ECOLOGICAL NETWORKS IN AGROECOSYSTEMS: APPROACHES AND APPLICATIONS

● Agricultural intensification reduced the complexity and connectance of soil food webs. ● Agricultural intensification impaired the robustness of pollination networks. ● High connectance in co-occurrence networks indicates efficient nutrient utilization. Complex network theory has been increasingly used in various research areas, including agroecosystems. This paper summarizes the basic concepts and approaches commonly used in complex network theory, and then reviews recent studies on the applications in agroecosystems of three types of common ecological networks, i.e., food webs, pollination networks and microbial co-occurrence networks. In general, agricultural intensification is considered to be a key driver of the change of agroecosystems. It causes the simplification of landscape, leads to the loss of biocontrol through cascading effect in food webs, and also reduces the complexity and connectance of soil food webs. For pollination networks, agricultural intensification impaired the robustness by reducing specialization and enhancing generality. The microbial co-occurrence networks with high connectance and low modularity generally corresponded to high efficiency in utilization of nutrients, and high resistance to crop pathogens. This review aims to show the readers the advances of ecological networks in agroecosystems and inspire the researchers to conduct their studies in a new network perspective.     

Direct and indirect effects of resource quality on food web structure

The diversity and complexity of food webs (the networks of feeding relationships within an ecological community) are considered to be important factors determining ecosystem function and stability. However, the biological processes driving these factors are poorly understood. Resource quality affects species interactions by limiting energy transfer to consumers and their predators, affecting life history and morphological traits. We show that differences in plant traits affect the structure of an entire food web through a series of direct and indirect effects. Three trophic levels of consumers were influenced by plant quality, as shown by quantitative herbivore-parasitoid-secondary parasitoid food webs. We conclude, on the basis of our data, that changes in the food web are dependent on both trait- and density-mediated interactions among species.     

The robustness and restoration of a network of ecological networks

Understanding species' interactions and the robustness of interaction networks to species loss is essential to understand the effects of species' declines and extinctions. In most studies, different types of networks (such as food webs, parasitoid webs, seed dispersal networks, and pollination networks) have been studied separately. We sampled such multiple networks simultaneously in an agroecosystem. We show that the networks varied in their robustness; networks including pollinators appeared to be particularly fragile. We show that, overall, networks did not strongly covary in their robustness, which suggests that ecological restoration (for example, through agri-environment schemes) benefitting one functional group will not inevitably benefit others. Some individual plant species were disproportionately well linked to many other species. This type of information can be used in restoration management, because it identifies the plant taxa that can potentially lead to disproportionate gains in biodiversity.     

Scale invariance in food web properties

The robustness of five common food web properties is examined by varying the resolution of the data through aggregation of trophic groupings. A surprising constancy in each of these properties is revealed as webs are collapsed down to approximately half their original size. This analysis of 60 invertebrate-dominated community food webs confirms the existence of all but one of these properties in such webs and addresses a common concern held by critics of food web theory that observed food web properties may be sensitive to trophic aggregation. The food web statistics (chain length; predator/prey ratio; fraction of top, intermediate, and bottom species; and rigid circuits) are scaling in the sense that they remain roughly invariant over a wide range of data resolution. As such, within present standards of reporting food web data, these statistics may be used to compare systems whose trophic data are resolved differently within a factor of 2.     

Evolutionary conservation of species' roles in food webs

Studies of ecological networks (the web of interactions between species in a community) demonstrate an intricate link between a community's structure and its long-term viability. It remains unclear, however, how much a community's persistence depends on the identities of the species present, or how much the role played by each species varies as a function of the community in which it is found. We measured species' roles by studying how species are embedded within the overall network and the subsequent dynamic implications. Using data from 32 empirical food webs, we find that species' roles and dynamic importance are inherent species attributes and can be extrapolated across communities on the basis of taxonomic classification alone. Our results illustrate the variability of roles across species and communities and the relative importance of distinct species groups when attempting to conserve ecological communities.     

海洋食物网结构稳定性及干扰响应研究综述

针对当前全球环境急剧变化的背景,本文归纳了海洋食物网结构复杂性常用的量化指标,梳理了食物网稳定性不同维度的概念并深入探究其与复杂性之间的内在联系。根据海洋生态系统的不同干扰类型,分析了各种类型干扰的发生方式以及食物网在应对干扰时的适应机理,最后总结了海洋生态系统数据获取分析以及食物网构建时有待解决的问题,并指出开展初级生产者与消费者之间的相互作用研究能更利于解析全球变化对海洋食物网结构和功能的影响。     

Network motifs: simple building blocks of complex networks

Complex networks are studied across many fields of science. To uncover their structural design principles, we defined "network motifs," patterns of interconnections occurring in complex networks at numbers that are significantly higher than those in randomized networks. We found such motifs in networks from biochemistry, neurobiology, ecology, and engineering. The motifs shared by ecological food webs were distinct from the motifs shared by the genetic networks of Escherichia coli and Saccharomyces cerevisiae or from those found in the World Wide Web. Similar motifs were found in networks that perform information processing, even though they describe elements as different as biomolecules within a cell and synaptic connections between neurons in Caenorhabditis elegans. Motifs may thus define universal classes of networks. This approach may uncover the basic building blocks of most networks.     

