Ecosystem health is a metaphor used to describe the condition of an ecosystem. Ecosystem condition can vary as a result of fire, flooding, drought, extinctions, invasive species, climate change, mining, overexploitation in fishing, farming or logging, chemical spills, and a host of other reasons. There is no universally accepted benchmark for a healthy ecosystem, rather the apparent health status of an ecosystem can vary depending upon which health metrics are employed in judging it and which societal aspirations are driving the assessment. Advocates of the health metaphor argue for its simplicity as a communication tool. "Policy-makers and the public need simple, understandable concepts like health." Critics worry that ecosystem health, a "value-laden construct", is often "passed off as science to unsuspecting policy makers and the public."
The health metaphor applied to the environment has been in use at least since the early 1800s and the great American conservationist Aldo Leopold (1887–1948) spoke metaphorically of land health, land sickness, mutilation, and violence when describing land use practices. The term "ecosystem management" has been in use at least since the 1950s. The term "ecosystem health" has become widespread in the ecological literature, as a general metaphor meaning something good, and as an environmental quality goal in field assessments of rivers, lakes, seas, and forests.
Recently however this metaphor has been subject of quantitative formulation using complex systems concepts such as criticality, meaning that a healthy ecosystem is in some sort of balance between adaptability (randomness) and robustness (order) . Nevertheless the universality of criticality is still under examination and is known as the Criticality Hypothesis, which states that systems in a dynamic regime shifting between order and disorder, attain the highest level of computational capabilities and achieve an optimal trade-off between robustness and flexibility. Recent results in cell and evolutionary biology, neuroscience and computer science have great interest in the criticality hypothesis, emphasizing its role as a viable candidate general law in the realm of adaptive complex systems (see  and references therein).
The term ecosystem health has been employed to embrace some suite of environmental goals deemed desirable. Edward Grumbine's highly cited paper "What is ecosystem management?" surveyed ecosystem management and ecosystem health literature and summarized frequently encountered goal statements:
Grumbine describes each of these goals as a "value statement" and stresses the role of human values in setting ecosystem management goals.
It is the last goal mentioned in the survey, accommodating humans, that is most contentious. "We have observed that when groups of stakeholders work to define … visions, this leads to debate over whether to emphasize ecosystem health or human well-being … Whether the priority is ecosystems or people greatly influences stakeholders' assessment of desirable ecological and social states." and, for example, "For some, wolves are critical to ecosystem health and an essential part of nature, for others they are a symbol of government overreach threatening their livelihoods and cultural values."
Measuring ecosystem health requires extensive goal-driven environmental sampling. For example, a vision for ecosystem health of Lake Superior was developed by a public forum and a series of objectives were prepared for protection of habitat and maintenance of populations of some 70 indigenous fish species. A suite of 80 lake health indicators was developed for the Great Lakes Basin including monitoring native fish species, exotic species, water levels, phosphorus levels, toxic chemicals, phytoplankton, zooplankton, fish tissue contaminants, etc.
Some authors have attempted broad definitions of ecosystem health, such as benchmarking as healthy the historical ecosystem state "prior to the onset of anthropogenic stress." A difficulty is that the historical composition of many human-altered ecosystems is unknown or unknowable. Also, fossil and pollen records indicate that the species that occupy an ecosystem reshuffle through time, so it is difficult to identify one snapshot in time as optimum or "healthy.".
A commonly cited broad definition states that a healthy ecosystem has three attributes:
While this captures significant ecosystem properties, a generalization is elusive as those properties do not necessarily co-vary in nature. For example, there is not necessarily a clear or consistent relationship between productivity and species richness. Similarly, the relationship between resilience and diversity is complex, and ecosystem stability may depend upon one or a few species rather than overall diversity. And some undesirable ecosystems are highly productive.
