Variation in per capita interaction strength: Thresholds due to nonlinear dynamics and nonequilibrium conditions

I measured the strength of interaction between a marine herbivore and its growing resource over a realistic range of absolute and relative abundances. The herbivores (hermit crabs: Pagurus spp.) have slow and/or weak functional and numerical responses to epiphytic diatoms (Isthmia nervosa), which show logistic growth in the absence of consumers. By isolating this interaction in containers in the field, I mimicked many of the physical and biological variables characteristic of the intertidal while controlling the densities of focal species. The per capita effects of consumers on the population dynamics of their resource (i.e., interaction strength) were defined by using the relationship between hermit crab density and proportional change in the resource. When this relationship is fit by a Weibull function, a single parameter distinguishes constant interaction strength from one that varies as a function of density. Constant interaction strength causes the proportion of diatoms to fall linearly or proportionally as hermit crab density increases whereas per capita effects that increase with density cause an accelerating decline. Although many mathematical models of species interactions assume linear dynamics and invariant parameters, at least near equilibrium, the per capita effects of hermit crabs on diatoms varied substantially, apparently crossing a threshold from weak to strong when consumption exceeded resource production. This threshold separates a domain of coexistence from one of local extinction of the resource. Such thresholds may help explain trophic cascades, resource compensation, and context-dependent interaction strengths, while indicating a way to predict trophic effects, despite nonlinearities, as a function of vital rates.

Modeling the dynamics of human hair cycles by a follicular automaton

The hair follicle cycle successively goes through the anagen, catagen, telogen, and latency phases, which correspond, respectively, to hair growth, arrest, shedding, and absence before a new anagen phase is initiated. Experimental observations collected over a period of 14 years in a group of 10 male volunteers, alopecic and nonalopecic, allowed us to determine the characteristics of scalp hair follicle cycles. On the basis of these observations, we propose a follicular automaton model to simulate the dynamics of human hair cycles. The automaton model is defined by a set of rules that govern the stochastic transitions of each follicle between the successive states anagen, telogen, and latency, and the subsequent return to anagen. The transitions occur independently for each follicle, after time intervals given stochastically by a distribution characterized by a mean and a variance. The follicular automaton model accounts both for the dynamical transitions observed in a single follicle and for the behavior of an ensemble of independently cycling follicles. Thus, the model successfully reproduces the evolution of the fractions of follicle populations in each of the three phases, which fluctuate around steady-state or slowly drifting values. We apply the follicular automaton model to the study of spatial patterns of follicular growth that result from a spatially heterogeneous distribution of parameters such as the mean duration of anagen phase. When considering that follicles die or miniaturize after going through a critical number of successive cycles, the model can reproduce the evolution to hair patterns similar to well known types of diffuse or androgenetic alopecia.

Spindle Dynamics and the Role of γ-Tubulin in Early Caenorhabditis elegans EmbryosV⃞

γ-Tubulin is a ubiquitous and highly conserved component of centrosomes in eukaryotic cells. Genetic and biochemical studies have demonstrated that γ-tubulin functions as part of a complex to nucleate microtubule polymerization from centrosomes. We show that, as in other organisms, Caenorhabditis elegans γ-tubulin is concentrated in centrosomes. To study centrosome dynamics in embryos, we generated transgenic worms that express GFP::γ-tubulin or GFP::β-tubulin in the maternal germ line and early embryos. Multiphoton microscopy of embryos produced by these worms revealed the time course of daughter centrosome appearance and growth and the differential behavior of centrosomes destined for germ line and somatic blastomeres. To study the role of γ-tubulin in nucleation and organization of spindle microtubules, we used RNA interference (RNAi) to deplete C. elegans embryos of γ-tubulin. γ-Tubulin (RNAi) embryos failed in chromosome segregation, but surprisingly, they contained extensive microtubule arrays. Moderately affected embryos contained bipolar spindles with dense and long astral microtubule arrays but with poorly organized kinetochore and interpolar microtubules. Severely affected embryos contained collapsed spindles with numerous long astral microtubules. Our results suggest that γ-tubulin is not absolutely required for microtubule nucleation in C. elegans but is required for the normal organization and function of kinetochore and interpolar microtubules.

