Antonio Bodini

Towards a sustainable use of water resources: a whole-ecosystem approach using network analysis

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.

Functional links and robustness in food webs.

The 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.

Detecting Stress at the Whole-Ecosystem Level: The Case of a Mountain Lake (Lake Santo, Italy)

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.

Representing Ecosystem Structure through Signed Digraphs. Model Reconstruction, Qualitative Predictions and Management: The Case of a Freshwater Ecosystem

Predictions occupy a central position in environmental management. To make predictions, models are constructed and analyzed. In this framework the quantitative approach based on large simulation models dominates. It has been shown in various ecological contexts that relevant aspects of the systems behavior may depend on the structure of the interactions. In this paper a strategy to-reconstruct the structure of a freshwater ecosystem is presented. It uses statistical patterns and field observations combined with a qualitative algorithm, that of the loop analysis. This procedure leads to unambiguous conclusions about system structure, although alternatives are possible. The models obtained are then qualitatively investigated and results discussed about management opportunities.

Using food web dominator trees to catch secondary extinctions in action

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.

Threshold extinction in food webs

Food web response to species loss has been investigated in several ways in the previous years. In binary food webs, species go secondarily extinct if no resource item remains to be exploited. In this work, we considered that species can go extinct before the complete loss of their resources and we introduced thresholds of minimum energy requirement for species survival. According to this approach, extinction of a node occurs whenever an initial extinction event eliminates its incoming links so it is left with an overall energy intake lower than the threshold value. We tested the robustness of 18 real food webs by removing species from most to least connected and considering different scenarios defined by increasing the extinction threshold. Increasing energy requirement threshold negatively affects food web robustness. We found that a very small increase of the energy requirement substantially increases system fragility. In addition, above a certain value of energy requirement threshold we found no relationship between the robustness and the connectance of the web. Further, food webs with more species showed higher fragility with increasing energy threshold. This suggests that the shape of the robustness–complexity relationship of a food web depends on the sensitivity of consumers to loss of prey.

Multicriteria analysis as a tool to investigate compatibility between conservation and development on Salina Island, Aeolian Archipelago, Italy

Several multicriteria evaluation techniques have been developed since the 1970s. The need to compare different territorial policies has justified their introduction into environmental research. These methods are based on the numerical manipulation of heterogeneous information, which varies in terms of reference scale and type of measure (continuous, ordinal, qualitative, binary, etc.).During recent years, diverse investigations have focused on general conditions on Salina, the “green island” of the Aeolian archipelago. Such studies, within an interdisciplinary project, aimed to explore the possibility of implementing conservation strategies that are compatible with human needs, landscape preservation, and sustainable economic development.Three different evaluation techniques are applied, namely multicriteria weighted concordance and discordance analysis and a qualitative procedure. They are used to compare four alternative plans for the socioeconomic development of Salina Island. These plans lie between extreme alternatives: total protection of natural resources and maximizing economic development based on tourism. The plans are compared to each other on the basis of 14 criteria that reflect the socioenvironmental perception of Salinas inhabitants.The approach used in this research seems particularly fruitful because of its flexibility: it offers decision makers the chance to manage heterogeneous data and information that is not easily quantifiable. Such “soft” information helps to evaluate environmental conditions more precisely, and to make a less damaging choice among alternative development plans.

Plant-ant-fungus communities investigated through qualitative modelling.

Qualitative models are applied to investigate aspects related to the activities of leaf-cutting ant populations, pernicious agents in the tropical areas of the New World for the productivity of forests and crop fields. The modelling approach allows global evaluation of interactions occurring in a natural system. In particular, loop analysis allows one to qualitatively handle the dynamic behaviour of complex systems subject to external actions that tend to modify their parameters. The peculiar mutualism between these insects and some Basidiomycetes is articulated on subtle behavioural and biochemical mechanisms which confer a primary role to the leaf-cutting ants in the processes of organic decomposition and mineral cycling. If one adds the diverse forms of interaction the ants undertake with other organisms to the variety of functions they carry out, one obtains a picture that would be reductive to analyse only from the point of view of the food chain, under the hypothesis of simple two trophic level structures. The complexity of the community system generates feedback processes which condition the dynamics of the components, that involve cause-effect processes easily missed when intuitive or linear interpretations prevail. Alterations of parameters carried on to control the harmful action of these organisms can generate situations opposite to the expectations, as shown in the models here investigated.

High Nature Value Farmland: Assessment of Soil Organic Carbon in Europe

High Nature Value Farmland (HNVF) is commonly associated with low intensity agricultural systems. HNVFs cover approximately 32% of the agricultural land in Europe and are of strategic importance for the European Union policy since they are reservoirs of biodiversity and provide several ecosystem services. Carbon sequestration is an important service that can be supplied by HNVFs as addressed in this study. Considering soil carbon content as a proxy for soil carbon storage, we compare HNVFs with soils that undergo more conventional land management (nHNVFs) and study the consequences of diverse land uses and geographic regions as additional explanatory variables. The results of our research show that, at the European level, organic carbon content is higher in HNVF than in nHNVF. However, this difference is strongly affected by the type of land use and the geographic region. Rather than seeing HNVF and nHNVF as two sharply distinct categories, as for carbon storage potential, we provide indications that the interplay between soil type (HNVF or nHNVF), land use and geographic region determines carbon content in soils.

What is the role of predation on stability of natural communities? A theoretical investigation

A basic question in ecology concerns the role of species interaction on dynamics of natural communities. In this framework, ecologists have considered predation, competition, mutualism, the three most important interactions, highlighting their specific effects on distribution and abundance of species, providing knowledge about phenomena like coexistence and extinction. This paper seeks to identify the effects of predation on stability of natural communities by mathematical models. Simple multispecies community models, organized in trophic levels, are analyzed by means of a qualitative technique, loop analysis, combined with a computer calculation procedure. Results do not support the hypothesis of predation as a stabilizing factor. Rather, the outcomes of the analysis suggest that predation may or may not stabilize a community. This depends on the predators behaviour and on the network of the community.