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Biodiversity and Human Health brings together leading thinkers on the global environment and biomedicine to explore the human health consequences of the loss of biological diversity. Contributors discuss the uses and significance of biodiversity to the practice of medicine today, and develop strategies for conservation of these critical resources.
Global Environmental Degradation and Biodiversity Loss: Implications for Human Health
"The Planet is not in jeopardy. We are in jeopardy. We haven't got the power to destroy the planet—or to save it. But we might have the power to save ourselves."
—Michael Crichton, Jurassic Park (1990)
There is abundant evidence that we are beginning to alter, for the first time in history, the chemistry and physics and physiology of the Earth. A basic understanding of biological systems and their dependence on the environment should alert people to the potential dangers these alterations pose for human beings. Yet, most people, including most policymakers, do not comprehend the human implications of global environmental change. Underlying this lack of comprehension is the widespread belief that human beings are separate from the environments in which they live, that they can change the atmosphere and oceans—and damage marine, aquatic, and terrestrial ecosystems in the process—without these changes affecting them.
In focusing on the human health dimensions of biodiversity loss, this volume helps people understand that human beings are an integral part of the environment, and that to protect their health and lives, and those of their children, people must learn to protect the environment.
This chapter will provide an overview about how environmental degradation leads to biodiversity loss, and what the implications of this loss are for human beings. This is an enormously complex topic, one that is in its infancy in terms of scientific understanding. Much of what can be said about it is uncertain and speculative. But the subject is also perhaps the most important one of all—namely the ways that global ecosystems support human health and make human life possible, and there is enough evidence available in the scientific and medical literature to justify looking at possible future scenarios, especially as they may serve as warnings.
Medicine brings a new perspective to discussions about global environmental change. It has a long tradition of acting decisively to prevent life threatening situations from occurring even when all the evidence is not in. The low threshold for performing appendectomies is a case in point (Gross 1956)—if surgeons waited until they were absolutely certain that their patients had appendicitis, it would often be too late to prevent serious illness and death. This is the situation we face today with global environmental degradation. If we wait until we have definite proof of its occurrence and of its consequences, it may be too late to avoid a medical catastrophe.
This chapter will cover the contributions to biodiversity loss from:
global climate change,
stratospheric ozone depletion,
toxic substances in the environment, and
and the possible effects on human health from this loss, including:
the loss of medicines,
the loss of medical models, and
the emergence and spread of infectious diseases.
Central to these discussions will be two themes:
1. That the study of species and biodiversity may be the best means we have for recognizing future danger signals to human health from global environmental degradation, as some species may be so uniquely sensitive to specific assaults on the environment that they may serve as our "canaries," or so-called "indicator species."
2. That we must focus much greater attention on biodiversity loss, which looms as a slowly evolving, potential medical emergency of unprecedented proportions, still largely unappreciated by policymakers and the public.
When Homo sapiens evolved, some 100,000 years ago, the number of species on Earth was the largest ever, but current rates of species extinction resulting from human activities, at least 1000 times those that would have occurred naturally, rivalling the great geologic extinctions of the past, may be reducing these numbers to the lowest levels since the end of the Age of Dinosaurs, 65 million years ago (Wilson 1993). Paul Ehrlich and E.O. Wilson (1991) and others have predicted that one-quarter of all species now alive may become extinct during the next 50 years if these rates of extinction continue.
There are 100 times more people on Earth than any land animal of comparable size that ever lived, and we are the most voracious and destructive species that ever existed. We consume or destroy or co-opt, for example, as much as 40% of all the solar energy trapped by land plants (Ehrlich and Wilson 1991). Not only is this behavior morally indefensible, endangering in the process countless other species, it is fundamentally and ultimately self-destructive.
Several aspects of global environmental degradation have an impact on species populations and biodiversity. This chapter will briefly review some of the evidence.
Global Climate Change
The Intergovernmental Panel on Climate Change (IPCC), the United Nations working group of 2500 of some of the world's most eminent atmospheric, physical, and biological scientists, has stated that projected worldwide CO2 (the main greenhouse gas) emissions will result in an increase in global mean surface temperatures of about 1.8 to 6.3°F by the year 2100 (IPCC 1995). The magnitude of these changes may not seem very large, but it must be remembered that the difference between the temperatures at present and those at the peak of the last ice age, 18,000 years ago, are only 50–9°F (Stevens 9/20/94), and that temperatures of 7°F higher than those of today have not been present since the Eemian interglacial period 130,000 years ago (Stevens 11/1/94), and perhaps not since the Eocene epoch, tens of millions of years ago (Webb 1992).
