水生生态系统食物网复杂性与多样性的关系

探索食物网的复杂结构是生态学的中心问题之一。基于构建的黄河口海草床食物网并耦合实际食物网的数据集,整理了包含河口、湖泊、海洋和河流四种水生生态系统类型的48个实际食物网案例。以食物网的节点数反映食物网多样性,物种之间的营养链接数、链接密度和连通度来表示食物网的复杂性,采用营养缩尺模型描述水生生态系统食物网的复杂性特征与节点数的普适性规律。结果表明：所涉及的48个水生生态系统食物网的多样性和复杂性跨度较大,其中,节点数的分布范围为4-124,链接数为3-1830,链接密度为0.75-15.71,连通度为0.06-0.25。不同类型水生生态系统间的连通度存在显著性差异（P=0.01）,节点数、链接数、链接密度不存在显著性差异。各类型生态系统的食物网链接数、链接密度均随节点数的增加而增加（R²=0.92,P<0.001和R²=0.82,P<0.001）。湖泊生态系统的连通度随节点数的变化不明显,围绕在0.20附近;而其他3种类型生态系统的食物网连通度随节点数的增加而降低（R²=0.06-0.41,P<0.001）。对全球尺度的水生食物网多样性和复杂性的定量化研究对于提升对食物网的复杂结构的科学认识,从系统尺度探究多样性和复杂性的关系提供数据支撑。     

Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.     

Using food network unfolding to evaluate food–web complexity in terms of biodiversity: theory and applications

Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (D_H), vertical diversity (D_V) and range diversity (D_R), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = D_H + D_V ? D_R. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone.     

Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.     

Seasonal dynamics in the community structure and trophic structure of testate amoebae inhabiting the Sanjiang peatlands,Northeast China

Peatlands cover 3% of the earth’s land surface but contain 30% of the world’s soil carbon pool. Microbial communities constitute a crucial detrital food web for nutrient and carbon cycling in peatlands. Heterotrophic protozoans are considered top predators in the microbial food web; however, they are not yet well understood. In this study, we investigated seasonal dynamics in the community and the trophic structure of testate amoebae in four peatlands. Testate amoebae density and biomass in August were significantly higher than those in May and October. The highest density, 6.7 × 10⁴ individual g⁻¹ dry moss, was recorded in August 2014. The highest biomass, 7.7 × 10² μg C g⁻¹ dry moss, was recorded in August 2013. Redundancy analyses showed that water-table depth was the most important factor, explaining over one third of the variance in fauna communities in all sampled seasons. High trophic position taxa dominated testate amoebae communities. The Shannon diversity index and community size structure index declined from August to October in 2013 and from May to October in 2014. These seasonal patterns of testate amoebae indicated the seasonal variations of the peatlands’ microbial food web and are possibly related to the seasonal carbon dynamics in Northeast Chinese peatlands.     

Age composition of winter irruptive Snowy Owls in North America

Patterns of winter irruptions in several owl species apparently follow the ‘lack of food’ hypothesis, which predicts that individuals leave their breeding grounds in search of food when prey populations do not allow breeding and are too small to ensure survival. Recent analyses, however, suggest an alternative mechanism dubbed the ‘breeding success’ hypothesis, which predicts that winter irruptions might instead be the result of a very successful breeding season, with a large pool of young birds subsequently migrating south from the breeding grounds. Here we assessed age‐class (juvenile vs. non‐juvenile) composition of winter irruptive Snowy Owls Bubo scandiacus over a 25‐year period (winter 1991–1992 to 2015–2016) between regular (North American Prairies and Great Plains) and irregular wintering areas (northeastern North America) using live‐trapped individuals and high‐resolution images of individual owls. Our results show that the proportion of juveniles (birds less than 1 year of age) varies considerably annually but is positively correlated with irruption intensity in both regions. In irregular wintering areas, it can constitute the majority (up to more than 90%) of winter irruptive Snowy Owls over a large geographical area. These results are consistent with the idea that large winter irruptions at temperate latitudes are not the result of adults massively leaving the Arctic in search of food after a breeding failure but are more likely to be a consequence of good reproductive conditions in the Arctic that create a large pool of winter migrants.     

Global change alters the stability of food webs

Recent research has generally shown that a small change in the number of species in a food web can have consequences both for community structure and ecosystem processes. However ‘change’ is not limited to just the number of species in a community, but might include an alteration to such properties as precipitation, nutrient cycling and temperature. How such changes might affect species interactions is important, not just through the presence or absence of interactions, but also because the patterning of interaction strengths among species is intimately associated with community stability. Interaction strengths encompass such properties as feeding rates and assimilation efficiencies, and encapsulate functionally important information with regard to ecosystem processes. Interaction strengths represent the pathways and transfer of energy through an ecosystem. We review the best empirical data available detailing the frequency distribution of interaction strengths in communities. We present the underlying (but consistent) pattern of species interactions and discuss the implications of this patterning. We then examine how such a basic pattern might be affected given various scenarios of ‘change’ and discuss the consequences for community stability and ecosystem functioning.     

