Stage-structured cycles promote genetic diversity in a predator-prey system of Daphnia and algae

Competition theory predicts that population fluctuations can promote genetic diversity when combined with density-dependent selection. However, this stabilizing mechanism has rarely been tested, and was recently rejected as an explanation for maintaining diversity in natural populations of the freshwater herbivore Daphnia pulex. The primary limitation of competition theory is its failure to account for the alternative types of population cycles that are caused by size- or stage-dependent population vital rates--even though such structure both explains the fluctuating dynamics of many species and may alter the outcome of competition. Here we provide the first experimental test of whether alternative types of cycles affect natural selection in predator-prey systems. Using competing Daphnia genotypes, we show that internally generated, stage-structured cycles substantially reduce the magnitude of selection (thereby contributing to the maintenance of genetic diversity), whereas externally forced cycles show rapid competitive exclusion. The change in selection is ecologically significant, spanning the observed range in natural populations. We argue that structured cycles reduce selection through a combination of stalled juvenile development and stage-specific mortality. This potentially general fitness-equalizing mechanism may reduce the need for strong stabilizing mechanisms to explain the maintenance of genetic diversity in natural systems.     

Variance in ecological consumer-resource interactions

Food-web models use the effect size of trophic interactions to predict consumer-resource dynamics. These models anticipate that strong effects of consumers increase spatial and temporal variability in abundance of species, whereas weak effects dampen fluctuations. Empirical evidence indicates that opposite patterns may occur in natural assemblages. Here I show that spatial variance in the distribution of resource populations is sensitive to changes in the variance of the trophic interaction, in addition to the mean effect of consumers, relative to other causes of spatial variability. Simulations indicate that both strong and weak direct effects of consumers can promote spatial variability in abundance of resources, but only trophic interactions with a large mean effect size can reduce variation. Predictions of the model agree with the results of repeated field experiments and are consistent with data from published consumer-resource interactions, proving to be robust across widely varying environmental conditions and species' life histories. Thus, food-web models that embody variance in trophic interactions may have increased capacity to explain the wide range of effects of consumers documented in empirical studies.     

High-amplitude fluctuations and alternative dynamical states of midges in Lake Myvatn

Complex dynamics are often shown by simple ecological models and have been clearly demonstrated in laboratory and natural systems. Yet many classes of theoretically possible dynamics are still poorly documented in nature. Here we study long-term time-series data of a midge, Tanytarsus gracilentus (Diptera: Chironomidae), in Lake Myvatn, Iceland. The midge undergoes density fluctuations of almost six orders of magnitude. Rather than regular cycles, however, these fluctuations have irregular periods of 4-7 years, indicating complex dynamics. We fit three consumer-resource models capable of qualitatively distinct dynamics to the data. Of these, the best-fitting model shows alternative dynamical states in the absence of environmental variability; depending on the initial midge densities, the model shows either fluctuations around a fixed point or high-amplitude cycles. This explains the observed complex population dynamics: high-amplitude but irregular fluctuations occur because stochastic variability causes the dynamics to switch between domains of attraction to the alternative states. In the model, the amplitude of fluctuations depends strongly on minute resource subsidies into the midge habitat. These resource subsidies may be sensitive to human-caused changes in the hydrology of the lake, with human impacts such as dredging leading to higher-amplitude fluctuations. Tanytarsus gracilentus is a key component of the Myvatn ecosystem, representing two-thirds of the secondary productivity of the lake and providing vital food resources to fish and to breeding bird populations. Therefore the high-amplitude, irregular fluctuations in midge densities generated by alternative dynamical states dominate much of the ecology of the lake.     