Even small changes in temperature can lead to enormous changes in global weather patterns and habitats. For example, during the years from about 1000 to 1350 A.D., known as the "Medieval Warm Period," when global mean surface temperatures were only slightly more than 1°F warmer than they are now, there were vineyards in England, and Greenland supported hundreds of farms (Ponting 1991). By contrast, from 1430 to 1850, the period known as the "Little Ice Age," when global mean surface temperatures were only slightly more than 1°F cooler than they are now, the Thames was often frozen in winter, and there were icebergs off the coast of Norway (Leggett 1990).
Since the late 19th century, global mean surface temperatures have already warmed on average approximately 1°F (Stevens 1995), consistent with the increases in atmospheric CO2 concentrations during the past century, from preindustrial revolution levels of 280 ppm to current levels of 350 ppm (Maskell et al. 1993). And 1995 was the warmest year since 1856, when mean global temperatures were first systematically recorded (Stevens 1996).
There is growing evidence that the seas are undergoing a similar warming, from measurements off the coast of California (Barry et al. 1995) and deep in the Atlantic Ocean (Parrilla et al. 1994), and that sea levels worldwide have risen about 10–20 cm over the last century (Warwick and Oerlemans 1990). The IPCC predicts that with greenhouse warming, sea levels will increase an additional 15-95 cm by the year 2100 (IPCC 1995).
These predicted changes in climate and sea levels will pose enormous and unprecedented threats to plant, microbial, and animal species, including human beings.
Many paleontologists believe that climate change, both global warming and cooling, was the dominant factor in the great extinctions of the past (Eldredge 1991), both directly, because of shifts in temperature outside the ranges to which species could adapt, and indirectly because of changes in habitats, for example, the formation of glaciers or changes in sea levels (Stanley 1987). Fossil records indicate that many species have been able to adapt to climate changes by shifting their ranges—during warming periods, for example, species colonized new habitats nearer the poles or at higher altitudes, while during cooling periods they retreated back toward the equator (Peters and Darling 1985). During several Pleistocene interglacial periods, when mean surface temperatures in North America were 3–5°F higher than they are at present, Cape Cod had forests like those found in present day North Carolina, manatees swam off the coast of New Jersey, and osage oranges grew near Toronto, several hundred kilometers north of their present ranges (Peters and Darling 1985). But many other species could not adapt and were lost, either because their rates of migration were too slow, or because geographical barriers like oceans, mountains, or unsuitable habitat conditions prevented their advance (Peters and Darling 1985).
By contrast to the past major changes in climate, however, when temperatures warmed or cooled over thousands of years, the changes predicted over the next century will be an order of magnitude or more faster, and it is not at all clear, even if there were no barriers to migration, whether species can migrate fast enough to avoid extinction. But there are barriers everywhere people live—cities, roads, agricultural lands, and other human constructions would further complicate species migration. It is calculated, for example, that for each 1°C rise in temperature, land plants would have to shift their ranges toward the poles by 100-150 km (Roberts 1989). The warming predicted by 2100, for example, would mean shifts of a few hundred kilometers. Some species, propagated by spores or dust seeds, might be able to achieve these rates (Perring 1965); most others would not (Peters and Darling 1985). Some spruce tree species, for example, even though they have light, wind-carried seeds, disperse them no farther than 200 m from the parent tree, corresponding to a potential maximum migration rate of only 20 km per century (Seddon 1971).
Animals, while more mobile, would be limited by the distributions of the plants they eat or otherwise depend on, by their ability to adapt behaviors to climate-altered habitats, or by changes in the populations of their predators or competitors, even if they could adapt physiologically (Peters and Darling 1985).
We may already be seeing evidence of species migrations and potential losses, paralleling the increases in recorded temperatures—in large reductions in red spruce trees in New Hampshire over the last 200 years (Hamburg and Cogbill 1988), in the upward climb of several vascular plant species in the Austrian alps over the past 70–90 years (Grabherr et al. 1994), in the shift to the north of Edith's checkerspot butterfly in the western United States (Parmesan 1996), and in the dramatic shift northward of a large number of marine invertebrate species off the coast of California over the past 60 years (Barry et al. 1995). In one particularly alarming study, warming seas off the coast of San Diego have been linked to an 80% reduction in zooplankton since 1951 and to similar declines in sea birds and fish, with the creation of a veritable "biological wasteland" (Roemmich and McGowan 1995).
