Resource productivity and availability impacts for food-chain length

Ecologists have focused on food-chain length (FCL) for the past eight decades as an index of food-web structure. Here, I review the hypotheses determining FCL with a focus on resource productivity and availability effects on FCL. First, I introduce the mainstream hypotheses to explain FCL variations: productivity, ecosystem size, and disturbance. For the existing productivity and productive space hypotheses, I stress the importance of using resource availability to estimate the productivity effect on FCL. Using a FCL dataset from 15 ponds, I tested the resource stoichiometry effect on FCL for ponds with between carbon:nitrogen ratio of primary producers and FCL. Moreover, I provide a perspective for studying resource availability and stoichiometry effects on FCL and of the alternative hypotheses of productive-space and ecosystem size. Finally, I suggest the future directions of the FCL study: a resource subsidy and climate change effects on FCL and food-web structure.     

Testing the productive-space hypothesis: rational and power

Understanding and explaining the causes of variation in food-chain length is a fundamental challenge for community ecology. The productive-space hypothesis, which suggests food-chain length is determined by the combination of local resource availability and ecosystem size, is central to this challenge. Two different approaches currently exist for testing the productive-space hypothesis: (1) the dual gradient approach that tests for significant relationships between food-chain length and separate gradients of ecosystem size (e.g., lake volume) and per-unit-size resource availability (e.g., g C m⁻¹ year⁻²), and (2) the single gradient approach that tests for a significant relationship between food-chain length and the productive space (product of ecosystem size and per-unit-size resource availability). Here I evaluate the efficacy of the two approaches for testing the productive-space hypothesis. Using simulated data sets, I estimate the Type 1 and Type 2 error rates for single and dual gradient models in recovering a known relationship between food-chain length and ecosystem size, resource availability, or the combination of ecosystem size and resource ability, as specified by the productive-space hypothesis. The single gradient model provided high power (low Type 2 error rates) but had a very high Type 1 error rate, often erroneously supporting the productive-space hypothesis. The dual gradient model had a very low Type 1 error rate but suffered from low power to detect an effect of per-unit-size resource availability because the range of variation in resource availability is limited. Finally, I performed a retrospective power analysis for the Post et al. (Nature 405:1047–1049, 2000) data set, which tested and rejected the productive-space hypothesis using the dual gradient approach. I found that Post et al. (Nature 405:1047–1049, 2000) had sufficient power to reject the productive-space hypothesis in north temperate lakes; however, the productive-space hypothesis must be tested in other ecosystems before its generality can be fully addressed.     

食物链长度理论研究进展

食物链长度（Food chain length,FCL）是生态系统中最重要的特点之一,它通过改变生物间的营养关系,影响着生物多样性,群落的结构以及稳定性;它是反映食物网物质转换与能量传递的综合指数,食物链及其动态特征是生态学许多重要理论的基础,食物链长度理论的研究进展,推动了人们对水域生态系统中生物和非生物相互作用的理解。回顾了食物链长度的3种度量方法及其详细的计算方法,在此基础上简述了各方法的特点。综述了食物链长度的决定因素的4种假说（资源可利用性假说、生产力空间假说、生态系统大小假说、动态稳定性假说）及其交互作用,重点总结了湖泊食物链长度的空间格局与决定因素的研究进展。最后,食物链长度研究展望,包括食物链长度决定因子研究存在的问题及发展方向的总结,以及在在水域生态学中的应用的研究进展,例如食物链长度在指示污染物的生物富集中的研究进展、食物链食物链长度在指导生物操作、以及在食物链长度在对气候变化响应方面的研究进展等等。     

Towards a mechanistic understanding of variation in aquatic food chain length

Ecologists have long sought to understand variation in food chain length (FCL) among natural ecosystems. Various drivers of FCL, including ecosystem size, resource productivity and disturbance, have been hypothesised. However, when results are aggregated across existing empirical studies from aquatic ecosystems, we observe mixed FCL responses to these drivers. To understand this variability, we develop a unified competition-colonisation framework for complex food webs incorporating all of these drivers. With competition-colonisation tradeoffs among basal species, our model predicts that increasing ecosystem size generally results in a monotonic increase in FCL, while FCL displays non-linear, oscillatory responses to resource productivity or disturbance in large ecosystems featuring little disturbance or high productivity. Interestingly, such complex responses mirror patterns in empirical data. Therefore, this study offers a novel mechanistic explanation for observed variations in aquatic FCL driven by multiple environmental factors.     

