Embedded in nature: human health and biodiversity.

A loss of global biodiversity, namely a reduction in the variety of life on Earth, is rarely given much attention by physicians or environmental scientists. Like most people, they do not spend much time thinking about their relationship to other life forms, and they generally act, unknowingly, as if human beings were separate from the rest of nature--as if we could change the composition of the atmosphere and degrade the land and the oceans without these alterations having much effect on us. It is this disconnect that is at the core of the global environmental crisis--that policy makers and the public by and large do not understand that their health and lives are ultimately dependent on other species and on the integrity of the planet's ecosystems, and, as a result, they do not appreciate the urgent need to protect the natural world.

Approximately 1.7 million species have been identified on Earth and given Linnaean names (United Nations Environment Programme 1996), but there may be 10 times that number in all, and perhaps many times more if we include microbial diversity (Pimm et al. 1995). Species interact with each other and with their physical and chemical environments to make up ecosystems such as forests and wetlands. Stratospheric ozone depletion, pollution, the introduction of alien species, the overharvesting of species, and increasingly global climate change (Walther et al. 2002) all threaten biodiversity and thus ecosystem function. However, the degradation, reduction, and fragmentation of habitats on land, in fresh water, and in the oceans are the greatest threats (Pimm and Raven 2000). All of these factors are the result of human activity and are driven by unsustainable consumption, especially in the industrialized world, and rising human populations. Together they have disrupted grassland, river, lake, coral reef, and other ecosystems at alarming levels, and have raised the rate of species extinctions to 100 and, by some estimates, even to 1,000 times natural background rates (Pimm et al. 1995).

The loss of species deprives us of invaluable tools for biomedical research that provide insights into how human cells and organ systems function in health and illness, and precludes our developing important new medicines for currently untreatable human diseases. Cone snails, a large genus of some 500 species that live mostly in tropical coral reefs and mangroves, are a case in point. These remarkable creatures capture their prey by lancing them with a harpoon coated with a cocktail of toxic peptides, which bind to an enormous variety of ion channels and receptors on cellular membranes throughout the animal kingdom (Olivera et al. 1990). Each species may make as many as 100 distinct toxins, so there may be as many as 50,000 different ones in all (Olivera 1990). One hundred or so of these toxins have been studied to date and have demonstrated such selectivity for specific receptors that some have been used, for example, to help characterize subtypes of nicotinic acetylcholine receptors in mammalian heart muscle, leading to a better understanding of the mechanisms that control heart rate and contractility (Bibevski et al. 2000). Others are being developed as medicines, including a painkiller possibly 1,000 times more potent than morphine but that does not cause tolerance or addiction (Bowersox et al. 1996). This painkiller may soon come on the market in Europe for the treatment of severe, chronic pain, a condition that often defies treatment with opiates such as morphine because of tolerance. Other cone snail toxins are being investigated for treating intractable epilepsy (McIntsosh et al. 2001), for preventing nerve cell death when there is inadequate circulation (Williams et al. 2002), and for the early diagnosis and treatment of small cell carcinomas of the lung, one of the most aggressive human cancers (Codignola et al. 1996; Sher et al. 2000). Cone snails may contain the largest and most clinically important pharmacopoeia of any genus in nature, and yet, as coral reefs and mangroves are in danger of being destroyed, so are they (Chivian et al. 2003).

The importance of biodiversity to human health is particularly well illustrated by some human infectious diseases. Lyme disease, the most common vectorborne disease in the United States, is a prime example. When high levels of vertebrate-species diversity exist in a Lyme disease area, the risk of getting Lyme disease is lessened. One reason is that some of the vertebrates that are bitten by infected ticks, the vectors which transmit the Lyme bacteria, are "dead end" hosts--poorly able or incapable of passing on the bacteria and continuing the disease cycle. This effectively "dilutes" the disease agent and makes it less likely for an infected tick to transmit the disease to a human (Ostfeld and Keesing 2000a). Another reason this diversity is beneficial is that some vertebrate species compete with the main Lyme reservoir host or carrier (the white-footed mouse in the eastern United States), whereas others are predators--in both cases keeping mice populations low and reducing disease risk. This buffering effect conferred by biodiversity may also apply to other human infectious diseases such as West Nile encephalitis, cutaneous and visceral leishmaniasis, African trypanosomiasis, and Chagas disease (Ostfeld and Keesing 2000b).

