What Effects do Diseases Have on Bird Populations?
"...the epizootic could have killed 50 million rabbits...reduction in the number of rabbits to only 5%..."
- quote from 1998 Journal of Wildlife Disease, Vol. 34, pp. 221-227
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
- 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:
back to Bird Population Studies Research web page
Copyright 2000, Cornell Laboratory or Ornithology