to managers. The techniques used here are broadly applicable to invasive populations worldwide.

Multi-disciplinary approach for sediment dynamics study of active floodplains

This study reviews multi-disciplinary approaches developed on active floodplains in order to formulate their evolution model and sediment budget, at different time scales, from historical evolution to hydrological event, and spatial scales (floodplain, active channels and fluvial units). The main objective of the different case studies is to emphasize the need for coupling tools and methods in analysing active floodplain patterns, deposition and erosion rates with regards to hydrological functioning and impact of human activities. The first part of the paper is dedicated to methods that help us to characterize the construction of floodplains at a historical time-scale. The second part examines the functioning of active floodplains at a detailed spatial and time scales. Different case studies selected in European and American floodplains aim at highlighting the contribution and the limits of methods developed at various temporal and spatial scales. Multi-temporal analysis of fluvial planform allows to determine the recycling time of a floodplain (Beni River in Amazonia) or to evaluate the impact of a large flood (Rhone River). Historical sedimentation of floodplain may be estimated on the basis of specific elements present in sediments such as ironwork waste products in Belgian rivers Different methods are used to evaluate sediment remobilization in fluvial beds at different time scales. Bathymetric and topographic surveys, digital elevation models document vertical movements of the fluvial bed (Beni, Moselle and Loire rivers) at pluri-annual and annual scales; they allow the calculation of sediment budgets. At a longer time scale, bedload progression has been evaluated in Ardenne rivers using slag elements.

A new approach and case study for estimating extent and rates of habitat loss for ecological communities

The pace and magnitude of biodiversity loss has led to wide recognition that efforts to conserve individual species must be complemented by assessment and planning at community and ecosystem levels. Emerging protocols for assessing the conservation status of communities include as central criteria the current extent, historical reduction and contemporary rate of decline in geographic distribution. Estimation of these parameters is confronted by methodological challenges, data limitations and uncertainties that may vary from case to case. We describe an approach to these issues comprising five steps: (1) classification of the community using an analysis of data from systematic ground surveys; (2) mapping to produce a contemporary baseline distribution (1980s); (3) interpolation to produce a historical distribution (pre-European settlement); (4) interpretation of satellite imagery to update the distribution (various dates up to 2004) and (5) assessment of change in extent over historical and contemporary time scales incorporating plausible bounds of uncertainty around best estimates. The bounds can be based on areas for which image interpretation produces uncertain diagnosis of clearing and differences between credible alternative base maps of the same area. We demonstrated the approach using a case study of Coolibah–Black Box Woodland, a declining semi-arid woodland community in Australia and found that 61% (plausible range 50–67%) of the woodland community had been cleared since European settlement and that during 1998–2004 the community continued to decline on average by 135.3(±21.7) km 2 each year, or 1.7(±0.3)% of each previous year’s distribution, apparently accelerating in recent years. Strengths of the approach include the use of biological data (cf. remote sensing) to distinguish the target assemblage from others, the use of historical and contemporary base lines to examine change over different time scales, and the use of bounded estimates to incorporate uncertainty into the assessment.

Coastal geomorphology: trends and challenges

This review highlights recent work in coastal geomorphology that challenges existing wisdom, breaks new ground or contributes in some way to wider principles. Sediment transport by currents and by waves remains a major focus of coastal process geomorphology, and the contribution of this work to our appreciation of landform dynamics is particularly evident at the beach scale. Reconstructive studies over Holocene to historical timescales are yielding new insights into the adjustment of coastal sedimentary systems to progressive environmental forcing, high-magnitude events and human activities. Coastal geomorphology is currently buoyed by opportunities afforded by the implied threats of climate change and sea-level rise, and by the conceptual and technical challenges of predicting morphodynamic behaviour at the all-important `engineering' scales. Of some concern is the lack of dialogue with other major strands of geomorphology. One of the main challenges, to create models capable of revealing emergent aspects of spatially distributed interactions between form and process, is common to all geomorphologists engaged with the meso-scale. Yet coastal specialists seem reluctant both to draw inspiration from and also to contribute ideas to what might be seen as a single, extremely broad, research front. More generally, there is little exchange of ideas or contribution to a shared intellectual endeavour. Coastal geomorphology is at the forefront of the discipline when it comes to the exploitation of new environmental instrumentation and remote sensing technologies. It can only be hoped that the plethora of high-resolution multitemporal data sets will contribute to the advance of geomorphological thinking rather than merely being used to constrain sediment budgets as a basis for the management of localized erosion problems.

Les conditions climatiques de la torrentialité au cours du Petit Age Glaciaire en Provence

The aim of this article is to characterize the climatic conditions of torrentiality in Provence during the Little Ice Age. The study is based on chronicles of floods and overflowings of the sub-alpine torrents (the Ubaye valley in particular) and on instrumental measures of rainfalls in Marseilles. Several periods marked by frequent floods in the Durancian hydrosystem follow periods of relative tranquillity. This study shows that streams, according to their rank, do not have the same regimes of floods, so testifying of different meteorological forcing, that the frequent occurrence of floods during the Little Ice Age is not linked to a global increase of pluviometrical but to an increased variability of interannual rains, periods of more frequent floods corresponding to series of rainy years (1750-1800, 1830-1860) and periods of rarefaction of floods corresponding to series of dry years (1800-1830). The crossing of these data with other paleoclimatic indicators such as the series of growth rings of mountain larches offers perspectives to characterize the regional nuances of the Provençal climate on the historical time scale.

