Numerical Evidence for Divergent Burnett Coefficients

In previous papers [Phys. Rev. A {\bf 41}, 4501 (1990), Phys. Rev. E {\bf 18}, 3178 (1993)], simple equilibrium expressions were obtained for nonlinear Burnett coefficients. A preliminary calculation of a 32 particle Lennard-Jones fluid was presented in the previous paper. Now, sufficient resources have become available to address the question of whether nonlinear Burnett coefficients are finite for soft spheres. The hard sphere case is known to have infinite nonlinear Burnett coefficients (ie a nonanalytic constitutive relation) from mode coupling theory. This paper reports a molecular dynamics caclulation of the third order nonlinear Burnett coefficient of a Lennard-Jones fluid undergoing colour flow, which indicates that this term is diverges in the thermodynamic limit.Comment: 12 pages, 9 figure

The role of bacterial protein tyrosine phosphatases in the regulation of the biosynthesis of secreted polysaccharides

SIGNIFICANCE: Tyrosine phosphorylation and associated protein tyrosine phosphatases are gaining prominence as critical mechanisms in the regulation of fundamental processes in a wide variety of bacteria. In particular, these phosphatases have been associated with the control of the biosynthesis of capsular polysaccharides and extracellular polysaccharides, critically important virulence factors for bacteria.RECENT ADVANCES: Deletion and over-expression of the phosphatases result in altered polysaccharide biosynthesis in a range of bacteria. The recent structures of associated auto-phosphorylating tyrosine kinases has suggested that the phosphatases may be critical for the cycling of the kinases between monomers and higher order oligomers. CRITICAL ISSUES: Additional substrates of the phosphatases apart from cognate kinases are currently being identified. These are likely to be critical to our understanding of the mechanism by which polysaccharide biosynthesis is regulated. FUTURE DIRECTIONS: Ultimately, these protein tyrosine phosphatases are an attractive target for the development of novel anti-microbials. This is particularly the case for the polymerase and histidinol phosphatase family, which are predominantly found in bacteria. Furthermore, the determination of bacterial tyrosine phosphoproteomes will likely help to uncover the fundamental roles, mechanism and critical importance of these phosphatases in a wide range of bacteria.Alistair James Standish and Renato Moron

Statistics of Certain Models of Evolution

In a recent paper, Newman surveys the literature on power law spectra in evolution, self-organised criticality and presents a model of his own to arrive at a conclusion that self-organised criticality is not necessary for evolution. Not only did he miss a key model (Ecolab) that has a clear self-organised critical mechanism, but also Newman's model exhibits the same mechanism that gives rise to power law behaviour as does Ecolab. Newman's model is, in fact, a ``mean field'' approximation of a self-organised critical system. In this paper, I have also implemented Newman's model using the Ecolab software, removing the restriction that the number of species remains constant. It turns out that the requirement of constant species number is non-trivial, leading to a global coupling between species that is similar in effect to the species interactions seen in Ecolab. In fact, the model must self-organise to a state where the long time average of speciations balances that of the extinctions, otherwise the system either collapses or explodes. In view of this, Newman's model does not provide the hoped-for counter example to the presence of self-organised criticality in evolution, but does provide a simple, almost analytic model that can used to understand more intricate models such as Ecolab.Comment: accepted in Phys Rev E.; RevTeX; See http://parallel.hpc.unsw.edu.au/rks/ecolab.html for more informatio

Dynamic modelling to predict the likelihood of plant species persisting in fragmented landscapes in the face of climate change

Many species are threatened by global climate change, but plants are particularly vulnerable because, as sessile organisms, they are unable to move to areas with more suitable conditions as the climate changes. Instead they must rely on their seeds dispersing far and often to keep pace with a changing climate. This problem is exacerbated by the fragmentation of natural landscapes by clearing for agricultural or urban development, or similarly by a species requirement for particular soil types or topography. Models can help predict how different species will be affected by climate change. Most previous modelling work on predicting the persistence of plant and other species under climate change has been static, regression style modelling, known as climate envelope modelling. This has focussed on predicting where suitable environments for a species will likely occur under possible future climatic conditions, based on the species’ distribution under current conditions. While the existence of suitable environments in a new climate is a necessary condition for a species’ persistence, for sessile organisms such as plants, the ability of a species to move and colonise these suitable environments is also likely to be a major limitation. There is therefore a need for models that account for the dynamic processes involved in plant species’ migration and colonisation in changing climates. This paper presents such a dynamic model, called PPunCC (Plant Persistence under Climate Change). We describe how the PPunCC model represents the important factors and processes likely to affect a plant species’ capacity to migrate across a landscape fast enough to keep pace with a changing climate, such as the rate of climate change, the degree of landscape fragmentation, and the plant species’ life history, seed production, dispersal, and establishment. We also discuss how the model could be used to inform management decisions regarding adaptation options such as assisted migration or the creation of large-scale corridors that increase the connectivity of fragmented landscapes in order to help species migrate naturally and find suitable environments in new climates

