The role of a transient potassium current in a bursting neuron model

Presented here is a biophysical cell model which can exhibit low-frequency repetitive activity and bursting behavior. The model is developed from previous models (Av-Ron et al. 1991, 1993) for excitability, oscillations and bursting. A stepwise development of the present model shows the contribution of a transient potassium current (I _A ) to the overall dynamics. By changing a limited set of model parameters one can describe different firing patterns; oscillations with frequencies ranging from 2–200 Hz and a wide range of bursting behaviors in terms of the durations of bursting and quiescence, peak firing frequency and rate of change of the firing frequency.     

Effects of a disease affecting a predator on the dynamics of a predator-prey system

We study the effects of a disease affecting a predator on the dynamics of a predator-prey system. We couple an SIRS model applied to the predator population, to a Lotka-Volterra model. The SIRS model describes the spread of the disease in a predator population subdivided into susceptible, infected and removed individuals. The Lotka-Volterra model describes the predator-prey interactions. We consider two time scales, a fast one for the disease and a comparatively slow one for predator-prey interactions and for predator mortality. We use the classical “aggregation method” in order to obtain a reduced equivalent model. We show that there are two possible asymptotic behaviors: either the predator population dies out and the prey tends to its carrying capacity, or the predator and prey coexist. In this latter case, the predator population tends either to a “disease-free” or to a “disease-endemic” state. Moreover, the total predator density in the disease-endemic state is greater than the predator density in the “disease-free” equilibrium (DFE).     

A generalized diffusion model for growth and dispersal in a population

A reaction-diffusion model is presented in which spatial structure is maintained by means of a diffusive mechanism more general than classical Fickian diffusion. This generalized diffusion takes into account the diffusive gradient (or gradient energy) necessary to maintain a pattern even in a single diffusing species. The approach is based on a Landau-Ginzburg free energy model. A problem involving simple logistic kinetics is fully analyzed, and a nonlinear stability analysis based on a multi-scale perturbation method shows bifurcation to non-uniform states.Part of this work was done while at the Mathematical Institute, Oxford University as a Senior Visiting Fellow supported by the Science Research Council of Great Britain under grant GR/B31378     

Sinks and sources of methylmercury in a boreal catchment

A simple, catchment-scale, cascade model was used toassess the importance of sinks and sources ofmethylmercury (MeHg) in a boreal catchment thatcontains a forested upland, a lowland peatland and asmall lake. The three compartment model was run usingrealistic flow rates and atmospheric loading of MeHg,and the model was constrained by observedconcentrations of MeHg in each compartment. Assumingno internal sinks and sources of MeHg, modelledcatchment yields showed reasonable agreement withfield observation, but the predicted internal MeHgconcentrations in each compartment were implausible. Only when sources and sinks of MeHg are added to thethree compartments do MeHg-pool concentrations fallinto the range of those measured in the field. Tomaintain both catchment-scale and compartment-scalecontinuity, the upland and peatland were net sourcesof MeHg (0.0007 and 0.1065 mg ha^-1 d^-1respectively), and the lake a net sink (-0.2215 mgha^-1 d^-1). These source/sink rates are 1.73,259 and -539 times the input of MeHg via wetprecipitation input for the modelled ice-free season. Sensitivity analysis revealed that the volume ofrunoff delivered to the peatland by the upland area,peatland size and porewater MeHg concentration in thepeatland are important controls on catchment MeHgyield, and that contemporary atmospheric deposition ofMeHg is insignificant compared to the sources of MeHgwithin the catchment.     

Long-term control of species abundances in a dry grassland: a spatially explicit model

Abstract. The long-term spatio-temporal dynamics of a sparse dry-grassland community (Thymo-Festucetum) is investigated by a spatially explicit individual-based simulation model and by analytical model equations. The community (investigated over 15 yr in permanent plots) is characterized by a permanently low cover (30–50 %), mainly of the perennial tuft grass Festuca cinerea. Seedling establishment and the fate of juveniles are strongly dependent on weather conditions. The simulation programme focuses on the mechanism of clonal growth of grasses and the reproduction of tufts by fragmentation. Questions answered by the modelling approach were (1) which life-history features of the species are responsible for their persistence and for the low vegetation cover of the community and (2) what are the main mechanisms of species interactions. Different sets of simulation runs, together with the evaluation of the analytical models, show: (1) long-term persistence of the main species is possible only by a combination of sexual and clonal reproduction; the low cover is due to low germination rate, low mortality and limited growth of tufts; (2) the intra and interspecific control of the community is performed mainly via a reduction by already established individuals; (3) persistence of uncommon species relies on a diaspore buffer in, or around, the community (‘spatial mass effect’).     

