SPECIES EXTINCTION, BIODIVERSITY LOSS AND HUMAN HEALTH
This article is adapted with permission from Chivian, E. 1993. Species Extinction and Biodiversity Loss: The Implications for Human Health. In “Critical Condition: Human Health and the Environment”, edited by E Chivian, M McCally, H Hu and A Haines. Cambridge, Mass. and London, England: MIT Press. With thanks to EO Wilson, Richard Schultes, Stephen Morse, Andrew Spielman, Paul Epstein, David Potter, Nan Vance, Rodney Fujita, Michael Balick, Suzan Strobel and Edson Albuquerque.
Human activity is causing the extinction of animal, plant and microbial species at rates that are a thousand times greater than those which would have occurred naturally (Wilson l992), approximating the largest extinctions in geological history. When homo sapiens evolved, some l00 thousand years ago, the number of species that existed was the largest ever to inhabit the Earth (Wilson l989). Current rates of species loss are reducing these levels to the lowest since the end of the Age of Dinosaurs, 65 million years ago, with estimates that one-fourth of all species will become extinct in the next 50 years (Ehrlich and Wilson l99l).
In addition to the ethical issues involved - that we have no right to kill off countless other organisms, many of which came into being tens of millions of years prior to our arrival - this behaviour is ultimately self-destructive, upsetting the delicate ecological balance on which all life depends, including our own, and destroying the biological diversity that makes soils fertile, creates the air we breathe and provides food and other life-sustaining natural products, most of which remain to be discovered.
The exponential growth in human population coupled with an even greater rise in the consumption of resources and in the production of wastes, are the main factors endangering the survival of other species. Global warming, acid rain, the depletion of stratospheric ozone and the discharge of toxic chemicals into the air, soil and fresh- and salt-water ecosystems - all these ultimately lead to a loss of biodiversity. But it is habitat destruction by human activities, particularly deforestation, that is the greatest destroyer.
This is especially the case for tropical rainforests. Less than 50% of the area originally covered by prehistoric tropical rainforests remains, but they are still being cut and burned at a rate of approximately l42,000 square kilometres each year, equal in area to the countries of Switzerland and the Netherlands combined; this is a loss of forest cover each second the size of a football field (Wilson l992). It is this destruction which is primarily responsible for the mass extinction of the world’s species.
It has been estimated that there are somewhere between l0 million and l00 million different species on Earth. Even if a conservative estimate of 20 million total world species is used, then l0 million species would be found in tropical rainforests, and at current rates of tropical deforestation, this would mean 27,000 species would be lost in tropical rainforests alone each year, or more than seventy-four per day, three each hour (Wilson l992).
This article examines the human health implications resulting from this widespread loss of biological diversity. It is the author’s belief that if people fully comprehended the effect these massive species extinctions will have - in foreclosing the possibility of understanding and treating many incurable diseases, and ultimately, perhaps, in threatening human survival - then they would recognize that the current rates of biodiversity loss represent nothing less than a slowly evolving medical emergency and would demand that efforts to preserve species and ecosystems be given the highest priority.
The Loss of Medical Models
Three groups of endangered animals, far apart in the animal kingdom - dart-poison frogs, bears and sharks - offer striking examples of how important models for biomedical science are in danger of being squandered by humans.
The entire family of dart-poison frogs, the Dendrobatidae, found in the American tropics, is threatened by destruction of its habitats - the lowland tropical rainforests of Central and South America (Brody l990). These brightly coloured frogs, which include more than l00 species, are particularly sensitive to deforestation, as they often live only in very specific areas of the forest and cannot live naturally anywhere else. Scientists have come to understand that the toxins they produce, used for centuries to poison arrows and blowgun darts by Central and South American Indians, are among the deadliest natural substances known. They are also enormously useful to medicine. The active ingredients of the toxins are alkaloids, nitrogen-containing ring compounds almost exclusively found in plants (morphine, caffeine, nicotine and cocaine are examples). The alkaloids bind selectively to specific ion channels and pumps in nerve and muscle membranes. Without them, knowledge of these basic units of membrane function, found throughout the animal kingdom, would be very incomplete.
