34 research outputs found
Strategy diversity stabilizes mutualism through investment cycles, phase polymorphism, and spatial bubbles
There is continuing interest in understanding factors that facilitate the evolution and stability of cooperation within and between species. Such interactions will often involve plasticity in investment behavior, in response to the interacting partner's investments. Our aim here is to investigate the evolution and stability of reciprocal invesment behavior in interspecific interactions, a key phenomenon strongly supported by experimental observations. In partcular, we present a comprehensive analysis of a continuous reciprocal investment game between mutualists, both in well-mixed and spatially structured populations, and we demonstrate a series of novel mechanisms for maintaining interspecific mutualism. We demonstrate that mutualistic partners invariably follow investment cycles, during which mutualism firt increases, before both partners eventually reduce their investments to zero, so that these cycles always conclude with full defection. We show that the key mechanism for stabilizing mutualism is phase polymorphism along the investment cycle. Although mutualistic partners perpetually change their strategies, the community-level distribution of investment levels bcomes stationary. In spatially structured populations, the maintenance of polymorphism is further facilitated by dynamic mosaic structures, in which mutualistic partners form expanding and collapsing spatial bubbles or clusters. Additionally, we reveal strategy diversity thresholds, both for well-mixed and spatially structured mutualistic communities, and discuss factors for meeting these thresholds, and thus maintaining mutualism. Our results demonstrate that interspecific mutualism, when considered as plastic investment behavior, can be unstable, and, in agreement with empirical observations, may involve a polymorphism of investment levels, varying both in space and in time. Identifying the mechanisms maintaining such polymorphism, and hence mutualism in natural communities, provides a significant step towards understnding the coevolution and population dynamics of mutualistic interactions
Charge independence breaking and charge symmetry breaking in the nucleon-nucleon interaction from effective field theory
We discuss charge symmetry and charge independence breaking in an effective
field theory approach for few-nucleon systems. We systematically introduce
strong isospin-violating and electromagnetic operators in the theory. The
charge dependence observed in the nucleon-nucleon scattering lengths is due to
one-pion exchange and one electromagnetic four-nucleon contact term. This gives
a parameter free expression for the charge dependence of the corresponding
effective ranges, which is in agreement with the rather small and uncertain
empirical determinations. We also compare the low energy phase shifts of the
and the system.Comment: 11 pp, LaTeX, 2 figures (uses epsfig), dedicated to Walter Gloeckle
on the occasion of his 60th birthda
Evaluating complex relationships between ecological indicators and environmental factors in the Baltic Sea : A machine learning approach
The state of marine ecosystems is increasingly evaluated using indicators. The indicator assessment results need to be understood in the context of the whole ecosystem in order to understand the key factors determining the status of these environmental components. Data available from the system’s different components are, however, often heterogeneous: they may represent different spatial and temporal scales, and different parameters can be measured with different accuracy. This makes it difficult to evaluate the relationship between these variables and status of the environment using indicators. We studied whether probabilistic, machine learning-based classifiers could provide for assessing the relationships between multiple environmental factors and ecological indicators. This paper demonstrates the use of Bayesian network classifiers (Tree-augmented Naive Bayes classifier, TAN as the specific case example), used together with structural learning from data and Entropy Minimization Discretization (IEMD) algorithm to study environment-indicator relationships within coastal fish communities in the Baltic Sea. By using two Baltic-wide indicators of coastal fish community status and a heterogeneous set of potentially influential natural and anthropogenic variables, we explore and discuss the potential of the approach. Given pre-defined cutting points for the indicators, such as the classification thresholds of the indicator, the method enables identifying relevant variables and estimating their relative importance. This information could be used in environmental management to demonstrate at which threshold value the state of an indicator is likely to respond to a pressure or a combination of pressures. In contrast to many other multivariate statistical methodologies, the presented approach can handle missing data as well as data of varying types, from fully quantitative to presence-absence, in the same analysis.peerReviewe
Neutral pion photoproduction off 3H and 3He in chiral perturbation theory
We calculate electromagnetic neutral pion production off three-nucleon bound
states (3H, 3He) at threshold to leading one-loop order in the framework of
chiral nuclear effective field theory. In addition, we analyze the dependence
of the nuclear S-wave amplitude E_{0+} on the elementary neutron amplitude
E_{0+}^{pi0 n} which in the case of 3He provides a stringent test of the
prediction based on chiral perturbation theory. Uncertainties from higher order
corrections are estimated.Comment: 6 pages, 3 figures, version accepted for publication in Phys. Lett.
