Simple rules yield complex food webs

Several of the most ambitious theories in ecology describe food webs that document the structure of strong and weak trophic links that is responsible for ecological dynamics among diverse assemblages of species. Early mechanism-based theory asserted that food webs have little omnivory and several properties that are independent of species richness. This theory was overturned by empirical studies that found food webs to be much more complex, but these studies did not provide mechanistic explanations for the complexity. Here we show that a remarkably simple model fills this scientific void by successfully predicting key structural properties of the most complex and comprehensive food webs in the primary literature. These properties include the fractions of species at top, intermediate and basal trophic levels, the means and variabilities of generality, vulnerability and food-chain length, and the degrees of cannibalism, omnivory, looping and trophic similarity. Using only two empirical parameters, species number and connectance, our 'niche model' extends the existing 'cascade model and improves its fit ten-fold by constraining species to consume a contiguous sequence of prey in a one-dimensional trophic niche.     

Allometric degree distributions facilitate food-web stability

In natural ecosystems, species are linked by feeding interactions that determine energy fluxes and create complex food webs. The stability of these food webs enables many species to coexist and to form diverse ecosystems. Recent theory finds predator-prey body-mass ratios to be critically important for food-web stability. However, the mechanisms responsible for this stability are unclear. Here we use a bioenergetic consumer-resource model to explore how and why only particular predator-prey body-mass ratios promote stability in tri-trophic (three-species) food chains. We find that this 'persistence domain' of ratios is constrained by bottom-up energy availability when predators are much smaller than their prey and by enrichment-driven dynamics when predators are much larger. We also find that 97% of the tri-trophic food chains across five natural food webs exhibit body-mass ratios within the predicted persistence domain. Further analyses of randomly rewired food webs show that body mass and allometric degree distributions in natural food webs mediate this consistency. The allometric degree distributions hold that the diversity of species' predators and prey decreases and increases, respectively, with increasing species' body masses. Our results demonstrate how simple relationships between species' body masses and feeding interactions may promote the stability of complex food webs.     

Ecosystem-level patterns of primary productivity and herbivory in terrestrial habitats

Ecosystems are structurally organized as food webs within which energy is transmitted between trophic levels and dissipated into the environment. Energy flow between two trophic levels is given by the amount of production at the lower level and by the proportion of production that is consumed, assimilated and respired at the higher level. Considerable evidence indicates that food-web structure varies predictably in different habitats, but much less is known about quantitative relationships among food web fluxes. Many of the energetic properties of herbivores in African game parks are associated with rainfall and, by inference, with net primary productivity. Respiratory costs per unit production at the consumer trophic level are higher for homeotherms than for heterotherms. Plant secondary chemicals affect herbivore dietary choices and the allocation of plant resources to those chemicals varies with resource availability. How these phenomena are translated into ecosystem fluxes is unknown. We present evidence that herbivore biomass, consumption and productivity are closely correlated with plant productivity, suggesting that the latter is a principal integrator and indicator of functional processes in food webs.     

考虑腹板弯曲的一维格栅夹层结构热变形

针对温度载荷作用下一维格栅夹层结构的柱面弯曲问题,提出了一个新的考虑了腹板弯曲效应的分析模型. 假设夹芯内变形后腹板依然垂直于上、下面板,并且腹板两端无相对转动. 将腹板和面板均视为处于弯曲和热伸长状态下的弹性梁,由此建立了所选胞元的平衡及几何方程,以及相邻胞元间受力和位移的相互关系. 利用传递矩阵方法得到了夹层结构受热变形. 作为一个算例,分析了几何与材料参数对悬臂夹层结构自由端最大挠度的影响. 本文结果与有限元方法得到的结果相一致,证实了本文所提出的分析模型的正确性.      

钢腹板-混凝土组合箱梁试验研究与有限元分析

进行波形钢腹板-混凝土组合箱梁和平钢腹板-混凝土组合箱梁的模型试验.提出模拟钢腹板-混凝土组合结构的有限元方法,并在大型通用程序ANSYS中实现.有限元计算结果得到了模型梁试验结果的验证,可用于钢腹板-混凝土组合结构的数值分析.试验与数值分析结果表明,两种组合箱梁的总体受力在弹性阶段和弹塑性阶段相似.相对于平钢腹板-混凝土组合箱梁,波形钢腹板-混凝土组合箱梁由于波形钢腹板的折迭效应,其抗变形能力和抗裂性能较相对较弱,但抗剪性能和抗屈曲能力较好.在破坏模式上,波形钢腹板-混凝土组合箱梁属于整体破坏,平钢腹板-混凝土组合箱梁属于平钢腹板局部屈曲破坏,其极限承载力小于波形钢腹板-混凝土组合箱梁.平钢腹板刚度小,在实际工程应用过程中应进行加劲,以防止局部屈曲破坏早于整体破坏的发生,同时也有利于避免施工过程的局部变形.     

Niche lability in the evolution of a Caribbean lizard community

Niche conservatism--the tendency for closely related species to be ecologically similar--is widespread. However, most studies compare closely related taxa that occur in allopatry; in sympatry, the stabilizing forces that promote niche conservatism, and thus inhibit niche shifts, may be countered by natural selection favouring ecological divergence to minimize the intensity of interspecific interactions. Consequently, the relative importance of niche conservatism versus niche divergence in determining community structure has received little attention. Here, we examine a tropical lizard community in which species have a long evolutionary history of ecological interaction. We find that evolutionary divergence overcomes niche conservatism: closely related species are no more ecologically similar than expected by random divergence and some distantly related species are ecologically similar, leading to a community in which the relationship between ecological similarity and phylogenetic relatedness is very weak. Despite this lack of niche conservatism, the ecological structuring of the community has a phylogenetic component: niche complementarity only occurs among distantly related species, which suggests that the strength of ecological interactions among species may be related to phylogeny, but it is not necessarily the most closely related species that interact most strongly.     

波形钢腹板抗剪性能的研究

为了研究波形钢腹板的抗剪受力性能,首先设计了4根波形钢腹板H型钢梁并进行屈曲加载试验,掌握了波形钢腹板屈曲的基本特征;统计国内外已建波形钢腹板组合桥的波形钢腹板尺寸参数、并对弹性屈曲强度的计算进行了分析,建议了弹性屈曲强度简化计算公式和适用范围;考虑局部弹性屈曲强度要小于整体弹性屈曲强度等参数范围,合理选取一批试验数据对Yi、聂建国等提出的波形钢板非线性剪切屈曲强度计算公式进行了对比分析,并通过ANSYS有限元程序对本文部分试验结果进行了分析验证.结果表明:波形钢腹板主要承担剪力且剪应力沿高度均匀分布;几何初始缺陷对其剪切屈曲强度的影响较明显;在工程应用范围内,文中建议的弹性屈曲强度和非线性剪切屈曲强度公式与试验值和有限元分析值吻合较好,精确度较高,可供工程设计参考.     

波形钢腹板组合箱梁横向受力有效分布宽度研究

为了研究波形钢腹板箱梁的桥面板有效分布宽度,制作了一片模型试验梁,对其进行了静载非破坏性试验,研究了此种结构桥面板的有效分布宽度变化规律.结合现行公路桥规值和有限元结果,在3种有效分布宽度计算值比较的基础上对现行公路桥规值进行修正,得到了不同工况下的有效分布宽度修正系数.结果表明:按现行公路桥规计算的有效分布宽度值相比试验值、有限元结果略小,应对桥规值乘以大于1.0的修正系数,使之适用于波形钢腹板箱梁的有效分布宽度计算.     

