3,352 research outputs found
General relationships between consumer dispersal, resource dispersal and metacommunity diversity
One of the central questions of metacommunity theory is how dispersal of
organisms affects species diversity. Here we show that the diversity-dispersal
relationship should not be studied in isolation of other abiotic and biotic
flows in the metacommunity. We study a mechanistic metacommunity model in which
consumer species compete for an abiotic or biotic resource. We consider both
consumer species specialized to a habitat patch, and generalist species capable
of using the resource throughout the metacommunity. We present analytical
results for different limiting values of consumer dispersal and resource
dispersal, and complement these results with simulations for intermediate
dispersal values. Our analysis reveals generic patterns for the combined
effects of consumer and resource dispersal on the metacommunity diversity of
consumer species, and shows that hump-shaped relationships between local
diversity and dispersal are not universal. Diversity-dispersal relationships
can also be monotonically increasing or multimodal. Our work is a new step
towards a general theory of metacommunity diversity integrating dispersal at
multiple trophic levels.Comment: Main text: 15 pages, 4 figures. Supplement: 25 pages, 12 figure
Potential energy curves for the interaction of Ag(5s) and Ag(5p) with noble gas atoms
We investigate the interaction of ground and excited states of a silver atom
with noble gases (NG), including helium. Born-Oppenheimer potential energy
curves are calculated with quantum chemistry methods and spin-orbit effects in
the excited states are included by assuming a spin-orbit splitting independent
of the internuclear distance. We compare our results with experimentally
available spectroscopic data, as well as with previous calculations. Because of
strong spin-orbit interactions, excited Ag-NG potential energy curves cannot be
fitted to Morse-like potentials. We find that the labeling of the observed
vibrational levels has to be shifted by one unit
Resilience, reactivity and variability : A mathematical comparison of ecological stability measures
In theoretical studies, the most commonly used measure of ecological
stability is resilience: ecosystems asymptotic rate of return to equilibrium
after a pulse-perturbation or shock. A complementary notion of growing
popularity is reactivity: the strongest initial response to shocks. On the
other hand, empirical stability is often quantified as the inverse of temporal
variability, directly estimated on data, and reflecting ecosystems response to
persistent and erratic environmental disturbances. It is unclear whether and
how this empirical measure is related to resilience and reactivity. Here, we
establish a connection by introducing two variability-based stability measures
belonging to the theoretical realm of resilience and reactivity. We call them
intrinsic, stochastic and deterministic invariability; respectively defined as
the inverse of the strongest stationary response to white-noise and to
single-frequency perturbations. We prove that they predict ecosystems worst
response to broad classes of disturbances, including realistic models of
environmental fluctuations. We show that they are intermediate measures between
resilience and reactivity and that, although defined with respect to persistent
perturbations, they can be related to the whole transient regime following a
shock, making them more integrative notions than reactivity and resilience. We
argue that invariability measures constitute a stepping stone, and discuss the
challenges ahead to further unify theoretical and empirical approaches to
stability.Comment: 35 pages, 7 figures, 2 table
Cold reactive and non-reactive collisions of Li and Rb with C: implications for hybrid trap experiments
We present a theoretical investigation of cold reactive and non-reactive
collisions of Li and Rb atoms with C. The potential energy surfaces
for the singlet and triplet states of the Li--C and Rb--C
systems have been obtained using the CASSCF/ic-MRCI+Q approach with extended
basis sets. The potential energy surfaces are then used to investigate the
associative detachment reaction and to calculate rotationally inelastic cross
sections by means of the close-coupling method. The effect of the core
correlation on the potential energy surfaces is discussed and we estimate the
error on the collisional cross sections induced by freezing the orbitals
of the carbon atoms. The results are compared to those obtained for the
Rb-OH system and the applications for hybrid trap experiments are
explored. Furthermore, we discuss the possibility to perform Doppler
thermometry on the C anion and investigate the collision process
involving excited states. The implications for sympathetic cooling experiments
are also discussed
Reactivity of hydrated hydroxide anion cluster OH(HO) with H and Rb: an ab initio study
We present a theoretical investigation of the hydrated hydroxide anion
clusters OH(HO) and of the collisional complexes
H-OH(HO) and Rb-OH(HO) (with n). The MP2
and CCSD(T) methods are used to calculate interaction energies, optimized
geometries and vertical detachment energies. Part of the potential energy
surfaces are explored with a focus on the autodetachment region. We point out
the importance of diffuse functions to correctly describe the latter. We use
our results to discuss the different water loss and electronic detachment
channels which are the main reaction routes at room temperature. In particular,
we have considered a direct and an indirect process for the electronic
detachment, depending on whether water loss follows or precedes the detachment
of the excess electron. We use our results to discuss the implication for
astrochemistry and hybrid trap experiments in the context of cold chemistry
Ab initio calculation of H + He charge transfer cross sections for plasma physics
The charge transfer in low energy (0.25 to 150 eV/amu) H() + He
collisions is investigated using a quasi-molecular approach for the as
well as the first two singlet states. The diabatic potential energy
curves of the HeH molecular ion are obtained from the adiabatic potential
energy curves and the non-adiabatic radial coupling matrix elements using a
two-by-two diabatization method, and a time-dependent wave-packet approach is
used to calculate the state-to-state cross sections. We find a strong
dependence of the charge transfer cross section in the principal and orbital
quantum numbers and of the initial or final state. We estimate the
effect of the non-adiabatic rotational couplings, which is found to be
important even at energies below 1 eV/amu. However, the effect is small on the
total cross sections at energies below 10 eV/amu. We observe that to calculate
charge transfer cross sections in a manifold, it is only necessary to
include states with , and we discuss the limitations of our
approach as the number of states increases.Comment: 14 pages, 10 figure
Relationships between Biodiversity and Production in Grasslands at Local and Regional Scales
Key points
1. Experimental manipulations of plant species diversity in unfertilised prairies and meadows has revealed that increasing diversity often leads to increased productivity (range of observed relationships varies from flat to log-linearly positive); driven by a combination of facilitation, niche-partitioning and sampling/selection effects.
2. The longer-term effects of diversity on ecosystem stability are not as clear and in need of further work.
3. Recent applied work, and a new review of the grassland literature, both show the potential for biodiversity to increase productivity under realistic field conditions.
4. The longer-term feedback of grazers on biodiversity gradients is unknown, and grassland biodiversity experiments that incorporate grazers will be needed to test whether patterns differ from those seen in ungrazed prairies and meadows.
5. The relationship between diversity and productivity seen in local experiments is often different from regional-scale correlations, and the scaling-up of experimental results remains a research priority
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Elastic scattering and rotational excitation of nitrogen molecules by sodium atoms
A quantal study of the rotational excitation of nitrogen molecules by sodium atoms is carried out. We present the two-dimensional potential energy surface of the NaN2 complex, with the N2 molecule treated as a rigid rotor. The interaction potential is computed using the spin unrestricted coupled-cluster method with single, double, and perturbative triple excitations (UCCSD(T)). The long-range part of the potential is constructed from the dynamic electric dipole polarizabilities of Na and N2. The total, differential, and momentum transfer cross sections for rotationally elastic and inelastic transitions are calculated using the close-coupling approach for energies between 5 cm−1 and 1500 cm−1. The collisional and momentum transfer rate coefficients are calculated for temperatures between 100 K and 300 K, corresponding to the conditions under which Na–N2collisions occur in the mesosphere.Astronom
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