10,683 research outputs found
Experimental demonstration of the antiherbivore effects of silica in grasses: impacts on foliage digestibility and vole growth rates
The impact of plant-based factors on the population dynamics of mammalian herbivores has been the subject of much debate in ecology, but the role of antiherbivore defences in grasses has received relatively little attention. Silica has been proposed as the primary defence in grasses and is thought to lead to increased abrasiveness of foliage so deterring feeding, as well as reducing foliage digestibility and herbivore performance. However, at present there is little direct experimental evidence to support these ideas. In this study, we tested the effects of manipulating silica levels on the abrasiveness of grasses and on the feeding preference and growth performance of field voles, specialist grass-feeding herbivores. Elevated silica levels did increase the abrasiveness of grasses and deterred feeding by voles. We also demonstrated, for the first time, that silica reduced the growth rates of both juvenile and mature female voles by reducing the nitrogen they could absorb from the foliage. Furthermore, we found that vole feeding leads to increased levels of silica in leaves, suggesting a dynamic feedback between grasses and their herbivores. We propose that silica induction due to vole grazing reduces vole performance and hence could contribute to cyclic dynamics in vole populations
The Massive Wolf-Rayet Binary SMC WR7
We present a study of optical spectra of the Wolf--Rayet star AzV 336a (= SMC
WR7) in the Small Magellanic Cloud. Our study is based on data obtained at
several Observatories between 1988 and 2001. We find SMC WR7 to be a double
lined WN+O6 spectroscopic binary with an orbital period of 19.56 days. The
radial velocities of the He absorption lines of the O6 component and the strong
He{\sc ii} emission at 4686\AA of the WN component describe antiphased
orbital motions. However, they show a small phase shift of 1 day. We
discuss possible explanations for this phase shift. The amplitude of the radial
velocity variations of He {\sc ii} emission is twice that of the absorption
lines. The binary components have fairly high minimum masses, 18
\modot and 34 \modot for the WN and O6 components, respectively.Comment: Accepted by MNRA
The Schur-Horn theorem for operators and frames with prescribed norms and frame operator
Let be a Hilbert space. Given a bounded positive definite
operator on , and a bounded sequence of non negative real numbers, the pair is
frame admissible, if there exists a frame on
with frame operator , such that , . We relate the existence of such frames with the Schur-Horn
theorem of majorization, and give a reformulation of the extended version of
Schur-Horn theorem, due to A. Neumann. We use it to get necessary conditions
(and to generalize known sufficient conditions) for a pair ,
to be frame admissible.Comment: To appear in Illinois Journal of Mat
Grasses and the resource availability hypothesis: the importance of silica-based defences
The resource availability hypothesis (RAH) predicts that allocation of resources to anti-herbivore defences differs between species according to their growth rate. We tested this hypothesis by assessing the growth and defence investment strategies of 18 grass species and comparing them against vole feeding preferences. In addition, we assessed the effectiveness of silica, the primary defence in many grasses, in influencing vole feeding behaviour. Across species, we found that there was a strong negative relationship between the overall investment in defence and growth rate, thus supporting predictions of the RAH. However, no such relationship was found when assessing the various individual anti-herbivore defences, suggesting that different grass species show significant variation in their relative investment in strategies such as phenolic concentration, silica concentration and leaf toughness. Silica was the most influential defensive factor in determining vole feeding preference. Experimentally induced increases in leaf silica concentration deterred vole feeding in three of the five species tested, and altered feeding preference ranks between species. The strong positive relationship between silica concentration and leaf abrasiveness, when assessed both within and between species, suggests that increased abrasiveness is the mechanism by which silica deters feeding. Although grasses are often considered to be tolerant of herbivore damage rather then defended against it, they do follow predictions of defence allocation strategy based on their growth rates, and this affects the feeding behaviour of generalist grass-feeding herbivores
Deconvolution with Shapelets
We seek to find a shapelet-based scheme for deconvolving galaxy images from
the PSF which leads to unbiased shear measurements. Based on the analytic
formulation of convolution in shapelet space, we construct a procedure to
recover the unconvolved shapelet coefficients under the assumption that the PSF
is perfectly known. Using specific simulations, we test this approach and
compare it to other published approaches. We show that convolution in shapelet
space leads to a shapelet model of order
with and being the maximum orders of the intrinsic
galaxy and the PSF models, respectively. Deconvolution is hence a
transformation which maps a certain number of convolved coefficients onto a
generally smaller number of deconvolved coefficients. By inferring the latter
number from data, we construct the maximum-likelihood solution for this
transformation and obtain unbiased shear estimates with a remarkable amount of
noise reduction compared to established approaches. This finding is
particularly valid for complicated PSF models and low images, which
renders our approach suitable for typical weak-lensing conditions.Comment: 9 pages, 9 figures, submitted to A&
Limitations for shapelet-based weak-lensing measurements
We seek to understand the impact on shape estimators obtained from circular
and elliptical shapelet models under two realistic conditions: (a) only a
limited number of shapelet modes is available for the model, and (b) the
intrinsic galactic shapes are not restricted to shapelet models.
We create a set of simplistic simulations, in which the galactic shapes
follow a Sersic profile. By varying the Sersic index and applied shear, we
quantify the amount of bias on shear estimates which arises from insufficient
modeling. Additional complications due to PSF convolution, pixelation and pixel
noise are also discussed.
Steep and highly elliptical galaxy shapes cannot be accurately modeled within
the circular shapelet basis system and are biased towards shallower and less
elongated shapes. This problem can be cured partially by allowing elliptical
basis functions, but for steep profiles elliptical shapelet models still depend
critically on accurate ellipticity priors. As a result, shear estimates are
typically biased low. Independently of the particular form of the estimator,
the bias depends on the true intrinsic galaxy morphology, but also on the size
and shape of the PSF.
As long as the issues discussed here are not solved, the shapelet method
cannot provide weak-lensing measurements with an accuracy demanded by upcoming
missions and surveys, unless one can provide an accurate and reliable
calibration, specific for the dataset under investigation.Comment: 8 pages, 5 figures, submitted to A&
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