Dimensionality of consumer search space drives trophic interaction strengths

Trophic interactions govern biomass fluxes in ecosystems, and stability in food webs. Knowledge of how trophic interaction strengths are affected by differences among habitats is crucial for understanding variation in ecological systems. Here we show how substantial variation in consumption-rate data, and hence trophic interaction strengths, arises because consumers tend to encounter resources more frequently in three dimensions (3D) (for example, arboreal and pelagic zones) than two dimensions (2D) (for example, terrestrial and benthic zones). By combining new theory with extensive data (376 species, with body masses ranging from 5.24 × 10(-14) kg to 800 kg), we find that consumption rates scale sublinearly with consumer body mass (exponent of approximately 0.85) for 2D interactions, but superlinearly (exponent of approximately 1.06) for 3D interactions. These results contradict the currently widespread assumption of a single exponent (of approximately 0.75) in consumer-resource and food-web research. Further analysis of 2,929 consumer-resource interactions shows that dimensionality of consumer search space is probably a major driver of species coexistence, and the stability and abundance of populations.     

Analysis and correction for systematic error of ionospheric monostatic system

In the ionospheric monostatic system the echo is affected by systematic error. A real-time method using the special characters of direct wave existing in monostatic radar to correct systematic error is advanced. In the condition of large-amplitude direct wave, time delay of maximum amplitude of echo is considered as system time delay; in the condition of small-amplitude direct wave, the mathematical model of time delay error is built to approach after direct wave test. And a method using large-amplitude direct wave to correct phase distortion is advanced. It achieves good effect using this method in practice, which can improve the quality of sound result obviously. Biography: WANG Feng (1982–), male, Ph. D. candidate, research direction: radar signal processing and radar software system design.     

太阳能热动力系统Brayton装置与Stirling装置分析与比较

空间太阳能热动力发电系统是非常有前景的未来空间能源供应系统,对适用于空间SD装置的Brayton循环和Stirling循环进行了详细的分析,并以7kW发电装置为对象,从质量、效率和对吸热器的要求等方面对2种循环进行了比较。结果表明,Brayton装置为目前太阳能热动力发电的首选方案,Stirling装置是未来最理想的空间发电系统。     

动力系统极限环计算方法的改进

对计算极限环的摄动－增量法作了改进,解的表达形式更一般化,更适合一般平面动力系统极限环及其分叉的计算。该方法的特点是用有限Fourier级数给出极限环的近似表达式,把微分方程化为线性增量方程,应用谐波平衡法建立Fourier系数的线性代数方程组,再用迭代法求解,计算方法直观、简单,求出了以前原方法难于计算的二次系统的4个极限环,也求出了其具有争议的算例的极限环的个数。算例表明该方法是有效的。求出了改进前的摄动－增量法所不能求出的极限环。     

Stochastic gene expression out-of-steady-state in the cyanobacterial circadian clock

Recent advances in measuring gene expression at the single-cell level have highlighted the stochastic nature of messenger RNA and protein synthesis. Stochastic gene expression creates a source of variability in the abundance of cellular components, even among isogenic cells exposed to an identical environment. Recent integrated experimental and modelling studies have shed light on the molecular sources of this variability. However, many of these studies focus on systems that have reached a steady state and therefore do not address a large class of dynamic phenomena including oscillatory gene expression. Here we develop a general protocol for analysing and predicting stochastic gene expression in systems that never reach steady states. We use this framework to analyse experimentally stochastic expression of genes driven by the Synechococcus elongatus circadian clock. We find that, although the average expression at two points in the circadian cycle separated by 12 hours is identical, the variability at these two time points can be different. We show that this is a general feature of out-of-steady-state systems. We demonstrate how intrinsic noise sources, owing to random births and deaths of mRNAs and proteins, or extrinsic noise sources, which introduce fluctuations in rate constants, affect the cell-to-cell variability. To distinguish experimentally between these sources, we measured how the correlation between expression fluctuations of two identical genes is modulated during the circadian cycle. This quantitative framework is generally applicable to any out-of-steady-state system and will be necessary for understanding the fidelity of dynamic cellular systems.     

Single-species models for many-species food webs

Most species live in species-rich food webs; yet, for a century, most mathematical models for population dynamics have included only one or two species. We ask whether such models are relevant to the real world. Two-species population models of an interacting consumer and resource collapse to one-species dynamics when recruitment to the resource population is unrelated to resource abundance, thereby weakening the coupling between consumer and resource. We predict that, in nature, generalist consumers that feed on many species should similarly show one-species dynamics. We test this prediction using cyclic populations, in which it is easier to infer underlying mechanisms, and which are widespread in nature. Here we show that one-species cycles can be distinguished from consumer resource cycles by their periods. We then analyse a large number of time series from cyclic populations in nature and show that almost all cycling, generalist consumers examined have periods that are consistent with one-species dynamics. Thus generalist consumers indeed behave as if they were one-species populations, and a one-species model is a valid representation for generalist population dynamics in many-species food webs.     