Application of self-absorption correction method in gamma spectroscopy for210Pb and137Cs sediment chronology on the continental slope off NW Africa

The uppermost 50cm of the high resolution core GeoB9501-4 from the continental slope off NW Africa has been used for improving routine technique of 210 Pb and 137 Cs sediment chronology in the Radioactivity measurement laboratory of the Bremen University. The experimental requirements for 210 Pb and 137 Cs determination were non-destructive analysis of wet sediment samples in cylindrical containers with different heights, unknown elemental composition and density. The method used in this study to deal with self attenuation is applying efficiencies calculated using the LabSOCS TM , Genie 2000 software calibration tool, validated by self-absorption test measurements of different materials with known composition. A mean sedimentation rate of 0.49cm · yr −1 was obtained from the investigated core using the CRS chronological model. This means a relatively increased recent sedimentation rate comparing to an average sedimentation rate at higher depths of an associated gravity core estimated to 0.15cm · yr −1 by 14 C chronology. A further increase of the sedimentation rate was observed in the uppermost part of the studied core (up to 0.90cm · yr −1 ). Life in the semiarid Sahel belt of tropical North Africa strongly depends on the availability of water and has been repeatedly affected by shifts to more arid climate. The most recent drought occurred in the early 1970's and 1980's, with partial recovery during the late 1990's. High resolution fluvial sediments off Senegal offer the opportunity to study the history of the Sahel drought and to assess its imprint on the composition of terrestrial materials deposited at the sea floor, if the material can be accurately dated on historical time scales.

Loss of Large Predatory Sharks from the Mediterranean Sea

Evidence for severe declines in large predatory fishes is increasing around the world. Because of its long history of intense fishing, the Mediterranean Sea offers a unique perspective on fish population declines over historical timescales. We used a diverse set of records dating back to the early 19th and mid 20th century to reconstruct long-term population trends of large predatory sharks in the northwestern Mediterranean Sea. We compiled 9 time series of abundance indices from commercial and recreational fishery landings, scientific surveys, and sighting records. Generalized linear models were used to extract instantaneous rates of change from each data set, and a meta-analysis was conducted to compare population trends. Only 5 of the 20 species we considered had sufficient records for analysis. Hammerhead (Sphyrna spp.), blue (Prionace glauca), mackerel (Isurus oxyrinchus and Lamna nasus), and thresher sharks (Alopias vulpinus) declined between 96 and 99.99% relative to their former abundance. According to World Conservation Union (IUCN) criteria, these species would be considered critically endangered. So far, the lack of quantitative population assessments has impeded shark conservation in the Mediterranean Sea. Our study fills this critical information gap, suggesting that current levels of exploitation put large sharks at risk of extinction in the Mediterranean Sea. Possible ecosystem effects of these losses involve a disruption of top-down control and a release of midlevel consumers.

Integrating paleoecology and genetics of bird populations in two sky island archipelagos

BackgroundGenetic tests of paleoecological hypotheses have been rare, partly because recent genetic divergence is difficult to detect and time. According to fossil plant data, continuous woodland in the southwestern USA and northern Mexico became fragmented during the last 10,000 years, as warming caused cool-adapted species to retreat to high elevations. Most genetic studies of resulting 'sky islands' have either failed to detect recent divergence or have found discordant evidence for ancient divergence. We test this paleoecological hypothesis for the region with intraspecific mitochondrial DNA and microsatellite data from sky-island populations of a sedentary bird, the Mexican jay (Aphelocoma ultramarina). We predicted that populations on different sky islands would share common, ancestral alleles that existed during the last glaciation, but that populations on each sky island, owing to their isolation, would contain unique variants of postglacial origin. We also predicted that divergence times estimated from corrected genetic distance and a coalescence model would post-date the last glacial maximum.ResultsOur results provide multiple independent lines of support for postglacial divergence, with the predicted pattern of shared and unique mitochondrial DNA haplotypes appearing in two independent sky-island archipelagos, and most estimates of divergence time based on corrected genetic distance post-dating the last glacial maximum. Likewise, an isolation model based on multilocus gene coalescence indicated postglacial divergence of five pairs of sky islands. In contrast to their similar recent histories, the two archipelagos had dissimilar historical patterns in that sky islands in Arizona showed evidence for older divergence, suggesting different responses to the last glaciation.ConclusionThis study is one of the first to provide explicit support from genetic data for a postglacial divergence scenario predicted by one of the best paleoecological records in the world. Our results demonstrate that sky islands act as generators of genetic diversity at both recent and historical timescales and underscore the importance of thorough sampling and the use of loci with fast mutation rates to studies that test hypotheses concerning recent genetic divergence.