Refuge increases food chain length: modeled impacts of littoral structure in lake food webs

Food chain length (FCL) represents a fundamental metric within ecology because it has implications for ecosystem function and responses to environmental change. Omnivory between linked food chains situated within large ecosystems can increase FCL, whereas overlap of food chains within small or spatially compressed ecosystems is generally thought to decrease FCL. Yet FCL varies widely in small ecosystems and the mechanisms underlying determinants of FCL in these systems is unclear. In small shallow lakes, littoral structure is a predictor of FCL but it is unclear whether this is due to productivity or refuge mechanisms. Here we provide evidence, using consumer resource food web modules parameterized with empirical data, that refuge in spatially compressed ecosystems has the ability on its own to increase the trophic position of top predators by increasing the biomass of top and intermediate predators across a range of common food web module structures. Our results suggest that refuge is an important driver of FCL in small ecosystems, which has implications for determining responses of these systems to environmental change.     

Does the morphology of beaver ponds alter downstream ecosystems?

Differences among lake morphologies often explain variation in characteristics of lentic ecosystems. Although beaver ponds also vary in morphology, previous studies have not examined the effects of such variation on downstream ecosystems. This study evaluated downstream effects of multiple beaver ponds in the Colorado Rocky Mountains during one low and one high-flow year. Beaver pond morphology was described as the natural log transformed ratio of beaver dam height (which determines hydraulic head) to pond surface area and related to pond spillover phytoplankton and characteristics of the ecosystem downstream (nutrient concentrations, limiting nutrients, periphyton, benthic organic matter (BOM), and benthic invertebrate consumers). Nitrate concentration increased systematically downstream of beaver ponds, but only in the low flow year when groundwater influences predominated. Effects of beaver ponds on soluble reactive phosphorus concentration depended on pond morphology, increasing downstream of small ponds with high dams, but only during the low-flow year. In situ experiments showed that neither beaver activity nor pond morphology predicted periphyton-limiting nutrients downstream. Both periphyton biomass and BOM decreased downstream of small ponds with high dams but pond morphology did not predict abundance of invertebrate grazers or detritus-feeding consumers. While suspension feeding invertebrates increased downstream from small ponds with high dams, variation in chlorophyll a from water spilling over beaver dams did not follow a similar pattern. We conclude that the effects of beaver ponds on downstream nutrients, resources and consumers are rarely systematic, but instead depend on variation in pond morphology and on annual hydrologic variation.     

Predicting Plant Diversity Response to Disturbance: Applicability of the Intermediate Disturbance Hypothesis and Mass Ratio Hypothesis

Predicting the relationships between disturbance, biodiversity and productivity of ecosystems continue to preoccupy ecologists and resource managers. Two hypotheses underpin many of the discussions. The Intermediate Disturbance Hypothesis (IDH), which proposes that biodiversity peaks at intermediate levels of disturbance, is often extended to predict that productivity follows the same response pattern. The Mass Ratio Hypothesis (MRH) proposes that the biological traits of the dominant species are the critical drivers of ecosystem function (e.g., productivity) and that these species increase in biomass rapidly after disturbance then stabilize. As a consequence, species diversity first peaks then declines after disturbance as a few species dominate the site. Both provide a conceptual link among disturbance, species diversity and productivity (an index of ecosystem function). We assessed the current state of empirical support for these two hypotheses with a literature survey and determined if their conformance is related to ecosystem type or site productivity. Conformance of IDH reported in past reviews (considering all ecosystems) ranged from 16 to 21%. This contrasts with our finding that in terrestrial ecosystems conformance to IDH was 46% (22 of 48 studies), 17% studies reported non-compliance, and 23% reported inconclusive results. Most studies explained their results with respect to IDH or MRH. Only two studies were specifically designed to test the validity of IDH or MRH. We conclude that (i) the IDH is mostly applicable to predict species diversity response to disturbance in upland sites of medium to high productivity and the MRH is applicable to organic sites of low productivity; (ii) there is a critical need for more studies specifically designed to test these hypotheses in natural ecosystems using common protocols; and (iii) enhanced understanding of these models will add value in refining management policies and in the selection of meaningful diversity indicators of sustainability.     

