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  • 1
    Language: English
    In: Ecological Modelling, 24 June 2015, Vol.306, pp.160-173
    Description: Robustness is a universal feature of ecological systems which promotes sustainability over time. Robustness of an aquatic ecosystem, specifically an estuarine system, is investigated here using indicators derived from ecological network analysis. Estuaries provide us with many ecosystem services and these are consequently prone to face anthropogenic stresses. In South Africa, temporarily open/closed estuaries occupy a significant percentage of coastal boundaries. One of the South African estuaries, namely Mdloti, is studied here using network-based, Ecopath software. The estuarine energy flow networks are perturbed following different scenarios, which are assumed to be a result of selected anthropogenic stresses (eutrophication, overfishing) to the system. Several network indices such as total system throughput (TST), redundancy ( ), Finn’s Cycling Index (FCI) and ascendency over development capacity ratio ( / ) are calculated and analyzed for the original field-based network and three perturbed networks under different scenarios (change of autotrophic biomass, fish yield, and detritus import). The change of ecosystem robustness from the unperturbed network is more pronounced in the perturbed networks of fish biomass change and detritus import than change in autotrophic biomass scenario. These indicators reliably reflected the relative change of flow pattern if any changes occur and magnitude in the networks in different scenarios. From the present study, we show that certain common network indices as mentioned above provide a measure of robustness and can be used for the assessment of ecosystem organization and function. ENA properties and also robustness change depending on the type and magnitude of stress imposed on the system.
    Keywords: Network Analysis ; Ascendency ; Development Capacity ; Redundancy ; Perturbation ; Mdloti Estuary ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 2
    Language: English
    In: Ecological Modelling, 2007, Vol.208(1), pp.56-67
    Description: Empirically observable energy and matter transfers in ecosystems create network structures commonly called food webs. The relation or interaction type associated with each link between pair-wise objects can be classified as (+, −) or (−, +) depending on the net gain or loss experienced by each object. If objects are not adjacent in the food web, then their observed direct interaction is neutralism (0, 0). From this perspective, a zero-sum balance exists between the number of positive and negative relations in the ecosystem. However, community-level relations arise from observable direct and unobservable indirect pathways within a food web, giving rise to indirectly mediated relations, mutualism (+, +) and competition (−, −). Determination of community-level relations requires a systemic or holistic approach. measures from in the broader frame of ecological network analysis (ENA) provide such a methodology to investigate the relations resulting from all observed and indirect transfers. This research demonstrates the methodology and shows three important results from the analysis. First, all objects in ecological networks are related either through their input and output environs, and therefore all objects interact with and influence the others in the web: there are no null community-level relations. Second, the community-level relations can and do differ from direct relations: what you see is not always what you get. Third, due to the web of trophic and non-trophic interactions, community-level relations usually have a greater occurrence of mutualism than competition making them more positive than the direct relations that produced them: this is the property called network mutualism.
    Keywords: Community Interactions ; Competition ; Food Webs ; Ecological Network Analysis ; Mutualism ; Synergism ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 3
    Language: English
    In: Ecological Modelling, 2009, Vol.220(22), pp.3210-3218
    Description: This research compares two existing methodologies, mixed trophic impact analysis and utility analysis, which use network analysis to evaluate the direct, pair-wise, and indirect, holistic, ecological relations between ecosystem compartments. The two approaches have many similarities, but differ in some key assumptions which affect both the final results and interpretations. Here, we briefly introduce both methodologies through a series of two simple examples; a 3-compartment competition model and a 3-compartment food chain model, and then apply the methodologies to a 15-compartment ecosystem model of the Chesapeake Bay. This example demonstrates how implementing the various conceptual and methodological assumptions lead to differing results. Notably, the overall number of positive relations is greatly affected by the treatment of the self-interactions and the handling of detritus compartments lead to a distinction between ecological or trophic relations. We recommend slight changes to both methodologies, not necessarily in order to bring them completely together, but because each has some points which are stronger and better defensible.