昆虫种群动态模型及其图形理论

<正>本文应用系统分析、控制论与图形理论研究了昆虫种群消长的动力过程,建立了带有控制项的昆虫种群动态预测预报方程,推导出反映一个世代的昆虫种群动态的时间序列模型,获得了昆虫种群动态时间序列的图形模型与昆虫种群动态预测预报方程的广义信号流图表示式,从而使昆虫种群消长的动力过程,在定量与定性上都能得到比较充分而直观的描述。经实例计算证明,上述诸动态模型,使用效果较好。     

门限自回归模型与非线性系统的极限环

本文根据武汉市某区人口死亡率和加拿大山猫数据建立了SETAR模型,并作出了相应系统的极限环。在此基础上指出,SETAR模型可描述一个确定极限环的某一邻域内的一“族”相轨线,此极限环是轨道稳定的。文中还根据现有结果分析指出,对于具有两个门限段的SETAR模型,门限值r表征观测时序{x_t}的均值特性;在一定情况下,延迟步数d可表征极限环的周期T的特性。利用这些特性,不仅可使建模中对r和d的寻优工作量大为减小,而且还从本质上揭示出SETAR模型参数的物理含义,便于分析研究。     

时间测度上具有时滞的互惠系统的周期解

互惠相互作用关系是生物种群之间相互作用的基本关系之一,是生态学、生物数学的研究热点。2种群互惠系统是指每一种群的存在对另一种群的增长都会起促进作用的系统。由于时滞对一系统所带来的影响,在自然现象中是屡见不鲜的。因此,生态系统中,为了更真实的反应自然,时滞是一种不应忽略的因素。时标理论的提出,整合和统一了连续与离散的分析。因此时标上的动力系统更为一般,包含微分方程与差分方程作为它的特例。在时间测度上研究了具有时滞的两种群互惠系统,利用重合度理论中的延拓定理讨论此系统周期解的存在性问题,从而使这一类系统的连续时间情形与离散时间情形的周期解存在性问题得到了统一研究。并且所获得的周期解存在性定理,推广了文献[15]的主要结果。     

“双碳”目标下多周期供应链碳减排协调决策研究

针对“双碳”目标下的供应链多周期碳减排问题,提出了单制造商、单零售商组成的多周期情形供应链模型,研究各周期碳减排水平和零售定价之间的协调优化问题。首先,建立单周期Stackelberg博弈模型并在此基础上构建了多周期供应链协调决策的动态规划模型,其次,利用Stackelberg博弈求解各周期产品零售价格与批发价格,之后运用动态规划求解出各周期最优碳减排水平并得出制造商与零售商的多周期最大利润;通过算例分析得出碳减排成本系数和低碳偏好程度等因素对碳减排、定价决策以及制造商、零售商利润的影响。研究发现,制造商各周期的碳减排量与周期剩余数以及低碳偏好程度呈正向关系,与减排成本系数呈负向关系;各周期批发价格、零售价格与消费者低碳偏好程度以及减排成本系数呈负向关系;各周期批发价格与零售价格随着周期推移逐渐增加。     

含时滞动态系统运行安全的柔性和可控性集成分析

许多动态过程系统柔性分析未考虑时滞因素的影响,系统原本设计的柔性区域不能保证系统实际操作过程的安全运行.为此,文中提出了过程系统对时滞的敏感度问题,通过两个案例分析了时滞因素对动态系统柔性的影响,对比不同过程系统对时滞的敏感度,对过程系统进行控制器优化仿真和可控性分析.最后通过一个典型的化工反应器案例对含时滞因素的动态过程系统进行柔性和可控性集成分析,提高了动态过程系统柔性分析的正确性,保证了系统实际运行的安全性和稳定性.     

