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Societal Dependence on Natural Ecosystems
By Gretchen C. Daily
ISLAND PRESSCopyright © 1997 Island Press
All rights reserved.
ECOSYSTEM SERVICES: A FRAGMENTARY HISTORY
Harold A. Mooney and Paul R. Ehrlich
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While explicit recognition of ecosystem services is a relatively new phenomenon, the notion that natural ecosystems help to support society probably traces back to the time when our ancestors were first able to have notions. For example, Plato understood that the deforestation of Attica led to soil erosion and the drying of springs.
One might consider the origins of modern concern for ecosystem services to trace to George Perkins Marsh's publication of Man and Nature in 1864. The book was the first to attack the idea that America's resources (or the world's) were infinite, an error that persists among the scientifically ignorant.
Marsh, a lawyer, politician, and scholar, knew the Mediterranean well, having traveled there extensively and served as ambassador in Turkey and Italy. He noted that much of the once-fertile Roman Empire "is either deserted by civilized man and surrendered to hopeless desolation, or at least greatly reduced in both productiveness and population" (p. 9). He described the deterioration of the services of retaining soil and supplying fresh water: "Vast forests have disappeared from mountain spurs and ridges, the vegetable earth ... [is] washed away; meadows, once fertilized by irrigation, are waste and unproductive, because ... the springs that fed them dried up; rivers famous in history and song have shrunk to humble brooklets" (p. 9). He recognized the connections of deforestation to climate: "With the disappearance of the forest, all is changed. At one season, the earth parts with its warmth by radiation to an open sky—receives, at another, an immoderate heat from the unobstructed rays of the sun. Hence the climate becomes excessive, and the soil is alternately parched by the fervors of summer, and seared by the rigors of winter. Bleak winds sweep unresisted over its surface, drift away the snow that sheltered it from the frost, and dry up its scanty moisture" (p. 186).
Marsh was also quite aware of the waste-disposal service of natural ecosystems. For example, he wrote, "The carnivorous, and often the herbivorous insects render an important service to man by consuming dead and decaying animal and vegetable matter, the decomposition of which would otherwise fill the air with effluvia noxious to health" (p. 95). He noted the pest-control service: "man has promoted the increase of the insect and the worm, by destroying the bird and the fish which feed upon them" (p. 96). He was more aware in 1864 of the services performed by microorganisms than the average politician or economist in 1996:
Earth, water, the ducts and fluids of vegetation and of animal life, the very air we breathe, are peopled by minute organisms which perform most important functions in both the living and the inanimate kingdoms of nature. It is evident that the chemical, and in many cases mechanical character of a great number of objects important in the material economy of human life, must be affected by the presence of so large an organic element in their substance, and it is equally obvious that all agricultural and all industrial operations tend to disturb the natural arrangements of this element. (p. 108)
Almost a century later, Aldo Leopold (1949) touched more poetically on ecosystem services. Writing of the loss of natural controls of herbivore herds, he said: "I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.... So also with cows. The cowman who cleans his range of wolves does not realize he is taking over the wolf's job of trimming the herd to fit the range. He has not learned to think like a mountain. Hence we have dustbowls, and rivers washing the future into the sea" (p. 132). Leopold recognized the basic impossibility of substituting satisfactorily for ecosystem services: "A land ethic changes the role of Homo sapiens from conquerer of the land community to plain member and citizen of it.... In human history we have learned (I hope) that the conquerer role is eventually self-defeating. Why? Because it is implicit in such a role that the conquerer knows, ex cathedra, just what makes the community clock tick, and just what and who is valuable, and what and who is worthless, in community life. It always turns out that he knows neither, and this is why his conquests eventually defeat themselves" (p. 204).
About the same time, two influential books appeared that helped reawaken interest in the sorts of ecosystem issues Marsh had discussed: Fairfield Osborn's Our Plundered Planet (1948) and William Vogt's Road to Survival (1948). Osborn summarized the situation simply and accurately: "As far as the habitable and cultivable portions of the earth's surface are concerned, there are four major elements that make possible not only our life but, to a large degree, the industrial economy upon which civilization rests: water; soil; plant life, from bacteria to forests; animal life, from protozoa to mammals" (pp. 48–49). Vogt pioneered the concept of natural capital. On the national debt he wrote: "By using up our real capital of natural resources, especially soil, we reduce the possibility of ever paying off the debt" (p. 44).
A little later, Paul Sears, distinguished botanist from Yale, explicitly recognized the recycling service: "Less obvious is the presence of a complex population of microorganisms and invertebrates which, among other functions, takes care of the breakdown of organic wastes and their return to chemical forms that can be reused to sustain life" (1956, p. 471).
