Predators, their ecology and impact on gamebird populations.
Before the 20th century, predation was universally regarded as a major factor controlling the numbers of organisms in nature. Scientists, naturalists, and game managers alike believed that since predators killed their prey they must reduce prey numbers. More recently, ecologists have shown that the influence of predators on prey populations may not always be straightforward, and have developed a more sophisticated understanding of the relationship between predator and prey. To begin with, did predators control their prey, or were predator numbers controlled by their food supply? In some situations, such as the relationship of lynx and snowshoe hares in Canada, the numbers of both prey and predators followed one another in cycles of abundance (Elton and Nicholson, 1942), apparently because each successively controlled the numbers of the other. Both mathematical models and laboratory populations of invertebrate predators and prey (e.g. Huffaker, 1958) could produce similar effects, but only under special circumstances. Errington (1946) proposed that predators may in fact be unimportant in the control of prey density. He observed mortality in muskrats (Errington, 1943) and in bobwhite quail (Errington, 1945) and saw each population as having a surplus number of individuals, whose death was assured, regardless of the means. The ultimate cause of death was, he thought, being surplus to the capacity of the environmental resources that muskrats or bobwhite quails use. More recently, field studies have shown that the importance of predation actually varies from one system to the next and in some instances can indeed have a major impact on prey dynamics (Sih et al., 1986) when the capacity of the environment to support a prey species is influenced by the abundance of predators.
Predation is just one of an array of interrelated factors which can influence the dynamics of gamebird populations - others include parasitism (Chapter 5) and habitat quality (Chapter 6). It is also a very complex process in itself, being founded on the individual behaviour of sophisticated organisms. Consequently, in order to understand the impact predators can have, it is frequently valuable to use models, both to simulate the dynamics of populations and to explore the theoretical considerations influencing individual behaviour. Such general models provide an important insight into the processes involved in complex field situations.
The aim of this chapter is to review our current knowledge of predation on gamebirds and to show how generalised models have enabled us to clarify many of the issues involved. Ways in which gamebird managers can influence the effect of predators are examined and various methods in addition to predator removal are considered and evaluated.