Uncertainty Evaluation of a Groundwater Conceptual Model by Using a Multimodel Averaging Method

ABSTRACT

A groundwater field is a complex and open system. Groundwater simulation and prediction often deviated from true values, which is attributed to the uncertainty of groundwater modeling. The conceptual model (model struture) is one of the main sources of groundwater modeling uncertianty. In this study, the mean Euclidean distance (MED) between model simulations and observations is proposed to assess the integrated likelihood value of a conceptual model in Bayesian model averaging (BMA). Moreover, this proposed BMA method is compared with the traditional generalized likelihood uncertainty estimation (GLUE) BMA method by a synthetical groundwater model, and the characteristics of these two BMA methods are summarized.     

Joint model for recurrent event data with a cured fraction and a terminal event

In a longitudinal study where the recurrence of an event and a terminal event such as death are observed, a certain portion of the subjects may experience no event during a long follow-up period; this often denoted as the cure group which is assumed to be the risk-free from both recurrent events and death. However, this assumption ignores the possibility of death, which subjects in the cure group may experience. In the present study, such misspecification is investigated with the addition of a death hazard model to the cure group. We propose a joint model using a frailty effect, which reflects the association between a recurrent event and death. For the estimation, an expectation-maximization (EM) algorithm was developed and PROC NLMIXED in SAS was incorporated under a piecewise constant baseline. Simulation studies were performed to check the performance of the suggested method. The proposed method was applied to leukemia patients experiencing both infection and death after bone marrow transplant.     

A comparison of a deterministic and stochastic model for Hepatitis C with an isolation stage

We formulate a deterministic epidemic model for the spread of Hepatitis C containing an acute, chronic and isolation class and analyse the effects of the isolation class on the transmission dynamics of the disease. We calculate the basic reproduction number R₀ and show that for R₀≤1, the disease-free equilibrium is globally asymptotically stable. In addition, it is shown that for a special case when R₀>1, the endemic equilibrium is locally asymptotically stable. Furthermore, an analogous stochastic epidemic model for Hepatitis C is formulated using a continuous time Markov chain. Numerical simulations are used to estimate the mean, variance and probability distributions of the discrete random variables and these are compared to the steady-state solutions of the deterministic model. Finally, the expected time to disease extinction is estimated for the stochastic model and the impact of isolation on the time to extinction is explored.     

Multi-dimensional sensitivity analysis and ecological implications of a nutrient uptake model

Mechanistic models of nutrient uptake are essential to the study of plant-soil interactions. In these models, uptake rates depend on the supply of the nutrient through the soil and the uptake capacity of the roots. The behaviour of the models is complex, although only six to ten parameters are used. Our goal was to demonstrate a comprehensive and efficient method of exploring a steady-state uptake model with variation in parameters across a range of values described in the literature. We employed two analytical techniques: the first a statistical analysis of variance, and the second a graphical representation of the simulated response surface. The quantitative statistical technique allows objective comparison of parameter and interaction sensitivity. The graphical technique uses a judicious arrangement of figures to present the shape of the response surface in five dimensions. We found that the most important parameters controlling uptake per unit length of root are the average dissolved nutrient concentration and the maximal rate of nutrient uptake. Root radius is influential if rates are expressed per unit root length; on a surface area basis, this parameter is less important. The next most important parameter is the effective diffusion coefficient, especially in the uptake of phosphorus. The interactions of parameters were extremely important and included three and four dimensional effects. For example, limitation by maximal nutrient influx rate is approached more rapidly with increasing nutrient solution concentration when the effective diffusion coefficient is high. We also note the ecological implications of the response surface. For example, in nutrient-limited conditions, the rate of uptake is best augmented by extending root length; when nutrients are plentiful increasing uptake kinetics will have greater effect.     

