Disturbance, Pollinator Predictability, and Pollination Success Among Costa Rican Cloud Forest Plants

Cloud forest at Monteverde, Costa Rica experiences frequent natural disturbance. To determine whether species interactions vary spatially due to physical heterogeneity produced by disturbance, we examined relationships between 22 plant species and 11 nectar—feeding bird species in 14 study plots distributed among three patch types: larger landslide—like gaps (hand—cleared areas along a trail), small gaps (formed by recent treefalls), and understory of closed—canopy forest. Species we describe here flowered in two or three patch types. The aspects of pollination we examined varied little with patch type. Mean frequency of pollinator visits varied with patch type in a few plant species but not in most, and there was no significant trend across species. Pollen loads carried by 314 mist—netted hummingbirds did not vary significantly with patch type, either in total number of grains or number of species represented. Cumulative pollen loads that hummingbirds deposited on stigmas of two species of Acanthaceae (Razisea spicata and Hansteinia blepharorachis) did not vary consistently with patch type, except that Hansteinia flowers in treefall gaps received fewer heterospecific pollen grains than flowers in the other two patch types. Frequency of fruit set varied significantly with patch type in three of the four species examined, but the direction of variation in one of these was opposite to the direction of the other two. The absolute frequency with which flowers were pierced by nectar—robbing hummingbirds did not vary significantly with patch type, although the frequency of piercing relative to legitimate pollinator visits did increase in the large gaps. We attribute the latter result to aggregation of the hummingbird Eupherusa eximia, a chronic nectar robber, at dense clumps of long—flowered plant species that occurred in large gaps. Only one feature we examined suggested that patch type might directly affect the nature of species interactions: in two different analyses, the level of variation in frequency of hummingbird visits to flowers declined from large gaps to small gaps to forest. Results suggest that, unless the disturbance initiating a patch is unusually severe or widespread, interactions between the plants and hummingbirds examined are insensitive to patch type. Such species, existing in naturally dynamic forests throughout their recent evolutionary histories, presumably have become accommodated to frequent small—scale disturbance. Results also suggest that those habitat—related contrasts in plant reproductive traits and plant—pollinator interactions documented in other studies, which compare habitats initiated by anthropogenic disturbances with undisturbed patches, may be artifacts to some extent. Anthropogenically generated disturbance mosaics may promote the spread of species whose reproductive traits evolved under very different circumstances from mosaics generated by natural disturbances

Site dilution of quantum spins in the honeycomb lattice

We discuss the effect of site dilution on both the magnetization and the density of states of quantum spins in the honeycomb lattice, described by the antiferromagnetic Heisenberg spin-S model. For this purpose a real-space Bogoliubov-Valatin transformation is used. In this work we show that for the S>1/2 the system can be analyzed in terms of linear spin wave theory. For spin S=1/2, however, the linear spin wave approximation breaks down. In this case, we have studied the effect of dilution on the staggered magnetization using the Stochastic Series Expansion Monte Carlo method. Two main results are to be stressed from the Monte Carlo method: (i) a better value for the staggered magnetization of the undiluted system, m=0.2677(6); (ii) a finite value of the staggered magnetization of the percolating cluster at the classical percolation threshold, showing that there is no quantum critical transition driven by dilution in the Heisenberg model. In the solution of the problem using linear the spin wave method we pay special attention to the presence of zero energy modes. Using a combination of linear spin wave analysis and the recursion method we were able to obtain the thermodynamic limit behavior of the density of states for both the square and the honeycomb lattices. We have used both the staggered magnetization and the density of states to analyze neutron scattering experiments and Neel temperature measurements on quasi-two- -dimensional honeycomb systems. Our results are in quantitative agreement with experimental results on Mn_pZn_{1-p}PS_3 and on the Ba(Ni_pMg_{1-p})_2V_2O_8.Comment: 21 pages (REVTEX), 16 figure

Evaluation of Character Displacement Among Plants in Two Tropical Pollination Guilds

