Interactions between macroparasites and wild animal populations.

Abstract

Despite the very large number of diseases seen to afflict wild animals, relatively few ecologists have investigated the role of parasites in regulating host abundance or distribution. This is surprising when one considers the numerous studies that have investigated the regulatory role of other natural enemies such as parasitoids and predators (Crawley, 1992). On the one hand, ecologists have traditionally seen parasites as benign symbionts living in a careful balance with their host and believed that those parasites that do not disturb this relationship by causing the death of the host were at a selective advantage. Occasionally epidemics were recorded but these were thought to be the exception when density independent factors disturbed the careful balance and resulted in massive host mortality. On the other hand, parasitologists carried a different perspective and saw pan sites as organisms that benefited at the expense of their host (Crofton, 1971). Even so their efforts were concentrated at the level of the individual host, usually human or domestic hosts, and they were rarely interested in the effects at the population level.
The views of ecologists have changed over the past fifteen years following the early work of Crofton (1971) and the synthesis of the principles of population biology with those of parasitology (Anderson and May, 1978). This work showed that parasites are not only capable of reducing the growth rate of the host population in a density dependent manner and thus able to regulate the population but also that certain processes would destabilize the association and could cause oscillations in parasite and host numbers. Benign parasites will not necessarily be at a selective advantage since fitness does not depend upon survival alone but on the success at establishing breeding offspring in other hosts.
While the framework for a clear understanding of parasite-host relationships is now available, few empirical studies have examined the processes in wild animal populations and linked theory with field data. This is partly because suitable systems have to be identified, partly because long term studies are required which fail to attract the attention of funding bodies and partly because of methodological problems in understanding the components of transmission.
Macroparasites are those parasite species that generally achieve transmission through a larval stage but do not usually have direct reproduction within the definitive host, a definition which embraces most parasitic arthropods and helminths (except the monogeneans) including the nematodes, cestodes, trematodes, lice, fleas and ticks. Many have complex life cycles involving several obligatory host species. Compared with the microparasites (protozoans, viruses and bacteria) the macroparasites are relatively large, have long generation times and are characterised by more antigens. Infections are generally chronic leading to morbidity rather than mortality so it is important to consider the sub-lethal effects of macroparasites on fitness and host population dynamics rather than more obvious lethal effects.
This paper aims to describe the observed patterns of macroparasites in wild animal populations, examining epidemiological patterns together with the impact of parasites on individuals and their consequences for the abundance and distribution of hosts. Within each section the approach has been to describe the general patterns observed, the factors that could generate these patterns and the limitations in assessing the field data. The paper is not a comprehensive review but an identification of the general patterns. Throughout, the general contention is that macropamsites frequently have an important sub-lethal impact on their hosts with interesting consequences for the dynamics of the host population.