Processes influencing the distribution of parasite numbers within host populations with special emphasis on parasite-induced host mortalities

Abstract

The paper examines the factors which generate various patterns of dispersion in the distribution of parasites within their host populations. Particular emphasis is placed on the role played by chance elements in the growth and decay of parasite populations and on the influence of different types of demographic processes. It is argued that observed distributions are dynamic, rather than static, entities generated by opposing forces, some acting to create over-dispersion and others acting to generate under-dispersion. Monte Carlo simulation experiments, based on probability models of the growth and decay of host and parasite populations, are used to study the dynamics of parasite dispersion. Attention is specifically focused on the role played by parasite-induced host mortality. It is shown that, for certain types of host-parasite associations, convex curves of mean parasite abundance in relation to age (age-intensity curves), concomitant with a decline in the degree of dispersion in the older age, classes of hosts, may be evidence of the induction in host mortality by parasite infection. Empirical evidence is examined in light of this prediction. In general, however, simulation studies highlight the technical difficulties inherent in establishing clear evidence of parasite-induced host mortality from ecological studies of hosts and parasites in their natural habitats.

Experimental population dynamics of Rhabdias bufonis (Nematoda) in toads (Bufo bufo): density-dependence in the primary infection

“…First, it is difficult to interpret field-collected data in terms of density-dependent regulation (e.g. Anderson & Gordon, 1982;Kennedy, 1984Kennedy, , 1987 and second, it is difficult to bring naturally occurring helminth systems into the laboratory for experimentation.…”

The characteristics of the infection of juvenile Atlantic salmon with glochidia of the freshwater pearl mussel in rivers of Northwest Russia

“…Parameter k is inversely proportionate to the degree of parasite aggregation (k → 0 as parasites are concentrated on a decreasing number of hosts, and k → ∞ as the distribution grows more even). Using k as a measure of aggregation, one finds aggregation is by far the predominant pattern across natural host-parasite systems (Anderson, 1982;Shaw and Dobson, 1995). Statistical analysis of Pearson correlation between the infection intensity and the infected salmon parr body weight was carried out with the PAST v. 2.17 software (Hammer et al, 2001).…”

Occurrence of metacercariae of <i>Diplostomum</i> and <i>Tylodelphys</i> species (diplostomidae) in <i>Clarias</i> species (clariidae) fishes from Lake Victoria

“…The over dispersion of metacercariae in fish host populations has been attributed to heterogeneity in susceptibility, and uneven exposure of fish to infection that is a result of non-random distribution of cercariae in the habitat caused by aggregated distribution of snail intermediate hosts and chance effects (Anderson and Gordon 1982, Esch and Fernandez 1993, Bush et al 2001.…”