In cloud forest at Monteverde, Costa Rica, two guilds of bird—pollinated plants exist; on guild pollinated by long—billed hummingbirds, primarily the Green Hermit (Phaethornis guy), and one guild pollinated by short—billed hummingbirds, primarily the Purple—throated Mountain—gem (Lampornis calolaema). Plants were assigned to guilds based on hummingbird visit patterns documented during >4000 plant—hours of field observations, and on identities of pollen grains collected from 600 mist—netted hummingbirds. Other studies indicated that pollination in these plants is often insufficient for maximum seed set. Each guild was examined for character displacement expected within a stable assemblage of plants structured by competition for pollination. (1) By comparing observed flowering phenologies with those obtained through a randomization procedure, we determined whether each species' phenology minimized overlap with the remainder of its guild. (2) We also examined complementarity between phenological displacement and morphological displacement in reproductive structures. Neither guild exhibited pronounced character displacement. (1) In most cases, flowering phenologies were indistinguishable from those generated at random; the few statistically significant departures mostly indicated aggregation, rather than displacement, of flowering seasons. (2) In most cases, morphological similarity was independent of phenological similarity. The only statistically significant result among the studied species was a positive correlation, among long—flowered species only, between rarity and uniqueness of flowering season. We do not conclude that this absence of expected pattern indicates that competition never occurs or that competition is an inconsequential ecological event. Rather, we attribute absence of pattern to the following aspects of biological variability, two of which we have demonstrated in other studies. (1) Within any one year, density—dependent competition for pollination is sporadic, and is not clearly related to flowering season or morphological similarity. (2) The nature of interspecific interactions varies among years, as neither the relative intensities of flowering nor the flowering seasons themselves are consistent from year to year. (3) The nature of interspecific interactions varies with changes in species composition, which occur over short distances. (4) The assemblage of species is probably not stable over long time spans; the species have Gleasonian ecologies that change distribution and abundance faster than natural selection or diffuse competition can screen out improper phenotypes or species, respectively

Mixed Support for Spatial Heterogeneity in Species Interactions: Hummingbirds in a Tropical Disturbance Mosaic

Many natural landscapes experience frequent disturbance on a small scale. Disturbance loosens or disrupts relations between species, or between species and resources, characteristic of intact communities. One result is the release of previously scarce resources, leading to increased productivity and increased intensity of species interactions in disturbed patches as compared with undisturbed patches. Additionally, populations in disturbed sites may exploit resources in a more haphazard and opportunistic fashion than populations in undisturbed sites. The altered ecological conditions of disturbed sites may favor species different from those occupying undisturbed sites, leading to spatial heterogeneity in community composition. Nectar-feeding birds (mainly hummingbirds) inhabiting the natural disturbance mosaic of a Costa Rican cloud forest responded to habitat heterogeneity in complex ways. Whereas most ecological traits of hummingbird assemblages varied among patch types (understory of canopied forest; treefall gaps; large, landslide-like gaps), the direction of variation differed for different traits. Density of hummingbird food (nectar) was highest in treefall gaps, and some characteristics of hummingbirds (e.g., species diversity) reflected this enrichment. Variables that involve collective foraging by the entire hummingbird assemblage (e.g., intensity of interspecific competition) suggest that species interactions in the forest are the least haphazard, those in treefall gaps more haphazard, and those in large gaps the most haphazard. Even the largest gaps examined, however, were rarely invaded by hummingbird "weeds" available in the regional species pool, and interactions in these gaps showed only faint resemblance to those in the tremendously fluctuating competitive environments that characterize nectar-feeding bird assemblages in large anthropogenic old fields nearby or at other tropical sites. Our results, and reconsideration of results from other studies involving natural disturbance mosaics, suggest that responses of consumers to disturbance mosaics may often be subtle and complex. Comparisons between patch types in a natural disturbance mosaic need not resemble comparisons between points in a successional sequence after anthropogenic disturbance

Peierls to superfluid crossover in the one-dimensional, quarter-filled Holstein model

We use continuous-time quantum Monte Carlo simulations to study retardation effects in the metallic, quarter-filled Holstein model in one dimension. Based on results which include the one- and two-particle spectral functions as well as the optical conductivity, we conclude that with increasing phonon frequency the ground state evolves from one with dominant diagonal order---2k_F charge correlations---to one with dominant off-diagonal fluctuations, namely s-wave pairing correlations. In the parameter range of this crossover, our numerical results support the existence of a spin gap for all phonon frequencies. The crossover can hence be interpreted in terms of preformed pairs corresponding to bipolarons, which are essentially localised in the Peierls phase, and "condense" with increasing phonon frequency to generate dominant pairing correlations.Comment: 11 pages, 5 figure

Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires

Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and 0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques

Transmission Imaging With Axially Overlapping Cone-Beams

We have shown that cone-beam transmission imaging of medium-energy photons that penetrate the parallel-hole collimators can be used to rapidly estimate attenuation maps for use in reconstruction of cardiac SPECT images. Such a transmission imaging geometry offers the advantages of eliminating the need to mechanically move the point-sources during imaging, and minimizes cross-talk between emission and transmission imaging. The axial extent over which artifact-free attenuation maps can be reconstructed is limited by the cone-beam geometry and source collimation. We investigated irradiation of a single head by multiple point-sources such that their asymmetric cone-beam fields overlap in the axial direction as a method of extending the axial coverage of the patient. This study reports on testing of a penalized-likelihood algorithm for transmission reconstruction of overlapping cone-beams. This algorithm was evaluated through MCAT simulations and applied to transmission measurements of an anthropomorphic phantom. The experimental work consisted of performing a series of flood and transmission measurements on the anthropomorphic phantom with shifted axial locations of point-sources. We summed the projection data from individual measurements to simulate the projection data for a multiple point-source system. With the proposed penalized-Iikelihood algorithm, the full axial extent (20.5 cm) of the anthropomorphic phantom was reconstructed for the overlapping cone-beam geometry with 2 point-sources per camera head.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85904/1/Fessler202.pd

Treatment of spontaneous preterm labour with retosiban: a phase 2 proof-of-concept study

This is the peer reviewed version of the article which has been published in final form at doi: 10.1111/bcp.12646. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.AIM: To investigate the efficacy and safety of intravenous retosiban in women with spontaneous preterm labour. METHODS: Randomised, double-blind, placebo-controlled, phase 2 trial. Retosiban was administered intravenously for 48 hours to women in spontaneous preterm labour between 30(0/7) and 35(6/7)  weeks' gestation with an uncomplicated singleton pregnancy in an in-patient obstetric unit. Outcome measures were uterine quiescence (primary endpoint), days to delivery, preterm delivery, and safety. RESULTS: Uterine quiescence was achieved in 62% of women who received retosiban (n = 30) compared with 41% who received placebo (n = 34). The relative risk (RR) was 1.53 (95% credible interval [CrI]: 0.98, 2.48; NS). Retosiban resulted in a significant increase in time to delivery compared with placebo (mean difference, 8.2 days; 95% CrI: 2.7, 13.74); this difference was consistent across all gestational ages. The proportion of preterm births in the retosiban and placebo groups was 18.7% (95% CrI: 7.4%, 33.7%) and 47.2% (95% CrI: 31.4%, 63.4%), respectively. The RR of preterm birth in women treated with retosiban was 0.38 (95% CrI: 0.15, 0.81). There were no deliveries within 7 days in the retosiban group, but there were six (17.6%) births in the placebo group. Maternal, fetal, and neonatal adverse events were similar in the retosiban and placebo groups. CONCLUSIONS: Intravenous administration of retosiban in women with spontaneous preterm labour was associated with a greater than 1-week increase in time to delivery compared with placebo, a significant reduction in preterm deliveries, a non-significant increase in uterine quiescence, and a favourable safety profile.GlaxoSmithKlin

Estimation of the rigid-body motion from three-dimensional images using a generalized center-of-mass points approach

We present an analytical method for the estimation of rigid-body motion in sets of three-dimensional (3-D) SPECT and PET slices. This method utilizes mathematically defined generalized center-of-mass points in images, requiring no segmentation. It can be applied to compensation of the rigid-body motion in both SPECT and PET, once a series of 3-D tomographic images are available. We generalized the formula for the center-of-mass to obtain a family of points comoving with the object\u27s rigid-body motion. From the family of possible points we chose the best three points which resulted in the minimum root-mean-square difference between images as the generalized center-of-mass points for use in estimating motion. The estimated motion was used to sum the sets of tomographic images, or incorporated in the iterative reconstruction to correct for motion during reconstruction of the combined projection data. For comparison, the principle-axes method was also applied to estimate the rigid-body motion from the same tomographic images. To evaluate our method for different noise levels, we performed simulations with the MCAT phantom. We observed that though noise degraded the motion-detection accuracy, our method helped in reducing the motion artifact both visually and quantitatively. We also acquired four sets of the emission and transmission data of the Data Spectrum Anthropomorphic Phantom positioned at four different locations and/or orientations. From these we generated a composite acquisition simulating periodic phantom movements during acquisition. The simulated motion was calculated from the generalized center-of-mass points calculated from the tomographic images reconstructed from individual acquisitions. We determined that motion-compensation greatly reduced the motion artifact. Finally, in a simulation with the gated MCAT phantom, an exaggerated rigid-body motion was applied to the end-systolic frame. The motion was estimated from the end-diastolic and end-systolic images, and used to sum them into a summed image without obvious artifact. Compared to the principle-axes method, in two of the three comparisons with anthropomorphic phantom data our method estimated the motion in closer agreement to the Polaris system than the principal-axes method, while the principle-axes method gave a more accurate estimation of motion in most cases for the MCAT simulations. As an image-driven approach, our method assumes angularly com plete data sets for each state of motion. We expert this method to be applied in correction of respiratory motion in respiratory gated SPECT, and respiratory or other rigid-body motion in PET. © 2006 IEEE