Ecosystem health is a metaphor used to describe the condition of an ecosystem.Rapport, David (1998). "Defining ecosystem health." Pages 18-33 in Rapport, D.J. (ed.) (1998). Ecosystem Health. Blackwell Scientific. Ecosystem condition can vary as a result of fire, flooding, drought, extinctions, invasive species, climate change, mining, 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." Some critics worry that ecosystem health, a "value-laden construct", can be "passed off as science to unsuspecting policy makers and the public." However, this term is often used in portraying the state of ecosystems worldwide and in conservation and management. For example, scientific journals and the UN often use the terms planetary and ecosystem health, such as the recent journal The Lancet Planetary Health.
History of the concept
The health metaphor applied to the environment has been in use at least since the early 1800sAnon (1839). "On the culture of potatoes". Framer's Magazine, 2(5):337-338. and the great American conservationist Aldo Leopold (1887–1948) spoke metaphorically of land health, land sickness, mutilation, and violence when describing land use practices.Leopold, Aldo (1946). "The land health concept and conservation." Pages 218-226 in Callicott, J. Baird, and Eric T.Freyfogle. (1999) For the Health of the Land. Washington DC: Island Press. The term "ecosystem management" has been in use at least since the 1950s.Lutz, H.J. (1957). "Applications of ecology in forest management." Ecology 38:46-64. 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,HELCOM (2010). Ecosystem health of the Baltic Sea 2003–2007 HELCOM Initial Holistic Assessment.Balt. Sea Environ. Proc. No. 122. and forests.
Meaning
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: Conserving viable populations of native species; Conserving ecosystem diversity; Maintaining evolutionary and ecological processes; Managing over long time frames to maintain evolutionary potential; Accommodating human use and occupancy within these constraints.
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.Horns, W.H., et al. (2003). Fish-community objectives for Lake Superior. Great Lakes Fish. Commission Special Publication. 03-01. 78 pages. 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: productivity, resilience, and "organization" (including biodiversity).
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. "If species richness is our major normative target, then we should convert the Amazon rainforest even faster into pasture."
"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.Costanza, R. 1992. "Toward an operational definition of ecosystem health." Pp 239-256 in Costanza, R., B. Norton, and B. Haskell. Ecosystem health. New Goals for Environmental Management. Washington DC: Island Press. 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."
Clement Loo argues that “if one becomes wedded to one particular guiding concept regarding the key factors … one runs the risk of failing to recognize other more relevant factors.” “There is little reason to suppose … that an account of ecosystem health – particularly one that focuses on as few factors as vigor, structure, and resilience – ought to be able to be particularly useful generally.” And is “failure to recognize how heterogeneous the biological world is.”
"Another way to measure ecosystem health" is 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).
Health indicators
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."
The very act of selecting indicators of any kind is biased by the observer's perspective and 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.