11,019 research outputs found
Density-and trait-mediated effects of a parasite and a predator in a tri-trophic food web
1. Despite growing interest in ecological consequences of parasitism in food webs, relatively little is known about effects of parasites on long-term population dynamics of non-host species or about whether such effects are density- or trait- mediated.
2. We studied a tri-trophic food chain comprised of: (i) a bacterial basal resource (Serratia fonticola), (ii) an intermediate consumer (Paramecium caudatum), (iii) a top predator (Didinium nasutum), and (iv) a parasite of the intermediate consumer (Holospora undulata). A fully-factorial experimental manipulation of predator and parasite presence/absence was combined with analyses of population dynamics, modelling, and analyses of host (Paramecium) morphology and behavior.
3. Predation and parasitism each reduced the abundance of the intermediate consumer (Paramecium), and parasitism indirectly reduced the abundance of the basal resource (Serratia). However, in combination, predation and parasitism had non-additive effects on the abundance of the intermediate consumer, as well as on that of the basal resource. In both cases, the negative effect of parasitism seemed to be effaced by predation.
4. Infection of the intermediate consumer reduced predator abundance. Modelling and additional experimentation revealed that this was most likely due to parasite reduction of intermediate host abundance (a density-mediated effect), as opposed to changes in predator functional or numerical response.
5. Parasitism altered morphological and behavioural traits, by reducing host cell length and increasing the swimming speed of cells with moderate parasite loads. Additional tests showed no significant difference in Didinium feeding rate on infected and uninfected hosts, suggesting that the combination of these modifications does not affect host vulnerability to predation. However, estimated rates of encounter with Serratia based on these modifications were higher for infected Paramecium than for uninfected Paramecium.
6. A mixture of density-mediated and trait-mediated indirect effects of parasitism on non- host species creates rich and complex possibilities for effects of parasites in food webs that should be included in assessments of possible impacts of parasite eradication or introduction
First Results from SPARO: Evidence for Large-Scale Toroidal Magnetic Fields in the Galactic Center
We have observed the linear polarization of 450 micron continuum emission
from the Galactic center, using a new polarimetric detector system that is
operated on a 2 m telescope at the South Pole. The resulting polarization map
extends ~ 170 pc along the Galactic plane and ~ 30 pc in Galactic latitude, and
thus covers a significant fraction of the central molecular zone. Our map shows
that this region is permeated by large-scale toroidal magnetic fields. We
consider our results together with radio observations that show evidence for
poloidal fields in the Galactic center, and with Faraday rotation observations.
We compare all of these observations with the predictions of a magnetodynamic
model for the Galactic center that was proposed in order to explain the
Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude
that the observations are basically consistent with the model.Comment: 11 pages, 2 figures, 1 table, submitted to ApJ Let
Finite temperature excitations of a trapped Bose gas
We present a detailed study of the temperature dependence of the condensate
and noncondensate density profiles of a Bose-condensed gas in a parabolic trap.
These quantitites are calculated self-consistently using the
Hartree-Fock-Bogoliubov equations within the Popov approximation. Below the
Bose-Einstein transition the excitation frequencies have a realtively weak
temperature dependence even though the condensate is strongly depleted. As the
condensate density goes to zero through the transition, the excitation
frequencies are strongly affected and approach the frequencies of a
noninteracting gas in the high temperature limit.Comment: 4 pages, Latex, 4 postscript figures. Submitted to Physical Review
Letter
Frame Dependence of Spin-One Angular Conditions in Light Front Dynamics
We elaborate the frame dependence of the angular conditions for spin-1 form
factors. An extra angular condition is found in addition to the usual angular
condition relating the four helicity amplitudes. Investigating the
frame-dependence of the angular conditions, we find that the extra angular
condition is in general as complicated as the usual one, although it becomes
very simple in the frame involving only two helicity amplitudes. It
is confirmed that the angular conditions are identical in frames that are
connected by kinematical transformations. The high behaviors of the
physical form factors and the limiting behaviors in special reference frames
are also discussed.Comment: 29 pages RevTeX. submitted to Phys. Rev.
