Composition of 'fast-slow' traits drives avian community stability over North America

1. Rapid biodiversity loss has triggered decades of research on the relationships between biodiversity and community stability. Recent studies highlighted the importance of species traits for understanding biodiversity-stability relationships. The species with high growth rates ('fast' species) are expected to be less resistant to environmental stress but recover faster if disturbed; in contrast, the species with slow growth rates ('slow' species) can be more resistant but recover more slowly if disturbed. Such a 'fast-slow' trait continuum provides a new perspective for understanding community stability, but its validity has mainly been examined in plant communities. Here, we investigate how 'fast-slow' trait composition, together with species richness and environmental factors, regulate avian community stability at a continental scale. 2. We used bird population records from the North American Breeding Bird Survey during 1988-2017 and defined avian community stability as the temporal invariability of total community biomass. We calculated species richness and the community-weighted mean (CWM) and functional diversity (FD) of four key life-history traits, including body size, nestling period (i.e. period of egg incubation and young bird fledging), life span and clutch size (i.e. annual total number of eggs). Environmental factors included temperature, precipitation and leaf area index (LAI). 3. Our analyses showed that avian community stability was mainly driven by the CWM of the 'fast-slow' trait. Communities dominated by 'fast' species (i.e. species with small body size, short nestling period and life span and large clutch size) were more stable than those dominated by 'slow' species (i.e. species with large body size, long nestling period and life span and small clutch size). Species richness and the FD of the 'fast-slow' trait explained much smaller proportions of variation in avian community stability. Temperature had direct positive effects on avian community stability, while precipitation and leaf area index affected community stability indirectly by influencing species richness and trait composition. 4. Our study demonstrates that composition of 'fast-slow' traits is the major biotic driver of avian community stability over North America. Temperature is the most important abiotic factor, but its effect is weaker than that of the 'fast-slow' trait. An integrated framework combining 'fast-slow' trait composition and temperature is needed to understand the response of avian communities in a changing environment.Peer reviewe

Self-excitation and energy recovery of air-core compulsators

As power supplies, compulsators are popular choices for high-end railgun power supplies. In order to increase power and energy density, air-core compulsators are proposed by using composite materials instead of traditional iron-core compulsators. Due to the absence of ferromagnetic material, the flux density in the air-core compulsator can reach to 4–6 T instantaneously, which is much higher than the saturation field strength in traditional iron-core machines. Therefore, self-excitation topology is essential for the air-core compulsator to obtain up to 100-kA field current. This paper carried out research on the key parameters of self-excitation efficiency first, and then focus on the large magnetic energy remained in the inductive field winding after one shot, an implementation scheme and control strategy of energy recovery of air-core compulsator was proposed and analyzed. By controlling the field rectifier working at active inverter state after one discharge process, the magnetic energy stored in the field winding can be converted to rotor kinetic energy again. The simulation results indicate that the energy recovery efficiency can reach to 70% for a reference air-core compulsator. The continuous discharge number of times increased from 3 to 4 during one kinetic energy charging, which means that the delivered energy density increases 33.3%

A fractional slot multiphase air-core compulsator with concentrated winding

Compulsator is a specially designed generator capa¬ble of delivering high current pulses to a low-impedance load, such as the electromagnetic railgun. In order to increase the tip speed of the rotor, advanced composite materials have been used in the recent compulsator prototype, which is mentioned as air core instead of the traditional iron core. For typical air-core compulsators, there are no slots and steel teeth to place the armature windings due to the nonmachinability of composite materials. Therefore, concentric windings in racetrack style are often adopted instead of traditional lap winding in most cases, since it is more convenient to be fixed by composite materials. However, overlap occurs at the end winding part for multiphase compulsators, which are not easy to be formed during the manufacture process. In this paper, a fractional slot multiphase air-core compulsator with concentrated windings is proposed and analyzed. The main advantage of fractional slot configuration is that it can offer a concentrated winding structure under certain conditions, which means each coil only spans one “tooth,” and will not cause any intersection between each phase at the end winding. Two referenced fractional slot air-core compulsators with two phases, six poles, and four “slots” or eight “slots” (q = 1/3 and q = 2/3, q is the “slot” per pole per phase) are analyzed and compared with the performance of a traditional integral slot machine. The results indicated that the output voltage and self-excitation performance of a fractional slot compulsator can reach the same level with an integral slot one, and the discharging performance can reach an acceptable level. Thus, the fractional slot multiphase concept can be further used to improve the manufacture process of the winding in the future

Functional traits and environment jointly determine the spatial scaling of population stability in North American birds

Understanding the spatial scaling of population stability is critical for informing conservation strategies. A recently proposed metric for quantifying how population stability varies across scales is the invariability-area relationship (IAR), but the underlying drivers shaping IARs remain unclear. Using 15-year records of 249 bird species in 1035 survey transects in North America, we derived the IAR for each species by calculating population temporal invariability at different spatial scales (i.e., number of routes) and investigated how species IARs were influenced by functional traits and environmental factors. We found that species with faster life history traits and reduced flight efficiency had higher IAR intercepts (i.e., locally more stable), whereas migratory species exhibited higher IAR slopes (i.e., a faster gain of stability with increasing spatial scale). In addition, spatial correlation in temperature and vegetation structure synchronized bird population dynamics over space and thus decreased IAR slopes. By demonstrating the joint influence of functional traits and environmental factors on bird population stability across scales, our results highlight the need for dynamic conservation strategies tailored to particular types of species in an era of global environmental changes.Peer reviewe