"Resilience is not desirable per se. There can be highly resilient states of ecosystems which are very undesirable from some human perspectives , such as algal-dominated coral reefs." Ecological resilience is a "capacity" that varies depending upon which properties of the ecosystem are to be studied and depending upon what kinds of disturbances are considered and how they are to be quantified. Approaches to assessing it "face high uncertainties and still require a considerable amount of empirical and theoretical research."
Other authors have sought a numerical index of ecosystem health that would permit quantitative comparisons among ecosystems and within ecosystems over time. One such system employs ratings of the three properties mentioned above: Health = system vigor x system organization x system resilience. Ecologist Glenn Suter argues that such indices employ "nonsense units," the indices have "no meaning; they cannot be predicted, so they are not applicable to most regulatory problems; they have no diagnostic power; effects of one component are eclipsed by responses of other components, and the reason for a high or low index value is unknown."
Health metrics are determined by stakeholder goals, which drive ecosystem definition. An ecosystem is an abstraction. "Ecosystems cannot be identified or found in nature. Instead, they must be delimited by an observer. This can be done in many different ways for the same chunk of nature, depending on the specific perspectives of interest."
Ecosystem definition determines the acceptable range of variability (reference conditions) and determines measurement variables. The latter are used as indicators of ecosystem structure and function, and can be used as indicators of "health".
An indicator is a variable, such as a chemical or biological property, that when measured, is used to infer trends in another (unmeasured) environmental variable or cluster of unmeasured variables (the indicandum). For example, rising mortality rate of canaries in a coal mine is an indicator of rising carbon monoxide levels. Rising chlorophyll-a levels in a lake may signal eutrophication.
Ecosystem assessments employ two kinds of indicators, descriptive indicators and normative indicators. "Indicators can be used descriptively for a scientific purpose or normatively for a political purpose."
Used descriptively, high chlorophyll-a is an indicator of eutrophication, but it may also be used as an ecosystem health indicator. When used as a normative (health) indicator, it indicates a rank on a health scale, a rank that can vary widely depending on societal preferences as to what is desirable. A high chlorophyll-a level in a natural successional wetland might be viewed as healthy whereas a human-impacted wetland with the same indicator value may be judged unhealthy.
Estimation of ecosystem health has been criticized for intermingling the two types of environmental indicators. A health indicator is a normative indicator, and if conflated with descriptive indicators "implies that normative values can be measured objectively, which is certainly not true. Thus, implicit values are insinuated to the reader, a situation which has to be avoided."
It can be argued that the very act of selecting indicators of any kind is biased by the observer's perspective but separation of goals from descriptions has been advocated as a step toward transparency: "A separation of descriptive and normative indicators is essential from the perspective of the philosophy of science … Goals and values cannot be deduced directly from descriptions … a fact that is emphasized repeatedly in the literature of environmental ethics … Hence, we advise always specifying the definition of indicators and propose clearly distinguishing ecological indicators in science from policy indicators used for decision-making processes."
And integration of multiple, possibly conflicting, normative indicators into a single measure of "ecosystem health" is problematic. Using 56 indicators, "determining environmental status and assessing marine ecosystems health in an integrative way is still one of the grand challenges in marine ecosystems ecology, research and management"
Another issue with indicators is validity. Good indicators must have an independently validated high predictive value, that is high sensitivity (high probability of indicating a significant change in the indicandum) and high specificity (low probability of wrongly indicating a change). The reliability of various health metrics has been questioned and "what combination of measurements should be used to evaluate ecosystems is a matter of current scientific debate." Most attempts to identify ecological indicators have been correlative rather than derived from prospective testing of their predictive value and the selection process for many indicators has been based upon weak evidence or has been lacking in evidence.
In some cases no reliable indicators are known: "We found no examples of invertebrates successfully used in [forest] monitoring programs. Their richness and abundance ensure that they play significant roles in ecosystem function but thwart focus on a few key species." And, "Reviews of species-based monitoring approaches reveal that no single species, nor even a group of species, accurately reflects entire communities. Understanding the response of a single species may not provide reliable predictions about a group of species even when the group is comprised of a few very similar species."
