Through humankind's reproductive and technological 'success', we have occupied, damaged or eliminated the natural habitat of many other species. Biologists estimate that this mass extinction may cause around one third of all species that were alive in the 19th century to be gone before the end of the 21st century.7-57 The loss of various key species weakens ecosystems, with many potential adverse consequences for humans as 'nature's goods and services' decline.i^ Examples include disturbing the ecology of vector-borne infections - for example, by reducing or eliminating mosquito predators - thereby enhancing transmission of infections such as yellow fever and malaria. Other examples include damage to food-producing systems that depend on pollinators and the predation of pests, and impairing the cleansing of water and the circulation of nutrients that normally pass through ecosystems.
We would also lose a rich repertoire of genetic and phenotypic material. To maintain the hybrid vigour and environmental resilience of 'food' species, a diversity of wild species needs to be preserved as a source of genetic additives. Similarly, a high proportion of modern medicinal drugs in western medicine has natural origins, and many defy synthesis in the laboratory. Scientists test thousands of novel natural chemicals each year, seeking new drugs to treat HIV, malaria, drug-resistant tuberculosis, cancers and so on.5®
The other side of this coin is the accelerating spread of 'invasive' species, as long-distance trade, tourism and migration increase in intensity.60 For example,
5 2 A.-L. Ponsonby, A.J. McMichael and I. van der Mei, Toxicology, 2001, in press.
53 H . Slaper, G. J. M . Velders,J . S . Daniel, F. R . de Gruijl and J. C . van der Leun, Nature, 1996,384,256.
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55 P.J. Neale, R.F. Davis and J.J. Cullen, Nature, 1998, 392, 585.
56 C.B. Field, M.J. Behrenfeld, J.T. Randerson and P. Falkowski, Science, 1998, 281, 237.
57 S.L. Pimm, G.J. Russell, J.L. Gittleman and T.M. Brooks, Science, 1995, 269, 347.
58 G. C. Daily, in Nature's Services: Societal Dependence on Natural Ecosystems, Washington DC, 1997. 5® P. A. Cox, Science, 2000, 287, 44.
60 T. Low, Feral Future: The Untold Story of Australia's Exotic Invaders, Viking, 1999.
the spread of water hyacinth in eastern Africa's Lake Victoria, introduced from Brazil as a decorative plant, has provided a micro-environment for the proliferation of diarrhoeal disease bacteria and water snails that transmit schistosomiasis.61
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