4,608 research outputs found
Granivory of invasive, naturalized, and native plants in communities differentially susceptible to invasion
Seed predation is an important biotic filter that can influence abundance and spatial distributions of native species through differential effects on recruitment. This filter may also influence the relative abundance of nonnative plants within habitats and the communities’ susceptibility to invasion via differences in granivore identity, abundance, and food preference. We evaluated the effect of postdispersal seed predators on the establishment of invasive, naturalized, and native species within and between adjacent forest and steppe communities of eastern Washington, USA that differ in severity of plant invasion. Seed removal from trays placed within guild-specific exclosures revealed that small mammals were the dominant seed predators in both forest and steppe. Seeds of invasive species (Bromus tectorum, Cirsium arvense) were removed significantly less than the seeds of native (Pseudoroegneria spicata, Balsamorhiza sagittata) and naturalized (Secale cereale, Centaurea cyanus) species. Seed predation limited seedling emergence and establishment in both communities in the absence of competition in a pattern reflecting natural plant abundance: S. cereale was most suppressed, B. tectorum was least suppressed, and P. spicata was suppressed at an intermediate level. Furthermore, seed predation reduced the residual seed bank for all species. Seed mass correlated with seed removal rates in the forest and their subsequent effects on plant recruitment; larger seeds were removed at higher rates than smaller seeds. Our vegetation surveys indicate higher densities and canopy cover of nonnative species occur in the steppe compared with the forest understory, suggesting the steppe may be more susceptible to invasion. Seed predation alone, however, did not result in significant differences in establishment for any species between these communities, presumably due to similar total small-mammal abundance between communities. Consequently, preferential seed predation by small mammals predicts plant establishment for our test species within these communities but not between them. Accumulating evidence suggests that seed predation can be an important biotic filter affecting plant establishment via differences in consumer preferences and abundance with important ramifications for plant invasions and in situ community assembly
Fundamental investigation of losses of skeletal mineral in young adult human males and collaterally in young adult male pigtail monkeys /macacus nemestrima/ through immobilization for varying periods of time, coupled with a study of methods of preven
Skeletal mineral losses in young adult humans and monkeys resulting from immobilizatio
Dust in 3C324
The results of a deep submillimetre observation using SCUBA of the powerful
radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns,
emission from the location of the host radio galaxy is marginally detected at
the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission
at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation
using the Effelsberg 100m telescope confirms that the sub-millimetre signal is
not associated with synchrotron emission. These observations indicate that both
the mass of warm dust within 3C324, and the star formation rate, lie up to an
order of magnitude below the values recently determined for radio galaxies at z
= 3 to 4. The results are compared with dust masses and star formation rates
derived in other ways for 3C324.Comment: 5 pages LaTeX, including 1 figure. Accepted for publication in MNRA
Four Dimensional CFT Models with Rational Correlation Functions
Recently established rationality of correlation functions in a globally
conformal invariant quantum field theory satisfying Wightman axioms is used to
construct a family of soluble models in 4-dimensional Minkowski space-time. We
consider in detail a model of a neutral scalar field of dimension 2. It
depends on a positive real parameter c, an analogue of the Virasoro central
charge, and admits for all (finite) c an infinite number of conserved symmetric
tensor currents. The operator product algebra of is shown to coincide
with a simpler one, generated by a bilocal scalar field of
dimension (1,1). The modes of V together with the unit operator span an
infinite dimensional Lie algebra whose vacuum (i.e. zero energy lowest
weight) representations only depend on the central charge c. Wightman
positivity (i.e. unitarity of the representations of ) is proven to be
equivalent to .Comment: 28 pages, LATEX, amsfonts, latexsym. Proposition 2.3, and Conjecture
in Sec. 6 are revised. Minor errors are correcte
The role of polysaccharide intercellular adhesin (PIA) in Staphylococcus epidermidis adhesion to host tissues and subsequent antibiotic tolerance
The aim of this study was to determine the role of polysaccharide intercellular adhesin (PIA) in Staphylococcus epidermidis adhesion to host tissues and subsequent antibiotic tolerance. The adherence of S. epidermidis 1457 and the mutant defective in PIA production (1457-M10) to urinary epithelium and endothelium was estimated by colony counting. Minimum bactericidal concentration and mean reduction of cellular activity (XTT) following antibiotic exposure was determined for planktonic and adhered bacteria. S. epidermidis 1457 adhered to a greater extent to both cells than the mutant strain. The adhered strains had a significantly higher antimicrobial tolerance than their planktonic counterparts. The mutant strain was, in general, the most susceptible to the antibiotics assayed. In conclusion, PIA may influence S. epidermidis adherence to host tissues and their antimicrobial susceptibility. Initial adhesion may be the main step for the acquisition of resistance in S. epidermidis
Is the EGRET source 3EG J1621+8203 the radio galaxy NGC 6251?