Dynamics of maize endosperm development and DNA endoreduplication.

Endosperm development in Zea mays is characterized by a period of intense mitotic activity followed by a period in which mitosis is essentially eliminated and the cell cycle becomes one of alternating S and G phases, leading to endoreduplication of the nuclear DNA. The endosperm represents a significant contribution to the grain yield of maize; thus, methods that facilitate the study of cellular kinetics may be useful in discerning cellular and molecular components of grain yield. Two mathematical models have been developed to describe the kinetics of endosperm growth. The first describes the kinetics of mitosis during endosperm development; the second describes the kinetics of DNA endoreduplication during endosperm development. The mitotic model is a modification of standard growth curves. The endoreduplication model is composed of six differential equations that represent the progression of nuclei from one DNA content to another during the endoreduplication process. Total nuclei number per endosperm and the number of 3C, 6C, 12C, 24C, 48C, and 96C nuclei per endosperm (C is the haploid DNA content per nucleus) for inbred W64A from 8 to 18 days after pollination were determined by flow cytometry. The results indicate that the change in number of nuclei expressed as a function of the number of days after pollination is the same from one yearly crop to another. These data were used in the model to determine the endosperm growth rate, the maximum nuclei number per endosperm, and transition rates from one C value to the next higher C value. The kinetics of endosperm development are reasonably well represented by the models. Thus, the models provide a means to quantify the complex pattern of endosperm development.

Nonequilibrium spin glass dynamics with Janus

The out of equilibrium evolution for an Edwards‐Anderson spin glass is followed for a tenth of a second, a long enough time to let us make safe predictions about the behaviour at experimental scales. This work has been made possible by Janus, an FPGA based special purpose computer. We have thoroughly studied the spin glass correlation functions and the growth of the coherence length for L = 80 lattices in 3D. Our main conclusion is that these spin glasses follow noncoarsening dynamics, at least up to the experimentally relevant time scales.

Interplay Between Structure, Stoichiometry, and Electron Transfer Dynamics in SILAR-based Quantum Dot-Sensitized Oxides

We quantify the rate and efficiency of picosecond electron transfer (ET) from PbS nanocrystals, grown by successive ionic layer adsorption and reaction (SILAR), into a mesoporous SnO2 support. Successive SILAR deposition steps allow for stoichiometry- and size-variation of the QDs, characterized using transmission electron microscopy. Whereas for sulfur-rich (p-type) QD surfaces substantial electron trapping at the QD surface occurs, for lead-rich (n-type) QD surfaces, the QD trapping channel is suppressed and the ET efficiency is boosted. The ET efficiency increase achieved by lead-rich QD surfaces is found to be QD-size dependent, increasing linearly with QD surface area. On the other hand, ET rates are found to be independent of both QD size and surface stoichiometry, suggesting that the donor–acceptor energetics (constituting the driving force for ET) are fixed due to Fermi level pinning at the QD/oxide interface. Implications of our results for QD-sensitized solar cell design are discussed.

Measuring the intermittent synchronicity of macroeconomic growth in Europe. ACES Cases No. 2010.1

Synchronization of growth rates are an important feature of international business cycles, particularly in relation to regional integration projects such as the single currency in Europe. Synchronization of growth rates clearly enhances the effectiveness of European Central Bank monetary policy, ensuring that policy changes are attuned to the dynamics of growth and business cycles in the majority of member states. In this paper a dissimilarity metric is constructed by measuring the topological differences between the GDP growth patterns in recurrence plots for individual countries. The results show that synchronization of growth rates were higher among the Euro area member states during the second half of the 1980s and from 1997 to roughly 2002. Apart from these two time periods, Euro area member states do not appear to be more synchronized than a group of major international countries, signifying that globalization was the major cause of international business cycle synchronization.