Rapid evolution drives ecological dynamics in a predator-prey system

Ecological and evolutionary dynamics can occur on similar timescales. However, theoretical predictions of how rapid evolution can affect ecological dynamics are inconclusive and often depend on untested model assumptions. Here we report that rapid prey evolution in response to oscillating predator density affects predator-prey (rotifer-algal) cycles in laboratory microcosms. Our experiments tested explicit predictions from a model for our system that allows prey evolution. We verified the predicted existence of an evolutionary tradeoff between algal competitive ability and defence against consumption, and examined its effects on cycle dynamics by manipulating the evolutionary potential of the prey population. Single-clone algal cultures (lacking genetic variability) produced short cycle periods and typical quarter-period phase lags between prey and predator densities, whereas multi-clonal (genetically variable) algal cultures produced long cycles with prey and predator densities nearly out of phase, exactly as predicted. These results confirm that prey evolution can substantially alter predator-prey dynamics, and therefore that attempts to understand population oscillations in nature cannot neglect potential effects from ongoing rapid evolution.     

中国北方农牧交错带全新世环境演变的若干特征

我国北方农牧交错带是环境变化的敏感地区之一.全新世以来,根据本区的沙地形成、演化和湖泊演化对比,得知近1万a来有过四五次较大的气候波动;自3000a以来干湿频率有加强的趋势。本地带具有准2000a周期及准22、准38、准83、准102和准400a周期,为预测未来环境变化提供依据,为制定开发规划时参考。     

三突花蛛的生活史、生物学特性及蜘蛛目生活史分类初论

本文于1986～1988年在武汉市对三突花蛛Misumenops tricuspidatus的生活史及生物学特性进行了研究。结果表明,三突花蛛在湖北武汉一年完成完整的2～3个世代,以第3代若蛛和第2代成蛛越冬。雌蛛保持一年内稳定的生活周期,而雄蛛在发育历期上变化较大,雄蛛和雌蛛从卵发育至成熟在发育历期上的差异排除了同胞蜘蛛在自然界交配的可能性。雌蛛在不同季节对产卵植物具有选择性。提出了蜘蛛生活史分类的一般原则。     

A test of general relativity using radio links with the Cassini spacecraft

According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass. The bending and delay are proportional to gamma + 1, where the parameter gamma is unity in general relativity but zero in the newtonian model of gravity. The quantity gamma - 1 measures the degree to which gravity is not a purely geometric effect and is affected by other fields; such fields may have strongly influenced the early Universe, but would have now weakened so as to produce tiny--but still detectable--effects. Several experiments have confirmed to an accuracy of approximately 0.1% the predictions for the deflection and delay of photons produced by the Sun. Here we report a measurement of the frequency shift of radio photons to and from the Cassini spacecraft as they passed near the Sun. Our result, gamma = 1 + (2.1 +/- 2.3) x 10(-5), agrees with the predictions of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models.     

基于Internet实时遥操作移动机器人

构建了能使操作者通过Internet远程实时控制的移动机器人系统。为了补偿网络时延和抵消其对遥操作系统的影响,基于我们以前提出的改进型Smith预估器原理,给出了预估控制策略。为了保证系统稳定性和透明性,基于主从端的传感器信息交换,设计了一个动态模型管理器,并给出了同步管理算法。除了力反馈外,为了增强系统控制的实时性,引入了预估的虚拟显示。为了降低时延抖动,提出了新的网络时延管理算法,并分析了时延缓冲器原理。最后长距离的网络遥操作实验验证了系统和控制策略的实用性和有效性。     

基于混杂系统技术的网络控制系统的建牛与稳定性分析

针对网络控制系统中网络带宽受限所产生的传输延时问题提出一种基于混杂系统技术的建模方法,采用动态输出反馈形式给出了闭环网络控制系统描述,将采样周期和传输延时为常数的情况推广到时变情况,使所建立的模型更具有一般性;并运用混杂系统理论分析了网络控制系统的稳定性,避免了将其转化成一般延时系统构造Lyapunov函数所带来的困难.增大系统稳定域的同时,使系统具有良好的动态性能.仿真结果说明了此方法的可行性和有效性.