We discuss the nature of the unidentified EGRET source 3EG J1621+8203. In an
effort to identify the gamma-ray source, we have examined X-ray images of the
field from ROSAT PSPC, ROSAT HRI, and ASCA GIS. Of the several faint X-ray
point sources in the error circle of 3EG J1621+8203, most are stars or faint
radio sources, unlikely to be counterparts to the EGRET source. The most
notable object in the gamma-ray error box is the bright FR I radio galaxy NGC
6251. If 3EG J1621+8203 corresponds to NGC 6251, then it would be the second
radio galaxy to be detected in high energy gamma rays, after Cen A, which
provided the first clear evidence of the detection above 100 MeV of an AGN with
a large-inclination jet. If the detection of more radio galaxies by EGRET has
been limited by its threshold sensitivity, there exists the exciting
possibility that new high energy gamma-ray instruments, with much higher
sensitivity, will detect a larger number of radio galaxies in the future.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical
Journal, August 2002 issu
Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices
We have fabricated electric double-layer field-effect devices to
electrostatically dope our active materials, either =0.015
GaMnAs or =3.2 GaBeAs. The devices
are tailored for interrogation of electric field induced changes to the
frequency dependent conductivity in the accumulation or depletions layers of
the active material via infrared (IR) spectroscopy. The spectra of the
(Ga,Be)As-based device reveal electric field induced changes to the IR
conductivity consistent with an enhancement or reduction of the Drude response
in the accumulation and depletion polarities, respectively. The spectroscopic
features of this device are all indicative of metallic conduction within the
GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show
enhancement of the far-IR itinerant carrier response and broad mid-IR resonance
upon hole accumulation, with a decrease of these features in the depletion
polarity. These later spectral features demonstrate that conduction in
ferromagnetic (FM) GaMnAs is distinct from genuine metallic
behavior due to extended states in the host VB. Furthermore, these data support
the notion that a Mn-induced impurity band plays a vital role in the electron
dynamics of FM GaMnAs. We add, a sum-rule analysis of the spectra
of our devices suggests that the Mn or Be doping does not lead to a substantial
renormalization of the GaAs host VB
Simple Space-Time Symmetries: Generalizing Conformal Field Theory
We study simple space-time symmetry groups G which act on a space-time
manifold M=G/H which admits a G-invariant global causal structure. We classify
pairs (G,M) which share the following additional properties of conformal field
theory: 1) The stability subgroup H of a point in M is the identity component
of a parabolic subgroup of G, implying factorization H=MAN, where M generalizes
Lorentz transformations, A dilatations, and N special conformal
transformations. 2) special conformal transformations in N act trivially on
tangent vectors to the space-time manifold M. The allowed simple Lie groups G
are the universal coverings of SU(m,m), SO(2,D), Sp(l,R), SO*(4n) and E_7(-25)
and H are particular maximal parabolic subgroups. They coincide with the groups
of fractional linear transformations of Euklidean Jordan algebras whose use as
generalizations of Minkowski space time was advocated by Gunaydin. All these
groups G admit positive energy representations. It will also be shown that the
classical conformal groups SO(2,D) are the only allowed groups which possess a
time reflection automorphism; in all other cases space-time has an intrinsic
chiral structure.Comment: 37 pages, 4 Table
Cascades with Adjoint Matter: Adjoint Transitions
A large class of duality cascades based on quivers arising from non-isolated
singularities enjoy adjoint transitions - a phenomenon which occurs when the
gauge coupling of a node possessing adjoint matter is driven to strong coupling
in a manner resulting in a reduction of rank in the non-Abelian part of the
gauge group and a subsequent flow to weaker coupling. We describe adjoint
transitions in a simple family of cascades based on a Z2-orbifold of the
conifold using field theory. We show that they are dual to Higgsing and produce
varying numbers of U(1) factors, moduli, and monopoles in a manner which we
calculate. This realizes a large family of cascades which proceed through
Seiberg duality and Higgsing. We briefly describe the supergravity limit of our
analysis, as well as a prescription for treating more general theories. A
special role is played by N=2 SQCD. Our results suggest that additional light
fields are typically generated when UV completing certain constructions of
spontaneous supersymmetry breaking into cascades, potentially leading to
instabilities.Comment: 29 pages, a few typos fixed, improved discussion, added figure; now
there is 1 figur
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