The ecology and control of parasites in gamebird populations.
Parasitic agents are ubiquitous; the majority of living species are parasites of one form or another, and there can be little doubt that this group of animals has been remarkably successful in exploiting other living organisms. It is therefore somewhat surprising that few ecological studies have concentrated on the role of true parasites in regulating animal numbers, while at the same time there have been extensive studies on the biology of the parasitoids (Hassell, 1978, 1981). The lack of detailed studies is reflected in the majority of contemporary ecological texts, with some recent exceptions (May, 1981; Begon et al., 1986). Most have chapters on the importance of predation and competition, but parasitism is usually considered a special case of predation where the parasite has little or no effect on the host population.
The parasite is often seen as benign, living in a fine balance with its host; if it becomes harmful and kills its host, it follows that the parasite will lose its home and die. Those parasites that maintain a harmless level of infection have traditionally been seen to be more successful and, in evolutionary terms, selected for. Epidemics, by their very nature, are considered the exception where the careful balance between host and parasite is temporarily disturbed and the effects of the parasite lead to massive host mortality. The parasite's actions are considered density-independent since the imbalance can be brought about by a wide range of factors such as climatic changes or human intervention.
A different perspective is put on the host-parasite relationship by the parasitologist, who almost by definition tends to see the parasite not as benign but as an organism which benefits at the expense of its host (Crofton, 1971; Whitfield, 1979). Epidemiologists who study the population behaviour of diseases (in many ways parasite ecologists) have tended to concentrate their studies on the biology of parasites of man and his livestock. Only recently have the principles of epidemiology and ecology been brought together and applied to natural animal populations. The synthesis of these two disciplines suggests that parasites operate in a density-dependent manner and are thus capable of regulating the size of the host population (Anderson and May, 1978, 1979; May and Anderson, 1978, 1979). Empirical evidence shows that the relative effects of parasites increase with density, decreasing the growth rate of the host population through reductions in fecundity and survival and thus acting in a way similar to predation or competition. In evolutionary terms the success of any parasite depends on transmission from one host to another, and the parasite that produces successful transmission stages will be at an advantage even if such action kills its host.
Although theoretical studies provide a sound framework and indicate that under certain conditions parasites can regulate the size of their host population, the extent to which they do so in natural populations is not yet clear. In the special case of managed as opposed to unmanaged populations, parasites are likely to be of greater importance. The traditional management procedures of habitat improvement and predator control are designed to increase population density; this will tend to increase the rate of parasite transmission, leading to a rise in parasite burdens. In other words, the removal of limiting factors such as predation and food quality may increase density but may also result in an increase in the importance of parasitism.
Parasites are deleterious to the survival and productivity of gamebirds; reducing their impact is an important objective of gamebird management in some areas. In the case of reared or rare species, management procedures can create artificially high densities which may result in parasites becoming established which would not have done so if the animals were in their natural, more dispersed state. In situations where the host has little or no previous exposure to the parasite the species may be particularly vulnerable if its resistance to infection is low. A further problem is that a managed population of animals may act as a reservoir for a disease which threatens humans or their livestock; for example, domestic poultry and gamebirds share a number of harmful parasites.
This chapter describes how parasites operate to influence individual animals, how these effects influence the growth rate of the population, and the implications of these for managing animal populations.