    Keywords: Ecological Network Analysis ; Flow Analysis ; Mutualism ; Competition ; Indirect Effects ; Food Webs ; Ecological Relations ; Trophic Relations ; Predator–Prey Relations ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 4
    Language: English
    In: Ecological Modelling, 2007, Vol.208(1), pp.17-24
    Description: Standard ecology textbooks typically maintain that nutrients cycle, but energy flows in unidirectional chains. However, here we use a new metric that allows for the identification and quantification of cyclic energy pathways. Some of these important pathways occur due to the contribution of dead organic matter to detrital pools and those organisms that feed on them, reintroducing some of that energy back into the food web. Recognition of these cyclic energy pathways profoundly impacts many aspects of ecology such as trophic levels, control, and the importance of indirect effects. Network analysis, specifically the maximum eigenvalue of the connectance matrix, is used to identify both the presence and strength of these structural cycles.
    Keywords: Cycling ; Detritus ; Energy Flow ; Food Webs ; Network Analysis ; Trophic Dynamics ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 5
    Language: English
    In: Ecological Modelling, 2007, Vol.208(1), pp.9-16
    Description: Food webs are constructed as structural directed graphs that describe “who eats whom,” but it is common to interpret them as energy flow diagrams where predation represents an energy transfer from the prey to the predator. It is the aim of this work to demonstrate that food webs are incomplete as energy flow diagrams if they ignore passive flows to detritus (dead organic material). While many ecologists do include detritus in conceptual and mathematical models, the detrital omission is still commonly found. Often detritus is either ignored or treated as an unlimited energy source, yet all organisms contribute to the detritus pool, which can be an energy source for other species in the system. This feedback loop is of high importance, since it increases the number of pathways available for energy flows, revealing the significance of indirect effects, and making the functional role of the top predators less clear. In this work we propose the by adding a detritus compartment to the . We demonstrate the effect of structural loops that result from feeding on detritus, by comparing empirical data sets to five different assembly models: (1) , (2) , (3) , (4) (original in this work), and (5) . Of these models, only the last two explicitly include detritus. We show that when passive flows to detritus are included in the food web structure, the structure becomes more robust to the removal of individual nodes or connections. In addition, we show that food web models that include the detritus feedback loop perform better with respect to several structural network metrics.
    Keywords: Niche Model ; Detritus ; Food Webs ; Energy Cycles ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 6
    Article
    Article
    Language: English
    In: Ecological Modelling, 2007, Vol.208(2), pp.391-394
    Description: Ecological network analysis allows for an investigation of the structural and functional interconnectedness in ecosystems. Typically, these interactions are seen to comprise a food web of “who eats whom”, but more generally applies to the transfer of energy-matter within the biotic and abiotic ecosphere. This web of transactions can be depicted as a digraph or an adjacency matrix in which the presence of direct transactions are represented as a 1 and no transactions as 0. Each transaction between system components leads to an overall network structural pattern. These structures cluster into different categories or regimes based on their cyclic nature. This paper demonstrates threshold effects of the placement or removal of links, such that certain changes essentially keep the structure in the same regime whereas others shift it to another regime in a non-linear manner.
    Keywords: Network Analysis ; Regime Changes ; Food Webs ; Cycling ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 7
    Language: English
    In: Ecological Modelling, 2004, Vol.171(4), pp.329-337
    Description: Network analysis is an important methodology that has been applied in systems ecology. Many theoretical insights have arisen from this approach, primarily the importance of indirect relations in ecosystems and the holistic determination of ecological relations. This approach, adopted from economic input–output analysis, treats systems as interconnected nodes and arcs of material or energy transfer. Due to the difficulty in quantifying large-scale, complex models, most ecological network models are aggregated or lumped models of a few general characteristics. Therefore, the resultant theory was tested using small-scale models with only a small number of compartments. Here, I present a way to develop large-scale cyber-models in order to test four of the main hypotheses of network analysis using models with a large number of compartments. The results show that the hypotheses are affected by model size.