食物链长度远因与近因研究进展综述

食物链长度是生态系统的基本属性,其变化决定着群落结构和生态系统功能。稳定同位素分析技术的进步推进了生态系统中食物链长度决定因子相关研究的开展。尽管近期的研究证明了食物链长度与资源可利用性、生态系统大小、干扰等远因之间的关系,但是对于食物网内部结构变化这一近因对食物链长度的影响作用关注较少。综述了边界明确和开放类型淡水生态系统中食物链长度的相关研究进展;探讨了远因和近因机制在决定食物链长度中的作用;给出了判断不同层次和尺度上决定食物链长度机制的概念框架;为今后更好的开展不同生态系统间食物链长度的比较研究提出了建议。     

Shrimp ponds lead to massive loss of soil carbon and greenhouse gas emissions in northeastern Brazilian mangroves

Mangroves of the semiarid Caatinga region of northeastern Brazil are being rapidly converted to shrimp pond aquaculture. To determine ecosystem carbon stocks and potential greenhouse gas emissions from this widespread land use, we measured carbon stocks of eight mangrove forests and three shrimp ponds in the Acaraú and Jaguaribe watersheds in Ceará state, Brazil. The shrimp ponds were paired with adjacent intact mangroves to ascertain carbon losses and potential emissions from land conversion. The mean total ecosystem carbon stock of mangroves in this semiarid tropical landscape was 413 ± 94 Mg C/ha. There were highly significant differences in the ecosystem carbon stocks between the two sampled estuaries suggesting caution when extrapolating carbon stock across different estuaries even in the same landscape. Conversion of mangroves to shrimp ponds resulted in losses of 58%–82% of the ecosystem carbon stocks. The mean potential emissions arising from mangrove conversion to shrimp ponds was 1,390 Mg CO₂e/ha. Carbon losses were largely from soils which accounted for 81% of the total emission. Losses from soils >100 cm in depth accounted for 33% of the total ecosystem carbon loss. Soil carbon losses from shrimp pond conversion are equivalent to about 182 years of soil carbon accumulation. Losses from mangrove conversion are about 10‐fold greater than emissions from conversion of upland tropical dry forest in the Brazilian Caatinga underscoring the potential value for their inclusion in climate change mitigation activities.     

Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams

We used standardized techniques to assemble eighteen food webs in streams. Our aim was to identify the determinants of food-web structure with particular reference to energy availability (related to land use), spatial heterogeneity and ecosystem size (both independent of land use). Forested streams displayed lower algal productivity and higher standing crops of organic matter than the grassland streams. The organic matter in the pine streams was probably of lower quality than that elsewhere. Measures of energy availability and spatial heterogeneity predicted species richness and connectance. A combination of energy availability, spatial heterogeneity and ecosystem size accounted for the representation of particular invertebrate feeding groups in the streams. Algal production and organic matter standing crop were important determinants of invertebrate biomass and overall food-web structure. Grassland sites showed a positive relationship between algal productivity and food chain length whereas forest sites displayed a positive relationship between ecosystem size and food chain length. Therefore, these results provide support for both Pimm's productivity hypothesis and Cohen and Newman's ecosystem size hypothesis.     

