1,043 research outputs found
Improved timber harvest techniques maintain biodiversity in tropical forests
Tropical forests are selectively logged at 20 times the rate at which they are cleared, and at least a fifth have already been disturbed in this way. In a recent pan-tropical assessment, Burivalova et al. demonstrate the importance of logging intensity as a driver of biodiversity decline in timber estates. Their analyses reveal that species richness of some taxa could decline by 50% at harvest intensities of 38 m3 ha-1. However, they did not consider the extraction techniques that lead to these intensities. Here, we conduct a complementary meta-analysis of assemblage responses to differing logging practices: conventional logging and reduced-impact logging. We show that biodiversity impacts are markedly less severe in forests that utilise reduced-impact logging, compared to those using conventional methods. While supporting the initial findings of Burivalova et al., we go on to demonstrate that best practice forestry techniques curtail the effects of timber extraction regardless of intensity. Therefore, harvest intensities are not always indicative of actual disturbance levels resulting from logging. Accordingly, forest managers and conservationists should advocate practices that offer reduced collateral damage through best practice extraction methods, such as those used in reduced-impact logging. Large-scale implementation of this approach would lead to improved conservation values in the 4 million km2 of tropical forests that are earmarked for timber extraction
A Surveillance of the Causes of Mortality in Three South Dakota Layer Flocks
A surveillance program was conducted during June 1968 through June 1969 to determine the important causes of mortality in certain South Dakota layer flocks that experienced reasonably normal mortality
PKS B1545-321: Bow shocks of a relativistic jet?
Sensitive, high resolution images of the double-double radio galaxy PKS
B1545-321 reveal detailed structure, which we interpret in the light of
previous work on the interaction of restarted jets with pre-existing relict
cocoons. We have also examined the spectral and polarization properties of the
source, the color distribution in the optical host and the environment of this
galaxy in order to understand its physical evolution. We propose that the
restarted jets generate narrow bow shocks and that the inner lobes are a
mixture of cocoon plasma reaccelerated at the bow shock and new jet material
reaccelerated at the termination shock. The dynamics of the restarted jets
implies that their hot spots advance at mildly relativistic speeds with
external Mach numbers of at least 5. The existence of supersonic hot spot Mach
numbers and bright inner lobes is the result of entrainment causing a reduction
in the sound speed of the pre-existing cocoon. The interruption to jet activity
in PKS B1545-321 has been brief - lasting less than a few percent of the
lifetime of the giant radio source. The host
galaxy is located at the boundary of a large scale filamentary structure, and
shows blue patches in color distribution indicative of a recent merger, which
may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA
Equation of state and transport processes in self--similar spheres
We study the effect of transport processes (diffusion and free--streaming) on
a collapsing spherically symmetric distribution of matter in a self--similar
space--time. A very simple solution shows interesting features when it is
matched with the Vaidya exterior solution. In the mixed case (diffusion and
free--streaming), we find a barotropic equation of state in the stationary
regime. In the diffusion approximation the gravitational potential at the
surface is always constant; if we perturb the stationary state, the system is
very stable, recovering the barotropic equation of state as time progresses. In
the free--streaming case the self--similar evolution is stationary but with a
non--barotropic equation of state.Comment: 9 pages, 2 figure
Interactions of a Light Hypersonic Jet with a Non-Uniform Interstellar Medium
We present three dimensional simulations of the interaction of a light
hypersonic jet with an inhomogeneous thermal and turbulently supported disk in
an elliptical galaxy. We model the jet as a light, supersonic non-relativistic
flow with parameters selected to be consistent with a relativistic jet with
kinetic power just above the FR1/FR2 break.
We identify four generic phases in the evolution of such a jet with the
inhomogeneous interstellar medium: 1) an initial ``flood and channel'' phase,
where progress is characterized by high pressure gas finding changing weak
points in the ISM, flowing through channels that form and re-form over time, 2)
a spherical, energy-driven bubble phase, were the bubble is larger than the
disk scale, but the jet remains fully disrupted close to the nucleus, 3) a
rapid, jet break--out phase the where jet breaks free of the last dense clouds,
becomes collimated and pierces the spherical bubble, and 4) a classical phase,
the jet propagates in a momentum-dominated fashion leading to the classical jet
+ cocoon + bow-shock structure.