Growth for Europe – Is the Juncker Plan the answer? EPC Discussion Paper, 20 March 2015

Executive Summary. The euro area is still suffering from low growth and high unemployment. For the recovery to become a reality, there needs to be a balance between fiscal discipline, supply side improvements and actions aimed at stimulating demand and growth. Increasing investment, both private and public, are important components in overcoming the recession. This becomes especially clear when comparing investment dynamics during the crisis with pre-crisis levels. Total investment is still much lower than before the crisis and public investment is well below its pre-crisis peak as well. In late November 2014, European Commission President Jean-Claude Juncker submitted a long-awaited proposal for a European Investment Plan that aims to stimulate private investment. Apart from the creation of the new European Fund for Strategic Investment (EFSI), through which private investors will receive public guarantees, the investment plan also aims to provide project assistance and improve the Single Market by removing sector-specific or other financial barriers to investment. While generally perceived as a first positive step towards increasing private investment, some commentators have expressed reservations about the plan. These include, among others, the lack of fresh money for the initial contributions to EFSI. Since a substantial amount of these contributions is reshuffled from other places in the European budget, the question was raised whether EFSI can fund additional projects or just replicates investment projects that would have happened without the plan. Other criticism relates to the high estimate of the expected leverage ratio of 1:15, and to the risk that the plan will only have a limited impact on stressed economies. The Juncker Plan addresses private investment, but so far there really is no clear strategy to stimulate productive public investment on the European and national level. Countries with fiscal space are reluctant to engage in higher spending, while those willing and in need of it the most are restricted by the rules. Member States and the Commission should therefore discuss options for further improving the euro area's economic governance. In addition to urging countries with fiscal space to increase investing in national public goods, investment could be treated with budget flexibility. One could, for instance, upgrade the importance of public investment in the European Semester. Additional deficit granted for public investment purposes could be attached to certain Country-Specific Recommendations. Another solution would be to allow some form of budget flexibility, such as the formulation of a new Golden Rule for productive public investment becoming part of the Stability and Growth Pact's application. Besides relying on a larger amount of flexibility in the rules, the Financial Transaction Tax (FTT) could be another solution to fund investment in European public goods. It will also be necessary to overcome the mistrust among Member States that is preventing further action. The political bargain of stronger conditionality, such as through contractual arrangements, could improve the situation. Increased trust will also be an important condition for tackling long-reaching economic governance reforms such as the creation of a Fiscal Capacity, which could take the form of a macroeconomic shock insurance. Such a Fiscal Capacity could make a real difference in providing the necessary funding to maintain productive public investment, even in times of deep recessions. The proposals presented do not attempt to be conclusive, but shall rather be an input for a wider debate on how to increase growth and employment in Europe. The paper draws heavily on the discussion of a Workshop on Growth and Investment, which the European Policy Centre (EPC) hosted on 10 December 2014 under Chatham-House Rule, with a group of economists and representatives from the European institutions.

Evolution of the landscape of Madeira Island: long-term vegetation dynamics

The aim of this thesis was to evaluate historical change of the landscape of Madeira Island and to assess spatial and temporal vegetation dynamics. In current research diverse “retrospective techniques”, such as landscape repeat photography, dendrochronology, and research of historical records were used. These, combined with vegetation relevés, aimed to gather information about landscape change, disturbance history, and vegetation successional patterns. It was found that landscape change, throughout 125 years, was higher in the last five decades manly driven by farming abandonment, building growth and exotic vegetation coverage increase. Pristine vegetation was greatly destroyed since early settlement and by the end of the nineteenth century native vegetation was highly devastated due to recurrent antropogenic disturbances. These actions also helped to block plant succession and to modify floristical assemblages, affecting as well as species richness. In places with less hemeroby, although significant growth of vegetation of lower seral stages was detected, the vegetation of most mature stages headed towards unbalance between recovery and loss, being also very vulnerable to exotic species encroachment. Recovery by native vegetation also occurred in areas formerly occupied by exotic plants and agriculture but it was almost negligible. Vegetation recovery followed the successional model currently proposed, attesting the model itself. Yet, succession was slower than espected, due to lack of favourable conditions and to recurrent disturbances. Probable tempus of each seral stage was obtained by growth rates of woody taxa estimated through dendrochronology. The exotic trees which were the dominant trees in the past (Castanea sativa and Pinus pinaster) almost vanished. Eucalyptus globulus, the current main tree of the exotic forest is being replaced by other cover types as Acacia mearnsii. The latter, along with Arundo donax, Cytisus scoparius and Pittosporum undulatum are currently the exotic species with higher invasive behaviour. However, many other exotic species have also proved to be highly pervasive and came together with the ones referred above to prevent native vegetation regeneration, to diminish biological diversity, and to block early successional phases delaying native forest recovery.