生态位理论在我国生态城市研究中的应用

本文从城市生态系统的经济发展、社会发展和自然生态环境三个方面构建了生态城市建设水平的评价指标体系,同时将生态位理论应用于生态城市研究,选取了我国35个省会城市和计划单列市进行城市生态建设状况的综合评价与比较分析,从中找出各地城市生态建设自身的优势与不足,并根据各城市自身的特点,解决突出限制城市生态建设水平提高的主要矛盾,促进我国城市在经济、社会、生态环境等方面地协调持续发展,同时针对不同的城市提出了今后的发展方向和发展重点.     

Universal scaling relations in food webs

The structure of ecological communities is usually represented by food webs. In these webs, we describe species by means of vertices connected by links representing the predations. We can therefore study different webs by considering the shape (topology) of these networks. Comparing food webs by searching for regularities is of fundamental importance, because universal patterns would reveal common principles underlying the organization of different ecosystems. However, features observed in small food webs are different from those found in large ones. Furthermore, food webs (except in isolated cases) do not share general features with other types of network (including the Internet, the World Wide Web and biological webs). These features are a small-world character and a scale-free (power-law) distribution of the degree (the number of links per vertex). Here we propose to describe food webs as transportation networks by extending to them the concept of allometric scaling (how branching properties change with network size). We then decompose food webs in spanning trees and loop-forming links. We show that, whereas the number of loops varies significantly across real webs, spanning trees are characterized by universal scaling relations.     

生态位构建的进化意义及其生态与行为响应

生态位构建是指有机体对局部环境中重要组成成分的建造、修复以及选择的一种能力.重点讨论了生态位构建的进化意义以及由生态位构建引起的生态和行为响应,主要结果包括:(1)生态继承作为与基因继承并行的遗传途径,扩展了对有机体适应性的机理解释;(2)环境变化与有机体对该变化导致的选择压的响应之间存在的时滞能够产生进化惯量与动量;(3)生态位构建与生态系统工程之间存在三个层面的差异;(4)Baldwin效应、自然选择和生态位构建分别体现了有机体对环境变化的中性、被动和主动等三种不同响应;(5)由于生态位构建可取决于学习的过程,因此它可以解释动物行为对策的进化.通过对生态位构建中这些热点问题以及其理论研究不足的讨论,提出了该领域中三个潜在的研究方向.     

Reconciling complexity with stability in naturally assembling food webs

Understanding how complex food webs assemble through time is fundamental both for ecological theory and for the development of sustainable strategies of ecosystem conservation and restoration. The build-up of complexity in communities is theoretically difficult, because in random-pattern models complexity leads to instability. There is growing evidence, however, that nonrandom patterns in the strengths of the interactions between predators and prey strongly enhance system stability. Here we show how such patterns explain stability in naturally assembling communities. We present two series of below-ground food webs along natural productivity gradients in vegetation successions. The complexity of the food webs increased along the gradients. The stability of the food webs was captured by measuring the weight of feedback loops of three interacting 'species' locked in omnivory. Low predator-prey biomass ratios in these omnivorous loops were shown to have a crucial role in preserving stability as productivity and complexity increased during succession. Our results show the build-up of food-web complexity in natural productivity gradients and pin down the feedback loops that govern the stability of whole webs. They show that it is the heaviest three-link feedback loop in a network of predator-prey effects that limits its stability. Because the weight of these feedback loops is kept relatively low by the biomass build-up in the successional process, complexity does not lead to instability.     