In addition to their value in basic neurophysiological research, dart-poison frogs also offer valuable biochemical clues for the production of new and potent analgesics that have a mechanism of action different from that of morphine, of new medicines for cardiac arrhythmias and of new treatments for the alleviation of some neurological diseases such as Alzheimer’s disease, myasthenia gravis and amyotrophic lateral sclerosis (Brody l990). If rainforest destruction continues at its present rate in Central and South America, these extremely valuable frogs will be lost.
The growing black market trade in Asia for bear parts, with bear gallbladders being sold for their reputed medicinal value (worth l8 times their weight in gold), and paws for gourmet food (Montgomery l992), coupled with continued hunting and the destruction of habitats, has imperilled bear populations in many parts of the world. If some species of bears become extinct, we will all be the poorer, not only because they are beautiful, fascinating creatures that fill important ecological niches, but also because some species possess several unique physiological processes that may provide important clues for treating various human disorders. “Hibernating” (or, more accurately, “denning”) black bears, for example, are immobile for up to five months in the winter, yet do not lose bone mass (Rosenthal 1993). (True hibernators, like the marmot, woodchuck and ground squirrel, show a marked lowering of body temperature during hibernation and are not easily aroused. Black bears, by contrast, “hibernate” at near normal body temperatures and can be fully responsive to defend themselves instantly.) In contrast to humans, who would lose almost one-fourth of their bone mass during a similar period of immobility (or lack of weight bearing), bears continue to lay down new bone, making use of circulating calcium in their blood (Floyd, Nelson and Wynne 1990). Understanding the mechanisms of how they accomplish this feat may lead to effective ways of preventing and treating osteoporosis in the elderly (an enormous problem leading to fractures, pain and disability), in those confined to bedrest for long periods and in astronauts subject to prolonged states of weightlessness.
In addition, “hibernating” bears do not urinate for months. Humans who cannot excrete their waste products in urine for several days build up high levels of urea in their blood and die from its toxicity. Somehow bears recycle their urea to make new proteins, including those in muscle (Nelson 1973). If we could determine the mechanism of this process, it might lead to successful, long-term treatments for those with kidney failure, who must now rely on regular detoxification by kidney dialysis machines, or on transplantation.
Like bears, many species of sharks are being decimated because of the demand for shark meat, especially in Asia, where shark fins for soup command prices as high as $l00 a pound (Stevens l992). Because sharks produce few offspring, grow slowly and take years to mature, they are highly vulnerable to overfishing.
Sharks have been around for almost 400 million years and have evolved highly specialized organs and physiological functions that have protected them against virtually all threats, except slaughter by humans. The wiping out of populations and extinction of some of the 350 species may become a major disaster for human beings.
The immune systems of sharks (and of their relatives, skates and rays) seem to have evolved so that the animals are almost invulnerable to developing cancers and infections. While tumours are often seen in other fish and molluscs (Tucker l985), they are rare in sharks. Preliminary investigations have supported this finding. It has proved impossible, for example, to produce tumour growth in Nurse Sharks with repeated injections of known potent carcinogenic substances (Stevens l992). And researchers at the Massachusetts Institute of Technology have isolated a substance, present in large amounts, from Basking Shark cartilage (Lee and Langer l983) that strongly inhibits the growth of new blood vessels towards solid tumours, and thereby prevents tumour growth.
Sharks may also provide valuable models for developing new types of medications to treat infections, especially important at the present time when infectious agents are developing increasing resistance to currently available antibiotics.
Countless other examples could be mentioned of unique plants, animals and micro-organisms holding the secrets of billions of evolutionary experiments that are increasingly threatened by human activity and in danger of being lost forever to medical science.
