1,721,001 research outputs found
L'ecosistema e le sue relazioni. Idee e strumenti per la valutazione di impatto ambientale e di incidenza
No full textFunctional links and robustness in food webs
No full textThe robustness of ecosystems to species losses is a central question in ecology, given the current pace
of extinctions and the many species threatened by human impacts, including habitat destruction and
climate change. Robustness from the perspective of secondary extinctions has been addressed in the
context of food webs to consider the complex network of species interactions that underlie responses
to perturbations. In-silico removal experiments have examined the structural properties of food webs
that enhance or hamper the robustness of ecosystems to species losses, with a focus on the role of
hubs, the most connected species. Here we take a different approach and focus on the role of the
connections themselves. We show that trophic links can be divided into functional and redundant
based on their contribution to robustness. The analysis of empirical webs shows that hubs are not
necessarily the most important species as they may hold many redundant links. Furthermore, the
fraction of functional connections is high and constant across systems regardless of size and
interconnectedness. The main consequence of this scaling pattern is that ecosystem robustness can
be considerably reduced by species extinctions even when these do not result in any secondary
extinctions. This introduces the possibility of tipping points in the collapse of ecosystems
Secondary extinctions in ecological networks: bottlenecks unveiled
No full textIn 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
Cities as ecosystems: Growth, development and implications for sustainability
No full textPrescriptions 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. (C) 2012 Elsevier B.V. All rights reserved
Using dominator trees to catch secondary extinction in action
No full textIn 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. 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 dominator tree. This structure has revealed simple. elegant, and highly informative. Aim of this research is to test the dominator tree model in cases where secondary extinction has been observed
Network science in ecology: the structure of ecological communities and the biodiversity question
No full textThe study of networks in ecology is rapidly expanding. Although network thinking is by no means new to ecologists, cross-fertilization from other fields, ranging from computer science to sociology, has recently furthered the field significantly. Here we examine some of the applications of network science to ecology with an emphasis on its potential to contribute to the preservation of biodiversity, an issue that has relevant social and policy implication. Two different forms in which ecological networks may appear are used: food webs and signed digraphs of dynamical systems. In the former, networks represent energy flow transfers form producers to consumers, while in the latter what is depicted is the effect that populations exert on each other.
The main objective is to enlighten how applying network science can contribute to some central questions concerning biodiversity such as the identification of keystone species, the response of population to environmental perturbations, the robustness or inertia of the system to external events in the form of loss of species and links and press perturbations that alter populations dynamics
Cities as ecosystems: functional similarities and the quest for sustainability
No full textEcosystems 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 as for entropy, flow diversity, conservation of medium, information, niche specialization, seem to hold also in the urban context.
By contrasting the three urban networks with alternative configurations of their flow structure inspired by sustainability thinking we highlight that single-issue criteria that commonly direct actions toward sustainability may have unexpected effects at the whole system scale
Using food web dominator trees to catch secondary extinctions in action
No full textIn 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
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