An Ecological Perspective on Population Change and Land Use
Steward T.A. Pickett
Is there a common vocabulary to underwrite studies of population and land use change? Can ecological and social sciences research on the topic be joined? This commentary has three parts. The first concerns the relationship of ecology to the study of human population; the second deals with recent changes in ecology that better dispose it to join with social science research; and the third points out opportunities for interaction between ecology and the social sciences.
American ecology has largely ignored humans. It is unusual in this stance, perhaps due to a combination of factors. First, the vastness of North America, coupled with the frontier mentality, have meant that ecologists have had apparently pristine or lightly human-impacted areas in which to work. But in addition to that opportunity, the dominant ecological world view, or paradigm, has predisposed the American discipline to work with apparently closed, self-regulating systems, for which human effects are external or temporary excursions from a biotic equilibrium. Such an approach reflects the long-lived cultural metaphor of the balance of nature. Yet, despite the dominance of the ''equilibrium paradigm,'' many scholars and scientists have pointed out the error of excluding humans. The battle to include humans in the scope of ecology is evidenced by the persistence of George Perkins Marsh's "great question": "Whether man is of nature or above her?" (Marsh, 1864).
The upstart of this situation is that ecology has, unfortunately, little to say about the subtle effects of human population and its constructions on
ecological systems or the ecology of populated areas. But human effects, both subtle and conspicuous, are being increasingly documented at all spatial scales. Geographers, environmental historians, paleoecologists, and landscape ecologists have accumulated an impressive body of information documenting the subtle, indirect, distant, or past effects of human populations throughout the world. The weight, pervasiveness, and ubiquity of the examples have stimulated ecologists to communicate with social scientists and historians. The empirical mass of human effects is beginning to be translated into mainstream ecology.
Ecologists are currently more open to the translation and import of information on human effects because of changes in their own discipline. Empirically, studies of natural disturbances, the extensive migrations of plant and animal species in the past, and studies of climate change force ecologists to see their systems in a different light. As a result, the dominant paradigm is changing in ecology. Ecological systems are now seen by most ecologists as being (1) open to outside influence; (2) often regulated from outside; (3) rarely at or near a single point equilibrium; (4) not strictly deterministic; and (5) inclusive of humans or their effects. This new world view can be called the nonequilibrium paradigm. It suggests a cultural metaphor of "the flux of nature." American ecology is, as a result of the paradigm shift, better prepared to communicate with ecologists in developing countries, whose mandate often causes them to address human population and to communicate with those specialists who study human populations.
An example of recognizing the new frontier in North American ecology is the study of urban rural gradients. Drs. Mark McDonnell, Margaret Carreiro, Richard Pouyat, Gary Lovett, Robert Parmelee, and I have initiated such studies in the New York Metropolitan region (McDonnell and Pickett, 1990). As an introduction into the ecology of this populated area, we wish to know how ecosystem function and forest regeneration are affected by the human population and its environmental effects. Essentially, the specific research questions and approaches are among those that ecologists would apply in any system they chose to study. But we are applying them along a previously neglected axis of environmental contrast. Already we have discovered some new environmental patterns that demand explanation. Although levels of heavy metals in the soil are two to four times higher in urban than rural sites (Pouyat and McDonnell, 1991), and urban soils are highly hydrophobic (White and McDonnell, 1988), the key ecosystem processes of nitrogen mineralization and litter decomposition rates actually increase in urban compared to rural sites (Pouyat et al., n.d.). Ultimately, comparison with other anthropogenic stress and disturbance gradients can expose generalizable and fundamental limits on ecological systems, as well as supply quantitative environmental measurements to evaluate the effects of human population and land use
change. Because so many of our treasured natural areas are near the expanding urban fringe, the work promises to have immense practical value as well.
The opportunities for interaction between social science and ecology are great and compelling. The information presented by social scientists at the workshop was at relatively coarse scales. Such scales are critical to understand, predict, and mitigate regional and global effects of human population change. Yet the models and empirical studies presented by the social scientists often used land use patterns as their environmental variable. Such a choice would not connect closely with the work of most ecologists, because the quantifiable environmental variables that are so important to predicting and explaining the workings of ecological systems are almost completely ignored by the focus on land use.