Finally, and most importantly, ecosystems provide the life support systems for all life, including human life, on Earth. Not only do they give us food and fuel, but ecosystems, among other things, purify air and fresh water, bind and detoxify poisonous substances, break down wastes and recycle nutrients on land and in the oceans, pollinate crops and natural vegetation, make soils fertile, and store carbon, mitigating human-caused climate change (Melillo and Sala 2002). We tend to take these services for granted and generally do not recognize that we cannot live without them. Nor do we understand many ecosystem services well enough to recreate them, not knowing what species are necessary for the services to work and in what proportions, or whether for some services there are essential or "keystone" species without which ecosystems would cease to function. Human activity may now be altering some ecosystems in destructive ways that we are unaware of and that could lead to a collapse of their functioning.

The importance of recognizing how biodiversity affects human health and how it is increasingly threatened by human activity will only increase in coming years. Physicians and environmental scientists will need to understand these interconnections because they will be called upon to explain them to policy makers and the public. Such knowledge will also be critically important in clinical medicine, particularly in relation to the emergence and spread of some human infectious diseases. Our center and others, such as the Consortium for Conservation Medicine, are working to better understand how human health depends on the health of other species and on the natural functioning of healthy ecosystems, and to help disseminate this understanding more widely.

REFERENCES

Bibevski S, Zhou Y, McIntosh JM, Zigmond RE, Dunlap ME. 2000. Functional nicotinic acetylcholine receptors that mediate ganglionic transmission in cardiac parasympathetic neurons. J Neurosci 20:5076-5082.

Bowersox SS, Gadbois T, Singh T, Pettus M, Wang YX, Luther RR. 1996. Selective N-type neuronal voltage-sensitive calcium channel blocker, SNX-111, produces spinal antinociception in rat models of acute, persistent and neuropathic pain. J Pharmacol Exp Ther 279:1243-1249.

Chivian E, Roberts CM, Bernstein AS. 2003. The threat to cone snails [Letter]. Science 302:391.

Codignola A, McIntosh JM, Cattaneo MG, Vicentini LM, Clementi F, Sher E. 1996. [alpha]-Conotoxin imperialis I inhibits nicotine-evoked hormone release and cell proliferation in human neuroendocrine carcinoma cells. Neurosci Lett 206:53-56.

McIntosh HM, Jones RM 2001. Cone venom--from accidental stings to deliberate injection. Texicon 39:1447-1451.

Melillo J, Sala O. 2002. Ecosystem services. In: Biodiversity: Its Importance to Human Health, Interim Executive Summary (Chivian E, ed). Boston, MA:Center for Health and the Global Environment, Harvard Medical School, 15-20. Available: http://www.med.harvard.edu/chge/resources.html [accessed 29 October 2003].

Olivera BM, Rivier J, Clark C, Ramilo CA, Corpuz GP, Abogadie FC, et al. 1990 Diversity of Conus neuropeptides. Science 249:257-263.

Ostfeld RS, Keesing F. 2000a. Biodiversity and disease risk: the case of Lyme disease. Conserv Biol 14:722-728

--. 2000b. The function of biodiversity in the ecology of vector-borne zoonotic diseases. Can J Zool 78:2061-2078.

Pimm SL, Raven R. 2000. Extinction by numbers. Nature 403:843-844.

Pimm SL, Russell GJ, Gittleman JL, Brooks TM. 1995. The future of biodiversity. Science 269:347-350.

Sher E, Giovannini F, Boot J, Lang B. 2000. Peptide neurotaxins, small-cell lung carcinoma and neurological paraneoplastic syndromes. Biochimie 82:927-936.

United Nations Environment Programme. 1996. Global Biodiversity Assessment. New York:Cambridge University Press.

Walther GR, Post E, Convey P, Menzel A, Parmesan C, Beebee TJC, et all. 2002 Ecological responses to recent climate change. Nature 416:389-395.

Williams AJ, Ling G, McCabe RT, Tortella FC. 2002. Intrathecal CGX-1007 is neuroprotective in a rat model of focal cerebral ischemia. Neuroreport 13:821-824.

Eric Chivian

Aaron S. Bernstein

Center for Health and the Global Environment

Harvard Medical School

Boston, Massachusetts

E-mail: [e-mail]

Eric Chivian is founder and director of the Center for Health and the Global Environment at Harvard Medical School. In 1985, he shared the Nobel Peace Prize for cofounding International Physicians for the Prevention of Nuclear War. He currently leads a project for the United Nations titled Biodiversity: Its Importance to Human Health.

Aaron S. Bernstein, a graduate of the Program in Human Biology at Stanford University, is currently on leave from the University of Chicago Pritzker School of Medicine to conduct research at the Center for Health and the Global Ervironment.