Conservation and Disease

Abstract

In developed countries over the past 50 years and more, a combination of better nutrition, better hygiene, antibiotics and other chemotherapeutic agents, and programs of immunization have almost eradicated morbidity and mortality resulting from infectious diseases. The smallpox virus is now extinct in the wild (and no conservationist mourned this act; our concerns change sign as we move from very large organisms to microorganisms). Scarlet fever and diphtheria, which carried off heroines in so many Victorian novels-and in Victorian real life-are rarely heard of today. Vaccination has removed polio from essentially all developed countries, and measles, whooping cough, rubella and other childhood infections seem on the road to local extinction in many. Whereas hookworm and other parasitic infections were endemic in the southern regions of the United States in the early years of this century, most people in developed countries today would react with horror to the thought of harboring a couple of intestinal or other parasites. Although heavy burdens of such parasites continue to afflict many people in developing countries, not the least of the reasons why HIV/AIDS so appalls us, I think, is that in developed countries we have come to believe a life free from any serious effects of viral, bacterial, protozoan or helminth infections is a natural state, to which we have some kind of entitlement. Nothing could be further from the truth. As emphasized by Haldane (1949), infectious diseases have undoubtedly been the main agents of morbidity and mortality (and thus the dominant selective forces) in human populations at least for the past 10,000 years. In combination with malnutrition, infectious diseases are still the main cause of the dramatic differences between survivorship curves in developed and developing countries (Bradley, 1974). To put it another way, current estimates are that 30% of HIV infections go on to produce death from AIDS (although this number may rise as longer runs of data accumulate) which can be compared with the 30% case-mortality associated with smallpox infections that were, until recently, endemic in most parts of the world. Given the conspicuous role that diseases have played, and in many parts of the world continue to play, in human demography, it is surprising that ecologists have given so little attention to the way diseases may affect the distribution and abundance of other animals and plants. Until recently, for example, ecology textbooks had chapters discussing how vertebrate and invertebrate predators may influence prey abundance, but in most cases you will search the index in vain for mention of infectious diseases. I think this is partly because ecologists find fourand six-legged predators more engaging, partly because pathogenic organisms (like decomposers, another category of organisms neglected by most ecologists) are relatively hard to see and thus to study, and partly because veterinarians and wildlife managers understandably tend to focus on individual sick animals rather than on population aspects of infection. The past few years have, however, seen an upsurge in efforts to document the effects that viral, bacterial, protozoan and helminth infections may have on the distribution and abundance of animal populations in the laboratory and-more importantly-in the field (for reviews, see Anderson and May, 1978, 1979 or Toft, 1986). These studies have many implications for conservation biology, some of which are widely recognized and others not. It therefore seemed a good idea to hold a symposium on and Disease at the first annual meeting for the Society for Conservation Biology, and to publish the four papers presented at that symposium in this issue of the journal. The first paper, by Marilyn Scott, reviews evidence for the ways in which infectious diseases may influence the ecology of animal populations. In particular, Scott gives