Anthony J. McMichael

London School of Hygiene and Tropical Medicine.

Debate on the paper by David Waltner-Toews

Debate sobre o artigo de David Waltner-Toews



This paper addresses a complex topic: the "ecosystem approach to health". We health scientists can readily lose our way among some of the concepts in this unfamiliar domain. We are, frankly, not used to thinking of health and disease within an ecosystem-based framework; we prefer to attribute disease causation to events or processes that arise and act at the level of the individual. This, for half a century, has been the dominant expectation of epidemiologists, and their methods are well-honed to this expectation.

Scientists in the Western world are the intellectual heirs to three centuries of a very different conceptual framework, that is, Newtonian science that views the world as a mechanical system amenable to reductive analysis. This has been the science of disaggregation, quantification, and assumed orderliness; the science that assumes a gradualist relationship, wherein a small change in the initial conditions of some process will cause a small effect, while larger changes will induce larger effects. This is the scientific tradition within which doctors "have sought to localize disease within the body, to identify specific pathological lesions, first in organs, then in tissues, then cells and now molecules... A germ, toxin, or gene can be held to account for most ailments, and the importance of predisposition, constitution, and social or environmental circumstances has diminished" (Anderson, 1999:49).

This conventional biomedical view is not wrong. But it is only part of the story - and perhaps not even the most important part. A larger question for epidemiologists is to understand why disease rates rise and fall over time, why some populations have different rates from others. Such questions are best answered when there is recognition that the health of a population is an ecological index - that is, it tells us about the balance between that population's biological needs and the capacity of the environment to meet those needs. In the longer term the level of a population's health reflects the adequacy of the carrying capacity of the environment. If we are slow to appreciate this, it is largely because humankind's cultural devices, especially technological adaptations and the buffering effects of imports, modulate the environmental carrying capacity; for humans (in contrast to other wild species) the carrying capacity is not externally fixed.

This paper alludes to the ecological dimension of human health. It describes, for example, how environmental interventions by human societies alter the risks of transmission of various infectious diseases. The theoretical base for "ecosystem health" is presented in terms of nested relationships - holarchies - and the interdependencies between levels and components. Nevertheless, the paper fails to make quite clear just what is the "ecosystem approach". Is this because that approach cannot be defined so much as intuited from a generic understanding of how ecological systems and processes work? Is this a phenomenon that is subject to multiple interpretations, one that takes us into the uncertain and chameleon-like realm of non-normal science (Funtowicz & Ravetz, 1994)?

Elsewhere, Waltner-Toews and colleagues have surveyed the historical progression of attempts to model influences on human states of health (VanLeeuwen et al., 1999). The earliest model, from the 1930s, was the classical triangular host-agent-environment model which elaborated the specific, one-agent-one-disease Germ Theory. In the 1950s, the British epidemiologist Jerry Morris propounded a social-ecological model in which the host was differentiated into genetic and experiential components, the environment was differentiated into physical and social, and the "agent" became behaviors of the person. Here was a pioneering integrative view of how diseases such as heart disease arose within modern urban society - this was social realism, but with no explicit recognition of the role of the natural environment and its life-support systems. In the 1970s and 1980s various attempts were made to extend these models, within a social-environmental context. These mandala-like models recognized nested layers of influence, interactive processes between those layered factors, and the several dimensions of "health" - physical, mental, and spiritual. In the 1990s a more elastic, permissive, "butterfly" model of biophysical and social-psychological influences on physical, mental, and spiritual health has been propounded (Bormann, 1996).

And so we enter the 21st century, having progressed from simplicity, linearity, and boxes-and-arrows to complexity, multidirectionality, and porous, interacting spheres of influence. This leads us to talk more comfortably of "life course epidemiology" and to seek an "ecosystem approach" to health. The Newtonian mold has been broken, though not discarded. This is what Wulff (1999:50) has referred to as the impending return of biomedicine towards the Aristotelian world view, "the theories about open self-organizing systems, chaos, fractals and non-linear dynamics [that] have already provided new and important insight into the mechanisms of nature".

We get further clues later in this paper. Waltner-Toews says we must use both current description and an evolutionary understanding. Positivism is inappropriate. We must have a "feel" for why the system is like it is, and a feel for what "health" (as an index of biological functioning within the system context) signifies. The issue is clarified as he explores the meaning of "ecosystem health" for infectious disease: "What distinguishes many so-called tropical diseases is that they occur in settings where the links between local ecosystems and local communities are intense and obvious." [quoted from a preliminary version of the article].We can, he says, replace Linnaean branching classification with generic groupings based on common ecological origins, e.g., housing-related infectious diseases in developing countries. We can see more clearly the interplay between local and global needs. We see the environment as an interactive medium; like Heraclitus' river, it is never the same twice. We also begin to see that disease and death are part of a larger, healthy, ongoing ecological narrative, with the succession of generations.

Finally we see more explicitly the political dimension to all of this, the dimension that goes beyond phenomenological description. This may shock the purist ecologist. But, then, we are talking about the human animal, the species that supplements "nature" (genes) with "nurture" (cumulative culture). Waltner-Toews says that: "The ecosystem approach is a way to bring together the socioeconomic and biophysical dimensions of health. Nevertheless, what it brings to our understanding of emerging diseases is not so much a way of identifying new variables - though it certainly forces us to cast our net much broader than in conventional research - but a new way of organizing how we think about them and respond to them."

We are not yet very good at thinking in terms of complex systems dynamics, in terms of holarchical relationships, nor within a long-term time frame. Striving for an ecosystem view of human health and disease requires us to understand these concepts. Faced by the challenge of finding ecologically sustainable ways of living for over six billion people, this understanding of human population health is essential. This paper helps us along that path - a path that requires some large-frame humility, not small-frame biomedical hubris. In Waltner-Toews' words: "Within an ecosystem approach, every policy decision is a hypothesis and every management plan is a test of that hypothesis."



ANDERSON, W. H., 1999. Perception of disease and its meanings. Lancet, 354(Sup.4):49.

BORMANN, F. H., 1996. Ecology: A personal history. Annual Review of Energetics and Environment, 21:1-29.

FUNTOWICZ, S. & RAVETZ, J. R., 1994. Emergent complex systems. Futures, 26:568-582.

VAN LEEUWEN, J. A.; WALTNER-TOEWS, D.; ABERNATHY, T. & SMIT, B., 1999. Evolving models of human health towards an ecosystem context. Ecosystem Health, 3:204-219.

WULFF, H. R., 1999. The concept of disease: From Newton back to Aristotle. Lancet, 354:SIV49.

Escola Nacional de Saúde Pública Sergio Arouca, Fundação Oswaldo Cruz Rio de Janeiro - RJ - Brazil