REVIEWS Assessing ecosystem health D.J. Rapport, R. Costanza and A.J. McMichael here is now abundant evi- Evaluating ecosystem health in relation to health in terms of ‘system organiz- dence that many human- the ecological, economic and human ation, resilience and vigor, as well dominated ecosystems, in- health spheres requires integrating human as the absence of signs of ecosys- Tcluding various biophysical values with biophysical processes, an tem distress24. The definition also systems at regional and global lev- integration that has been explicitly includes the presence of essential els, have become highly stressed avoided by conventional science. The field functions and key attributes that and dysfunctional1. The ‘services’ is advancing with the articulation of the sustain life systems (Box 1). provided by these ecosystems are linkages between human activity, regional A healthy ecosystem is defined extremely important to human wel- and global environmental change, as being ‘stable and sustainable’; fare2–5. As stressed ecosystems reduction in ecological services and the maintaining its organization and have become highly degraded1,6–9, consequences for human health, autonomy over time and its re- they have also become incapable economic opportunity and human silience to stress24. Assessment of of supplying services to the same communities. Increasing our these properties in large-scale sys- level as in the past5,10. The ca- understanding of these interactions will tems through specific indicators pacity of the environment to sus- involve more active collaboration between of resilience, organization and tain economic activity4,10 and hu- the ecological, social and health vigor has been attempted for the man health11–15 is, therefore, being sciences. In this, ecologists will have Chesapeake Bay25 (USA) and other reduced. substantive and catalytic roles. marine ecosystems26, freshwater Ecosystems will continue to ecosystems27, forested ecosys- degrade under pressure of in- tems18, arctic ecosystems28 and creased demands unless we apply D.J. Rapport is at the Faculty of Environmental Sciences, arid grasslands29. Boxes 2 and 3 preventative and restorative strat- University of Guelph, Canada N1G 2W1 illustrate some of the relation- ([email protected]) and at the Dept of egies to achieve the health and Pharmacology and Toxicology, Faculty of Medicine ships between biophysical change integrity of regional ecosystems. and Dentistry, The University of Western Ontario, and societal goals and human This was one of the main objec- Canada N6A 5B7; R. Costanza is at the Center for activities with three case studies tives outlined in the Rio Declar- Environmental Science, the Biology Dept and the drawn from contrasting ecosys- ation on Environment and Devel- Institute for Ecological Economics, tems. In each example, stress has opment16. Principle Seven reads University of Maryland, PO Box 38, Solomons, resulted in biotic impoverish- ‘States shall cooperate in a spirit MD 20688, USA ([email protected]); ment, impaired productivity, al- of global partnership to conserve, A.J. McMichael is at the Dept of Epidemiology and tered biotic composition to favor protect and restore the health and Population Health, London School of Hygiene and opportunistic species, reduced re- integrity of the Earth’s ecosys- Tropical Medicine, Keppel Street, London, silience, increased disease preva- UK WC1E 7HT ([email protected]). tems. In view of the different con- lence, reduced economic oppor- tributions to global environmental tunity and risks to human and degradation, states have common animal health. but differentiated responsibilities’. Many of the earth’s ecosystems are ‘unhealthy’. Their Socioeconomic consequences functions, particularly those that are vital to sustaining the The existence of multiple dynamic semistable states for human community, have become impaired. An ‘ecosystem natural and human-dominated ecosystems complicates the distress syndrome’ (EDS)17 is widely prevalent in both aquatic task of determining the extent to which ecosystem struc- and terrestrial ecosystems18. Linking ecosystem health to ture and function have been altered by human activity30. the provision of ecosystem services (those functions that However, careful studies leave little doubt that degradation are recognized as satisfying human needs) and determining has occurred in many ecosystems, including forests6, ma- how ecosystem dysfunction relates to these services are rine systems7, agroecosystems31,32 and freshwater33. Indeed, major challenges at the interface of the health, social and natural sciences. Box 1. Indicators of ecosystem health Ecosystem health from a biophysical perspective Ecosystem health can be assessed using measures of