Temporal activity of spiders and earwigs during winter in apple trees under a Mediterranean climate

It is well known that spiders are present in high numbers in orchards and may contribute to biocontrol. Some recent studies in central Europe further showed that some spiders are active year-round and consume pests even in winter. Using cardboard traps laid every two weeks, we carried out a survey to determine which spider and earwig species are active from September to May in an experimental, pesticide-free, apple orchard under a Mediterranean climate. We observed that spider activity was never completely absent. The structure of the spider communities showed a marked seasonality in three periods (so-called ‘autumn’, ‘winter’ and ‘spring’). Only two spider genera, Philodromus and Trachelas, were highly active in winter (percentage of catches during this season above 40%) and six others (Lathys, Clubiona, Gnaphosa, Theridion, Phrurolithus) had moderate activity (between 20 and 40%). The two earwig species had different patterns of winter activity with Forficula auricularia almost absent whereas F. pubescens was moderately active on trees. Spider community abundance, diversity and evenness significantly decreased between autumn and winter and remained low in the following spring probably because the attractiveness of the traps is much lower at this time of year due to mild temperatures and the presence of leaves on the trees. Winter-active spiders could contribute to pest biocontrol during the cold season and we advocate that the use of broad-spectrum pesticides at the end of winter, as classically applied in orchards, may be counter-productive for pest control.     

Complexity and structural properties of food webs in the Barents Sea

A food web topology describes the diversity of species and their trophic interactions, i.e. who eats whom, and structural analysis of food web topologies can provide insight into ecosystem structure and function. It appears simple, at first sight, to list all species and their trophic interactions. However, the very large number of species at low trophic levels and the impossibility to monitor all trophic interactions in the ocean makes it impossible to construct complete food web topologies. In practice, food web topologies are simplified by aggregating species into groups termed trophospecies. It is not clear though, how much simplified versions of food webs retain the structural properties of more detailed networks. Using the most comprehensive Barents Sea food web to date, we investigate the performance of methods to construct simplified food webs using three approaches: taxonomic, structural and regular clustering. We then evaluate how topological properties vary with the level of network simplification. Results show that alteration of food web structural properties due to aggregation are highly sensitive to the methodology used for grouping species and trophic links. In the specific case of the Barents Sea, we show that it is possible to preserve key structural properties of the original complex food web in simplified versions when using taxonomic or structural clustering combined with intermediate 25% linkage for trophic aggregation.     

Changes in plant species richness induce functional shifts in soil nematode communities in experimental grassland

Background

Changes in plant diversity may induce distinct changes in soil food web structure and accompanying soil feedbacks to plants. However, knowledge of the long-term consequences of plant community simplification for soil animal food webs and functioning is scarce. Nematodes, the most abundant and diverse soil Metazoa, represent the complexity of soil food webs as they comprise all major trophic groups and allow calculation of a number of functional indices.

Methodology/Principal Findings

We studied the functional composition of nematode communities three and five years after establishment of a grassland plant diversity experiment (Jena Experiment). In response to plant community simplification common nematode species disappeared and pronounced functional shifts in community structure occurred. The relevance of the fungal energy channel was higher in spring 2007 than in autumn 2005, particularly in species-rich plant assemblages. This resulted in a significant positive relationship between plant species richness and the ratio of fungal-to-bacterial feeders. Moreover, the density of predators increased significantly with plant diversity after five years, pointing to increased soil food web complexity in species-rich plant assemblages. Remarkably, in complex plant communities the nematode community shifted in favour of microbivores and predators, thereby reducing the relative abundance of plant feeders after five years.

Conclusions/Significance

The results suggest that species-poor plant assemblages may suffer from nematode communities detrimental to plants, whereas species-rich plant assemblages support a higher proportion of microbivorous nematodes stimulating nutrient cycling and hence plant performance; i.e. effects of nematodes on plants may switch from negative to positive. Overall, food web complexity is likely to decrease in response to plant community simplification and results of this study suggest that this results mainly from the loss of common species which likely alter plant – nematode interactions.     

