1. A great deal of semantic confusion has surrounded the concept of density-dependence, which was first introduced by H. S. Smith (Journal of Economic Entomology, vol. 28, p. 873, 1935) to describe a mortality factor which destroys an increasing percentage of a population as its density increases. In this book I use the term in its broadest sense to mean a feedback mechanism which responds to the density of the population. This does not necessarily imply negative feedback, as Smith’s definition does, and is, therefore, more in line with Haldane’s concept (New Biology, no. 15, Penguine Books, London, 1953). The various interpretations of the term “density-dependence” are summarized by M. E. Solomon in the book Natural Regulation of Animal Populations, edited by I. A. McLaren, Atheron Press, New York, 1971. This book also deals with some of the more recent theories for the natural regulation of animal populations, including genetic feedback and co-evolution. A good summary of the great debate of the fifties, concerning the role of physical versus biological factors, can be found in the book The Ecology of Insect Populations in Theory and Practice, by L. R. Clark, P. W. Geier, R. D. Hughes, and R. F. Morris, Methuen and Co., Ltd., London, 1967.
2. The comprehensive theory of natural control resulted from the work of many ecologists. However, the paper by C. B. Huffaker, which he presented at the Tenth International Congress of Entomology in 1958, is one of the first definitive statements leading to the contemporary viewpoint (the paper f’c ii“ be found in the Congress Proceedings, vol. 2, p. 625 ).
3. Examples of unusual behavior brought on through physiological and psychological stress amongst animals living under crowded conditions can be found in Desmond Morris’ article “Homosexuality in the ten-spined stickleback” (Behavior vol. 4, p. 233, 1952), and John Calhoun’s “Population density and social pathology” (Scientific American February 1962). The latter is based on experiments with crowded rats. Some typical responses to overcrowding were hypersexual and homosexual behavior, cannibalism of young, and continuous fighting amongst dominant males. Robert Ardrey, in his book The Social Contract (Athenum Press, New York, 1970), drew parallels between these experiments and the behavior of crowded humans. Although these works have received considerable criticism, their insights should not be dismissed lightly. As Thomas Malthus emphasized so strongly in his Essay on the Principle of Population (see Ann Arbor Paperbacks, University of Michigan Press, 1959, for a recent edition) almost 200 years ago, both misery and vice result from the struggle for scarce resources. Ecologists and demographers have generally been more concerned with misery in the form of starvation, disease, warfare, and such. And rightly so, because modern technology can temporarily alleviate these miseries. However, the experimental work of Morris, Calhoun, and their colleagues, has given weight to the second dimension of the Malthusian thesis, that crowding can cause social stress and lead to abnormal social behavior or, in Malthus’ own terms, vice. It is interesting that discussion of vice, and the moral and social issues this gives rise to, is almost as risky today as it was in Malthus’ times (see also Note 3.6).
4. Dennis Chitty has proposed that antagonistic interactions amongst crowded voles cause changes in the genetic properties of succeeding generations which makes them less resistant to normal mortality factors, and that this delayed genetic feedback is responsible for population cycles. For those interested in Chitty’s views, his paper in the Canadian Journal of Zoology (vol. 38, p. 99, 1960), and the excellent summary by C. J. Krebs in the book Population Ecology, edited by L. Adams (Dickerson Publishing Co., Delmont, California, 1970) are recommended. Chitty also gives a summary of these ideas in the book Natural Regulation of Animal Populations, edited by I. A. McLaren ( Atherton Press, New York, 1971 ).
5. The view that stability between co-evolving populations of plants, herbivores, and carnivores is maintained by genetic feedback was proposed by David Pimentel. For example, see his contribution to the book Natural Regulation of Animal Populations (Note 3.3 for reference). In this sense, the efficient predator (herbivore or carnivore) puts strong selective pressure on its food species to evolve resistance to attack, and this feeds back to the predator population to limit its numbers. Pimentel proposes that, after many such cycles, a stable equilibrium between predator and prey populations is attained; that is, the system approaches equilibrium with damped-stable oscillations. Pimentel also reports on laboratory experiments that support this argument.