Wetting of bio-rejuvenator nanodroplets on bitumen: A molecular dynamics investigation

Wetting is the first step during the mix process between rejuvenator and bitumen, which is important for mix efficiency and performance recovery. The wetting of bio-rejuvenator nanodroplets on bitumen was investigated by molecular dynamics (MD) simulations in this research. The bitumen molecule model and bio-rejuvenator nanodroplets were firstly built, then bio-rejuvenator nanodroplets/bitumen interface wetting model were assembled and constructed. Different simulated temperatures were applied to reach equilibrium in the wetting process. Dynamic wetting phenomenon, contact angle of nanodroplets, dynamic movement of nanodroplets, interaction between nanodroplets and bitumen, and hysteresis of contact angle were characterized respectively. The results show that the bio-rejuvenator nanodroplets will first approach the bitumen quickly, and then slow down to an equilibrium state in the wetting process, which delayed 1 ns with energy equilibrium independently. Its contact angle would decrease crossing 90° with time, the equilibrium contact angle of which varies linearly with simulated temperature. The time of nanodroplets reaching partial wetting state decreased with the increments of temperature, but complete wetting state was hard to reach even if the temperature was 433 K. During the nanodroplets movement, contact linear velocity of precursor film and cosine of contact angle was linearly related after nanodroplets and bitumen had caught each other. It was also found that the increasing mix degree was supported by the combination of wetting and infiltration before 373 K and by wetting mainly after 373 K. Finally, the application of external force on bio-rejuvenator nanodroplets will cause hysteresis phenomenon and it can be weakened by higher temperature

A molecular dynamics analysis of the influence of iron corrosion products on the healing process of bitumen

Corrosion of iron materials in the asphalt concrete pavement occurs commonly when the bitumen film peels off, and the generation of corrosion products would affect the healing performance of bitumen. To identify the affection, this research focuses on the influence of iron corrosion products on the healing process of bitumen by molecular dynamics simulation. Firstly, bitumen model and iron corrosion products model were built. Then the healing systems of sandwich structure were constructed, and the simulated temperature were applied to reach equilibrium in the healing process with NVT ensemble (constant number of atoms, volume, and temperature). Dynamic movements of bitumen were characterized by appearance qualitatively. Healing rate of crack and healing rate of bitumen aggregation were held to evaluate the healing effect. Diffusion behaviors, internal force of motivation and interaction effect were also analyzed. The results indicate the duplicity of iron corrosion products in the healing process including the ease for bitumen climbing and the obstruction of bitumen movement. The comprehensive healing index demonstrated that iron corrosion products would reduce the healing degree, which was mainly caused by the obstruction effect and large internal stress generated by severe aggregation of bitumen in the limited space. From the perspective of crack closure and bitumen aggregation degree in the corrosion area, FeO healing systems were healed best, followed by Fe3O4, Fe2O3 and FeOOH. Furthermore, diffusion period of bitumen molecules on the surface of iron corrosion products during the healing process should be regarded as the important period affecting healing

Horizontal and vertical diversity jointly shape food web stability against small and large perturbations

The biodiversity of food webs is composed of horizontal (i.e. within trophic levels) and vertical diversity (i.e. the number of trophic levels). Understanding their joint effect on stability is a key challenge. Theory mostly considers their individual effects and focuses on small perturbations near equilibrium in hypothetical food webs. Here, we study the joint effects of horizontal and vertical diversity on the stability of hypothetical (modelled) and empirical food webs. In modelled food webs, horizontal and vertical diversity increased and decreased stability, respectively, with a stronger positive effect of producer diversity on stability at higher consumer diversity. Experiments with an empirical plankton food-web, where we manipulated horizontal and vertical diversity and measured stability from species interactions and from resilience against large perturbations, confirmed these predictions. Taken together, our findings highlight the need to conserve horizontal biodiversity at different trophic levels to ensure stability

Interfacial characteristics between bitumen and corrosion products on steel slag surface from molecular scale

Corrosion commonly happened on the surface of steel slag during the weathering and accumulation process, whose products would form weak points and affect the interface between bitumen and steel slag. To clear its characteristics in the atomic scales, the interface between bitumen and corrosion products was investigated by molecular dynamics (MD) simulations. Firstly, bitumen model, corrosion products model and bitumen-corrosion products systems were constructed. Different simulated temperatures were applied on the systems to reach equilibrium with NVT (constant number of atoms, volume, and temperature) ensemble. The interaction effect in the interface were evaluated by geometric adsorption index, interaction energy, adhesion work and surface free energy. Diffusion coefficient and relative concentration were used to evaluate the diffusion and aggregation. Finally, the pull-out test was conducted on the equilibrium models to determine the debonding behaviors at the interface. The results show that the interaction effect in Bitumen-FeO system was the strongest while that in Bitumen-FeOOH system was the weakest, which can be proved by surface free energy and debonding behaviors. The temperature changing would affect van der Waals energy but had no obvious association with coulombic energy. The adhesion between bitumen and corrosion products was contributed by non-bond interaction energy which consisted of van der Waals interaction for Fe3O4, Fe2O3 and FeOOH, and van der Waals and electrostatic interaction for FeO. The most severe aggregation of bitumen occurred in Bitumen-FeO system, which was more likely caused by electrostatic interaction. Furthermore, the change of velocity and thickness led to the failure transformation from cohesion to adhesion. The strong interaction in Bitumen-FeO system increase the possibility of cohesion failure in the debonding process