Epiphyte metapopulation dynamics are explained by species traits, connectivity, and patch dynamics

The colonization-extinction dynamics of many species are affected by the dynamics of their patches. For increasing our understanding of the metapopulation dynamics of sessile species confined to dynamic patches, we fitted a Bayesian incidence function model extended for dynamic landscapes to snapshot data on five epiphytic lichens among 2083 mapped oaks (dynamic patches). We estimate the age at which trees become suitable patches for different species, which defines their niche breadth (number of suitable trees). We show that the colonization rates were generally low, but increased with increasing connectivity in accordance with metapopulation theory. The rates were related to species traits, and we show, for the first time, that they are higher for species with wide niches and small dispersal propagules than for species with narrow niches or large propagules. We also show frequent long-distance dispersal in epiphytes by quantifying the relative importance of local dispersal and background deposition of dispersal propagules. Local stochastic extinctions from intact trees were negligible in all study species, and thus, the extinction rate is set by the rate of patch destruction (tree fall). These findings mean that epiphyte metapopulations may have slow colonization-extinction dynamics that are explained by connectivity, species traits, and patch dynamics

Empirical calibrations of optical absorption line indices based on the stellar library MILES

Stellar population models of absorption line indices are an important tool for the analysis of stellar population spectra. They are most accurately modelled through empirical calibrations of absorption line indices with the stellar parameters effective temperature, metallicity, and surface gravity, the so-called fitting functions. Here we present new empirical fitting functions for the 25 optical Lick absorption line indices based on the new stellar library MILES. The major improvements with respect to the Lick/IDS library are the better sampling of stellar parameter space, a generally higher signal- to-noise, and a careful flux calibration. In fact we find that errors on individual index measurements in MILES are considerably smaller than in Lick/IDS. Instead we find the rms of the residuals between the final fitting functions and the data to be dominated by errors in the stellar parameters. We provide fitting functions for both Lick/IDS and MILES spectral resolutions, and compare our results with other fitting functions in the literature. A Fortran 90 code is available online in order to simplify the implementation in stellar population models. We further calculate the offsets in index measurements between the Lick/IDS system to a flux calibrated system. For this purpose we use the three libraries MILES, ELODIE, and STELIB. We find that offsets are negligible in some cases, most notably for the widely used indices Hbeta, Mgb, Fe5270, and Fe5335. In a number of cases, however, the difference between flux calibrated library and Lick/IDS is significant with the offsets depending on index strengths. Interestingly, there is no general agreement between the three libraries for a large number of indices, which hampers the derivation of a universal offset between the Lick/IDS and flux calibrated systems.Comment: 19 pages, MNRAS in press, online material available at http://www.icg.port.ac.uk/~johanss

A New Algorithm for Solving Ring-LPN with a Reducible Polynomial

The LPN (Learning Parity with Noise) problem has recently proved to be of great importance in cryptology. A special and very useful case is the RING-LPN problem, which typically provides improved efficiency in the constructed cryptographic primitive. We present a new algorithm for solving the RING-LPN problem in the case when the polynomial used is reducible. It greatly outperforms previous algorithms for solving this problem. Using the algorithm, we can break the Lapin authentication protocol for the proposed instance using a reducible polynomial, in about 2^70 bit operations

Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios

We present new stellar population models of Lick absorption-line indices with variable element abundance ratios. The models are based on our new calibrations of absorption-line indices with stellar parameters derived from the MILES stellar library. The key novelty compared to our previous models is that they are now available at the higher spectral resolution of MILES (~2.7A FWHM) and flux-calibrated, hence not tied anymore to the Lick/IDS system. This is essential for the interpretation of galaxy spectra where calibration stars are not available, such as large galaxy redshift surveys or other high-redshift observations. We note that the MILES resolution appears to be comparable to SDSS resolution, so that our models can be applied to SDSS data without any corrections for instrumental spectral resolution. For the first time we provide random errors for the model predictions based on the uncertainties in the calibration functions and the underlying stellar parameter estimates. We show that random errors are small except at the edges of the parameter space (high/low metallicities and young ages <1 Gyr) where the stellar library is under-sampled. We calibrate the base model for the parameters age, metallicity and alpha/Fe ratio with galactic globular cluster and galaxy gradient data. We discuss two model flavours with different input stellar evolutionary tracks from the Frascati and Padova groups. The new model release now includes abundance variations of the elements C, N, Mg, Na, Si, Ca, Ti, Cr, and Fe. The individual elements that are best accessible with these models and the standard set of Lick absorption features are C, N, Mg, Ca, Ti, and Fe. The model data is available at www.icg.port.ac.uk/~thomasd.Comment: MNRAS, re-submitted including referee's comments (moderate revision