Ecologists can say how the animal populations are regulated, how the communities are distributed in space and time, and how the nutrients and carbon are partitioned and processed in local forest stands, watersheds, or regional landscapes. But the appropriate and important focus of social scientists at the very coarse regional, national, or continental scales resides across a quite broad chasm from most ecology. The two different scales must be bridged by terminology, concepts, measurement techniques, data, models, and ultimately complete theories. From an ecological perspective, the chasm suggests a fundamental question: What are the quantitative environmental effects and biotic consequences of human population and land use change? This question is as important and begging in the United States as in developing countries.
There are two things that social scientists should know about ecology to proceed toward the important interaction with ecologists that can bridge the chasm between human population and land use data and environmental data. One is the structure and intent of modern ecology. I suspect that most established social scientists may have encountered, during their education, a brand of ecology that has been Superseded. The classic textbooks from which so many of us were trained took ecology to be strictly the study of the biogeochemistry of ecosystems, or the study of the growth, control, and interaction of populations, or the distribution and limitation of biotic communities. Fortunately, modern ecology is all of these things, along with new bridge disciplines or approaches that hardly existed 20 years ago. Modern ecology is the scientific study of the processes influencing the distribution and abundance of organisms, the interactions among organisms, the interactions between organisms, and the transformation and flux of energy and matter. Within ecology, specialists focus on either distinct entities (e.g., organisms, landscape patch types) or quantitative fluxes (e.g., energy, nutrients) and do so in either a historical and particularist perspective or an instantaneous and ergodic perspective. Furthermore, ecologists can initiate studies by focusing on a particular system or site, or on interactions that
may range broadly and extend beyond the boundaries of specific systems. The lesson for social scientists is that ecology is not monolithic (Figure 1).
There is a second message for social scientists about ecology. This lesson emerges from comparing the fundamental structure of ecological versus economic development models. These two disciplines represent two great paradigms that reflect radically different assumptions about how systems work. Put most crudely, ecologists, based on their empirical experience at the usual ecological time and space scales, see systems as ultimately limited. This paradigm is reflected in the generalization that no ecological system (organism, population, etc.) grows without limit. Ecological theories are structured around the negative feedbacks and other limits that constrain the basic positive feedback that underlies the growth of systems (Figure 2A; J. Fellows, personal communication, 1991). One basic assumption of economic and development models appear to be that systems are ultimately without limit. Thus, the economic studies and models are cast in terms of overcoming negative feedbacks, and permitting the positive feed-backs to operate without constraint (Figure 2B).
These two great paradigms, the ecological and economic, demand joint analysis. Rather than argue whether, in the universal sense, Malthus was wrong (apparently, an economic conclusion) or right (arguably, a conclusion acceptable to most ecologists when such thinking is applied to their scales and systems), the question should be cast differently. At what temporal and spatial scales, for what parameters, and for what kind of models
do limits exist or not? What statements in each discipline are meant to be idealizations for guidance of comparison and reference? What ways are there to constrain idealized generalization and expose particular exceptions? Finally, how can slippery but terribly important concepts, such as the quality of human life and the sustainability of ecosystems, be incorporated into the urgent question at the boundary between social and ecological science of whether "man is of nature or above her"?
Marsh, G.P. 1864 Man and Nature: Or Physical Geography as Modified by Human Action. New York: Scribner.
McDonnell, M.J., and S.T.A. Pickett 1990 The study of ecosystem structure and function along gradients of urbanization: an unexploited opportunity for ecology. Ecology 71:1232–1237.
Pouyat, R.V., and M.J. McDonnell 1991 Heavy-metal accumulations in forest soils along an urban-rural gradient in southeastern New York, USA. Water, Air and Soil Pollution 57–58:797–807.
Pouyat, R.V., M.J. McDonnell, and S.T.A. Pickett No date The effect of urban environments on soil characteristics in oak stands along an urban-rural land use gradient .
White, C.S., and M.J. McDonnell 1988 Nitrogen cycling processes and soil characteristics in an urban versus rural environment. Journal of Biogeochemistry 5:243–262.