Population Dynamics on Complex Food Webs

In this work we analyse the topological and dynamical properties of a simple model of complex food webs, namely the niche model. In order to underline competition among species, we introduce "prey" and "predators" weighted overlap graphs derived from the niche model and compare synthetic food webs with real data. Doing so, we find new tests for the goodness of synthetic food web models and indicate a possible direction of improvement for existing ones. We then exploit the weighted overlap graphs to define a competition kernel for Lotka-Volterra population dynamics and find that for such a model the stability of food webs decreases with its ecological complexity.Comment: 11 Pages, 5 Figures, styles enclosed in the submissio

Condition numbers and scale free graphs

In this work we study the condition number of the least square matrix corresponding to scale free networks. We compute a theoretical lower bound of the condition number which proves that they are ill conditioned. Also, we analyze several matrices from networks generated with the linear preferential attachment model showing that it is very difficult to compute the power law exponent by the least square method due to the severe lost of accuracy expected from the corresponding condition numbers.Comment: Submitted to EP

Power-law distributions in empirical data

Power-law distributions occur in many situations of scientific interest and have significant consequences for our understanding of natural and man-made phenomena. Unfortunately, the detection and characterization of power laws is complicated by the large fluctuations that occur in the tail of the distribution -- the part of the distribution representing large but rare events -- and by the difficulty of identifying the range over which power-law behavior holds. Commonly used methods for analyzing power-law data, such as least-squares fitting, can produce substantially inaccurate estimates of parameters for power-law distributions, and even in cases where such methods return accurate answers they are still unsatisfactory because they give no indication of whether the data obey a power law at all. Here we present a principled statistical framework for discerning and quantifying power-law behavior in empirical data. Our approach combines maximum-likelihood fitting methods with goodness-of-fit tests based on the Kolmogorov-Smirnov statistic and likelihood ratios. We evaluate the effectiveness of the approach with tests on synthetic data and give critical comparisons to previous approaches. We also apply the proposed methods to twenty-four real-world data sets from a range of different disciplines, each of which has been conjectured to follow a power-law distribution. In some cases we find these conjectures to be consistent with the data while in others the power law is ruled out.Comment: 43 pages, 11 figures, 7 tables, 4 appendices; code available at http://www.santafe.edu/~aaronc/powerlaws

Incorporation Of Deepwater Horizon Oil In A Terrestrial Bird

Carbon isotopic evidence revealed Deepwater Horizon (DWH) oil entering coastal planktonic and lower terrestrial food webs. The integration of spilled oil into higher terrestrial trophic levels, however, remains uncertain. Wemeasured radiocarbon (C-14) and stable carbon (C-13) in seaside sparrow (Ammodramus maritimus) feathers and crop contents. Lower C-14 and C-13 values in feathers and crop contents of birds from contaminated areas indicated incorporation of carbon from oil. Our results, although based on a small sample of birds, thus reveal a food-web link between oil exposure and a terrestrial ecosystem. They also suggest that the reduction in reproductive success previously documented in the same population might be due to the (direct) toxic effect of oil exposure, rather than to (indirect) ecological effects. Werecommend future studies test our results by using larger samples of birds from a wider area in order to assess the extent and implications ofDWHoil incorporation into the terrestrial food web

Dicarbonyl­dichloridobis(trimethyl­phosphane)iron(II)–carbonyl­dichlorido­tris(trimethyl­phosphane)iron(II)–tetra­hydro­furan (1/1/2)

The asymmetric unit of the title crystal, [FeCl2(C3H9P)3(CO)]·[FeCl2(C3H9P)2(CO)2]·2C4H8O, contains half mol­ecules of the two closely related FeII complexes lying on mirror planes and a tetra­hydro­furan solvent mol­ecule, one C atom of which is disordered over two sets of sites with site occupancy factors 0.633 (9) and 0.367 (9). In both FeII complex mol­ecules, a distorted octa­hedral coordination geometry has been observed around the Fe atoms. Weak intermolecular C—H⋯O inter­actions are observed in the crystal structure

Lutein-fortified Infant Formula Fed to Healthy Term Infants: Evaluation of Growth Effects and Safety