Dynamics of the Developing Chick Chorioallantoic Membrane Assessed by Stereology, Allometry, Immunohistochemistry and Molecular Analysis.

The chick chorioallantoic membrane (CAM) is a widely used model for the study of angiogenesis, tumour growth, as well as drug efficacy. In spite of this, little is known about the developmental alteration from its appearance to the time of hatching. In the current study the CAM has been studied by classical stereology and allometry. Expression levels of selected angiogenesis-related molecules were estimated by RT-PCR and cell dynamics assessed by proliferation and apoptosis assays. Absolute CAM volume increased from a low of 0.47 ± 0.11 cm3 at embryonic day 8 (E8) to a high of 2.05 ± 0.27 cm3 at E18, and then decreased to 1.6 ± 0.47 cm3 at E20. On allometric analysis, three growth phases were identifiable. Between E8-13 (phase I), the CAM grew fastest; moderately in phase II (E13-18) but was regressing in phase III (E18-20). The chorion, the mesenchyme and the allantoic layers grew fastest in phase I, but moderately in phase II. The mesenchyme grew slowly in phase III while the chorion and allantois were regressing. Chorionic cell volume increased fastest in phase I and was regressing in phase III. Chorionic capillaries grew steadily in phase I and II but regressed in phase III. Both the chorion and the allantois grew by intrinsic cell proliferation as well as recruitment of cells from the mesenchyme. Cell proliferation was prominent in the allantois and chorion early during development, declined after E17 and apoptosis started mainly in the chorion from E14. VEGFR2 expression peaked at E11 and declined steadily towards E20, VEGF peaked at E13 and E20 while HIF 1α had a peak at E11 and E20. Studies targeting CAM growth and angiogenesis need to take these growth phases into consideration.

Great Himalaya: Tourism and the dynamics of change in Nepal

Based on extensive field work and years of personal experience, the authors discuss the development of tourism in the Great Himalaya of Nepal from the early days of mountaineering to present-day trekking. Tourism and its potentials, drawbacks, and risks are illustrated with a focus on the Khumbu/Everest and Annapurna regions, the most popular mountain destinations in the country. The themes addressed include: growth and expansion of mountain tourism; employment and income generation; wealth, poverty, and livelihood as reflected in statistics and personal accounts by local people; the revival of trade with Tibet; tourism and the role of women; the crucial role played by institutions, policies and political stability; and environmental issues such as forest degradation, garbage management, and trail damage. With its numerous illustrations, text boxes and quotes, the book is intended for a broad readership of policy- and decision-makers in tourism and development, scholars, and tourists and mountaineers who take an interest in mountain development in the Great Himalaya and elsewhere.