A trade-off between human health and the "health" of nature has been termed the "health paradox" and it illuminates how human values drive perceptions of ecosystem health.
Human health has benefited by sacrificing the "health" of wild ecosystems, such as dismantling and damming of wild valleys, destruction of mosquito-bearing wetlands, diversion of water for irrigation, conversion of wilderness to farmland, timber removal, and extirpation of tigers, whales, ferrets, and wolves.
There has been an acrimonious schism among conservationists and resource managers over the question of whether to "ratchet back human domination of the biosphere" or whether to embrace it. These two perspectives have been characterized as utilitarian vs protectionist.
The utilitarian view treats human health and well-being as criteria of ecosystem health. For example, destruction of wetlands to control malaria mosquitoes "resulted in an improvement in ecosystem health." The protectionist view treats humans as an invasive species: "If there was ever a species that qualified as an invasive pest, it is Homo sapiens,"
Proponents of the utilitarian view argue that "healthy ecosystems are characterized by their capability to sustain healthy human populations," and "healthy ecosystems must be economically viable," as it is "unhealthy" ecosystems that are likely to result in increases in contamination, infectious diseases, fires, floods, crop failures and fishery collapse.
Protectionists argue that privileging of human health is a conflict of interest as humans have demolished massive numbers of ecosystems to maintain their welfare, also disease and parasitism are historically normal in pre-industrial nature. Diseases and parasites promote ecosystem functioning, driving biodiversity and productivity, and parasites may constitute a significant fraction of ecosystem biomass.
The very choice of the word "health" applied to ecology has been questioned as lacking in neutrality in a BioScience article on responsible use of scientific language: "Some conservationists fear that these terms could endorse human domination of the planet … and could exacerbate the shifting cognitive baseline whereby humans tend to become accustomed to new and often degraded ecosystems and thus forget the nature of the past."
Criticism of ecosystem health largely targets the failure of proponents to explicitly distinguish the normative dimension from the descriptive dimension, and has included the following:
Alternatives have been proposed for the term ecosystem health, including more neutral language such as ecosystem status, ecosystem prognosis, and ecosystem sustainability. Another alternative to the use of a health metaphor is to "express exactly and clearly the public policy and the management objective", to employ habitat descriptors and real properties of ecosystems. An example of a policy statement is "The maintenance of viable natural populations of wildlife and ecological functions always takes precedence over any human use of wildlife." An example of a goal is "Maintain viable populations of all native species in situ." An example of a management objective is "Maintain self-sustaining populations of lake whitefish within the range of abundance observed during 1990-99."
Kurt Jax presented an ecosystem assessment format that avoids imposing a preconceived notion of normality, that avoids the muddling of normative and descriptive, and that gives serious attention to ecosystem definition. (1) Societal purposes for the ecosystem are negotiated by stakeholders, (2) a functioning ecosystem is defined with emphasis on phenomena relevant to stakeholder goals, (3) benchmark reference conditions and permissible variation of the system are established, (4) measurement variables are chosen for use as indicators, and (5) the time scale and spatial scale of assessment are decided.
Ecological health has been used as a medical term in reference to human allergy and multiple chemical sensitivity and as a public health term for programs to modify health risks (diabetes, obesity, smoking, etc.). Human health itself, when viewed in its broadest sense, is viewed as having ecological foundations. It is also an urban planning term in reference to "green" cities (composting, recycling), and has been used loosely with regard to various environmental issues, and as the condition of human-disturbed environmental sites. Ecosystem integrity implies a condition of an ecosystem exposed to a minimum of human influence. Ecohealth is the relationship of human health to the environment, including the effect of climate change, wars, food production, urbanization, and ecosystem structure and function. Ecosystem management and ecosystem-based management refer to the sustainable management of ecosystems and in some cases may employ the terms ecosystem health or ecosystem integrity as a goal.