New modified Square Root and Schoolfield models for predicting bacterial growth rate as a function of temperature

Summary A new modified Square Root model and two new modified Schoolfield models were evaluated for their ability to predict the growth rate ofYersinia enterocolitica as a function of temperature. The new Square Root model fits the data better than both the original Square Root model and the Zwietering Square Root model. Both new Schoolfield models, a six-and a four-parameter equation, fit the data better than the original Schoolfield model. The new four-parameter Schoolfield model was developed by removing the term describing low temperature inactivation from the new six-parameter Schoolfield model. Inclusion of the two extra parameters in the new six-parameter Schoolfield model (F=318) did not significantly improve the fit compared to the new fourparameter Schoolfield model (F=488).     

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.     

FAREAST: a forest gap model to simulate dynamics and patterns of eastern Eurasian forests

Aim This paper describes the forest gap model, FAREAST, its testing and its application to simulating the distribution, composition and dynamics of forests in eastern Eurasia. Location The FAREAST model is tested in north‐eastern China, initially for forests on the elevational gradient of Changbai Mountain, which is located on the border of the People's Republic of China and the Democratic People's Republic of Korea. Subsequently, the model is inspected regionally for other northern Chinese mountains and, finally, it is applied to predict subcontinental forest communities in the Russian Far East. Boreal larch (Larix spp.) forests cover much of the 6 million km² of eastern Eurasia. Mixed broad‐leaved tree species/Korean pine (Pinus koraiensis) forests and spruce/fir (Picea/Abies) forests also occupy considerable areas. Methods The model is tested using three types of information: (1) direct species composition comparisons between simulated and observed mature forests at the same locations; (2) forest type comparisons between simulated and observed forests along altitudinal gradients of several different mountains; and (3) comparison with forest stands in different succession stages of simulated forests. Results Model comparisons with independent data indicate that the FAREAST model is capable of representing many of the broad features of the forests of north‐eastern China. After regional model validation in the north‐eastern region of China, geographical model applications were developed for the forests of the Russian Far East. In simulations at 31 different sites distributed across the entire Russian Far East and including a wide variety of natural forests, the model demonstrates an ability to reproduce observed vegetation pattern. The model simulations are correct with respect to our criteria for 23 of the 31 sites, and there are close results for three other sites. Among the five sites that are incorrectly predicted, four simulations can be corrected by adding a simple assumption to the model for permafrost effects on water balance. Main conclusions Continental‐scale forest cover can be simulated using a forest gap model to represent individual–plant interactions with one another, and their environment, and with parameters that describe the biology of each tree species. It appears that such a model, validated relatively locally (in this case, in north‐eastern China), can then be applied over a much larger region. These results further imply that the Russian Far East forests can be regarded as a natural geographical expansion of north‐eastern Chinese forests. In both regions, forests share not only similar species compositions, but also similar underlying causes of forest successional dynamics.     

BALB/c小鼠子宫内膜异位症痛经模型的建立

目的建立近交系BALB/c小鼠子宫内膜异位症痛经模型。方法采用自体移植法将小鼠自体子宫组织块移植到腹膜,复制子宫内膜异位症模型,将术后的小鼠随机分为4组,手术+雌激素+缩宫素组、手术+雌激素组、手术+缩宫素组、手术组,并设假手术组和雌激素+缩宫素组,术后1~12 d采用不同方案诱发小鼠扭体反应,记录扭体潜伏期及扭体次数,并取异位灶行HE染色和病理组织学观察,筛选建立内异症痛经模型的最佳方案。结果除假手术组、雌激素+缩宫素组外,各组小鼠移植物均生长良好,镜下可见子宫内膜腺体及间质细胞,证实内异症造模成功,与手术+缩宫素组、手术组相比,手术+雌激素+缩宫素组、手术+雌激素组移植物体积明显增大,差异有显著性(P0.01);手术+雌激素+缩宫素组扭体发生率100%,雌激素+缩宫素组扭体发生率80%,手术+缩宫素组扭体发生率50%,其余组未出现扭体反应,组间差异有显著性(P0.01);与手术+缩宫素组、雌激素+缩宫素组相比,手术+雌激素+缩宫素组扭体潜伏期明显缩短(P0.01,P0.05),扭体次数明显增多(P0.01,P0.05),差异有显著性。结论手术+雌激素+缩宫素组为建立内异症痛经模型的最佳方案,方法简单易行,可用于内异症痛经发病机制及药物治疗研究。     