resilience, vigor and The notion of ‘health’ has generally been used to denote organization: the vitality of individuals and, more recently, of populations Vigor is measured in terms of ‘activity, metabolism or primary productivity’25. An (humans, domesticated animals and wildlife). The extension example of reduced vigor, from a study of the Great Lakes Basin (North America)10, is the decline in the abundance of fish and infertility of agricultural soils within the of health to describe regional ecosystems is a response to the basin (Box 3). accumulating evidence that human-dominated ecosystems Organization can be assessed as the diversity and number of interactions between have become highly dysfunctional1. Extending the notion of system components. An example, also from the Great Lakes, is reduced morpho- health to regional levels (ecosystems, catchment areas, ba- logical and functional diversity of fish associations that occurs under multiple sins and landscapes) provides new opportunities to integrate stresses (Box 3). the social, natural and health sciences19. What is needed are Resilience (counteractive capacity)23, is measured in terms of a system’s capac- ity to maintain structure and function in the presence of stress. When resilience is methods for identifying dysfunction and evaluating causes exceeded, the system can ‘flip’ to an alternate state. A prime example is the shift and potential solutions. from benthic to pelagic dominated fish associations in the Laurentian Lower Great Definitions of ecosystem health20,21 have been closely al- Lakes Basin10 (Box 3). lied with the concepts of stress ecology19,22,23, which define TREE vol. 13, no. 10 October 1998 Copyright © 1998, Elsevier Science Ltd. All rights reserved. 0169-5347/98/$19.00 PII: S0169-5347(98)01449-9 397 REVIEWS welfare, both directly and Box 2. Health of a Ponderosa pine ecosystem indirectly, and therefore rep- resent a significant portion INCREASED of the total economic value • Tree density of the planet. Because these • Tree mortality services are not fully cap- • Fuel loads tured in markets or ad- • Fire hazard equately quantified in terms • Rates of disease comparable with economic • Pest outbreaks Heavy grazing services and manufactured capital, they are often given too little weight in policy de- Stressed forest ecosystem cisions. This neglect might ultimately compromise the sustainability of humans in the biosphere. Several stud- ies in the past few decades DECREASED have tried to estimate the • Rate of decomposition ‘incremental’ or ‘marginal’ • Rate of nutrient cycling value of ecosystem services – • Biodiversity the estimated rate of change • Fire hazard • Landscape diversity of value with changes in Healthy forest ecosystem • Aesthetic value ecosystem services from their current levels. Combin- ing these studies, the current Fire suppression economic value of 17 ecosys- tem services was estimated An important element in the assessment of ecosystem health is the need for an analysis of land-use history. A dominant for 16 biomes and extrapo- ecosystem in the Western USA, that of Ponderosa pine (Pinus ponderosa), provides a dramatic example of anthropogenic lated to estimate a value for 18 change on a large scale (as illustrated). Most signs of Ecosystem Distress Syndrome (EDS) are present in stands of 4 Ponderosa pine today. The indicators of ecosystem health include tree density and productivity, nutrient cycles, disease the entire biosphere . This prevalence and the destructive potential of wildfires. Criteria for the successful rehabilitation of the Ponderosa pine eco- value (most of which is out- system are based on monitoring for signs of EDS. Preventative approaches to ecosystem health imply more careful atten- side the market economy) tion to stress pressures and devising means to manage these pressures before degradation occurs (a complete diagnosis was estimated to be in the of EDS relies on both the stress pressures from human activity that are, at times, combined with natural events, and the range of US $16–54 trillion impacts of ecosystem behavior). In their study, Yazvenko and Rapport18 concluded that the stress pressure exerting the most impact on the ecosystem was fire suppression. Ponderosa pine forests have coevolved with the presence of periodic per year, with an average of fires. With fire suppression, the ecosystem displayed signs of EDS, including decreased productivity and increased mortality US $33 trillion per year. of Ponderosa pine, stagnated nutrient cycles, increased rates and magnitude of pest and disease outbreaks, and the Because of the nature of the increased destructive potential of wildfires resulting from higher understorey tree densities (thickets). This study reinforces uncertainties,