M.J. Adams

Quantitative evidence for the effects of multiple drivers on continental-scale amphibian declines

Since amphibian declines were first proposed as a global phenomenon over a quarter century ago, the conservation community has made little progress in halting or reversing these trends. The early search for a “smoking gun” was replaced with the expectation that declines are caused by multiple drivers. While field observations and experiments have identified factors leading to increased local extinction risk, evidence for effects of these drivers is lacking at large spatial scales. Here, we use observations of 389 time-series of 83 species and complexes from 61 study areas across North America to test the effects of 4 of the major hypothesized drivers of declines. While we find that local amphibian populations are being lost from metapopulations at an average rate of 3.79% per year, these declines are not related to any particular threat at the continental scale; likewise the effect of each stressor is variable at regional scales. This result - that exposure to threats varies spatially, and populations vary in their response - provides little generality in the development of conservation strategies. Greater emphasis on local solutions to this globally shared phenomenon is needed.

Discrete particle simulation of agglomerate impact coalescence

Abstract This paper describes computer simulations of pendular state wet agglomerates undergoing pair-wise collisions. The simulation method is based upon a ‘soft’ discrete particle formulation. Each agglomerate comprised 1000 primary particles with the interparticle interactions modelled as the combination of the solid–solid contact forces and also the forces developed at discrete liquid bridges between neighbouring particles. For the range of collisional velocities implemented, the agglomerates invariably coalesced. The energy dissipated was associated primarily with the viscous resistance of the fluid and the interparticle friction rather than by liquid bridge bond rupture. The structure of the resultant coalesced agglomerate was highly disordered and depended on the impact velocity. As the impact velocity approached zero, the agglomerates behaved like two rigid bodies bonded together. When the impact velocity was increased, the size of the circumscribing sphere of the coalesced agglomerate decreased and reached a minimum value at a critical velocity above which an increase in the circumscribing sphere size occurred due to extensive flattening. An increase in the viscosity of the interstitial fluid resulted in an increase in the proportion of energy dissipated by viscous resistance and a decrease in the proportion dissipated due to interparticle friction. An increase in the fluid viscosity also resulted in an increase in the critical impact velocity at which the size of the circumscribing sphere of the coalesced agglomerate was a minimum.

The effect of indenter geometry on the elastic response to indentation

The paper describes various analytical procedures that account for the influence of the geometry of a rigid indenter upon the measured contact compliance of a smooth perfectly elastic half space. The analytical solutions provide a means of interrelating the reaction force, P, and the displacement, h, characteristics in terms of the contact geometry and the reduced elastic modulus E*. The general form is P=gE*hn, where g and n are functions only of the indenter geometry. This relationship is incorporated into a curve-fitting procedure and used to evaluate the influence of the indenter geometry upon the computed modulus of a poly(isobutylene) rubber. The method described provides a viable means of incorporating the imperfections, associated with the geometry of an indenter, into the interpretation of contact compliance data.

A comparison of active and passive optical bistability in semiconductors

A comparison is presented between optical bistability in laser amplifiers and in passive Fabry-Perot cavities. The basis for comparison is afforded by a new analysis of optical amplifiers which encompasses the cases of passive refractive and absorptive bistability as special limiting cases. The results indicate that amplifiers have advantages of lower input intensity requirements (by a factor of 103) and reduced sensitivity to wavelength by comparison with passive cavities; experimental results indicate an input power of -30 dBm is required for active bistability. Facet coating requirements for active and passive optimum configurations are also discussed.

Nonlinear dynamics induced by parallel and orthogonal optical injection in 1550 nm Vertical-Cavity Surface-Emitting Lasers (VCSELs)

We report a first experimental study of the nonlinear dynamics appearing in a 1550 nm single-mode VCSEL subject to parallel and to orthogonal optical injection. For the first time to our knowledge we report experimentally measured stability maps identifying the boundaries between regions of different nonlinear dynamics for both cases of polarized injection. A rich variety of nonlinear behaviours, including periodic (limit cycle, period doubling) and chaotic dynamics have been experimentally observed.