Predicted Colors and Flux Densities of Protostars in the Herschel PACS and SPIRE Filters
Upcoming surveys with the Herschel Space Observatory will yield far-IR
photometry of large samples of young stellar objects, which will require
careful interpretation. We investigate the color and luminosity diagnostics
based on Herschel broad-band filters to identify and discern the properties of
low-mass protostars. We compute a grid of 2,016 protostars in various physical
congurations, present the expected flux densities and flux density ratios for
this grid of protostars, and compare Herschel observations of three protostars
to the model results. These provide useful constraints on the range of colors
and fluxes of protostar in the Herschel filters. We find that Herschel data
alone is likely a useful diagnostic of the envelope properties of young starsComment: Part of HOPS KP papers to the Herschel special A&A issu
A review of recent perspectives on biomechanical risk factors associated with anterior cruciate ligament injury
There is considerable evidence to support a number of biomechanical risk factors associated with non-contact anterior cruciate ligament (ACL) injury. This paper aimed to review these biomechanical risk factors and highlight future directions relating to them. Current perspectives investigating trunk position and relationships between strength, muscle activity and biomechanics during landing/cutting highlight the importance of increasing hamstring muscle force during dynamic movements through altering strength, muscle activity, muscle length and contraction velocity. In particular, increased trunk flexion during landing/cutting and greater hamstring strength are likely to increase hamstring muscle force during landing and cutting which have been associated with reduced ACL injury risk. Decision making has also been shown to influence landing biomechanics and should be considered when designing tasks to assess landing/cutting biomechanics. Coaches should therefore promote hamstring strength training and active trunk flexion during landing and cutting in an attempt to reduce ACL injury risk.Peer reviewe
Quantum Kinetic Theory of BEC Lattice Gas:Boltzmann Equations from 2PI-CTP Effective Action
We continue our earlier work [Ana Maria Rey, B. L. Hu, Esteban Calzetta,
Albert Roura and Charles W. Clark, Phys. Rev. A 69, 033610 (2004)] on the
nonequilibrium dynamics of a Bose Einstein condensate (BEC) selectively loaded
into every third site of a one-dimensional optical lattice. From the
two-particle irreducible (2PI) closed-time-path (CTP) effective action for the
Bose- Hubbard Hamiltonian, we show how to obtain the Kadanoff-Baym equations of
quantum kinetic theory. Using the quasiparticle approximation, we show that the
local equilibrium solutions of these equations reproduce the second- order
corrections to the self-energy originally derived by Beliaev. This work paves
the way for the use of effective action methods in the derivation of quantum
kinetic theory of many atom systems.Comment: 21 pages, 0 figures, minor editorial changes were mad
Residual strength of equine bone is not reduced by intense fatigue loading: Implications for stress fracture
Fatigue or stress fractures are an important clinical problem in humans as well as racehorses. An important question in this context is, when a bone experiences. fa!igue damage ~uring e~treme use, how much is it weakened compared to its original state? Since there are very ltmtted data on thts quesuon and stress fractures are common in racehorses, we sought to determine the effect of fatigue loading on the monotonic l:trength of equine cortical bone. Beams were machined from the dorsal, medial and lateral cortices of the third metacarpal bones of six thoroughbred racehorses. Beams from left and right bones were assigned to control and fatigue groups. respectively (N- 18 each). The fatigue group was cyclicully loaded in three-point bending at 2Hz for 100,000 cycles at 0- 5000 microstrain while submerged in saline at 37°C. These beams. as well as those in the control group. were then monotonically loaded to failure in three-point bending. The monotonic load-deflection curves were analyzed for differences using three-factor (fatigue loading, ~natotni~ region. and horse)_ analysis o_f variance .. The mean failure load was 3% less in the fatigue group, but thts reduction was only margmally stgmficant. Netther elastic modulus nor yield strength was significantly affected by the fatigue loading. The principal effects of fatigue loading were on post-yield behavior (yield being based on a 0.02% offset criter!on). The work don~ and the load increase between yield and failure were both significantly reduced. All the vanables except post-yteld deflecuon were significantly affected by anatomic region. In summary, loading equivalent to a lifetime of racing does not significantly weaken equine cortical bone ex vivo. The clinical implication of this may be that the biological repair of fatigue damage can actually contribute to stress fracture if pressed too far
Course-based Science Research Promotes Learning in Diverse Students at Diverse Institutions
Course-based research experiences (CREs) are powerful strategies for spreading learning and improving persistence for all students, both science majors and nonscience majors. Here we address the crucial components of CREs (context, discovery, ownership, iteration, communication, presentation) found across a broad range of such courses at a variety of academic institutions. We also address how the design of a CRE should vary according to the background of student participants; no single CRE format is perfect. We provide a framework for implementing CREs across multiple institutional types and several disciplines throughout the typical four years of undergraduate work, designed to a variety of student backgrounds. Our experiences implementing CREs also provide guidance on overcoming barriers to their implementation
First Results from Viper: Detection of Small-Scale Anisotropy at 40 GHZ
Results of a search for small-scale anisotropy in the cosmic microwave
background (CMB) are presented. Observations were made at the South Pole using
the Viper telescope, with a .26 degree (FWHM) beam and a passband centered at
40 GHz. Anisotropy band-power measurements in bands centered at l = 108, 173,
237, 263, 422 and 589 are reported. Statistically significant anisotropy is
detected in all bands.Comment: 5 pages, 4 figures, uses emulateapj.sty, submitted to ApJ Letter
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