Resource availability and ecosystem size predict food-chain length in pond ecosystems

Food-chain length (FCL) in ecosystems has been studied extensively, and numerous hypotheses to predict FCL, productivity, ecosystem size, and productive space have been proposed. For example, the productivity hypothesis suggests that resource availability limits FCL, whereas the productive-space hypothesis predicts that per-unit-size resource availability and ecosystem size equally limit FCL. However, previous studies have only measured total productivity to test FCL and have never tested the impact of resource availability within a system on FCL. Therefore, we estimated FCL in 15 ponds using stable isotope techniques to test the most common hypotheses for predicting FCL. We also measured total productivity, pond volume (ecosystem size), and edible microalgal carbon (resource availability) in each pond. We found that productive-space (edible carbon+pond volume) was the best model to predict FCL, and FCL was significantly correlated with edible carbon and pond volume. However, factors such as total productivity did not directly predict FCL of the ponds. Therefore, our results suggest that both resource availability and ecosystem size predict FCL in pond ecosystems and play significant roles in maintaining longer FCLs. Furthermore, the productive-space hypothesis appears to be particularly important for determining the FCL of ponds.     

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。     

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.     

Disturbance, resource supply, and food-web architecture in streams

We studied food webs comprising fish, macroinvertebrates, and algae (identified to species or morphospecies) in small streams using a consistent methodology at the same spatial and temporal scales. Our aim was to test a priori hypotheses derived from dynamic-demographic and energetics models concerning the effects of disturbance and resource availability on food-web attributes. The regime of bed disturbance affecting the organisms in the webs was measured in 10 streams. We also derived measures of the supply of resources for animals in the webs in terms of algal primary productivity and detritus standing crop. Both web size and number of links per species were significantly negatively related to mean intensity of bed disturbance. Mean chain length had a significant positive relationship with algal primary productivity but not disturbance. No food-web attribute was related to detritus standing crop.     

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.     

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。     

Food Webs in Relation to Variation in the Environment and Species Assemblage: A Multivariate Approach

The abiotic environment has strong influences on the growth, survival, behavior, and ecology of aquatic organisms. Biotic interactions and species life histories interact with abiotic factors to structure the food web. One measure of food-web structure is food-chain length. Several hypotheses predict a linear relationship between one environmental variable (e.g., disturbance or ecosystem size) and food-chain length. However, many abiotic and biotic variables interact in diverse ways to structure a community, and may affect other measures of food web structure besides food-chain length. This study took a multivariate approach to test the influence of several important environmental variables on four food-web characteristics measured in nine ponds along a hydroperiod gradient over two years. This approach allowed for testing the ecosystem size and dynamic constraints hypotheses while in context of other possibly interacting environmental variables. The relationship between amphibian and invertebrate communities and pond habitat variables was assessed to understand the underlying food-web structure. Hydroperiod and pond area had a strong influence on amphibian and invertebrate communities, trophic diversity and δ¹⁵N range. The range in δ¹³C values responded strongly to dissolved oxygen. Food-chain length responded to multiple environmental variables. Invertebrate and amphibian communities were structured by pond hydroperiod which in turn influenced the trophic diversity of the food web. The results of this study suggest food-chain length is influenced by environmental variation and species assemblage and that a multivariate approach may allow us to better understand the dynamics within and across aquatic food webs.     

Warming and nitrogen affect size structuring and density dependence in a host–parasitoid food web

Body size is a major factor constraining the trophic structure and functioning of ecological communities. Food webs are known to respond to changes in basal resource abundance, and climate change can initiate compounding bottom-up effects on food-web structure through altered resource availability and quality. However, the effects of climate and co-occurring global changes, such as nitrogen deposition, on the density and size relationships between resources and consumers are unknown, particularly in host–parasitoid food webs, where size structuring is less apparent. We use a Bayesian modelling approach to explore the role of consumer and resource density and body size on host–parasitoid food webs assembled from a field experiment with factorial warming and nitrogen treatments. We show that the treatments increased resource (host) availability and quality (size), leading to measureable changes in parasitoid feeding behaviour. Parasitoids interacted less evenly within their host range and increasingly focused on abundant and high-quality (i.e. larger) hosts. In summary, we present evidence that climate-mediated bottom-up effects can significantly alter food-web structure through both density- and trait-mediated effects.     

Proximate structural mechanisms for variation in food-chain length

Food-chain length is a central characteristic of ecological communities because of its strong influence on community structure and ecosystem function. While recent studies have started to better clarify the relationship between food-chain length and environmental gradients such as resource availability and ecosystem size, much less progress has been made in isolating the ultimate and proximate mechanisms that determine food-chain length. Progress has been slow, in part, because research has paid little attention to the proximate changes in food web structure that must link variation in food-chain length to the ultimate dynamic mechanism. Here we outline the structural mechanisms that determine variation in food-chain length. We explore the implications of these mechanisms for understanding how changes in food-web structure influence food-chain length using both an intraguild predation community model and data from natural ecosystems. The resulting framework provides the mechanisms for linking ultimate dynamic mechanisms to variation in food-chain length. It also suggests that simple linear food-chain models may make misleading predictions about patterns of variation in food-chain length because they are unable to incorporate important structural mechanisms that alter food-web dynamics and cause non-linear shifts in food-web structure. Intraguild predation models provide a more appropriate theoretical framework for understanding food-chain length in most natural ecosystems because they accommodate all of the proximate structural mechanisms identified here.     

