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Forest soils harbour a vast array of organisms that govern the processing of organic matter. Through their trophic interactions, they give rise to carbon flows that sustain soil ecosystem functioning. Understanding how soil food webs shape carbon flows may enhance our knowledge about the role of biodiversity on soil processes. In this work, we assembled trophic networks representing soil food webs of two beech forests during spring and autumn, and compiled mass-balanced models quantifying carbon flows between their components. These models were investigated using network analysis to identify the role of the components on carbon flow, cycling and functional trophic relationships. Moreover, we explored how the structure of carbon exchanges promote efficiency and stability. Results indicate the importance of indirect interactions. Most trophic groups exhibit a diffuse dependency on all the compartments for their carbon requirement although certain groups such as Collembola play the role of hubs in distributing carbon. Indirect interactions often reverse the impacts of direct trophic relationships being antagonistic to the direction of change predicted based on predator–prey interactions. The high incidence of generalist feeding habits increases the redundancy of energy channels thereby making such food webs more resilient against perturbations but at the expense of carbon transfer efficiency. Although differences can be observed across sites and periods, food web structure rather than environmental variability seems to be the main factor responsible for patterns of carbon flows in the two beech forests. Read the free Plain Language Summary for this article on the Journal blog.

The regulation of benthic nitrogen (N) cycling by multiple interactions among bacteria, macrofauna, and primary producers is poorly understood. We hypothesized that a biodiverse benthic system should better exploit the benthic N-availability and retain N than a simpler one. Retention occurs by avoiding losses both to the water column via increased recycling and to the atmosphere via decreased N2 fluxes and by limiting energy-costly processes as N-fixation. We also hypothesized that primary producer-bacterial competition is reduced in the presence of macrofauna due to mobilization of refractory N pools. To this purpose, the effects of two bioturbators (the detritivorous Sparganophilus tamesis and the filter-feeding Corbicula spp.) and two primary producer growth forms (the rooted macrophyte Vallisneria spiralis and microphytobenthos) on benthic N cycling were studied. An array of N-processes were measured along a complexity gradient (from bare sediments to all combinations of the above mentioned organisms), and experimental outcomes were analyzed via ecological network analysis (ENA). This suite of algorithms, applied to the microscale, revealed differential partitioning of N fluxes among bare sediments (highest denitrification rates), sediments with macrofauna (highest recycling), and sediments with rooted plants (highest N-fixation). N2 losses and inputs were significantly reduced when all components were represented, and N requirements by primary producers were to a large extent supported by the activity of macrofauna. Ecological interactions in biodiverse benthic systems promoted an efficient exploitation of sedimentary N pools, increased the coupling between recycling and uptake, and maximized N use efficiency at the expenses of losses and imports.

Links in food webs can be classified as functional, which affect robustness, and redundant, which do not affect robustness. We explore whether this topologically based distinction may still be informative when interaction strength is considered. In 81 weighted food webs we identified functional and redundant links by exploiting the method of generalized dominators, and tested whether the two groups could be patterned according to link strength. Overall, redundant connections are weaker than functional links. However this pattern does not hold for every food web. Also, the difference in strength between functional and redundant links seems to depend on food web topology. In fact the systems in which redundant connections are significantly weaker than functional ones show greater connectance than systems in which the difference in strength between the two groups is not significant. We observed that redundant connections are fundamental in the formation of omnivory modules that have been described as being crucial for the stability/persistence of food webs. Although stability can be promoted by the specific arrangement of links magnitude within these modules, it does not necessarily imply that redundant links have to be weaker than functional connections at the whole web scale.

Prescriptions for a more sustainable society are usually piecemeal. They are inspired by single issue criteria, no matter if sustainability is, rather, a whole system trait as it pertains to growth and development, that are overall system attributes. In this paper urban sustainability is discussed in a whole system perspective using the ecosystem approach as a framework. This required that urban systems were described as flow networks and investigated through ecological network analysis. Three cities are discussed as a case study and their network representation concerned water flows that were identified knowing water exchanges between city components (i.e. sectors of human activity). Network analysis yielded system level indices that condense the complexity of the flow structure (representing system's metabolism) in a few measures that provide information on how systems grow and develop; as such they allow to explore sustainability at the whole system scale. For every system the present network is compared with an alternative scenario envisioned considering policies that foster sustainability. The results show that although all the alternative scenarios would improve sustainability through reducing water consumption, effects at the whole system level may diverge from the expectation. Because system sustainability depends on the balance between organization of flows (order and coherence of flows) and flexibility (redundancy of connections), network reshaping may bring about a reduction in both these fundamental properties, with negative effects on system's propensity to be sustainable. System level indices are holistic measures that unveil the relation between internal processes and whole system performance. Understanding this relation is crucial because the former are the target of environmental policies but sustainability, the objective of such policies, is an overall system trait.