    Keywords: Ecological Models ; Food Webs ; Scale ; Network Analysis ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 8
    Language: English
    In: Ecological Indicators, December 2014, Vol.47, pp.80-88
    Description: Embodied water in a socio-economic system refers to the hidden water contained in products traded from one region or one sector to another and has been the center of concern for water management in recent years. However, most models developed for water system analysis ignore cycling and indirect flows, making it difficult to explain the effects of structure on these factors among sectors. Therefore, those models fail to examine the water utilization efficiency from an integral view. In this study, we investigate an embodied socio-economic water system using network analysis developed originally for ecological systems. In this manner, we identify structural and throughflow indicators, such as Finn Cycling Index, Indirect effects ratio, and aggradation, to show the efficiency of water utilization. The three indicators show different perspectives of the system's efficiency change over time, indicating that only the combination of these three indicators can provide a holistic portray about efficiency. Results showed that the structure influenced the cycling and indirect flows, and from a throughflow perspective, the system depends on large boundary inputs of fresh water. Furthermore, the values of Cycling Index and Indirect effect ratio are much lower than for natural food webs, implying that the policies that led to the structural change and reduction of boundary fresh water inputs do not lead to positive water utilization seen in natural systems. This study provides a novel perspective and methodology for assessing the structure and efficiency of water utilization system with a whole perspective.
    Keywords: Network Environ Analysis ; Water System ; Embodied Water ; Structural Analysis ; Throughflow Analysis ; Utilization Efficiency ; Environmental Sciences
    ISSN: 1470-160X
    E-ISSN: 1872-7034
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  • 9
    Language: English
    In: Ecological Modelling, 2007, Vol.208(1), pp.49-55
    Description: Ecological network analysis (ENA) is a systems-oriented methodology to analyze within system interactions used to identify holistic properties that are otherwise not evident from the direct observations. Like any analysis technique, the accuracy of the results is as good as the data available, but the additional challenge is that the data need to characterize an entire ecosystem's flows and storages. Thus, data requirements are substantial. As a result, there have, in fact, not been a significant number of network models constructed and development of the network analysis methodology has progressed largely within the purview of a few established models. In this paper, we outline the steps for one approach to construct network models. Lastly, we also provide a brief overview of the algorithmic methods used to construct food web typologies when empirical data are not available. It is our aim that such an effort aids other researchers to consider the construction of such models as well as encourages further refinement of this procedure.
    Keywords: Ascendency ; Ecological Network Analysis ; Food Webs ; Systems Analysis ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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  • 10
    Language: English
    In: Ecological Modelling, 2004, Vol.174(1), pp.25-35
    Description: Ecological indices are used to provide summary information about a particular aspect of ecosystem behavior. Many such indices have been proposed and here we investigate two: exergy and Fisher Information. Exergy, a thermodynamically based index, is a measure of the maximum amount of useable work that can be extracted when a system is brought into equilibrium with a reference state. The measure for exergy used herein, also includes a factor to weigh the “complexity” of the ecological species. Fisher Information is an old statistical measure that has recently been applied as a way to detect change in system regime and as a measure of system order. These two indices are compared on a 10-compartment food web model undergoing five different perturbation scenarios. This food web model, although simple, allows for some interesting insight into the two indices. The results show that generally, although not always, exergy and Fisher Information respond differently, such that when one increases due to a perturbation the other decreases and vice versa. We provide a discussion as to the usefulness of these metrics as ecological indices and as their potential use as ecological goal functions in light of these findings.
    Keywords: Ecological Indices ; Ecological Goal Functions ; Ecological Modeling ; Exergy ; Fisher Information ; Food Webs ; Environmental Sciences ; Ecology
    ISSN: 0304-3800
    E-ISSN: 1872-7026
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