Traffic matrix estimation on a large IP backbone: a comparison on real data

This paper considers the problem of estimating the point-to-point traffic matrix in an operational IP backbone. Contrary to previous studies, that have used a partial traffic matrix or demands estimated from aggregated Netflow traces, we use a unique data set of complete traffic matrices from a global IP network measured over five-minute intervals. This allows us to do an accurate data analysis on the time-scale of typical link-load measurements and enables us to make a balanced evaluation of different traffic matrix estimation techniques. We describe the data collection infrastructure, present spatial and temporal demand distributions, investigate the stability of fan-out factors, and analyze the mean-variance relationships between demands. We perform a critical evaluation of existing and novel methods for traffic matrix estimation, including recursive fanout estimation, worst-case bounds, regularized estimation techniques, and methods that rely on mean-variance relationships. We discuss the weaknesses and strengths of the various methods, and highlight differences in the results for the European and American subnetworks

Positional Concerns with Multiple Reference Points: Optimal Income Taxation and Public Goods in an OLG Model

This paper concerns optimal income taxation and provision of a state-variable public good under asymmetric information in a two-type overlapping generations model, where people care about their relative consumption. Each individual may compare his/her own current consumption with his/her own past consumption as well as with other people’s current and past consumption. The appearance of positional concerns affects the policy choices via two channels: (i) the size of the average degree of positionality and (ii) positionality differences between the (mimicked) low-ability type and the mimicker. Under plausible empirical estimates, the marginal labor income tax rates become substantially larger, and the absolute value of the marginal capital income tax rate of the low-ability type becomes substantially smaller, compared to the conventional optimal income tax model. The extent by which the rule for public provision should be modified depends crucially on the preference elicitation format.Optimal income taxation; asymmetric information; public goods; relative consumption; status; positional goods

Veblen’s Theory of the Leisure Class Revisited: Implications for Optimal Income Taxation

Almost all previous studies on public policy under relative consumption concerns have ignored the role of leisure for status comparisons. Inspired by Veblen (1899), this paper considers a two-type optimal income tax model, where people care about their relative consumption, and where the importance of relative consumption increases with the use of leisure due to increased consumption visibility. We show that increased consumption positionality typically implies higher marginal income tax rates for both ability-types. Using a leisure-weighted measure of reference consumption, rather than a measure where leisure plays no role as in the previous literature, increases the marginal income tax rate implemented for the low-ability type and decreases the marginal income tax rate implemented for the high-ability type, i.e., it gives rise to a regressive tax component.optimal taxation; redistribution; public goods; relative consumption; status; positional goods

State-Variable Public Goods When Relative Consumption Matters: A Dynamic Optimal Taxation Approach

This paper concerns the optimal provision of a state-variable public good, where the global climate is the prime example. The analysis is based on a two-type optimal income tax model with overlapping generations, where people care about their relative consumption. We consider both keeping-up-with-the-Joneses preferences (where people compare their own current consumption with others’ current consumption) and catching-up-with-the-Joneses preferences (where people compare their own current consumption with others’ past consumption). The extent to which the rule for public provision ought to be modified is shown to depend crucially on the preference elicitation format.State variable public goods; asymmetric information; relative consumption; status; positional preferences; climate policy

Conspicuous Leisure: Optimal Income Taxation when both Relative Consumption and Relative Leisure Matter

Previous studies on public policy under relative consumption concerns have ignored the role of leisure comparisons. This paper considers a two-type optimal nonlinear income tax model where people care both about their relative consumption and their relative leisure. Increased consumption positionality typically implies higher marginal income tax rates for both the high-ability and the low-ability type, whereas leisure positionality has an offsetting role. However, this offsetting role is not symmetric; concern about relative leisure implies a progressive income tax component, i.e., a component that is larger for the high-ability than for the low-ability type. Moreover, leisure positionality does not modify the policy rule for public good provision when the income tax is optimally chosen.Optimal taxation; redistribution; public goods; relative consumption; status; positional goods

Discounting and Relative Consumption

This paper analyzes optimal social discount rates where people derive utility from relative consumption. We identify and compare three separate discount rates -- the social rate (taking positional externalities into account), the private rate, and the conventional Ramsey rate. Two main findings resulted for the standard case with a positive growth rate -- 1) the social discount rate exceeds the private discount rate if the degree of positionality increases with consumption, and 2) the social discount rate is smaller than the Ramsey rate if preferences are quasi-concave in own and reference consumption, and exhibit risk aversion with respect to reference consumption. Numerical calculations demonstrate that the latter difference may be substantial and economically important for such issues as global warming.discounting, relative consumption, Ramsey rule, degree of positionality, keeping up with the Joneses