Link arrangement in food webs is determined by the species’ feeding habits. This work investigates whether food web topology is organized in a gradient of trophic positions from producers to consumers. To this end, we analyzed 26 food webs for which the consumption rate of each species was specified. We computed the trophic positions and the link densities of all species in the food webs. Link density measures how much each species contributes to the distribution of energy in the system. It is expressed as the number of links species establish with other nodes, weighted by their magnitude. We computed these two metrics using various formulations developed in the ecological network analysis framework. Results show a positive correlation between trophic position and link density across all the systems, regardless the specific formulas used to measure the two quantities. We performed the same analysis on the corresponding binary matrices (i.e. removing information about rates). In addition, we investigated the relation between trophic position and link density in: a) simulated binary webs with same connectance as the original ones; b) weighted webs with constant topology but randomized link strengths and c) weighted webs with constant connectance where both topology and link strengths are randomized. The correlation between the two indices attenuates, vanishes or becomes negative in the case of binary food webs and simulated data (weighted and unweighted). According to our analysis, link density in food webs decreases with trophic position so that it is greatly reduced toward the top of the trophic hierarchy. This outcome, that seems to challenge previous conclusions based on null models, strongly depends on link quantification. Including interaction strengths may improve substantially our understanding of food web organization, and possibly contradict results based on the analysis of binary webs.

In the present work we investigate whether the distribution of energy flows in ecosystems responds to criteria of trophic organization. We analyzed weighted and unweighted food webs estimating, for each node, trophic position (TP), Shannon's index of inflow diversity ( H ) and individual contribution to the whole average mutual information (AMI). Finally, we performed the same analysis on simulated webs that were constructed using the following criteria: (a) preserving topology and varying link strength; (b) modifying position of links and their intensities. In real ecosystems, moving toward top species, higher pathway redundancy coupled to a strong specialistic trophic behavior was observed. This means that, beside the availability of multiple topological routes, top predators tend to establish “bipolar” predator–prey interactions feeding on preferred preys that, in turn, are mainly preyed by that specialized consumer. Although the analysis shows that, in qualitative food webs, links should be more numerous at the top of the trophic hierarchy, with magnitude more evenly distributed among the interactions, in weighted networks, the tendency of sharing the main flow between donor and receiving compartments (i.e. “bipolar” predator–prey interaction) increases from basal to top species. Patterns displayed by empirical data vanish when indices are calculated for simulated networks, pointing out how topology of energy delivery and flow intensities may be read as a function of the trophic hierarchy.

The Ecological Footprint is a synthetic index useful to assess sustainability of anthropic systems. Its operational use, however, has been hampered by some difficulties, especially at a local scale. Being conceived as a measure of the biologically productive area requested to sustain individual consumptions in a human community, it leaves out the impacts associated to economic activities. Accordingly, the index cannot contribute much to define local policies, whose target are economic activities, and only marginally affect citizens' behaviour. Ecological Footprint calculation scheme can be modified to include the depletion of natural capital due to local activities such as industry, agriculture, tertiary sector, transport, waste and water management. We provide here an approach which takes into account these different aspects, while we discuss its application to a municipal area as a case study.

In the tropho-dynamic analysis of ecosystems the heuristic, discrete concept of trophic level has been replaced by the more realistic, continuous definition of trophic position. In ecological network analysis (ENA) the suite of matrix manipulations called canonical trophic aggregation (CTA) apportions each species’ feeding activity to a series of discrete trophic levels sensu Lindeman. The effective trophic position is computed as the sum of the fractions of trophic activity that each species performs at different trophic levels. In this paper we present an extension of the CTA that combines matrix manipulation and sensitivity analysis. Applying this “extended” CTA to an hypothetical network and to real ecosystems we show how trophic position can be computed taking into account the contribution of external inflows, making it scale-insensitive. Moreover “extended” CTA solves ambiguities related to trophic position in the presence of multiple non-living nodes, considering them as imports.