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.     

Are there real differences among aquatic and terrestrial food webs?

Recently, aquatic and terrestrial ecologists have put forward several hypotheses regarding similarities and differences in food-web structure and function among these ecosystem types. Although many of these hypotheses explore why strong top-down effects and trophic cascades might be less common in terrestrial than in aquatic ecosystems, there is little theoretical or empirical evidence available to support or refute these hypotheses. Many unanswered questions remain about potential differences across ecosystem types: progress will require empirical studies designed within a broader context that allows for more direct comparisons.     

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.     

A new graphical model for untangling complex relationships among environment, biodiversity, and ecosystem functioning

Recent advances in the research field of ‘biodiversity-ecosystem functioning’ have successfully begun to reconcile the apparent controversy on relationships between productivity and species richness. By unifying new advances into a single framework, I propose a 3D graphical model connecting the relationships among resource availability, species richness, and ‘community productivity.’ An emergent pattern from this model predicts that the effect of species richness on community productivity is maximized at intermediate levels of resource availability. This model will contribute to better understanding the relationships among environment, biodiversity, and ecosystem functioning.     

Predicting the outcome of trophic manipulation in lakes—a comment on Harris (1994)

1. Harris ( Freshwater Biology , 32 , 143–160, 1994) asserts that while empirical modelling of lake ecosystem properties has yielded general predictions and useful explanations, research into the effects of food-web interactions has not. I provide an explicit example of a useful, quantitative prediction gleaned from experiments on food-web effects—the relationship between total phosphorus and the magnitude of response of algal biomass.
2. I further argue that Harris errs in asserting that food-web effects will be ineffective in eutrophic lakes. This error stems from a misconception about the relevance of ecosystem behaviour in unmanipulated systems for predictions of how an ecosystem will respond to manipulation. I cite empirical evidence demonstrating that, contrary to Harris's contention, the response of algal biomass to a change in food-web structure increases as lakes are enriched.     

Adaptive movement and food-chain dynamics: towards food-web theory without birth–death processes

Population density can be affected by its prey [resource] and predator [consumer] abundances through two different mechanisms: the alternation of birth [or somatic growth] or death rate and inter-habitat movement. While the food-web theory has traditionally been built on the former mechanism, the latter mechanism has formed the basis of a successful theory explaining the spatial distribution of organisms in the context of behavioral and evolutionary ecology. Yet, few studies have compared these two mechanisms, leaving the question of how similar (or different) predictions derived from birth–death-based and movement-based food-web theories unanswered. Here, theoretical models of the tri-trophic (resource–consumer-top predator) food chain were used to compare food-web patterns arising from these two mechanisms. Specifically, we evaluated the response of the food-chain structure to inter-patch differences in productivity for movement-based models and birth–death-based models. Model analysis reveals that adaptive movements give rise to positively correlated responses of all trophic levels to increased productivity; however, this pattern was not observed in the corresponding birth–death-based model. The movement-based model predicts that the food chain response to productivity is determined by the sensitivity of animal movement to the environmental conditions. More specifically, increasing sensitivity of a consumer or top predator leads to smaller inter-patch variance of the resource or consumer density, while increasing inter-patch variance in the consumer or resource density. In conclusion, adaptive movement provides an alternative mechanism correlating the food-web structure to environmental conditions.     

Tree Size Inequality Reduces Forest Productivity: An Analysis Combining Inventory Data for Ten European Species and a Light Competition Model

Plant structural diversity is usually considered as beneficial for ecosystem functioning. For instance, numerous studies have reported positive species diversity-productivity relationships in plant communities. However, other aspects of structural diversity such as individual size inequality have been far less investigated. In forests, tree size inequality impacts directly tree growth and asymmetric competition, but consequences on forest productivity are still indeterminate. In addition, the effect of tree size inequality on productivity is likely to vary with species shade-tolerance, a key ecological characteristic controlling asymmetric competition and light resource acquisition. Using plot data from the French National Geographic Agency, we studied the response of stand productivity to size inequality for ten forest species differing in shade tolerance. We fitted a basal area stand production model that included abiotic factors, stand density, stand development stage and a tree size inequality index. Then, using a forest dynamics model we explored whether mechanisms of light interception and light use efficiency could explain the tree size inequality effect observed for three of the ten species studied. Size inequality negatively affected basal area increment for seven out of the ten species investigated. However, this effect was not related to the shade tolerance of these species. According to the model simulations, the negative tree size inequality effect could result both from reduced total stand light interception and reduced light use efficiency. Our results demonstrate that negative relationships between size inequality and productivity may be the rule in tree populations. The lack of effect of shade tolerance indicates compensatory mechanisms between effect on light availability and response to light availability. Such a pattern deserves further investigations for mixed forests where complementarity effects between species are involved. When studying the effect of structural diversity on ecosystem productivity, tree size inequality is a major facet that should be taken into account.     

Predicting the outcome of trophic manipulation in lakes—a comment on Harris (1994)

1. Harris ( Freshwater Biology , 32 , 143–160, 1994) asserts that while empirical modelling of lake ecosystem properties has yielded general predictions and useful explanations, research into the effects of food-web interactions has not. I provide an explicit example of a useful, quantitative prediction gleaned from experiments on food-web effects—the relationship between total phosphorus and the magnitude of response of algal biomass.
2. I further argue that Harris errs in asserting that food-web effects will be ineffective in eutrophic lakes. This error stems from a misconception about the relevance of ecosystem behaviour in unmanipulated systems for predictions of how an ecosystem will respond to manipulation. I cite empirical evidence demonstrating that, contrary to Harris's contention, the response of algal biomass to a change in food-web structure increases as lakes are enriched.     

Litter elemental stoichiometry and biomass densities of forest soil invertebrates

To maintain constant chemical composition, i.e. elemental homeostasis, organisms have to consume resources of sufficient quality to meet their own specific stoichiometric demand. Therefore, concentrations of elements indicate resource quality, and rare elements in the environment may act as limiting factors for individual organisms scaling up to constrain population densities. We investigated how the biomass densities of invertebrate populations of temperate forest soil communities depend on 1) the stoichiometry of the basal litter according to ecological stoichiometry concepts and 2) the population average body mass as predicted by metabolic theory. We used a large data set on biomass densities of 4959 populations across 48 forests in three regions of Germany. Following various ecological stoichiometry hypotheses, we tested for effects of the carbon‐to‐element ratios of 10 elements. Additionally, we included the abiotic litter characteristics habitat size (represented by litter depth), litter diversity and pH, as well as forest type as an indicator for human management. Across 12 species groups, we found that the biomass densities scaled significantly with population‐averaged body masses thus supporting metabolic theory. Additionally, 10 of these allometric scaling relationships exhibited interactions with stoichiometric and abiotic co‐variables. The four most frequent co‐variables were 1) forest type, 2) the carbon‐to‐phosphorus ratio (C:P), 3) the carbon‐to‐sodium ratio (C:Na), and the carbon‐to‐nitrogen ratio (C:N). Hence, our analyses support the sodium shortage hypothesis for microbi‐detritivores, the structural elements hypothesis for some predator groups (concerning N), and the secondary productivity hypothesis (concerning P) across all trophic groups in our data. In contrast, the ecosystem size hypothesis was only supported for some meso‐ and macrofauna detritivores. Our study is thus providing a comprehensive analysis how the elemental stoichiometry of the litter as the basal resource constrain population densities across multiple trophic levels of soil communities.     

Ecosystem engineering by invasive exotic beavers reduces in-stream diversity and enhances ecosystem function in Cape Horn,Chile

Species invasions are of global significance, but predicting their impacts can be difficult. Introduced ecosystem engineers, however, provide an opportunity to test the underlying mechanisms that may be common to all invasive engineers and link relationships between changes in diversity and ecosystem function, thereby providing explanatory power for observed ecological patterns. Here we test specific predictions for an invasive ecosystem engineer by quantifying the impacts of habitat and resource modifications caused by North American beavers (Castor canadensis) on aquatic macroinvertebrate community structure and stream ecosystem function in the Cape Horn Biosphere Reserve, Chile. We compared responses to beavers in three habitat types: (1) forested (unimpacted) stream reaches, (2) beaver ponds, and (3) sites immediately downstream of beaver dams in four streams. We found that beaver engineering in ponds created taxonomically simplified, but more productive, benthic macroinvertebrate assemblages. Specifically, macroinvertebrate richness, diversity and number of functional feeding groups were reduced by half, while abundance, biomass and secondary production increased three- to fivefold in beaver ponds compared to forested sites. Reaches downstream of beaver ponds were very similar to natural forested sections. Beaver invasion effects on both community and ecosystem parameters occurred predominantly via increased retention of fine particulate organic matter, which was associated with reduced macroinvertebrate richness and diversity (via homogenization of benthic microhabitat) and increased macroinvertebrate biomass and production (via greater food availability). Beaver modifications to macroinvertebrate community structure were largely confined to ponds, but increased benthic production in beaver-modified habitats adds to energy retention and flow for the entire stream ecosystem. Furthermore, the effects of beavers on taxa richness (negative) and measures of macroinvertebrate biomass (positive) were inversely related. Thus, while a generally positive relationship between diversity and ecosystem function has been found in a variety of systems, this work shows how they can be decoupled by responding to alterative mechanisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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...