Why large-scale climate indices seem to predict ecological processes better than local weather

Large-scale climatic indices such as the North Atlantic Oscillation are associated with population dynamics, variation in demographic rates and values of phenotypic traits in many species. Paradoxically, these large-scale indices can seem to be better predictors of ecological processes than local climate. Using detailed data from a population of Soay sheep, we show that high rainfall, high winds or low temperatures at any time during a 3-month period can cause mortality either immediately or lagged by a few days. Most measures of local climate used by ecologists fail to capture such complex associations between weather and ecological process, and this may help to explain why large-scale, seasonal indices of climate spanning several months can outperform local climatic factors. Furthermore, we show why an understanding of the mechanism by which climate influences population ecology is important. Through simulation we demonstrate that the timing of bad weather within a period of mortality can have an important modifying influence on intraspecific competition for food, revealing an interaction between climate and density dependence that the use of large-scale climatic indices or inappropriate local weather variables might obscure.     

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.     

New evidence on the earliest human presence at high northern latitudes in northeast Asia

The timing of early human dispersal to Asia is a central issue in the study of human evolution. Excavations in predominantly lacustrine sediments at Majuangou, Nihewan basin, north China, uncovered four layers of indisputable hominin stone tools. Here we report magnetostratigraphic results that constrain the age of the four artefact layers to an interval of nearly 340,000 yr between the Olduvai subchron and the Cobb Mountain event. The lowest layer, about 1.66 million years old (Myr), provides the oldest record of stone-tool processing of animal tissues in east Asia. The highest layer, at about 1.32 Myr, correlates with the stone tool layer at Xiaochangliang, previously considered the oldest archaeological site in this region. The findings at Majuangou indicate that the oldest known human presence in northeast Asia at 40 degrees N is only slightly younger than that in western Asia. This result implies that a long yet rapid migration from Africa, possibly initiated during a phase of warm climate, enabled early human populations to inhabit northern latitudes of east Asia over a prolonged period.     

Somatic diversification of variable lymphocyte receptors in the agnathan sea lamprey

Although jawless vertebrates are apparently capable of adaptive immune responses, they have not been found to possess the recombinatorial antigen receptors shared by all jawed vertebrates. Our search for the phylogenetic roots of adaptive immunity in the lamprey has instead identified a new type of variable lymphocyte receptors (VLRs) composed of highly diverse leucine-rich repeats (LRR) sandwiched between amino- and carboxy-terminal LRRs. An invariant stalk region tethers the VLRs to the cell surface by means of a glycosyl-phosphatidyl-inositol anchor. To generate rearranged VLR genes of the diversity necessary for an anticipatory immune system, the single lamprey VLR locus contains a large bank of diverse LRR cassettes, available for insertion into an incomplete germline VLR gene. Individual lymphocytes express a uniquely rearranged VLR gene in monoallelic fashion. Different evolutionary strategies were thus used to generate highly diverse lymphocyte receptors through rearrangement of LRR modules in agnathans (jawless fish) and of immunoglobulin gene segments in gnathostomes (jawed vertebrates).     

Quantum magnetic excitations from stripes in copper oxide superconductors

In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized--one per copper site--by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.     

A frequency-dependent switch from inhibition to excitation in a hippocampal unitary circuit

The hippocampus, a brain structure essential for memory and cognition, is classically represented as a trisynaptic excitatory circuit. Recent findings challenge this view, particularly with regard to the mossy fibre input to CA3, the second synapse in the trisynaptic pathway. Thus, the powerful mossy fibre input to CA3 pyramidal cells might mediate both synaptic excitation and inhibition. Here we show, by recording from connected cell pairs in rat entorhinal-hippocampal slice cultures, that single action potentials in a dentate granule cell evoke a net inhibitory signal in a pyramidal cell. The hyperpolarization is due to disynaptic feedforward inhibition, which overwhelms monosynaptic excitation. Interestingly, this net inhibitory synaptic response changes to an excitatory signal when the frequency of presynaptic action potentials increases. The process responsible for this switch involves the facilitation of monosynaptic excitatory transmission coupled with rapid depression of inhibitory circuits. This ability to immediately switch the polarity of synaptic responses constitutes a novel synaptic mechanism, which might be crucial to the state-dependent processing of information in associative hippocampal networks.     

Coral mucus functions as an energy carrier and particle trap in the reef ecosystem

Zooxanthellae, endosymbiotic algae of reef-building corals, substantially contribute to the high gross primary production of coral reefs, but corals exude up to half of the carbon assimilated by their zooxanthellae as mucus. Here we show that released coral mucus efficiently traps organic matter from the water column and rapidly carries energy and nutrients to the reef lagoon sediment, which acts as a biocatalytic mineralizing filter. In the Great Barrier Reef, the dominant genus of hard corals, Acropora, exudes up to 4.8 litres of mucus per square metre of reef area per day. Between 56% and 80% of this mucus dissolves in the reef water, which is filtered through the lagoon sands. Here, coral mucus is degraded at a turnover rate of at least 7% per hour. Detached undissolved mucus traps suspended particles, increasing its initial organic carbon and nitrogen content by three orders of magnitude within 2 h. Tidal currents concentrate these mucus aggregates into the lagoon, where they rapidly settle. Coral mucus provides light energy harvested by the zooxanthellae and trapped particles to the heterotrophic reef community, thereby establishing a recycling loop that supports benthic life, while reducing loss of energy and nutrients from the reef ecosystem.     

Cavity cooling of a single atom

All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom-cavity systems for quantum information processing. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules (which do not have a closed transition) and collective excitations of Bose condensates, which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom.