Patterns of species interaction strength in assembled theoretical competition communities

Strength of interactions between species may be an important tool in our effort to understand community structure. Recent theoretical and empirical findings suggest that despite the presence of some strong interactions, weak interactions prevail in communities. Here, we examine how mean interaction strengths change as theoretical competition communities assemble and what the distribution of interaction coefficients is in the communities that are formed during the assembly process. Our results show that the mean competition strengths fall as assembly progresses and that most interactions in the communities formed are weak. Communities that are invulnerable to further invasions are those where interspecific interactions are weaker than the average interaction strength between species in the pool. If these results can be generalized to more than one trophic level, implications for management and conservation of natural communities are substantial.     

Overyielding and species diversity: what should we expect?

Recent empirical studies have found evidence of increased biomass production ('overyielding') in species mixtures relative to monoculture, but the interpretation of these results remains controversial, in part, because of the lack of a theoretical expectation. Here, we examined the expected frequency and stability of overyielding species mixtures using Lotka-Volterra models of species dynamics in two- and four-species systems in conjunction with community, population, and specific rate of biomass production (SRP) definitions of overyielding. Overyielding plant mixtures represented > 55% of potential species assemblages under community definitions and approximately 100% of species were either overyielding or underyielding under the population definition. Our species simulations approached their equilibria in 1-2 yr, supporting the relevancy of an equilibrial analysis. The range of parameter space that we explored produced realistic values of plot biomass, supporting their biological relevance. We show that overyielding is expected to be common under community definitions and population definitions. Overyielding, under community or population definitions, does not imply an actual increase in the specific rate of biomass production. In addition, assemblages of overyielding and underyielding species under all three definitions can be stable over time with underyielding species persisting in the presence of overyielding species.     

Power law rank-abundance models for marine phage communities

Metagenomic analyses suggest that the rank-abundance curve for marine phage communities follows a power law distribution. A new type of power law dependence based on a simple model in which a modified version of Lotka-Volterra predator-prey dynamics is sampled uniformly in time is presented. Biologically, the model embodies a kill the winner hypothesis and a neutral evolution hypothesis. The model can match observed power law distributions and uses very few parameters that are readily identifiable and characterize phage ecosystems. The model makes new untested predictions: (1) it is unlikely that the most abundant phage genotype will be the same at different time points and (2) the long-term decay of isolated phage populations follows a power law.     

Modelling metapopulations with stochastic membrane systems

Metapopulations, or multi-patch systems, are models describing the interactions and the behavior of populations living in fragmented habitats. Dispersal, persistence and extinction are some of the characteristics of interest in ecological studies of metapopulations. In this paper, we propose a novel method to analyze metapopulations, which is based on a discrete and stochastic modelling framework in the area of Membrane Computing. New structural features of membrane systems, necessary to appropriately describe a multi-patch system, are introduced, such as the reduction of the maximal parallel consumption of objects, the spatial arrangement of membranes and the stochastic creation of objects. The role of the additional features, their meaning for a metapopulation model and the emergence of relevant behaviors are then investigated by means of stochastic simulations. Conclusive remarks and ideas for future research are finally presented.     

南亚热带森林群落种-多度的对数正态分布模型研究

通过对地处南亚热带的广东省黑石顶自然保护区森林群落的定点研究结果的分析表明：当用１,２,３,…分组每种个体数ｒ时,５个不同类型的群落样地的种一多度分布的直方图都呈明显的倒Ｊ－形;经Ｐｒｅｓｔｏｎ“倍程（ｏｃｔａｖｅｓ）”法分组ｒ后,其种－多度都服从对数正态分布。由种－多度模型可以推出另一新的模型一个体一多度分布模型,即Ｉ（Ｒ）＝２R0ＳＯＥＸＰ［［１ｎ２）２／４ａ２〕ＥＸＰ｛－ａ２（Ｒ－（ＲＯ＋１ｎ２／２ａ２）］２｝,它也符合对数正态分布。另外,还运用积分方法推导出估计总体（整个群落）中总种数Ｓ*和总个体数Ｉ*理论值的公式,用此公式估计的结果较为合理。     

刈割留茬高度和不同播种组合对人工草地初级生产力和物种丰富度的影响

采用7种播种方式(3种单播和4种混播)和3种刈割留茬高度裂区试验,测定了青藏高原高寒地区人工建植3年的垂穗披碱草(Elymus nutans)、中华羊茅(Festuca sinensis)和羊茅(Festuca ovina)草地地上生物量、物种丰富度以及杂草生物量,以分析刈割对青藏高原人工草地初级生产力和物种丰富度的影响。结果显示:(1)在草地建植第2年,不刈割与刈割留茬60和20 mm的草地初级生产力均有显著差异;留茬60和20 mm刈割使单播草地的平均初级生产力分别降低20%和27%,使混播草地的平均初级生产力分别降低29%和37%。(2)草地建植第3年,不刈割、留茬60和20 mm 3个处理间的草地生产力均差异极显著;留茬60和20 mm刈割使单播草地的平均生产力分别降低19%和36%,使混播草地的平均生产力分别降低4%和18%。研究表明,刈割显著降低了人工草地的初级生产力,同时显著增加了垂穗披碱草单播草地的物种丰富度和杂草生物量以及3种牧草混播草地物种丰富度,其他播种草地的物种丰富度和杂草生物量与不刈割草地均无显著差异。     