Embeddedness in action: Saffron and the making of the local in southern Tuscany

Despite the widespread use of the concept of embeddedness in the literature on agri-food networks, not much has been written on the process through which a food economy becomes embedded. To explore this dynamic and contribute to a more critical perspective on the meanings and implications of embeddedness in the context of food, this paper analyzes the emergence of saffron as a local food network in southern Tuscany. By adopting a constructivist approach, the analysis shows that embeddedness assumes simultaneously a social, spatial, and temporal dimension that are dynamically created by participants in the saffron economy as a response to specific market requirements. The paper concludes that a focus on how embeddedness is achieved in the context of food has both theoretical and empirical implications. Theoretically, it supports the need for a more holistic and actor-oriented approach that takes into consideration the tensions inherent in the process of embedding and also its ramifications outside of the social realm. Practically, a focus on how a food network comes to be embedded complicates the notion of food relocalization – an issue that raises empirical questions about the sustainability of local food networks and their contribution to rural development.     

Influence of productivity on the stability of real and model ecosystems

The lengths of food chains within ecosystems have been thought to be limited either by the productivity of the ecosystem or by the resilience of that ecosystem after perturbation. Models based on ecological energetics that follow the form of Lotka-Volterra equations and equations that include material (detritus) recycling show that productivity and resilience are inextricably interrelated. The models were initialized with data from 5-to 10-year studies of actual soil food webs. Estimates indicate that most ecological production worldwide is from ecosystems that are themselves sufficiently productive to recover from minor perturbations.     

Energetics, patterns of interaction strengths, and stability in real ecosystems

Ecologists have long been studying stability in ecosystems by looking at the structuring and the strengths of trophic interactions in community food webs. In a series of real food webs from native and agricultural soils, the strengths of the interactions were found to be patterned in a way that is important to ecosystem stability. The patterning consisted of the simultaneous occurrence of strong "top down" effects at lower trophic levels and strong "bottom up" effects at higher trophic levels. As the patterning resulted directly from the energetic organization of the food webs, the results show that energetics and community structure govern ecosystem stability by imposing stabilizing patterns of interaction strengths.     

Pollen feeding in an orb-weaving spider

Juvenile orb-weaving spiders appear in spring, when insect prey are scarce but when aerial plankton, such as pollen and fungus spores, is abundant. Microscopic organic matter may be the main food of orb-weaving spiderlings, with insects providing only a dietary supplement. Pollen, which is caught on the sticky spirals of Araneus diadematus orb webs, doubles the life expectancy of spiderlings and alters their web-spinning behavior, so that they spin more frequently than do fasting controls. Fungus spores do not have the same nutritional value as pollen and may be deleterious to the spiderlings.     

Management of multispecies fisheries

With the overexploitation of many conventional fish stcocks, and growing interest in harvesting new kinds of food from the sea, there is increasing need for managers of fisheries to take account of interactions among species. In particular, as Antarctic krill-fishing industries grow, there is a need to agree upon sound principles for managing the Southern Ocean ecosystem. Using simple models, we discuss the way multispecies food webs respond to the harvesting of species at differrent trophic levels. These biological and economic insights are applied to a discussion of fisheries in the Southern Otean and the North Sea and to enunciate some for harvesting in multispecies systems.     

Environmental correlates of food chain length

In 113 community food webs from natural communities, the average and maximal lengths of food chains are independent of primary productivity, contrary to the hypothesis that longer food chains should arise when more energy is available at their base. Environmental variability alone also does not appear to constrain average or maximal chain length. Environments that are three dimensional or solid, however, such as a forest canopy or the water column of the open ocean, have distinctly longer food chains than environments that are two dimensional or flat, such as a grassland or lake bottom.     

Foraging adaptation and the relationship between food-web complexity and stability

Ecological theory suggests that complex food webs should not persist because of their inherent instability. "Real" ecosystems often support a large number of interacting species. A mathematical model shows that fluctuating short-term selection on trophic links, arising from a consumer's adaptive food choice, is a key to the long-term stability of complex communities. Without adaptive foragers, food-web complexity destabilizes community composition; whereas in their presence, complexity may enhance community persistence through facilitation of dynamical food-web reconstruction that buffers environmental fluctuations. The model predicts a linkage pattern consistent with field observations.     