Regulation of dendrite growth, placement, and patterning in Drosophila melanogaster class 4 dendrite arborization neurons

Thesis (Ph.D.)--University of Washington, 2016-06

Prediction of the growth of wear-type rail corrugation

The growth behaviour of the vibrational wear phenomenon known as rail corrugation is investigated analytically and numerically using mathematical models. A simplified feedback model for wear-type rail corrugation that includes a wheel pass time delay is developed with an aim to analytically distil the most critical interaction occurring between the wheel/rail structural dynamics, rolling contact mechanics and rail wear. To this end, a stability analysis on the complete system is performed to determine the growth of wear-type rail corrugations over multiple wheelset passages. This analysis indicates that although the dynamical behaviour of the system is stable for each wheel passage, over multiple wheelset passages, the growth of wear-type corrugations is shown to be the result of instability due to feedback interaction between the three primary components of the model. The corrugations are shown analytically to grow for all realistic railway parameters. From this analysis an analytical expression for the exponential growth rate of corrugations in terms of known parameters is developed. This convenient expression is used to perform a sensitivity analysis to identify critical parameters that most affect corrugation growth. The analytical predictions are shown to compare well with results from a benchmarked time-domain finite element model. (C) 2004 Elsevier B.V. All rights reserved.

The effect of timing and severity of water deficit on growth, development, yield accumulation and nitrogen fixation of mungbean

Mungbean (Vigna radiata L.), as a dryland grain legume, is exposed to varying timing and severity of water deficit, which results in variability in grain yield, nitrogen accumulation and grain quality. In this field study, mungbean crops were exposed to varying timing and severity of water deficit in order to examine: (1) contribution of the second flush of pods to final grain yield with variable timing of relief from water deficit, (2) the sensitivity to water deficit of the accumulation of biomass and nitrogen (N) and its partitioning to grain, and (3) how the timing of water deficit affects the pattern of harvest index (HI) increase through pod filling. The results showed that the contribution of the second flush to final yield is highly variable (1-56%) and can be considerable, especially where mid-season stress is relieved at early pod filling. The capacity to produce a second flush of pods did not compensate fully for yield reduction due to water stress. Relief from mid-season stress also resulted in continued leaf production, N-2 fixation and vegetative biomass accumulation during pod filling. Despite the wide variation in the degree of change in vegetative biomass and N during pod filling, there were strong relationships between grain yield and net-above-ground biomass at maturity, and grain N and above-ground N at maturity. Only in the extreme situations were HI and nitrogen HI affected noticeably. In those treatments where there was a large second flush of pods, there was a pronounced biphasic pattern to pod number production, with HI also progressing through two distinct phases of increase separated by a plateau. The proportion of grain yield contributed to by biomass produced before pod filling varied from 0 to 61% with the contribution greatest under terminal water deficit. There was a larger effect of water deficit on N accumulation, and hence N-2 fixation, than on biomass accumulation. The study confirmed the applicability of a number of long-standing physiological concepts to the analysis of the effect of water deficit on mungbean, but also highlighted the difficulty of accounting for timing effects of water deficit where second flushes of pods alter canopy development, biomass and yield accumulation, and N dynamics. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.

Of sheep and rain: large-scale population dynamics of the red kangaroo

1. We analysed time-series data from populations of red kangaroos (Macropus rufus, Desmarest) inhabiting four areas in the pastoral zone of South Australia. We formulated a set of a priori models to disentangle the relative effects of the covariates: rainfall, harvesting, intraspecific competition, and domestic herbivores, on kangaroo population-growth rate. 2. The statistical framework allowed for spatial variation in the growth-rate parameters, response to covariates, and environmental variability, as well as spatially correlated error terms due to shared environment. 3. The most parsimonious model included all covariates but no area-specific parameter values, suggesting that kangaroo densities respond in the same way to the covariates across the areas. 4. The temporal dynamics were spatially correlated, even after taking into account the potentially synchronizing effect of rainfall, harvesting and domestic herbivores. 5. Counter-intuitively, we found a positive rather than negative effect of domestic herbivore density on the population-growth rate of kangaroos. We hypothesize that this effect is caused by sheep and cattle acting as a surrogate for resource availability beyond rainfall. 6. Even though our system is well studied, we must conclude that approximating resources by surrogates such as rainfall is more difficult than previously thought. This is an important message for studies of consumer-resource systems and highlights the need to be explicit about population processes when analysing population patterns.