Ecosystem health is a concept that has progressively gained ground; meanwhile, a coherent scientific framework to tackle environmental problems was taking shape. The idea to frame in a unique perspective the many forms of environmental degradation has inevitably led ecologists to use the ecosystem as the unit of study and management.
Accordingly, environmental stress, in its many forms, should be translated into modifications in the mechanisms that govern the ecosystem's natural tendency to grow and develop. Ascendency, thus, as a measure of growth and development, can be a useful index to assess whether an ecosystem is in a healthy condition. However, one must be aware that increasing ascendency means any combination ofgrowth and development. If, for example, a rise in system activity (TST) compensates for a concomitant fall in system organization (AMI), overall ascendency will increase but the system cannot be considered in a healthy status as retrogression toward earlier stages of development has occurred. This example describes what occurs during eutrophication. The accessibility to new resources by the producers results in greater activity; however, this activity is concentrated more at the lower trophic levels. A possible outcome is a reduced diversity of species and their associated transfers, which, in turn, imposes a drop in the level offlow organization. The fact that a form ofstress increases an attribute of the system, namely TST, it is not, per se, a contradiction; eutrophication in fact is a change in which ecosystem receives 'too much of a good thing'. In general, however, if a perturbation acts so as to disrupt ecosystem internal mechanisms, one should expect both system activity and development capacity to fall. To unveil these mechanisms, one needs to study how unimpacted ecosystems compare with their stressed counterpart. This can be carried out, in principle, through long-term monitoring or by reconstructing the ecosystem past history. The number of applications in this field is rapidly growing.
A precise definition ofecosystem health has not yet been proposed, although several attributes have been used as proxies: homeostasis, vigor or scope for growth, resilience, ability to self-repair, minimal external support, stability, sustainability, capability to maintain organization over time, to cite some. None of them in isolation allows a complete grasp ofthe essence ofecosystem health but each tells a 'part of the story'. Ascendency provides a synthesis of some of these indicators, but, still, it does not incorporate all the facets of ecosystem health. This happens because the different proxies naturally divide in two categories: indicators of system performance and indices of ability to maintain this performance over time. Ifecosystem health mostly addresses how well the system is functioning (performance), the other indices that measure the 'temporal functionality' ofthe ecosystem are better summarized by the concept of integrity. Integrity encompasses a system's entire trajectory ofpast and future configurations and refers to (1) capacity to withstand stress (resilience, resistance, and so forth); (2) maintaining the capability for the greatest possible developmental options; and (3) continued ability for ongoing change and development, unconstrained by human perturbations. In this scenario, ascendency becomes a proxy for ecosystem health while other indices must be utilized to measure the various components ofecosystem integrity.
In the realm of ecological flow networks, a good proxy for system's capacity to withstand stress seems to be the excess redundancy of internal flows, imports, and exports with respect to the optimal (maximally developed) configuration of flows in which redundancy is kept to a minimum. Such optimal configuration, with extreme specialization in exchanging medium, is however brittle and subject to collapse. A system could overcome the effect of perturbation only if it maintains a sufficient amount of degrees of freedom in terms of choices for channeling medium within the system and with the outside environment. The combination (summation) of organized complexity (ascendency) and encumbered complexity (excess redundancy or overhead) quantifies the entire capacity of an ecosystem to develop. The interplay between organized and encumbered complexity offers a system the capability for ongoing change and development. The analysis of networks of flows, by quantifying these attributes, gives the opportunity to assess ecosystem health and integrity in a complete and quantitative way.
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