棉田叶螨亚群落营养结构初步研究

本文报道了新疆棉田螨类群落中的叶螨科（Ｔｅｔｒａｎｙｃｈｉｄａｅ）亚群落营养结构。从采到的标本中已鉴定出棉叶螨５种，天敌２６种。初步探索了叶螨亚群落的食物网络关系，比较了棉田不同施药水平与天敌数量关系。     

钢桁腹式混凝土组合箱梁的挠度计算

为了研究钢桁腹式混凝土组合箱梁的挠度计算方法和影响其挠度变化的因素,将钢桁腹杆换算为具有等效厚度的换算钢腹板,对悬臂板纵向位移函数进行修正,再利用变分法原理推导综合考虑腹杆剪切变形和剪力滞效应的挠度计算公式.运用有限元软件ANSYS建立组合箱梁的有限元模型,对有限元数值计算值和理论计算值进行比较分析,并在此基础上研究高跨比和腹杆水平倾角对组合箱梁由腹杆剪切变形和剪力滞效应产生的附加挠度的影响.研究结果表明:对组合箱梁悬臂板纵向位移函数进行修正可提高挠度计算精度;对于处于合理高跨比的组合箱梁而言,其腹杆的剪切变形和剪力滞效应产生的附加挠度不可忽略;组合箱梁腹杆水平倾角仅会对腹杆剪切变形引起的附加挠度产生影响.     

基于小世界网络模型的学术创新力演化分析

研究了基于小世界网络模型下的创新体系中学术创新力的演化问题.学术创新体系包含研究人员与管理体制两大要素,而学术创新力是组成创新体系各要素之间相互作用的结果.以组织科学中的NK影响矩阵,描述高校创新体系的小世界组织结构,其中N表示系统中个体的数目,K表示个体的邻域尺度.以自组织理论中极值动力学机制来模拟创新体系的演化.最后,通过计算机进行模拟演化.结果表明,对于给定的N和K,基于小世界网络的创新体系的极值动力学演化结果与网络的拓扑结构无关,而仅N给定时,系统创新力演化结果与个体的邻域尺度K紧密相关.     

Trajectory P system

Membrane computing is a branch of natural computing aiming to abstract computing ideas for the structure and the functioning of living cells as well as from the way the cells are organized in tissues or higher-order structures. Trajectories are used as a tool for modeling language operations and other related objects. A trajectory P system consists of a membrane structure in which the object in each membrane is a collection of words and the evolutionary rules are given in terms of trajectories. In this paper, we present some properties of trajectory P systems.     

基于生态位适宜度的辽宁省生态环境负荷水平测度

辽宁省作为东北老工业基地的重点区域,自上世纪新中国建立以来,产业结构一直处于重化工行业压倒性发展态势,并且在盲目追求经济高增长的发展目标指引下,环境污染愈发严重。长久以来,畸形化的产业结构与生态保护认知盲区,使得辽宁经济发展福祉被生态环境负荷逐渐侵蚀掉,从而造成整个省域范围内生态系统的脆弱性。将生态学中的生态位模型应用于辽宁省生态环境负荷的适宜度研究,通过适宜度水平的测度,准确把握辽宁省1995~2014年生态系统的运行状况以及环境污染问题的症结所在,最后从产业结构、公众认知和技术创新等角度,提出提升辽宁省生态环境负荷适宜度的具体措施。     

基于核聚类算法的高校定位研究

提出一个基于核聚类算法的高校定位模型.引入核函数,将原始数据由数据空间映射到特征空间中进行聚类.核聚类算法经过了核函数的非线性映射,使原始数据的特征更完整地显现出来,从而使聚类结果更客观、有效,可以解决传统方法主观性强、偏差大的缺陷.将核聚类算法应用于我国16所高校定位的研究,结果表明该方法可行且有效.通过聚类结果的分析,提出高校可分为教学科研生态位协调型、低教学生态位高科研生态位型、高教学生态位低科研生态位型3类,并对不同类型高校提出发展建议.     

基于生态位理论对福建省土地利用结构的分析与预测

以福建省为例,用生态元的势与态分析预测了土地利用结构的变化,构建了土地利用类型的数量生态位和生态位扩充压缩度的数学模型,从而反映了各土地利用类型生态元的变化程度与趋势.同时,对各类用地的年度数量生态位进行了横向与纵向分析,得出了各类用地规模的变化趋势及其在土地生态系统中的地位功能,并从土地利用类型的生态位扩充压缩分析进...