Population-Level Consequences of Parasite Aggregation

Aggregations of parasites among hosts are generally thought to reduce the net deleterious effect of the parasites on populations of their hosts, but to increase the intensity of density-dependent suppression of parasite population growth. Here I show that whether this is actually the case depends on the shape of the fitness function, i.e., how host or parasite fitness changes as a function of parasite number per host. For sigmoid fitness functions, mean host fitness can decrease and mean parasite fitness increase as parasite aggregation increases for a given mean number of parasites per host. A survey of the literature reveals that such sigmoid fitness functions are not uncommon. Parasite aggregation, therefore, does not always have the population-level effects generally attributed to it.