Bottom-up effects of species diversity on the functioning and stability of food webs

1. The importance of species diversity for the stability of populations, communities and ecosystem functions is a central question in ecology. 2. Biodiversity experiments have shown that diversity can impact both the average and variability of stocks and rates at these levels of ecological organization in single trophic-level ecosystems. Whether these impacts hold in food webs and across trophic levels is still unclear. 3. We asked whether resource species diversity, community composition and consumer feeding selectivity in planktonic food webs impact the stability of resource or consumer populations, community biomass and ecosystem functions. We also tested the relative importance of resource diversity and community composition. 4. We found that resource diversity negatively affected resource population stability, but had no effect on consumer population stability, regardless of the consumer's feeding selectivity. Resource diversity had positive effects on most ecosystem functions and their stability, including primary production, resource biomass and particulate carbon, nitrogen and phosphorus concentrations. 5. Community composition, however, generally explained more variance in population, community and ecosystem properties than species diversity per se. This result points to the importance of the outcomes of particular species interactions and individual species' effect traits in determining food web properties and stability. 6. Among the stabilizing mechanisms tested, an increase in the average resource community biomass with increasing resource diversity had the greatest positive impact on stability. 7. Our results indicate that resource diversity and composition are generally important for the functioning and stability of whole food webs, but do not have straightforward impacts on consumer populations.     

Persistence increases with diversity and connectance in trophic metacommunities

Background

We are interested in understanding if metacommunity dynamics contribute to the persistence of complex spatial food webs subject to colonization-extinction dynamics. We study persistence as a measure of stability of communities within discrete patches, and ask how do species diversity, connectance, and topology influence it in spatially structured food webs.

Methodology/Principal Findings

We answer this question first by identifying two general mechanisms linking topology of simple food web modules and persistence at the regional scale. We then assess the robustness of these mechanisms to more complex food webs with simulations based on randomly created and empirical webs found in the literature. We find that linkage proximity to primary producers and food web diversity generate a positive relationship between complexity and persistence in spatial food webs. The comparison between empirical and randomly created food webs reveal that the most important element for food web persistence under spatial colonization-extinction dynamics is the degree distribution: the number of prey species per consumer is more important than their identity.

Conclusions/Significance

With a simple set of rules governing patch colonization and extinction, we have predicted that diversity and connectance promote persistence at the regional scale. The strength of our approach is that it reconciles the effect of complexity on stability at the local and the regional scale. Even if complex food webs are locally prone to extinction, we have shown their complexity could also promote their persistence through regional dynamics. The framework we presented here offers a novel and simple approach to understand the complexity of spatial food webs.     

Long‐term dynamics of winter and summer annual communities in the Chihuahuan Desert

Abstract. Winter and summer annuals in the Chihuahuan Desert have been intensively studied in recent years but little is known about the similarities and differences in the dynamics between these two communities. Using 15 yr of census data from permanent quadrats, this paper compared the characteristics and temporal dynamics of these two distinct, spatially co‐existent but temporally segregated communities. Although the total number of summer annual species recorded during our 15 yr of observation was higher than winter annuals, the mean number of species observed each year was higher in the winter community. The winter community exhibited lower temporal variation in total plant abundance and populations of individual species, lower species turnover rate and higher evenness than the summer community. The rank abundances of species in winter were significantly positively correlated for a period of up to 7 yr while in summer significant positive correlations in rank abundance disappeared after 2 to 3 yr. The higher seasonal species diversity (i.e. number of species observed in each season) in winter rather than the overall special pool (over 15 yr) may be responsible for the greater community stability of winter annuals. The difference in long‐term community dynamics between the two communities of annual plants are likely due to the differences in total species pool, life history traits (e.g. seed size), and seasonal climatic regimes.     

Winter hardiness of Miscanthus (I): Overwintering ability and yield of new Miscanthus ×giganteus genotypes in Illinois and Arkansas

Miscanthus ×giganteus (M×g) is an important bioenergy feedstock crop. However, biomass production of Miscanthus has been largely limited to one sterile triploid cultivar, M×g ‘1993‐1780’, which we demonstrate can have insufficient overwintering ability in temperate regions with cold winters. Key objectives for Miscanthus breeding include greater biomass yield and better adaptation to different production environments than M×g ‘1993‐1780’. In this study, we evaluated 13 M×g genotypes, including ‘1993‐1780’, in replicated field trials conducted for three years at Urbana, IL; Dixon Springs, IL; and Jonesboro, AR. Entries were phenotyped for first‐winter overwintering ability and plant hardiness (ratio of new tillers to old), yield in years 2 and 3, and first heading date, plant height, and culm number in years 1 and 2. We observed substantial variation for overwintering ability and biomass yield among the M×g genotypes tested and identified ones with better overwintering ability and/or higher biomass yield than ‘1993‐1780’. Most entries at Urbana were damaged during the first winter, whereas few or no entries were damaged at Dixon Springs or Jonesboro. However, M×g ‘Nagara’ was entirely undamaged during the first winter and produced high biomass yields at Urbana (19.7 Mg/ha in year 2 and 20.9 Mg/ha in year 3), whereas M×g ‘1993‐1780’ exhibited an overwintering loss of 29%, had severely damaged survivors (hardiness score of 25%), and reduced biomass yield (8.1 Mg/ha in year 2 and 16.2 Mg/ha in year 3), indicating that M×g ‘Nagara’ could be a better choice in hardiness zone 5 (average annual minimum air temperature of ?23.3 to ?28.9°C) or lower. In Dixon Springs, where M×g ‘1993‐1780’ was undamaged by the first winter, it yielded highest among all the entries (21.6 Mg/ha in year 3), though not significantly higher than M×g ‘Nagara’ (18.2 Mg/ha in year 3).     