Before the era of Leopold and Sears, the basic foundations for ecosystem ecology had been laid, providing a scientific basis for their views of the impact of human activities on earth's life-support systems. Those foundations can be traced as far back as Stephen Forbe's famous 1887 paper "The Lake as a Microcosm," which explicitly characterized one biological community within its physical context. But ecosystem ecology itself perhaps is best viewed as starting with the work of Henry Chandler Cowles (1899) on succession in the Indiana dunes. In that work the plant community and its physical environment were clearly tied together. The term ecosystem was first used by Tansley in his article (1935) "The Use and Abuse of Vegetational Concepts and Terms," a festschrift contribution for H. C. Cowles. The stature of Tansley as a scientist helped establish the ecosystem as a fundamental concept in ecology (Golley 1993).
The modern era of ecosystem ecology was ushered in by Raymond Lindeman's brilliant paper on a small lake ecosystem, published posthumously in 1942, shortly after his tragic death at the age of twenty-seven. He pointed out in his summary that, "Analysis of food-cycle relationships indicate that a biotic community cannot be clearly differentiated from its abiotic environment; the ecosystem is hence regarded as the more fundamental ecological unit" (p. 415).
The quantitative study of food chains was greatly stimulated during the early days of the nuclear age when intensive studies of the pathways of radionuclides in the environment were pursued. An energy-based approach to ecosystem studies was consolidated with the publication of Odum's classic textbook in 1953. Somewhat later, Bormann and Likens (1979) summarized their pioneering experiments that had begun in 1962 on whole watersheds. These studies demonstrated the crucial role of ecosystems in modulating the nutrient, sediments, and water budgets of landscapes. These studies, along with the efforts during the International Biological Program (IBP), in the late 1960s and early 1970s, to quantify the earth's productive capacity, firmly established the ecosystem as an important unit of study (Golley 1993). The investigation of the functioning of ecosystems centered primarily on the cycling of carbon, water, and nutrients between the biota and the soil and the atmosphere. During, and just subsequent to the IBP, enough momentum had been achieved in this area that research institutions were formed and government agencies began to organize in a manner that would allow long-term planning and funding of ecosystem research.
The environmental movement began with the publication of Rachel Carson's Silent Spring in 1962. Concern for preserving ecosystem functioning was expressed explicitly soon thereafter: "[Ecologists] realize how easily disrupted are ecological systems (called ecosystems), and they are afraid of both the short- and long-range consequences for these ecosystems of many of mankind's activities" (Ehrlich 1968, p. 47). In the first widely used environmental science text there is a chapter entitled "Ecosystems in Jeopardy," which defines ecosystems and then begins: "The most subtle and dangerous threat to man's existence ... is the potential destruction, by man's own activities, of those ecological systems upon which the very existence of the human species depends (Ehrlich and Ehrlich 1970, p. 157).
As far as we can determine, the functioning of ecosystems in terms of delivering services to humanity was first described in the report of the Study of Critical Environmental Problems (SCEP 1970). It listed (pp. 122–125) the following "environmental services" that would decline if there were a "decline in ecosystem function":
cycling of matter
composition of the atmosphere.
This was expanded upon under the rubric "public-service functions of the global environment" (Holdren and Ehrlich 1974) to include:
maintenance of soil fertility
maintenance of a genetic library.
With this, the normally cited list of services was essentially complete. These were subsequently referred to as "'public services of the global ecosystem" (Ehrlich et al. 1977) and "nature's services" (Westman 1977) and elaborated upon simply as "ecosystem services" (Ehrlich and Ehrlich 1981).
Two questions about ecosystem services have been clear from the start (Ehrlich and Ehrlich 1981, pp. 95–96). One is how the loss of biodiversity will affect ecosystem services, and the other is whether it will be possible to find and deploy technological substitutes for the services. The first attempt to approach these questions systematically (Ehrlich and Mooney 1983) concluded: "The loss of services to humanity following extinctions ranges from trivial to catastrophic, depending on the number of elements (populations, species, guilds) deleted and the degree of control each exerted in the system. Most attempts to substitute other organisms for those lost have been unsuccessful, to one degree or another, and prospects for increasing the success rate in the foreseeable future are not great. Attempts to supply the lost services by other means tend to be expensive failures in the long run" (p. 248). Overall, however, the quantification of how ecosystems provide societal services has developed slowly, principally because ecosystem-level experiments are difficult, and costly, and need to be pursued for long periods of time (Carpenter et al. 1995).