The Loss of New Medicines
Plant, animal and microbial species are themselves the sources for some of today’s most important medicines and make up a significant proportion of the total pharmacopoeia. Farnsworth (1990), for example, has found that 25% of all prescriptions dispensed from community pharmacies in the United States from l959 to l980 contained active ingredients extracted from higher plants. A much higher percentage is found in the developing world. As many as 80% of all people living in developing countries, or roughly two thirds of the world’s population, rely almost exclusively on traditional medicines using natural substances, mostly derived from plants.
The knowledge held by traditional healers, often passed down orally over centuries, has led to the discovery of many medicines that are widely used today - quinine, physostigmine, d-tubocurarine, pilocarpine and ephedrine, to name a few (Farnsworth et al. l985). But that knowledge is fast disappearing, particularly in the Amazon, as native healers die out and are replaced by more modern medical practitioners. Botanists and pharmacologists are racing to learn these ancient practices, which, like the forest plants they employ, are also endangered (Farnsworth l990; Schultes l99l; Balick l990).
Scientists have analysed the chemistry of less than 1% of known rainforest plants for biologically active substances (Gottlieb and Mors l980) - as well as a similar proportion of temperate plants (Schultes l992) and even smaller percentages of known animals, fungi and microbes. But there may be tens of millions of species as yet undiscovered in the forests, in soils, and in lakes and oceans. With the massive extinctions currently in progress, we may be destroying new cures for incurable cancers, for AIDS, for arteriosclerotic heart disease and for other illnesses that cause enormous human suffering.
Disturbing Ecosystem Equilibria
Finally, the loss of species and the destruction of habitats may upset delicate equilibria among ecosystems on which all life depends, including our own.
Food supplies, for one, may be seriously threatened. Deforestation, for example, can result in significantly reduced rainfall in adjacent agricultural areas and even in regions at some distance (Wilson l988; Shulka, Nobre and Sellers l990), compromising crop productivity. The loss of topsoil from erosion, another consequence of deforestation, can have an irreversible negative impact on crops in forested regions, particularly in areas of hilly terrain, such as in regions of Nepal, Madagascar and the Philippines.
Bats and birds, among the major predators of insects that infest or eat crops, are being lost in record numbers (Brody l99l; Terborgh 1980), with untold consequences for agriculture.
Recently in Brazil, malaria has reached epidemic proportions as a consequence of massive settlement and environmental disruption of the Amazon basin. Largely under control in Brazil during the l960s, malaria has exploded 20 years later, with 560,000 cases reported in l988, 500,000 in Amazonia alone (Kingman l989). In large part, this epidemic was a consequence of the influx of huge numbers of people who had little or no immunity to malaria, who lived in make-shift shelters and wore little protective clothing. But it was also an outgrowth of their disturbing the environment of the rainforest, creating in their wake stagnant pools of water everywhere - from road construction, from silt runoff secondary to land clearing, and from open mining - pools where Anopheles darlingi, the most important malaria vector in the area, could multiply unchecked (Kingman l989).
The story of “emerging” viral illnesses may hold valuable clues for understanding the effects of habitat destruction on human beings. Argentine haemorrhagic fever, for example, a painful viral disease having a mortality of between 3 and l5% (Sanford 1991) has occurred in epidemic proportions since l958 as a result of the widespread clearing of the pampas of central Argentina and the planting of corn (Kingman l989).
The emerging viral illness which has had the greatest impact on human health, and which may be a harbinger of future viral outbreaks, is AIDS, caused by the human immunodeficiency virus - types l (HIV-l) and 2 (HIV-2). There is general agreement that the current AIDS epidemic originated from non-human primates in Africa, which have acted as natural, asymptomatic hosts and reservoirs for a family of immunodeficiency viruses (Allan l992). Good genetic evidence exists for the links of HIV-l to a simian immunodeficiency virus in African chimpanzees (Huet and Cheynier l990) and of HIV-2 to another simian virus in African sooty mangabeys (Hirsch and Olmsted l989; Gao and Yue l992). Are these cross-species viral transmissions from primates to humans the result of human encroachment into degraded forest environments?