After the fence: Soil and vegetation condition in grazed, fenced and benchmark eucalypt woodlands of fragmented agricultural landscapes

Ecological theory predicts that vegetation changes caused by introduction of livestock grazing may be irreversible after livestock are removed, especially in regions such as Australia that have a short evolutionary exposure to ungulate grazing. Despite this, fencing to exclude livestock grazing is the major tool used for restoring biodiversity in remnant vegetation degraded by grazing in Australian agricultural landscapes. To characterise benefits and limitations of livestock exclusion for enhancing biodiversity in forb-rich York gum (Eucalyptus loxophleba subsp. loxophleba) – jam (Acacia acuminata) woodlands, we compared 29 fenced remnants from across the central Western Australian wheatbelt with 29 adjacent grazed remnants and 11 little-grazed ‘benchmark’ woodlands. We explored two hypotheses: (1) fencing to exclude livestock facilitates recovery of grazed woodlands towards benchmark conditions, and (2) after fencing, recovery of grazed woodlands to benchmark conditions is constrained by ecological or other limits. Our first hypothesis was supported, with fenced remnants more similar to benchmark woodlands in tree recruitment, exotic cover, native cover, native plant richness and plant species composition than grazed remnants were. Further, exotic cover decreased and frequency of jam increased with time since fencing (2–22 years). However we found no evidence for recovery of nutrient-enriched topsoils due to fencing. Our second hypothesis was also supported, with higher soil nutrients and exotic cover, lower native plant richness and different plant species composition in fenced compared with benchmark woodlands. Regression analyses suggested recovery of native species richness can be constrained by exotic species that persist after fencing, which in turn are more persistent at higher soil nutrient concentrations. We conclude that fencing to exclude livestock grazing is valuable for biodiversity conservation. However, consistent with ecological theory, additional interventions are likely to be necessary to achieve some conservation goals or to promote recovery in more productive (nutrient-enriched) sites

Effects of removal of Tradescantia fluminensis on Powelliphanta traversi and other invertebrates

This study was initiated owing to concern that removal of the invasive ground cover weed Tradescantia fluminensis may adversely effect the rare giant predatory land snail Powelliphanta traversi. From field trials using hand removal and herbicide spraying, it was considered that graduated control of Tradescantia with concomitant replacement of native ground cover could be of benefit to P. traversi and other ground-dwelling invertebrates. Grazon® herbicide (active ingredient triclopyr) appears suitable for controlling Tradescantia at sites where P. t. traversi occurs, since the effects of triclopyr on ground-dwelling invertebrates in the field and first generation P. t. traversi in the laboratory were minimal. However, possible effects of triclopyr on subsequent P. t. traversi generations remain untested, and evidence from the literature suggests that there could be some detrimental effects. Similarly, the effects of triclopyr on the presumed earthworm prey of P. t. traversi remain untested

Mapping the extent and spread of multiple plant invasions can help prioritise management in Galapagos National Park

Mapping is an important tool for the management of plant invasions. If landscapes are mapped in an appropriate way, results can help managers decide when and where to prioritize their efforts. We mapped vegetation with the aim of providing key information for managers on the extent, density and rates of spread of multiple invasive species across the landscape. Our case study focused on an area of Galapagos National Park that is faced with the challenge of managing multiple plant invasions. We used satellite imagery to produce a spatially-explicit database of plant species densities in the canopy, finding that 92% of the humid highlands had some degree of invasion and 41% of the canopy was comprised of invasive plants. We also calculated the rate of spread of eight invasive species using known introduction dates, finding that species with the most limited dispersal ability had the slowest spread rates while those able to disperse long distances had a range of spread rates. Our results on spread rate fall at the lower end of the range of published spread rates of invasive plants. This is probably because most studies are based on the entire geographic extent, whereas our estimates took plant density into account. A spatial database of plant species densities, such as the one developed in our case study, can be used by managers to decide where to apply management actions and thereby help curtail the spread of current plant invasions. For example, it can be used to identify sites containing several invasive plant species, to find the density of a particular species across the landscape or to locate where native species make up the majority of the canopy. Similar databases could be developed elsewhere to help inform the management of multiple plant invasions over the landscape

Bound on the Dark Matter Density in the Solar System from Planetary Motions

High precision planet orbital data extracted from direct observation, spacecraft explorations and laser ranging techniques enable to put a strong constraint on the maximal dark matter density of a spherical halo centered around the Sun. The maximal density at Earth's location is of the order $10^5$ ${\rm GeV/cm^3}$ and shows only a mild dependence on the slope of the halo profile, taken between 0 and -2. This bound is somewhat better than that obtained from the perihelion precession limits.Comment: 7 pages, 1 figur