Detecting the early signs of stress is imperative for the conservation of natural ecosystems. They may, however, go unrecognized because ecosystems, when disturbed, may act as sinks that absorb the external impact without showing any evident changes. This seems to be the case for Lake Santo, a small water body located in a mountainous area of northern Italy. Tourism activity in this area began to develop in the early 1970s and grew continuously over the following 20 years. This activity caused a continually increasing nutrient load into the waters, but surprisingly the lake has remained oligo-mesotrophic, as it was before human pressure became a stressor to the lake. To anticipate possible severe damage to the ecosystem, we searched for early signs of stress by carrying out a retrospective analysis based on a whole-ecosystem approach using trophic flow networks. Ecosystem properties of the lake as calculated from network analysis for the disturbed (year 1991) and unimpacted (year 1973) configurations were compared, with the support of sensitivity analysis and statistical tests. We found evidence that in the period 1970–90 nutrient enrichment did change the course of normal development as the observed increase in system throughput was accompanied by a drop in the level of mutual organization of flows, which instead would be expected to increase during the natural progression of the ecosystem. The scenario that emerged from the comparison of system-level indices, cycling activity, trophic structure, and trophic efficiency indicates that the ecosystem has been subjected to stress. In particular, the type of disturbance corresponds to a quantitative definition of eutrophication.

In ecosystems, a single extinction event could eventually precipitate in a mass extinction, involving species that may be several connections away from the target of the perturbation. This topic has been illuminated by recent studies on network mechanics, thanks to the concepts of hub, error and targeted removal, attack sensitivity, small world, and so forth. To forecast the effects of a species removal one can use an algorithm that unfolds a complex food web into a topologically simpler scheme, called its dominator tree. This structure is simple, elegant, and highly informative; all the bottlenecks and the effects of species removal are clearly traceable. While food web studies are mostly qualitative, in this paper the use of the dominator tree is extended to weighted food webs, in which link magnitude is specified. These structures were obtained from ecological flow networks. In eight of these food webs, the analysis consisted in removing links that were weaker than a threshold of magnitude and building the dominator tree associated to the remaining structure. By progressively increasing the threshold up to the value that would make the graph disconnected, we had the opportunity to investigate patterns of dominance as a function of link magnitude.

Ecological networks are quantitative, graph-based descriptions of ecosystems, consisting of compartments (trophospecies and nutrient pools) that exchange fluxes of nutrients or energy. Previous research pointed out how the model's design is a crucial task that can heavily influence analyses results, and how merging compartments for the purpose of comparing two or more different ecosystems can significantly alter the indices on which the comparison is based. All these works have been focused on the aggregation of trophospecies, whereas networks may comprise several nutrient compartments that may be lumped as well, either for lack of information or for comparison constraints. We show how the aggregation of these non-living compartments can have a greater influence on network analysis results than trophospecies clustering. This problem should on the one hand encourage modelers to make an effort to test the possible effects of aggregations, and on the other show how the role of non-living compartments could be very important in determining network dynamics.

In this paper we investigate ecological flow networks via graph theory in search of the real sequential chains through which energy passes from producers to consumers in complex food webs. We obtain such fundamental pathways by identifying strongly connected components (SCCs), subsystems that groups species that take part in cycling, and performing topological sorting on the acyclic graphs that are obtained. Topological sorting identifies preferential directions for energy to flow from sources to sinks, while recycling remains confined within each SCC. Resolving food web networks for SCC highlights the possibility that compartments can be found in ecosystems, but this does not seem a general rule. The four aquatic food webs described in detail show a rather clear subdivision between benthic and pelagic subcommunities, a result that is discussed in the light of other studies. Should further research confirm these results, new insight into the way ecosystems use energy will be provided, with implications on cycling, reciprocal dependency of variables and indirect effects.

Ecological network analysis is a modelling approach that requires one to represent ecosystems as networks wherein matter/energy enters compartments (species-trophospecies), is exchanged between them and finally leaves the system as dissipation or usable export. The systematic analysis of the ecosystem flow networks is comprised of several techniques, aiming to interpret ecosystems structure and functioning and to estimate their size and developmental stage. The implementation and improvement of this modelling technique depends strongly upon the availability of appropriate software and access to technical expertise to easily and speedily execute the huge amount of computation required. With the package WAND presented in this manuscript the authors intended to promulgate a new, more user-friendly version of some existing environmental software, NETWRK, to analyze networks of trophic exchanges in ecosystems. A Windows—compatible version of NETWRK has been developed, along with ancillary software to facilitate the estimation of new networks and the translation of existing data sets between text and Excel formats.