If this is the case, we may be witnessing with AIDS the beginning of a series of viral epidemics originating from tropical rainforests where there may be thousands of viruses that could infect humans, some of which may be as lethal as AIDS (approaching l00%) but spread more easily, for instance by airborne droplets. These potential viral diseases could become the most serious public health consequence from environmental disruption of the rainforests.
But it may be the disruption of other interrelationships among organisms, ecosystems and the global environment, about which almost nothing is known, that may prove the most catastrophic of all for human beings. What will happen to global climate and to the concentration of atmospheric gases, for example, when some critical threshold of deforestation has been reached? Forests play crucial roles in the maintenance of global precipitation patterns and in the stability of atmospheric gases.
What will be the effects on marine life if increased ultraviolet radiation causes massive ocean phytoplankton kills, particularly in the rich seas beneath the Antarctic ozone “hole”? These organisms, which are at the base of the entire marine food chain and which produce a significant portion of the world’s oxygen and consume a significant portion of its carbon dioxide, are highly vulnerable to ultraviolet damage (Schneider l99l; Roberts l989; Bridigare l989).
What will be the consequences for plant growth if acid rain and toxic chemicals poison soil fungi and bacteria essential for soil fertility? There has already been a 40-50% loss in species of fungi in Western Europe during the past 60 years, including many symbiotic mycorhizal fungi (Wilson l992), crucial to the absorption of nutrients by plants. No one understands what the effects of this loss will be.
Scientists do not know the answers to these and other critically important questions. But there are worrisome biological signals which suggest that major damage to global ecosystems has already occurred. The rapid simultaneous drop in populations of many species of frogs worldwide, even in pristine environments far from people, indicates that they may be dying as a consequence of some global environmental change (Blakeslee l990). Recent studies (Blaustein 1994) suggest that increased ultraviolet-B radiation from thinning of the ozone layer may be the cause in some of these cases.
Closer to humans, marine mammals such as striped dolphins in the Mediterranean, European harbour seals off the coast of Scandinavia and of northern Ireland, and Beluga whales in the Saint Lawrence River are also dying in record numbers. In the case of the dolphins and the seals, some of the deaths seem to be due to infections by morbilli viruses (the family of viruses including measles and canine distemper virus) causing pneumonias and encephalitides (Domingo and Ferrer l990; Kennedy and Smyth l988), perhaps also the consequence of compromised immune systems. In the case of the whales, chemical pollutants such as DDT, the insecticide Mirex, PCBs, lead and mercury seem to be involved, suppressing the Belugas’ fertility and causing their deaths ultimately by a variety of tumours and pneumonias (Dold l992). The Beluga carcasses were often so filled with these pollutants that they could be classified as hazardous waste.
Are these “indicator species”, like canaries that die in coal mines containing poisonous gases, warning us that we are upsetting fragile ecosystem balances that support all life, including our own? The 50% drop in sperm counts in healthy men worldwide during the period l938-l990 (Carlsen et al. l992), the marked increases in the rate of congenital malformations of the external genitalia in males in England and Wales from l964 to l983 (Matlai and Beral l985), the dramatic rise in some cancer incidence rates for white children from l973 to l988 (Angier l99l) and for white adults from l973 to l987 (Davis, Dinse and Hoel l994) in the United States, and the steady growth in the mortality rates for several cancers worldwide for the last three to four decades (Kurihara, Aoki and Tominaga l984; Davis and Hoel l990a, 1990b; Hoel l992) all suggest that environmental degradation may be starting to compromise not only the survival of frogs, marine mammals and other animal, plant and microbial species, but that of the human species as well.
Human activity is causing the extinction of animal, plant and microbial organisms at rates that may well eliminate one-fourth of all species on Earth within the next 50 years. There are incalculable human health consequences from this destruction:
· the loss of medical models to understand human physiology and disease
· the loss of new medicines that may successfully treat incurable cancers, AIDS, arteriosclerosis and other diseases that cause great human suffering.