Predator-prey dynamics in P systems ruled by metabolic algorithm

P systems are used to compute predator-prey dynamics expressed in the traditional formulation by Lotka and Volterra. By governing the action of the transition rules in such systems using the regulatory features of the metabolic algorithm we come up with simulations of the Lotka-Volterra equations, whose robustness is comparable to that obtained using Runge-Kutta schemes and Gillespie's Stochastic Simulation Algorithm. Besides their reliability, the results obtained using the metabolic algorithm on top of P systems have a clear biochemical interpretation concerning the role, of reactants or promoters, of the species involved.     

Native Cuscuta campestris restrains exotic Mikania micrantha and enhances soil resources beneficial to natives in the invaded communities

Nutrients in exotic species and invaded communities play a key role in determining the dynamics of invaders and the invasibility of a receipt community. This study focused on the effects of the native holoparasite Cuscuta campestris (for short Cuscuta) on nutrients in the exotic invasive Mikania micrantha (for short Mikania) and stands invaded by Mikania. We conducted a set of field investigations on Mikania with Cuscuta parasitism for 1–4 years, and measured soil properties, community composition, and the growth and nutrient content of Mikania and Cuscuta in two types of sub-communities (i.e. with Mikania only, or with Mikania and Cuscuta). Cuscuta dramatically reduced the cover, biomass, and nutrients (i.e. N, P, and K content) of Mikania, significantly enhanced soil water, pH and nutrient content (i.e. organic matter, total N and P, available P and K), and greatly increased the cover and species richness of native plants. In addition, N and K of Cuscuta were positively correlated with N of Mikania, which was negatively associated with soil total N, available P and K. These findings suggest that Cuscuta may be an effective measure against Mikania and be beneficial to the restoration of invaded communities.     

Paleoecology of benthic community replacement

The literature of community paleoecology is filled with examples in which long-term environmentally-controlled faunal transitions are misidentified as forms of ecologic succession. This has obscured a fundamental community-level process - community replacement - involving gradual to abrupt substitution of one benthic community for another as a result of subtle to sharp changes in habitats over subevolutionary time. In gradually changing environments, replacement takes place through conformational reorganization of species-abundance distributions within established communities, yielding sequences of slightly different fossil associations. Environments that change very rapidly drastically feature a different type of community replacement involving species turnover, wherein environmental tolerance limits of community members are closely approached or exceeded. Paleoecologists should be alert to the strong likelihood that many temporal transitions involving autochthonous fossil associations are, in fact, community replacement sequences.     

Will a large complex system be productive?

While the relationship between food web complexity and stability has been well documented, how complexity affects productivity remains elusive. In this study, we combine food web theory and a data set of 149 aquatic food webs to investigate the effect of complexity (i.e. species richness, connectance, and average interaction strength) on ecosystem productivity. We find that more complex ecosystems tend to be more productive, although different facets of complexity have contrasting effects. A higher species richness and/or average interaction strength increases productivity, whereas a higher connectance often decreases it. These patterns hold not only between realized complexity and productivity, but also characterize responses of productivity to simulated declines of complexity. Our model also predicts a negative association between productivity and stability along gradients of complexity. Empirical analyses support our predictions on positive complexity-productivity relationships and negative productivity-stability relationships. Our study provides a step forward towards reconciling ecosystem complexity, productivity and stability.     

美国纽约州阔叶林物种多度格局的研究(英文)

物种多度格局是群落结构的重要反映 ,近年来又受到生态学家的重视。用 5种生态位模型研究美国纽约州“城 -郊 -乡”生态样带温性落叶阔叶林的物种多度格局。其中 3个模型 ,即几何级数模型、生态位重迭模型和分割线段模型拟合效果很好 ,它们既适合于简单群落 ,又适合于复杂群落结构的研究。另外两个模型 ,即随机分配模型和优势优先模型拟合效果不如前三个模型 ,可能仅适合于简单的群落研究。森林群落不同功能层的物种多度格局相似 ;沿“城 -郊 -乡”生态样带 ,森林群落的结构一致 ;物种多度格局与物种多样性指数结合使用可以更好地研究群落的结构特征。     

Statistical mechanics of relative species abundance

Statistical mechanics of relative species abundance (RSA) patterns in biological networks is presented. The theory is based on multispecies replicator dynamics equivalent to the Lotka–Volterra equation, with diverse interspecies interactions. Various RSA patterns observed in nature are derived from a single parameter related to productivity or maturity of a community. The abundance distribution is formed like a widely observed left-skewed lognormal distribution. It is also found that the “canonical hypothesis” is supported in some parameter region where the typical RSA patterns are observed. As the model has a general form, the result can be applied to similar patterns in other complex biological networks, e.g. gene expression.