The focus of this paper is on the relationship between eutrophication and radionuclide circulation at the whole ecosystem scale in the shallow estuarine environment of the Sacca di Goro (Po River Delta, Italy). This lagoon is frequently affected by dystrophic crises, due to decomposition of huge amounts of macroalgae (mainly Ulva rigida), and critical conditions created at the interface between sediment and water are such that Cs-137 accumulated in the sediment can be mobilized and made available in the water column. The release of cesium from sediment in this ecosystem has been evaluated through a field experiment in which chemical conditions typical of anoxic crises were artificially created in enclosures. Also a lab experiment was carried out to shed light on possible cesium release by decomposing macroalgae. The two experiments allowed drawing conclusions on crucial factors controlling cesium release in the Sacca di Goro, the first objective of this research. The second objective was understanding the fate of radiocesium once transported in the water column. To this end ecological information gathered during the experiments and a yearly sampling campaign, has been converted into whole-system seasonal networks describing ecosystem flow structure for the Sacca di Goro. Analyzed by network analysis this model has provided clues about the dynamics of Cs-137 in terms of preferential pathways, sinks, sources, and cycling activity. Sediment, together with seston and dissolved cesium, appear to be the most significant components in the circulation of Cs-137; while macroalgal biomasses play a crucial role as an indirect causal factor.

Network analysis (NA) is used to compare two ecosystems with different spatial extents to understand the different patterns and dynamics that arise. NA allows one to study the system at different scales: At the level of bilateral interactions, input-output structure matrices are calculated to look at the direct and indirect effects that one flow has on another; at the functional level, the food web is mapped into a concatenated trophic chain, and all simple, directed biogeochemical cycles are identified and separated from the supporting dissipative flows; and at the system's level global variables describe the state of development of the total network. The systems in question are the Everglades graminoid marsh and the adjacent cypress swamp. The graminoid marsh is essentially a two-dimensional system, with reduced diversity of primary producers, and a more focussed dependency of higher trophic levels on one particular primary producer, the periphyton. Although the cypress swamp system contains most of the same flora and fauna as the graminoids, it extends into a third dimension, and contains additional forms of terrestrial vegetation that increase the diversity of primary production, and thereby the resilience of the ecosystem. The importance of detritus to both systems is marked, although recycling within detritus is far more important in the graminoids than in the cypress. The linkages to higher trophic levels are relatively fewer in the graminoids, and the diversity of interactions between the detritus and higher trophic levels is much higher in the cypress. Overall, the presence of a third dimension imparts diversity and resilience to the cypress system, although the faster turnover rates of the graminoids make them more productive.

Aim The aim of this research was to investigate the potential of the South Florida Cypress Wetlands as a carbon-accumulating system. Location This ecosystem is part of the Big Cypress Natural Preserve, located in the south-west part of Florida (USA) between the Mangrove Swamps that border the Gulf of Mexico and the Everglades. Methods This investigation was carried out by constructing networks of carbon exchange between the living and nonliving components that comprise this ecosystem. By means of these networks potential for carbon accumulation has been assessed by identifying and quantifying pathways for the transfer of carbon, assessing the efficiency between trophic levels, and evaluating the importance of material cycling. These analyses are commonly referred to as network analysis. Results Results obtained show that dependency on detritus by higher trophic levels is rather low and so is the trophic efficiency. Yet, less than 10% of the carbon that flows through the system is recycled and the magnitude of internal ascendency reaches only 40% of the total system ascendency. Main conclusions All these results support the hypothesis that the South Florida Cypress Wetlands are predominately flow-through in nature and that carbon accumulation in this environment is noticeable.