Food web-specific biomagnification of persistent organic pollutants

Substances that accumulate to hazardous levels in living organisms pose environmental and human-health risks, which governments seek to reduce or eliminate. Regulatory authorities identify bioaccumulative substances as hydrophobic, fat-soluble chemicals having high octanol-water partition coefficients (K(OW))(>/=100,000). Here we show that poorly metabolizable, moderately hydrophobic substances with a K(OW) between 100 and 100,000, which do not biomagnify (that is, increase in chemical concentration in organisms with increasing trophic level) in aquatic food webs, can biomagnify to a high degree in food webs containing air-breathing animals (including humans) because of their high octanol-air partition coefficient (K(OA)) and corresponding low rate of respiratory elimination to air. These low K(OW)-high K(OA) chemicals, representing a third of organic chemicals in commercial use, constitute an unidentified class of potentially bioaccumulative substances that require regulatory assessment to prevent possible ecosystem and human-health consequences.     

广东省医学科技查新委托中存在的问题及其改进措施

以调查问卷的方式对广东省医学情报研究所查新用户的查新委托中存在的问题进行了调查。结果显示,目前科技查新中存在查新时间不能满足用户要求、委托方式落后、网络宣传不足等问题。提出了建立医学科技查新服务门户网站提高委托效率,加强业务的协调和互助满足用户对查新时间的要求,加强网络宣传和用户培训等建议。同时,结合广东省医学情报研究所查新服务的实际,提出应重视和开展引文咨询和新技术推广等查新咨询新业务。

     

Effects of fish in river food webs

Experimental manipulations of fish in a Northern California river during summer base flow reveal that they have large effects on predators, herbivores, and plants in river food webs. California roach and juvenile steelhead consume predatory insects and fish fry, which feed on algivorous chironomid larvae. In the presence of fish, filamentous green algae are reduced to low, prostrate webs, infested with chironomids. When the absence of large fish releases smaller predators that suppress chironomids, algal biomass is higher, and tall upright algal turfs become covered with diatoms and cyanobacteria. These manipulations provide evidence that the Hairston, Smith, Slobodkin-Fretwell theory of trophic control, which predicts that plants will be alternately limited by resources or herbivores in food webs with odd and even numbers of trophic levels, has application to river communitics.     

基于TOE框架的县乡村三级养老服务网络优化研究——以安徽省为例

基于TOE框架及原理,结合县乡村三级养老服务网络的特点,运用NVivo12pro软件对安徽省56个区县的一手资料进行关联性编码并进行扎根理论分析,得到技术、组织、环境层面的模型。为解决县乡村三级养老服务网络资源与养老需求不匹配、养老行业人力资源匮乏,以及三级养老服务网络各主体、各模式、各组织间联动性不足问题,促进县乡村三级养老服务各主体动力激发、能力提升、多层面联动发展,建议从资源整合、人才补给、党建引领、社会力量四个方面优化县乡村三级养老服务网络。     

农户销售模式选择影响因素研究——以海南果农为例

分析农户在农产品销售过程中选择不同销售模式的原因及其影响因素,并用实证数据加以验证,为提高农产品运销效率,增加农民收入提供参考。以海南芒果果农为例,通过抽样调查和实地考察,总结当前芒果果农销售农产品的主要模式,并用计划行为理论分析农户选择销售模式的影响因素;通过问卷调查法收集数据,运用Logit模型验证影响因素。结果发现,教育程度、风险态度、进取心态、获取信息难度、社会认同五个因素对农户选择不同销售模式的行为有显著影响,农户选择"地头市场"模式而不选择"批发市场"模式的主要原因在于对新模式不够了解、惧怕承担风险和受到舆论压力。因此,政府应在农村地区加强宣传教育,扶持农业带头人,以促进新型销售模式的推广。     

A common scaling rule for abundance, energetics, and production of parasitic and free-living species

The metabolic theory of ecology uses the scaling of metabolism with body size and temperature to explain the causes and consequences of species abundance. However, the theory and its empirical tests have never simultaneously examined parasites alongside free-living species. This is unfortunate because parasites represent at least half of species diversity. We show that metabolic scaling theory could not account for the abundance of parasitic or free-living species in three estuarine food webs until accounting for trophic dynamics. Analyses then revealed that the abundance of all species uniformly scaled with body mass to the -¾ power. This result indicates "production equivalence," where biomass production within trophic levels is invariant of body size across all species and functional groups: invertebrate or vertebrate, ectothermic or endothermic, and free-living or parasitic.