Mass transport in the simulations is investigated, and we propose a model for
the morphology and component proper motions in the well-studied Compact
Symmetric Object 4C31.04.Comment: 66 pages, 22 figures, PDFLaTeX, aastex macros, graphicx and amssymb
packages, Accepted, to be published 2007 ApJ
Eukaryotic initiation factor-4E in superficial and muscle invasive bladder cancer and its correlation with vascular endothelial growth factor expression and tumour progression
Vascular endothelial growth factor (VEGF) is an important factor mediating tumour angiogenesis. VEGF mRNA is differentially expressed in bladder cancer with high expression in superficial tumours (stage pTaand pT1) contrasting with low expression in muscle invasive tumours (stage ≥ pT2). To investigate mechanisms regulating VEGF expression in bladder cancer, VEGF mRNA and protein were measured in normal bladder (n = 12) and primary bladder cancers (n = 57). VEGF protein levels correlated with mRNA expression in normal bladder (r = 0.68, P = 0.02) and bladder cancer (r = 0.46, P = 0.0007). Whilst VEGF mRNA expression was threefold higher in superficial compared to muscle invasive bladder cancers (P = 0.0001) there was no difference in VEGF protein (P = 0.81). Accordingly, the median protein:mRNA ratios increased more than 15-fold with increasing tumour stage (P< 0.0001) suggesting translational regulation. Expression of the eukaryotic initiation factor-4E (eIF-4E), a factor implicated in the translational regulation of VEGF, was greater in tumours than normal bladder (P< 0.0001) and correlated with VEGF protein:mRNA ratios (n = 43, r = 0.54, P = 0.0004) pointing to its role in the regulation of VEGF. In superficial tumours (n = 37) high expression of eIF-4E was associated with a poor prognosis and reduced stage progression-free survival (P = 0.04, Cox proportional hazards model). The study demonstrates that eIF-4E may be involved in translational regulation of VEGF in bladder cancer and might have a role as a prognostic factor in bladder cancer. © 2000 Cancer Research Campaig
Jet-Induced Emission-Line Nebulosity and Star Formation in the High-Redshift Radio Galaxy 4C41.17
The high redshift radio galaxy 4C41.17 consists of a powerful radio source in
which previous work has shown that there is strong evidence for jet-induced
star formation along the radio axis. We argue that nuclear photoionization is
not responsible for the excitation of the emission line clouds and we construct
a jet-cloud interaction model to explain the major features revealed by the
data. The interaction of a high-powered jet with a dense cloud in the halo of
4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s
shocks and induces star formation. The CIII to CIV line ratio and the CIV
luminosity emanating from the shock, imply that the pre-shock density in the
line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star
formation could proceed on a timescale of order a few x 10^6 yrs, well within
the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s)
emission lines are attributed to the disturbance of the gas cloud by a partial
bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular
CIV) arise in precursor emission in relatively low metallicity gas. The implied
baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies
that Milky Way size condensations existed in the environments of forming radio
galaxies at a redshift of 3.8. Our interpretation of the data provides a
physical basis for the alignment of the radio, emission-line and UV continuum
images in some of the highest redshift radio galaxies and the analysis
presented here may form a basis for the calculation of densities and cloud
masses in other high redshift radio galaxies.Comment: 18 pages, 5 figures; uses astrobib.sty and aaspp4.sty. Better
versions of figures available via anonymous from
ftp://mso.anu.edu.au:pub/pub/geoff/4C41.1
A complete classification of spherically symmetric perfect fluid similarity solutions
We classify all spherically symmetric perfect fluid solutions of Einstein's
equations with equation of state p/mu=a which are self-similar in the sense
that all dimensionless variables depend only upon z=r/t. For a given value of
a, such solutions are described by two parameters and they can be classified in
terms of their behaviour at large and small distances from the origin; this
usually corresponds to large and small values of z but (due to a coordinate
anomaly) it may also correspond to finite z. We base our analysis on the
demonstration that all similarity solutions must be asymptotic to solutions
which depend on either powers of z or powers of lnz. We show that there are
only three similarity solutions which have an exact power-law dependence on z:
the flat Friedmann solution, a static solution and a Kantowski-Sachs solution
(although the latter is probably only physical for a1/5, there are
also two families of solutions which are asymptotically (but not exactly)
Minkowski: the first is asymptotically Minkowski as z tends to infinity and is
described by one parameter; the second is asymptotically Minkowski at a finite
value of z and is described by two parameters. A complete analysis of the dust
solutions is given, since these can be written down explicitly and elucidate
the link between the z>0 and z<0 solutions. Solutions with pressure are then
discussed in detail; these share many of the characteristics of the dust
solutions but they also exhibit new features.Comment: 63 pages. To appear in Physical Review
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