The quote above is from research conducted in southern Australia on the effects of an introduced disease that scientists are hoping will control the introduced populations of rabbits that are causing widespread habitat destruction. The effect of this introduced disease - rapid death of a large proportion of an animal population - typifies many peoples' perceptions of the effects of diseases on wild populations of birds and other animals. However, there are reasons to believe that rapid and massive mortality may not be the general rule, and that diseases generally have subtler effects on the sizes of animal populations. Among the reasons are:

• Difficulty in detecting many diseases Researchers must be able to identify diseased animals in order to study the effects of disease. The presence of diseases is often difficult to detect (requiring intensive laboratory work); as a result, the prevalence of relatively benign diseases may be under-reported.

• Diseases have always been there How do we study the effects of a disease when there are no "control" (non-diseased) populations? Creating non-diseased populations to use for comparison with diseased populations can be extremely difficult, if not impossible. Thus, studying the effects of diseases can be equally intractable, again leading to under reporting of effects of diseases.

• Theory Theoretical research suggests that diseases could have a wide range of effects on animals, from the severe mortality sometimes observed to regular fluctuations in abundance (i.e. cycles) to maintaining animal populations at consistent and lower numbers than would be the case had there been no disease.
• Common sense If typical diseases of wild animals are at all similar in severity to human diseases, then for every extremely severe epidemic (recall "The Black Plague" in the Middle Ages in Europe), there will be other diseases with lower rates of fatality, and still other diseases that almost never cause deaths.

In a study published in the Proceedings of the National Academy of Sciences, USA (2000, Vol. 97, pp. 5303-5306), data from the Cornell Lab of Ornithology's House Finch Disease Survey and the Christmas Bird Count have allowed researchers to do what has previously been difficult if not impossible:

• study an animal population both before and after the presence of a disease,

• estimate the extent to which this disease has lowered population sizes on a continental scale,

• suggest that a disease may actually be able to consistently prevent an animal population from growing beyond a certain size.

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Copyright 2000, Cornell Laboratory or Ornithology