Seasonality of cavitation and frost fatigue in Acer mono Maxim.

Although cavitation is common in plants, it is unknown whether the cavitation resistance of xylem is seasonally constant or variable. We tested the changes in cavitation resistance of Acer mono before and after a controlled cavitation–refilling and freeze–thaw cycles for a whole year. Cavitation resistance was determined from ‘vulnerability curves’ showing the percent loss of conductivity versus xylem tension. Cavitation fatigue was defined as a reduction of cavitation resistance following a cavitation–refilling cycle, whereas frost fatigue was caused by a freeze–thaw cycle. A. mono developed seasonal changes in native embolisms; values were relatively high during winter but relatively low and constant throughout the growing season. Cavitation fatigue occurred and changed seasonally during the 12‐month cycle; the greatest fatigue response occurred during summer and the weakest during winter, and the transitions occurred during spring and autumn. A. mono was highly resistant to frost damage during the relatively mild winter months; however, a quite different situation occurred during the growing season, as the seasonal trend of frost fatigue was strikingly similar to that of cavitation fatigue. Seasonality changes in cavitation resistance may be caused by seasonal changes in the mechanical properties of the pit membranes.     

基于土壤食物网的生态系统复杂性-稳定性关系研究进展

复杂性-稳定性关系是生态学核心问题之一。作为模式食物网,土壤食物网在探索生态系统复杂性-稳定性关系中起了极大的作用。总结了以Moore、de Ruiter、Neutel等为代表的理论生态学家以土壤食物网为工具研究生态系统复杂性-稳定性关系的方法、结论及不足之处,并展望了未来的发展方向。Moore、de Ruiter、Neutel等将土壤食物网功能群生物量数据、土壤食物网Lotka-Volterra模型和面向过程模型三者结合起来,描述相互作用强度大小格局、分室、能流组织形式等复杂性特征;将土壤食物网Lotka-Volterra模型与群落矩阵结合起来分析局域稳定性,进而探讨生态系统复杂性-稳定性关系的一般规律。在此基础上,Moore、de Ruiter、Neutel等证明了与随机食物网相比,真实食物网的相互作用强度格局、分室等复杂性特征提高了生态系统稳定性,生产力与稳定性共同决定了食物链的长度,并指出建立在平衡态基础上的静态土壤食物网模型在探索生态系统复杂性-稳定性关系方面具有较大的不足,动态土壤食物网是未来以土壤食物网为工具研究生态系统复杂性-稳定性关系的发展方向。     

Variable littoral‐pelagic coupling as a food‐web response to seasonal changes in pelagic primary production

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Food web stability and weighted connectance: the complexity-stability debate revisited

How the complexity of food webs relates to stability has been a subject of many studies. Often, unweighted connectance is used to express complexity. Unweighted connectance is measured as the proportion of realized links in the network. Weighted connectance, on the other hand, takes link weights (fluxes or feeding rates) into account and captures the shape of the flux distribution. Here, we used weighted connectance to revisit the relation between complexity and stability. We used 15 real soil food webs and determined the feeding rates and the interaction strength matrices. We calculated both versions of connectance, and related these structural properties to food web stability. We also determined the skewness of both flux and interaction strength distributions with the Gini coefficient. We found no relation between unweighted connectance and food web stability, but weighted connectance was positively correlated with stability. This finding challenges the notion that complexity may constrain stability, and supports the ‘complexity begets stability’ notion. The positive correlation between weighted connectance and stability implies that the more evenly flux rates were distributed over links, the more stable the webs were. This was confirmed by the Gini coefficients of both fluxes and interaction strengths. However, the most even distributions of this dataset still were strongly skewed towards small fluxes or weak interaction strengths. Thus, incorporating these distribution with many weak links via weighted instead of unweighted food web measures can shed new light on classical theories.