Determination of optical fiber refractive index profiles by a near‐field scanning technique

A simple and rapid method is described for determining the refractive index profile of an optical fiber by observation of the near‐field intensity distribution. It is shown that in many cases the presence of tunnelling leaky modes is unavoidable and that these cause a length‐dependent error in the measurement. A correction factor is developed which may be applied to the measured intensity profile once the fiber length, core diameter, and numerical aperture are known. Examples are given of measurements made on both step and graded‐index fibers.

Performance predictions from a new optical amplifier model

We present results of a new model of semiconductor laser amplifiers which differs from previous analyses in that it includes the spectral dependence of material gain and spontaneous emission. The implications of low facet reflectivities are explored in some detail. For reflectivities below about 1 percent, the increased spontaneous emission imposes more stringent limits on current density than realized hitherto. If thermal runaway is to be avoided and gains in the range of 20-30 dB are to be achieved without excessive currents, then facet reflectivities on the order of 0.1-1 percent are probably optimal. Another consequence of including the spectral dependence is that wavelengths longer than that corresponding to the unsaturated gain peak are predicted to experience enhanced amplification at high input powers by comparison to shorter wavelengths.

The friction and lubrication of human stratum corneum.

In vivo measurements are reported of the lubrication imparted by water, various aqueous media, and some hydrophobic oils, to a glass probe sliding on the inner human forearm. The primary aim of the paper is to show that the tribology of the human skin surface may be satisfactorily interpreted on the basis of precedents developed for the rationalization of frictional data for elastomers and thermoplastics, and selected data are interpreted on this basis. Water itself plasticises the stratum corneum and, consequently, has profound effects on the frictional behaviour. The effective lubrication observed in surfactant solutions can be explained in terms of electrostatic double-layer repulsion between charged layers adsorbed on the contacting surfaces. Viscosity effects associated with conventional hydrodynamic lubrication are demonstrated using topically applied silicone oils.

Occurrence of the Amphibian Pathogen Batrachochytrium Dendrobatidis in the Pacific Northwest

Abstract Chytridiomycosis (infection by the fungus Batrachochytrium dendrobatidis) has been associated with amphibian declines in at least four continents. We report results of disease screens from 210 pond-breeding amphibians from 37 field sites in Oregon and Washington. We detected B. dendrobatidis on 28% of sampled amphibians, and we found ≥ 1 detection of B. dendrobatidis from 43% of sites. Four of seven species tested positive for B. dendrobatidis, including the Northern Red-Legged Frog (Rana aurora), Columbia Spotted Frog (Rana luteiventris), and Oregon Spotted Frog (Rana pretiosa). We also detected B. dendrobatidis in nonnative American Bullfrogs (Rana catesbeiana) from six sites in western and central Oregon. Our study and other recently published findings suggest that B. dendrobatidis has few geographic and host taxa limitations among North American anurans. Further research on virulence, transmissibility, persistence, and interactions with other stressors is needed to assess the potential impact of B. dendrobatidis on Pacific Northwestern amphibians.

Electromagnetic theory of heterostructure injection lasers

Abstract A theoretical study has been made of the wave-guiding properties of GaAs-Al x Ga 1−x As heterostructure injection lasers. The electromagnetic field distributions and far-field radiation patterns of these devices have been calculated by solving Maxwells equations for a three-layer wave-guide with complex dielectric constants in each region. The theory also yields threshold currents and external quantum efficiencies as functions of the principal laser parameters—temperature, doping, alloy composition x in the Al x Ga 1−x As regions, active region thickness, and cavity length. This permits detailed comparison of the theoretical results with experiment, and hence further estimates of optimal laser structures. In particular the variation of laser threshold with active region thickness (and the existence of an optimum thickness) in single heterostructures is sucessfully accounted for by the theory. For double heterostructures with the same alloy composition in each passive region, there exists no optimum active region thickness, and hence the lowest thresholds can be obtained by making these structures as thin as possible.