阿霉素心肌损伤大鼠模型的制备与评价

目的制备阿霉素心肌损伤大鼠模型,并对其进行评价。方法24只雄性SD大鼠随机分2组：正常对照组（CON,n=9）和阿霉素模型组（ADR,n=15）。ADR组腹腔注射阿霉素2 mg/kg,每周3次,连续2周,CON组注射相同体积的生理盐水,注射完毕后饲养5周;实验期间观察大鼠一般情况及死亡率;7周后检测心脏血流动力学及组织形态学变化,并进行心肌氧化损伤生化测定。结果ADR组大鼠死亡率为40%,CON组无死亡。与CON组比较,ADR组大鼠左室舒张末压（LVEDP）及左室内压最大下降速率（-LVdP/dtmax）显著升高（P〈0.001,P〈0.05）;组织学检查结果符合心肌损伤病理学改变的典型特征;心肌丙二醛（MDA）含量明显增加（P〈0.001）;谷胱甘肽过氧化物酶（GSH-Px）活性显著降低（P〈0.01）。结论按12 mg/kg的ADR总剂量,以每周3次,共两周,每次2mg/kg腹腔注射方式给药,7周后大鼠心脏产生明显功能及形态学异常,可成功建立ADR心肌损伤大鼠模型。     

Competition among plants: Concepts, individual-based modelling approaches, and a proposal for a future research strategy

Competition is a key process in plant populations and communities. We thus need, if we are to predict the responses of ecological systems to environmental change, a comprehensive and mechanistic understanding of plant competition. Considering competition, however, only at the population level is not sufficient because plant individuals usually are different, interact locally, and can adapt their behaviour to the current state of themselves and of their biotic and abiotic environment. Therefore, simulation models that are individual-based and spatially explicit are increasingly used for studying competition in plant systems. Many different individual-based modelling approaches exist to represent competition, but it is not clear how good they are in reflecting essential aspects of plant competition. We therefore first summarize current concepts and theories addressing plant competition. Then, we review individual-based approaches for modelling competition among plants. We distinguish between approaches that are used for more than 10 years and more recent ones. We identify three major gaps that need to be addressed more in the future: the effects of plants on their local environment, adaptive behaviour, and below-ground competition. To fill these gaps, the representation of plants and their interactions have to be more mechanistic than most existing approaches. Developing such new approaches is a challenge because they are likely to be more complex and to require more detailed knowledge and data on individual-level processes underlying competition. We thus need a more integrated research strategy for the future, where empirical and theoretical ecologists as well as computer scientists work together on formulating, implementing, parameterization, testing, comparing, and selecting the new approaches.     

子宫内膜异位症大鼠模型的建立及其病理学改变

目的构建子宫内膜异位症（内异症）大鼠动物模型,为阐明内异位症发病机理以及寻找有效的治疗方法提供理想的动物模型。方法取性成熟雌性Sprague-Dawley（SD）大鼠30只,通过手术将大鼠自体子宫组织移植到子宫旁韧带上,建立诱发型内异症大鼠动物模型。术后8周,再次剖腹观察异位组织的存活情况、病灶大小、与周围组织的粘连程度以及病理学变化。结果25只大鼠有明显的异位病灶。所有病灶都与周围组织有不同程度的粘连,病灶外观呈囊泡状。光镜观察见大部分异位子宫内膜形态和结构与在位子宫内膜基本相同,但内膜细胞、间质细胞、腺体,与在位内膜相比较少。少数病灶只有上皮组织或只有问质组织。结论自体子宫移植法可成功建立内异症大鼠模型。     

河湖生态系统生态用水优化研究——以滇池流域为例

生态需水是河流与湖泊生态系统健康的重要基础。湖泊流域的河流与湖泊生态系统之间存在密切的水量联系,目前对流域内生态用水的研究多为单一生态系统生态需水简单相加,忽略了河流和湖泊之间复杂的水量联系。基于河湖复合生态系统之间的水量联系构建了河湖生态系统生态用水优化模型,并以滇池为例分析了河湖生态系统生态用水规律。结果表明：湖泊流域中单一河流或湖泊生态需水计算结果不能满足复合生态系统的生态用水要求,需要综合考虑河流和湖泊之间的水量联系;在当前水质状况下,牛栏江每年的调水量不能满足滇池流域的生态用水要求;滇池流域水体污染对流域内生态用水影响较大,随着水体污染程度的下降,流域生态用水量和调水量呈指数下降,河流生态用水呈线性下降。