Résumé
Parasites are found in all living organisms. Parasite diversity is reflected in the number of parasite species (probably half of all living species), but also in terms of their life cycles (direct vs. indirect) or their size (from a few micrometres to metres). As a result of their impact on host reproduction, growth, and survival, parasites can be important agents regulating populations, communities, and ecosystems. The aim of this chapter is to illustrate, using several simple theoretical approaches, the role of parasites in the regulation of host populations.
It is first necessary to introduce some basic facts. First, we describe the heterogeneity in parasite distribution among host populations or communities. Secondly, we consider parasite virulence as a measure of the impact of parasites on hosts.