17,074 research outputs found
Tropical rainforest bird community structure in relation to altitude, tree species composition, and null models in the Western Ghats, India
Studies of species distributions on elevational gradients are essential to
understand principles of community organisation as well as to conserve species
in montane regions. This study examined the patterns of species richness,
abundance, composition, range sizes, and distribution of rainforest birds at 14
sites along an elevational gradient (500-1400 m) in the Kalakad-Mundanthurai
Tiger Reserve (KMTR) of the Western Ghats, India. In contrast to theoretical
expectation, resident bird species richness did not change significantly with
elevation although the species composition changed substantially (<10%
similarity) between the lowest and highest elevation sites. Constancy in
species richness was possibly due to relative constancy in productivity and
lack of elevational trends in vegetation structure. Elevational range size of
birds, expected to increase with elevation according to Rapoport's rule, was
found to show a contrasting inverse U-shaped pattern because species with
narrow elevational distributions, including endemics, occurred at both ends of
the gradient (below 800 m and above 1,200 m). Bird species composition also did
not vary randomly along the gradient as assessed using a hierarchy of null
models of community assembly, from completely unconstrained models to ones with
species richness and range-size distribution restrictions. Instead, bird
community composition was significantly correlated with elevation and tree
species composition of sites, indicating the influence of deterministic factors
on bird community structure. Conservation of low- and high-elevation areas and
maintenance of tree species composition against habitat alteration are
important for bird conservation in the southern Western Ghats rainforests.Comment: 36 pages, 5 figures, two tables (including one in the appendix)
Submitted to the Journal of the Bombay Natural History Society (JBNHS
Stability of Naked Singularity arising in gravitational collapse of Type I matter fields
Considering gravitational collapse of Type I matter fields, we prove that,
given an arbitrary - mass function and a -
function (through the corresponding - metric function
), there exist infinitely many choices of energy distribution
function such that the `true' initial data () leads
the collapse to the formation of naked singularity. We further prove that the
occurrence of such a naked singularity is stable with respect to small changes
in the initial data. We remark that though the initial data leading to both
black hole and naked singularity form a "big" subset of the true initial data
set, their occurrence is not generic. The terms `stability' and `genericity'
are appropriately defined following the theory of dynamical systems. The
particular case of radial pressure has been illustrated in details
to get clear picture of how naked singularity is formed and how, it is stable
with respect to initial data.Comment: 16 pages, no figure, Latex, submitted to Praman
An improved magnetic field simulator - MAGFLD.
An improved two-dimensional simulator MAGFLD has been developed which is useful for the design and simulation of periodic permanent magnet (PPM) focusing system for linear beam tubes. At present, input is possible only through the input file, which is very simple and user friendly. A complete PPM circuit is generated using the coordinates of first pole piece, first magnet, gun adapter (if the structure is a-periodic) and the region of computation. Small mesh units of either square or rectangular shapes can be used with mesh refinement capability in one or more regions in any or both directions for better accuracy of the solution. Materials with different magnetic permeability can be modeled by defining a characteristic value for each mesh point of the geometry. The effective potential value at each point in the region of interest is calculated based on the vector potential model by using the 5-point finite difference method and the solution is achieved by over relaxation technique for faster convergence. This package has an interface with EGUN to model the electron gun and collector under the influence of magnetic field. Versatile color graphics are capable of plotting both axial magnetic field and flux lines along with the magnetic circuit. MAGFLD has been validated against some published data and experimental results
Genericity aspects in gravitational collapse to black holes and naked singularities
We investigate here the genericity and stability aspects for naked
singularities and black holes that arise as the final states for a complete
gravitational collapse of a spherical massive matter cloud. The form of the
matter considered is a general Type I matter field, which includes most of the
physically reasonable matter fields such as dust, perfect fluids and such other
physically interesting forms of matter widely used in gravitation theory. We
first study here in some detail the effects of small pressure perturbations in
an otherwise pressure-free collapse scenario, and examine how a collapse
evolution that was going to the black hole endstate would be modified and go to
a naked singularity, once small pressures are introduced in the initial data.
This allows us to understand the distribution of black holes and naked
singularities in the initial data space. Collapse is examined in terms of the
evolutions allowed by Einstein equations, under suitable physical conditions
and as evolving from a regular initial data. We then show that both black holes
and naked singularities are generic outcomes of a complete collapse, when
genericity is defined in a suitable sense in an appropriate space.Comment: 24 pages, 6 figures, some changes in text and figures to match the
version accepted for publication by IJMP
Evaluations of a Mechanistic Hypothesis for the Influence of Extracellular Ions on Electroporation Due to High-Intensity, Nanosecond Pulsing
The effect of ions present in the extracellular medium on electroporation by high-intensity, short-duration pulsing is studied through molecular dynamic simulations. Our simulation results indicate that mobile ions in the medium might play a role in creating stronger local electric fields across membranes that then reinforce and strengthen electroporation. Much faster pore formation is predicted in higher conductivity media. However, the impact of extracellular conductivity on cellular inflows, which depend on transport processes such as electrophoresis, could be different as discussed here. Our simulation results also show that interactions between cations (Na+ in this case) and the carbonyl oxygen of the lipid headgroups could impede pore resealing
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