B, improved discussion of higher-order corrections, results unchange
Hadron Structure in the Non-Perturbative Regime of QCD: Isospin Symmetry and its Violation
I discuss recent progress made in calculating electromagnetic corrections in
the framework of the effective field theory of QCD. In the case of elastic
pion-pion scattering, strong interaction predictions have been worked out to
two loop accuracy. I present first results for the electromagnetic corrections
in the case of neutral pions. Here, the only sizeable effect comes from the
charged to neutral pion mass difference. In the presence of nucleons, isospin
violation can be measured in threshold pion photoproduction. I review the
present status of the theoretical predictions and the experimental data. I
argue that a deeper understanding of isospin violation based on a more precise
study of such reactions can be achieved.Comment: 10 pp, LaTeX file, 3 figures, uses espcrc1.sty and epsf, plenary
talk, QULEN '97, Osaka, May 1997, to be published in the proceeding
On neutral pion electroproduction off deuterium
Threshold neutral pion electroproduction on the deuteron is studied in the
framework of baryon chiral perturbation theory at next-to-leading order in the
chiral expansion. To this order in small momenta, the amplitude is finite and a
sum of two- and three-body interactions with no undetermined parameters. We
calculate the S-wave multipoles for threshold production and the deuteron
S-wave cross section as a function of the photon virtuality. We also discuss
the sensitivity to the elementary neutron amplitudes.Comment: 6 pp, revtex, 3 figs, corrected version, to appear in Phys. Rev.
Improved description of threshold pion electroproduction in chiral perturbation theory
We investigate neutral pion electroproduction off protons in the framework of
heavy baryon chiral perturbation theory. The chiral expansion of the S--wave
multipoles and is carried out to three orders. There appear
several undetermined low--energy constants. Three are taken from a recent study
of the new TAPS threshold photoproduction data, one is fixed from the
proton Dirac form factor and the novel S--wave constants appearing at orders
and are determined from a best fit (constrained by resonance
exchange) to the recent NIKHEF and MAMI data at photon momentum transfer
squared . The inclusion of a particular set of
dimension five operators is forced upon the fit by a soft--pion theorem which
severely constrains the momentum dependence of the longitudinal S--wave
multipole at order . We give predictions for lower photon
virtualities for the various multipoles and differential cross sections.
Further improvements are briefly touched upon.Comment: 25 pp, LaTeX, uses epsf and elsart12.sty, 8 figures in 10 PS file
Threshold neutral pion photoproduction off the tri-nucleon to O(q^4)
We calculate electromagnetic neutral pion production off tri-nucleon bound
states (3H, 3He) at threshold in chiral nuclear effective field theory to
fourth order in the standard heavy baryon counting. We show that the fourth
order two-nucleon corrections to the S-wave multipoles at threshold are very
small. This implies that a precise measurement of the S-wave cross section for
neutral pion production off 3He allows for a stringent test of the chiral
perturbation theory prediction for the S-wave electric multipole E_{0+}^{pi0
n}.Comment: 17 pages, 5 figures, title changed, final version to appear in EPJA.
arXiv admin note: substantial text overlap with arXiv:1103.340
Pion-nucleon scattering in chiral perturbation theory I: Isospin-symmetric case
We construct the complete effective chiral pion-nucleon Lagrangian to third
order in small momenta based on relativistic chiral perturbation theory. We
then perform the so-called heavy baryon limit and construct all terms
up-to-and-including order with fixed and free coefficients. As an
application, we discuss in detail pion-nucleon scattering. In particular, we
show that the expansion of the Born graphs calculated relativistically
can be recovered exactly in the heavy baryon approach without any additional
momentum-dependent wave function renormalization. We fit various empirical
phase shifts for pion laboratory momenta between 50 and 100 MeV. This leads to
a satisfactory description of the phase shifts up to momenta of about 200 MeV.
We also predict the threshold parameters, which turn out to be in good
agreement with the dispersive analysis. In particular, we can sharpen the
prediction for the isovector S-wave scattering length, . We also consider the subthreshold parameters
and give a short comparison to other calculations of scattering in
chiral perturbation theory or modifications thereof.Comment: 38 pp, LaTeX, uses epsf, 7 figs, additional remarks, one figure
added, version to be published in Nucl.Phys.