Ecosystem engineering by beavers affects mayfly life histories

1. The North American beaver has been studied as a model ecosystem engineer for many decades. Previous studies have documented physical, chemical and biological impacts attributed to beaver engineering in both aquatic and terrestrial environments. This study focused on the effects of ecosystem engineering by beavers on life histories of a common mayfly and on the potential consequences for mayfly populations. 2. We studied 18 montane beaver ponds of varying size and shape in western Colorado near the Rocky Mountain Biological Laboratory. Our goal was to test whether variation in beaver pond morphology (pond size and shape) explains downstream changes in stream temperature, mayfly size and timing of emergence. 3. Downstream water temperatures varied predictably with pond morphology, being colder downstream of high‐head dams and warmer downstream of low‐head dams. Pond morphology was also a significant predictor of variation in the size of mature female Baetis bicaudatus (the most abundant mayfly), with larger females emerging downstream of high‐head dams and smaller females downstream of low‐head dams. The size of male B. bicaudatus was not significantly related to pond morphology or stream temperature. There was no relationship between pond morphology and variation in the timing of emergence of Baetis (males or females) between upstream and downstream reaches. 4. Our results have implications for the effects of beaver ponds on Baetis individual fitness because large Baetis females are more fecund. Therefore, predictable female size variation associated with beaver pond morphology makes it possible to model the effects of beaver activity on local contributions of Baetis to the regional pool of reproductive adults at the catchment scale. Additionally, predictable changes in the size of emerging mayflies may have important consequences for the magnitude of aquatic to terrestrial resource subsidies in beaver‐modified systems.     

Trophic plasticity,environmental gradients and food‐web structure of tropical pond communities

相似文献   

Water- and Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

Background

Understanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.

Methodology/Principal Findings

In-situ canopy CO₂ exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO₂ exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.

Conclusion/Significance

Ecosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.     

Biodiversity and the productivity and stability of ecosystems

Attempts to unveil the relationships between the taxonomic diversity, productivity and stability of ecosystems continue to generate inconclusive, contradictory and controversial conclusions. New insights from recent studies support the hypothesis that species diversity enhances productivity and stability in some ecosystems, but not in others. Appreciation is growing for the ways that particular ecosystem features, such as environmental variability and nutrient stress, can influence biotic interactions. Alternatives to the diversity-stability hypothesis have been proposed, and experimental approaches are starting to evolve to test these hypotheses and to elucidate the mechanisms underlying the functional role of species diversity.     

Global patterns of aquatic food chain length

Food chain length is a fundamental ecosystem property, and plays a central role in determining ecosystem functioning. Recent advances in the field of stable isotope ecology allow the estimation of food chain length (FCL) from stable nitrogen isotope (δ¹⁵N) data. We conducted a global literature synthesis and estimated FCL for 219 lake, stream, and marine ecosystems. Streams had shorter food chains (∼3.5 trophic levels) than marine and lake ecosystems (∼4.0 trophic levels). In marine systems, inclusion of marine mammals increased FCL by 2/3 of a trophic level. For each ecosystem type, estimates of FCL were normally distributed and spanned two full trophic levels. Comparison with published connectance food webs revealed similar mean FCL values, though stable isotope-derived FCL estimates were less variable. At the global scale, FCL showed weak or no relationships with ecosystem size, mean annual air temperature, or latitude. Our study highlights the utility of stable isotopes for quantifying among-system food web variability, and the application of this approach for assessing global-scale patterns of food chain length.     

极端降水和极端干旱事件对草原生态系统的影响

当前人类活动的加剧显著地影响着全球大气循环的格局。大气循环的多个模型均预测未来全球气候变化的显著特征是极端降水事件和极端干旱事件发生的频率会显著增加。水分是干旱、半干旱区草原植物生长发育的限制性资源, 而草原生态系统是陆地生态系统中对降水格局变化非常敏感的系统。但是, 关于极端降水事件和极端干旱事件对草原生态系统结构和功能的影响还是以分散的个案研究为主, 甚至关于极端气候事件的定义迄今也不尽相同。为此, 该文在分析极端气候事件定义及其研究方法的基础上, 总结了极端降水事件和极端干旱事件对草原生态系统土壤水分和养分状况、植物生长发育和生理特性、群落结构、生产力和碳循环过程的影响, 并提出了未来极端气候事件研究中应重点关注的5个重要方向, 以及控制试验研究的2个关键科学问题, 对开展全球变化背景下草原生态系统对极端气候事件响应机制的研究具有指导意义。     

Carbon allocation in boreal black spruce forests across regions varying in soil temperature and precipitation

A common hypothesis for northern ecosystems is that low soil temperatures inhibit plant productivity. To address this hypothesis, we reviewed how separate components of ecosystem carbon (C) cycling varied along a soil temperature gradient for nine well-drained, relatively productive boreal black spruce ( Picea mariana Mill. [B.S.P.]) forests in Alaska, USA, and Saskatchewan and Manitoba, Canada. Annual soil temperature [expressed as soil summed degree days (SDD)] was positively correlated with aboveground net primary productivity (ANPP), while negatively correlated with total belowground carbon flux (TBCF). The partitioning of C to ANPP at the expense of root processes represented a nearly 1 : 1 tradeoff across the soil temperature gradient, which implied that the amount of C cycling through these black spruce ecosystems was relatively insensitive to variation in SDD. Moreover, the rate at which C accumulated in the ecosystem since the last stand replacing fire was unrelated to SDD, but SDD was positively correlated to the ratio of spruce-biomass : forest-floor-mass. Thus, plant partitioning of C and the distribution of ecosystem C were apparently affected by soil temperature, although across regions, precipitation co-varied with soil temperature. These two factors likely correlated with one another because of precipitation's influence on soil heat balance, suggesting that a soil temperature–precipitation interaction could be responsible for the shifts in C allocation. Nonetheless, our results highlight that for this boreal ecosystem, ANPP and TBCF can be negatively correlated. In tropical and temperate forests, TBCF and ANPP have been reported as positively correlated, and our results may reflect the unique interactions between soil temperature, forest floor accumulation, rooting depth, and nutrient availability that characterize the black spruce forest type.     

Carbon allocation in forest ecosystems

Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component. Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either. Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient. How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF. Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.     

Human transformation of ecosystems: Comparing protected and unprotected areas with natural baselines

Protected areas serve as reserves of biological diversity and conserve the naturalness of characteristic regional ecosystems. Numerous approaches have been applied to estimate the level of transformation of ecosystems and to compare trends inside and outside of protected areas. In this study, we apply aggregate indicators of anthropogenic pressures on ecosystems and biodiversity in a fine-scale spatial analysis to compare the level of human influence within protected and unprotected areas. The actual state of ecosystems is compared to a natural baseline that is intact or potential natural state. The results show that in a non-protected Central-European landscape, humans appropriate a considerable share of natural ecosystem productivity and carbon stocks, and significantly reduce natural biodiversity and ecosystem services. Human appropriation of net primary production reached more than 60% in total, humans reduced original biodiversity levels by 69%, and net carbon storage was considerably decreased by intensive types of land use. All three indicators significantly differed between protected areas and unprotected areas, suggesting that protected areas maintain higher biodiversity levels, store more carbon and are in total less influenced by human exploitation than average non-protected landscape. Furthermore, we bring evidence that human appropriation of net primary production is negatively related both to biodiversity and ecosystem services indicated by mean species abundance and net carbon storage at the national level. Our results contribute to the quantitative evidence of the impacts of anthropogenic transformation of natural ecosystems on the ecosystem condition, supporting the hypothesis that protected areas significantly reduce anthropogenic pressures and contribute to maintaining critical ecosystem services and biodiversity.     

生物多样性-生产力关系研究进展——基于文献计量分析

生态系统的结构和功能是生态学研究的核心内容。早期基于野外调查的生态学研究强调生产力表征的环境梯度对生态系统结构的影响,而基于控制试验的生态学研究则强调生态系统结构变化对生态系统功能的影响。围绕这两类研究所支持理论间的争论是当前生态学的前沿、热点和难点,其中最具代表性的科学问题是生物多样性与以生产力为代表的生态系统功能间是否存在一般性关系。为深入了解生物多样性-生产力关系研究脉络,分析其对生态学研究范式与理论发展的影响以及对未来研究方向的启示,以Web of Science核心合集数据库中的相关文献为数据源,结合文献计量分析和文献综述,系统总结了多样性-生产力关系研究进展。结果表明：(1)生物多样性-生产力关系研究推动了生态学研究范式由以样带调查为主的观察性研究向以控制试验为主的实验性研究的转变,促进了全球联网控制试验研究的发展。(2)研究聚焦的生态系统类型由最初的北美普列利草原逐渐向其它草地、灌丛、森林等多样的生态系统过渡,研究结论及其生态学理论的普适性逐渐增强。(3)该研究推动了对生物多样性不同维度(如功能多样性和系统发育多样性)在生态系统中作用的认识,促进了学界对除生产功能外的生态...