Gradually, however, the extent to which species can compensate for one another in their roles in the delivery of ecosystem services has become an active area of ecological research (Ehrlich and Ehrlich 1981, B. Walker 1991, Schulze and Mooney 1993). A stimulus to this study area has been the recent concern about the consequences of the predicted massive losses of species in general but in particular on the functioning of ecosystems, and hence to the provision of ecosystem services. SCOPE (Scientific Committee on Problems of the Environment) launched a program in 1991 to assess our state of knowledge in this area in order to prepare the way for explicit experimentation. The initial activity of this assessment was a meeting in Bayreuth, Germany, in October 1991 (Schulze and Mooney 1993) where hypotheses were formulated and a plan to gather information on the following two issues was consolidated. The program focused on two basic but complex questions:
1. Does biodiversity "count" in system processes (e.g., nutrient retention, decomposition, production, etc.) including atmospheric feedbacks, over short- and long-term time spans and in face of global change (climate change, land-use change, invasions)?
2. How is system stability and resistance affected by species diversity and how will global change affect these relationships?
Vitousek and Hooper (1993) proposed a number of possible responses of ecosystem functioning to changes in species numbers in terms of model types. The data available were so poor that it was not possible to give support to any one of these models. However, they became the center of discussion and elaboration over the next several years.
The principal approach of the assessment was to look at the major biomes of the world and to examine surrogate data on the two questions posed by the program. Records are available, for example, on biological invasions, epidemics, and economic alteration of ecosystems to maximize harvesting, forces that tend to add or delete species. Such "experiments" could be used to assess the general impact of changing species diversity. The program broadened somewhat as it became part of the Global Biodiversity Assessment of UNEP (United Nations Environment Programme). New biomes were added to the assessment and the concept of diversity was broadened from looking at species only to also considering genetic, community, and landscape diversity and their roles in providing ecosystem services.
Detailed evidence was given of the ecosystem services provided by biodiversity in a number of biomes including arctic and alpine (Chapin and Körner 1995), Mediterranean (Hobbs 1992, Davis and Richardson 1995), Savannas (Solbrig et al. 1996), tropical forests (Orians et al. 1996), and islands (Vitousek et al. 1995). More comprehensive coverage of biomes, but in less detail, was given in Mooney et al. (in press). Even broader but less detailed information was given in two chapters of the Global Biodiversity Assessment (Mooney et al. 1995). This latter effort included the work of hundreds of scientists and was provided in a format that enabled cross-biome comparisons of processes. Clearly, the early ideas of ecosystem services had moved to the mainstream of ecological research.
The general conclusions of these assessments were that in many cases, we can make clear predictions of the ecosystem consequences of losses of certain types of species that possess specialized traits. For others such as keystone species, however, our knowledge base is limited and we have to rely on direct experimentation. Since keystones play such a vital role in ecosystem integrity, this calls for precaution in ecosystem management. Further, it was concluded that losses of populations reduce ecosystem flexibility to changing environments and to habitat rehabilitation. It was noted that species diversity is vital in the resilience of ecosystems to perturbation and presumably to changing environmental conditions. It was found that simple ecosystems, which have few representatives of major functional groups, such as the arctic and deserts, are particularly vulnerable to disruption from species losses. For virtually all ecosystem services it was found that species diversity was important although some services, such as primary productivity, were less sensitive to diversity than were other processes. It was concluded that our knowledge base is very poor at the moment yet suggestive of the fundamental requirement of diversity for providing ample free services to society. Moreover, since local diversity is very difficult to restore and global biodiversity loss is irreversible on a time scale of interest to humanity, we should exert great caution in our husbanding of our global biotic resources.
We are now entering a period of experimental refinement of our knowledge in the area of ecosystem functioning and biodiversity. The International Geosphere Biosphere Program is initiating a project on the role of ecological complexity in earth system functioning. Already there have been important contributions in this area utilizing model ecosystems (Naeem et al. 1994, Lawton 1995), natural climatic perturbations on a gradient of diversity imposed by nutrient variability (Tilman and Downing 1994), and most recently direct tests of ecosystem functioning in field gardens where species and functional type diversity has been manipulated (Tilman et al. 1996). All of these studies have supported the generalizations that arose from the assessment—diversity "counts" in ecosystem functioning (see also Perrings et al. 1995).
Excerpted from Nature's Services by Gretchen C. Daily. Copyright © 1997 Island Press. Excerpted by permission of ISLAND PRESS.
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