Indirect trophic effects play important roles in ecosystem dynamics and can at times oppose and dominate the action of direct feeding linkages. Each predator directly exerts a negative effect upon its prey, but predators may also provide indirect benefits to their prey. In ecosystems, such benefits are effected via indirect trophic pathways that can provide a more than compensating positive influence. The ecosystem of the Big Cypress National Preserve (southwest Florida) appears to contain an unusually high number of such predators—most notably, the American alligator, Alligator mississippiensis. The trophic exchanges of carbon among the 68 principal taxa comprising the cypress wetland ecosystem have been quantified during both wet and dry seasons. The network analysis program IMPACTS identified predators that potentially have a positive influence on some of their prey. A total of 64 of these instances were recorded for the wet season and 44 for the dry. Taxa that, on balance, have positive effects upon their prey include fishes, turtles, snakes, birds, and, most significantly, alligators. The feeding habits of alligators benefit a conspicuous number (11) of their prey (invertebrates, frogs, mice, and rats). Further trophic analysis reveals that the predation by alligators on snakes and turtles accounts for most of the trophic benefits bestowed. The actions of alligators in modifying their physical environment has been cited elsewhere as contributing to the maintenance of biotic diversity. It appears that the trophic influence of this species adds further evidence to the important role it plays in the functional ecology of the cypress wetland.

Ecosystems and urban systems share similarities. To grasp them we exploit the ecosystem nature of cities by applying ecological network analysis to three urban systems represented as water flow networks. Urban expansion may bring about transformations that are coherent with ecosystem development. Trends predicted in ecosystems for entropy, flow diversity, conservation of medium, information, and niche specialization seem to hold in the urban context also. By contrasting the three urban networks with alternative configurations of their flow structure inspired by sustainability thinking, we highlight that a single-issue criteria that commonly direct actions toward sustainability may have unexpected effects on the whole system scale.

In ecosystems, a single extinction event can give rise to multiple ‘secondary’ extinctions. Conservation effort would benefit from tools that help forecast the consequences of species removal. One such tool is the dominator tree, a graph-theoretic algorithm that when applied to food webs unfolds their complex architecture, yielding a simpler topology made of linear pathways that are essential for energy delivery. Each species along these chains is responsible for passing energy to the taxa that follow it and, as such, it is indispensable for their survival. To assess the predictive potential of the dominator tree, we compare its predictions with the effects that followed the collapse of the capelin ( Mallotus villosus ) in the Barents Sea ecosystem. To this end, we first compiled a food web for this ecosystem, then we built the corresponding dominator tree and, finally, we observed whether model predictions matched the empirical observations. This analysis shows the potential and the drawbacks of the dominator trees as a tool for understanding the causes and consequences of extinctions in food webs.

The impact of macrophyte communities on benthic fluxes has been analyzed in three shallow coastal environments: Etang du Prevost (Mediterranean coast of France), characterized by the large floating macro-alga Ulva rigida; Certes fishponds (Bassin d’Arcachon), covered by Ruppia cirrhosa; and the inner intertidal mud-flat in the Arcachon Bay (French Atlantic coast), which has extensive Zostera noltii meadows. In these bodies of water, primary production is dependent primarily on the dominant seagrasses and macroalgae that are also responsible for the large quantity of organic matter deposited on the sediment surface. In 1993 and 1994, fluxes of oxygen, sulphide and nutrients were measured in early and late summer, which were selected in order to represent the production and decomposition phases of the dominant macrophytes. Experimental work was undertaken to measure: (1) standing crop of dominant macroalgae and rooted phanerogams and the elemental and macromolecular composition of plant biomass; (2) benthic fluxes of oxygen, sulphide, nitrogen and phosphorus using incubation of multiple dark and light benthic chambers; (3) water-sediment profiles of free-sulphide in sediment cores with rooted phanerogams (Ruppia) as well as with floating seaweeds (Ulva).

Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply. Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida. Two types of field experiments were conducted by incubating Ulva thalli inside large cages. In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage.

The object of this paper is to investigate the sustainability of water use in human settlements. The water system of a small municipality in northern Italy is discussed as a case study. By measuring water exchanges between different sectors of activity within the municipal borders, and considering underground and surface waters as natural components of this system, a network of flows is built and successively investigated by network analysis, a tool that is comprised in the apparatus of ecosystem ecology. By calculating the reciprocal dependence of compartments, the amount of resource that is involved in cycling, the length of exchange pathways, and the organisation of flows, network analysis shows that the system does not use water in a sustainable manner and provides general criteria for improving sustainability. In this respect, options to improve sustainability are also discussed. The results of this study support the idea that the ecosystem approach can provide an interesting conceptual perspective in which sustainability issues can be framed, and that network analysis is a promising tool to handle these issues in practice.