Control of parasites in natural populations: nematode and virus infections of Red Grouse.

Abstract

Many of the long-term studies on bird populations were stimulated by the pioneering publications of David Lack (1954, 1966, 1968). While Lack (1954) considered a range of density-dependent factors that could regulate populations and included a chapter on disease, he came to the conclusion that 'while further evidence is needed, it seems unlikely that disease is an important factor regulating numbers of most birds'. He came to this conclusion following the general view, held by most ecologists at that time, that parasites evolved to become benign symbionts that maintained high survival by reducing their deleterious effects on the host. Parasitologists, on the other hand, investigating parasite systems in man and domestic livestock viewed parasites as organisms that benefited at the expense of their host (Crofton 1971).
These apparently conflicting views were reconciled when the principles of epidemiology and ecology were combined and applied to wild animal populations. Empirical evidence suggested the relative effects of parasite increase with density, thereby decreasing growth rate of the host population by reducing the host's survival and fecundity (Anderson and May 1978; May and Anderson 1978). In evolutionary terms, the success of a parasite depends not on life expectancy alone but on successful transmission between hosts. Hence a successful parasite is one that produces transmission stages that become established and contribute to future generations, although in the process both host and parasite may die.
The synthesis produced by Anderson and May (op. cit.) has provided a sound framework for a series of studies on the population dynamics of disease agents in wild animals (e.g. Anderson 1982a; Rollinson and Anderson 1985). The essential model is of two linked differential equations which describe changes in the numbers of hosts and parasites. The attraction of these models over the simulation approach are threefold:
(1) They can often be solved analytically for equilibrium conditions;
(2) they can be developed and applied to a wide range of parasite-host systems whereas simulation models tend to be unique; and
(3) they provide an insight into stability and threshold properties and are thus useful in the development of control strategies. Not surprisingly, the majority of studies relating to parasite control have concentrated on man and livestock. However, there is increasing awareness that parasites have a significant impact in problems relating to conservation (Dobson and May1986; Dobson 1988; Scott 1988; Thorne and WilIiams 1988) and in the management of wild animals (Hudson and Dobson 1988). In this paper we describe two parasite systems of Red Grouse and the possible development of control techniques. We discuss the Red Grouse system because there is intensive and extensive information on changes in Grouse numbers and levels of parasite infection. First we consider the intestinal nematode Trichostrongylus tenuis which causes rapid declines in the density of Red Grouse by reducing fecundity. Second we consider the tick-borne viral infection, Louping-ill which is principally a disease of sheep but has a marked effect on the survival of Red Grouse. The two systems are very different and raise different issues relating to management and conservation. In the control of T. tenuis, the main problem is simply to develop a technique for reducing the build-up of parasite numbers by applying an anthelmintic drug. The application of anthelmintics to domestic livestock is relatively simple but treating wild animals raises a number of technical problems. With the Louping-ill system the analysis indicates that the problem relates to livestock acting as a reservoir host and amplifying the disease, a problem not uncommon in other wildlife systems (e.g. Boshell l969; Rogers 1988).