Background/Objectives: Breast milk contains lutein derived from the mother\u27s diet. This carotenoid is currently not added to infant formula, which has a small and variable lutein content from innate ingredients. This study was conducted to compare the growth of infants fed lutein-fortified infant formula with that of infants fed infant formula without lutein fortification. Subjects/Methods: This 16-week study was prospective, randomized, controlled, and double-blind with parallel groups of healthy term infants fed either control formula (Wyeth S-26 Gold, designated as Gold) or experimental formula (Wyeth S-26 Gold fortified with lutein at 200 mcg/l, designated as Gold + Lutein). Two hundred thirty-two (232) infants ≤ 14 days postnatal age were randomized and 220 (94.8%) completed the study. Weight (g), head circumference (cm), and length (cm) were measured at Weeks 4, 8, 12, and 16. The primary endpoint was weight gain (g/day) from baseline to Week 16. Safety was assessed through monitoring of study events (SEs) throughout the study and evaluation of selected blood chemistry tests performed at Week 16. Results: Infants in both treatment groups demonstrated appropriate growth. No differences between treatment groups were found in any of the measures of growth at any of the measurement time points. Both study formulas were well tolerated. The mean values of all measured blood chemistry parameters fell within the modified normal ranges for infants, and the values for both groups for any measured parameter were similar. Conclusions: Infants fed lutein-fortified S-26 Gold demonstrated growth equivalent to that of infants fed unfortified lutein formula

What can mixed-species flock movement tell us about the value of Amazonian secondary forests? Insights from spatial behavior

The value of secondary forest for rain forest species remains an important question for conservation in the 21st century. Here, we describe the spatial behavior of understory mixed species flocks in a heterogeneous landscape in central Amazonia. Understory mixed species flocks represent a diverse, highly organized component of the rich Amazonian avifauna. We recorded movements within 26 flock home ranges in primary forest, secondary forest, interfaces between forest types, and forest fragments. We describe frequency and movement orientation in relation to forest edges, movement patterns and proportion of use between secondary and primary forest, the relation between home range sizes and vegetation height, and home range configuration. Flocks visited only a small portion of forest edges, and showed a tendency for moving parallel to edges next to less developed secondary forest. Movement patterns in secondary forests did not show significant differences compared to primary forests. Time spent in secondary forests increased in proportion to mean canopy height. Flocks were consistently present in secondary forests where vegetation height averaged over 15 m, but home ranges were nearly twice as large compared to primary forest. Home range limits tended to be aligned with disturbed vegetation, essentially rearranging a territorial configuration normally adjusted by topography. The spatial behavior of this important subset of the Amazonian avifauna shows that secondary forests are tolerated above a certain development threshold, but perceived as sub optimal habitat until canopy height closely matches primary forests

A universal model for mobility and migration patterns

Introduced in its contemporary form by George Kingsley Zipf in 1946, but with roots that go back to the work of Gaspard Monge in the 18th century, the gravity law is the prevailing framework to predict population movement, cargo shipping volume, inter-city phone calls, as well as bilateral trade flows between nations. Despite its widespread use, it relies on adjustable parameters that vary from region to region and suffers from known analytic inconsistencies. Here we introduce a stochastic process capturing local mobility decisions that helps us analytically derive commuting and mobility fluxes that require as input only information on the population distribution. The resulting radiation model predicts mobility patterns in good agreement with mobility and transport patterns observed in a wide range of phenomena, from long-term migration patterns to communication volume between different regions. Given its parameter-free nature, the model can be applied in areas where we lack previous mobility measurements, significantly improving the predictive accuracy of most of phenomena affected by mobility and transport processes.Comment: Main text and supplementary informatio

Mesoscopic structure and social aspects of human mobility

The individual movements of large numbers of people are important in many contexts, from urban planning to disease spreading. Datasets that capture human mobility are now available and many interesting features have been discovered, including the ultra-slow spatial growth of individual mobility. However, the detailed substructures and spatiotemporal flows of mobility - the sets and sequences of visited locations - have not been well studied. We show that individual mobility is dominated by small groups of frequently visited, dynamically close locations, forming primary "habitats" capturing typical daily activity, along with subsidiary habitats representing additional travel. These habitats do not correspond to typical contexts such as home or work. The temporal evolution of mobility within habitats, which constitutes most motion, is universal across habitats and exhibits scaling patterns both distinct from all previous observations and unpredicted by current models. The delay to enter subsidiary habitats is a primary factor in the spatiotemporal growth of human travel. Interestingly, habitats correlate with non-mobility dynamics such as communication activity, implying that habitats may influence processes such as information spreading and revealing new connections between human mobility and social networks.Comment: 7 pages, 5 figures (main text); 11 pages, 9 figures, 1 table (supporting information