Other aspects of global climate change that could have major impacts on species and biodiversity include: algal blooms (fertilized by the discharge of sewage and by agricultural run-off) (Epstein and Colwell 1993); rising seas that may threaten species in coastal wetlands, mangrove swamps, and coral reefs (Grigg and Epp 1989); the very worrisome possibility of major alterations of ocean currents from sea warming and changes in salinity, with potentially enormous changes in climate and in marine ecosystems (Broecker 1987); and finally the increase in CO2 itself, which may threaten ecosystems by altering carbon and nitrogen cycles fundamental to the interactions between plants, the atmosphere, and the soil (Hilchey 1993) [for example, by slowing photosynthesis in some land plants (Korner and Arnone 1992)]. Furthermore, global warming may increase turnover in tropical forests, favoring rapidly growing, light-demanding plants (that take up less CO2) over denser, slower-growing, shade-tolerant plants (Phillips and Gentry 1994), thereby accelerating global warming (Pimm and Sugden 1994).
If one considers the human species as a part of biodiversity, it is clear that greenhouse gas warming will also eventually reduce human numbers, as human beings are exquisitely sensitive to high temperatures (Rogot and Padgett 1976, Kilbourne 1992, Kalkstein 1993). During the two-week summer heat wave in the eastern United States in July 1993, for example, 84 people died in Philadelphia as a result of the increased temperatures (Morbidity and Mortality Weekly Reports 1993). And during the five days in mid-July 1995 when temperatures over 100°F swept over the central plains of the United States, there were 700 excess heat—related deaths in Chicago alone (Morbidity and Mortality Weekly Reports 1995, Semenza et al. 1996, Kellermann and Todd 1996).
The indirect effects on human health secondary to global climate change—from infectious diseases caused by a spread of disease vectors (Shope 1991) or by changes in habitats [resulting, for example, in outbreaks of cholera (Epstein 1995)]; from crop failure; from violent weather patterns; and from the unavailability of drinking water—are likely to extract an even heavier toll on human beings.
With global climate change, we are clearly conducting a gigantic experiment with life on this planet, knowing almost nothing about the potential consequences, endangering perhaps not only countless microbial, plant, and animal species, but ultimately perhaps ourselves as well. No human subjects committee at any hospital or medical research facility would ever approve this experiment to be performed.
Stratospheric Ozone Depletion
Stratospheric ozone depletion may also threaten species, both on land and in the sea. It was the formation of the ozone layer 450 million years ago that permitted marine life forms to colonize the land, as it protected them from the lethal effects of ultraviolet radiation.
In the mid 1970s Mario Molina and Sherwood Rowland (1974) predicted that the continued use of chlorofluorocarbons (CFCs) would lead to a decrease in stratospheric ozone, but it was not until 1985 that the first conclusive evidence of ozone depletion was reported (Farman et al. 1985), when ozone levels over the Antarctic were observed to have declined by 40% between 1975 and 1984 compared to 1960 baseline levels. Since then, record low levels have been recorded over the Antarctic in 1993 [down by more than 70% (Wilford 1993)], and over the Arctic in 1995, when readings 40% below normal values were recorded (Zurer 1995). Significant ozone depletion has also been observed by NASA and NOAA scientists over the heavily populated middle latitudes of the Northern Hemisphere, with early 1993 levels down by 10 to 20% (New York Times 4/23/93), and with corresponding increases in ground level ultraviolet-B radiation (UV-B) (Kerr and McElroy 1993).
The increased UV-B reaching the ground as a result of ozone thinning will damage the DNA and proteins of all living things (Leaf 1993), and could become a cause of species extinction and of a loss of biodiversity. Food crops (Worrest and Caldwell 1986), wild plants, and marine phytoplankton (Bridigare 1989) may all be affected, with possible major implications for terrestrial and marine food chains. Animals would also be vulnerable. Laboratory studies have shown, for example, that UV-B can lead to suppression of the immune response in mice, rats, and guinea pigs (Kripke 1990). Some amphibians seem particularly sensitive (see below), and there are anecdotal reports of sheep developing cataracts in Punta Arenas, Chile, at the tip of South America, beneath the ozone "hole" (Sims 1995).
Again, human beings are also at substantial risk, with increased rates expected of nonmelanocytic skin cancers (Suarez-Varela et al. 1992), malignant melanomas (Kricker et al. 1994), and cataracts (Taylor et al. 1988). The incidence of malignant melanomas has been increasing faster than that of any other cancer in the United States and worldwide (Rigel 1994), growing in Western populations by 20 to 50% every five years over the past two decades, particularly in young adults (Coleman et al. 1993). Every 11% decrease in stratospheric ozone is expected to increase melanoma mortality by 0.8 to 1.5% (Hoffman and Longstreth 1987).
Of great concern also is the potential for UV-B to weaken the systemic immune response in humans (Kripke 1990, Jeevan and Kripke 1993), potentially impairing peoples' ability to fight infections and cancers.
Excerpted from Biodiversity and Human Health by Francesca Grifo, Joshua Rosenthal. Copyright © 1997 Island Press. Excerpted by permission of ISLAND PRESS.
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