Competition

Density-dependent transmission allows parasites to disproportionately affect common species. This helps maintain rarer competitors, thereby promoting coexistence and stability. In addition, when hosts share parasites, parasites can be competitive weapons. For example, two competing species ofamphipod may coexist in nature because a trematode reverses their relative population growth rates. Competitive weapons, such as parasites, can also reduce biodiversity ifsubordinate species are more susceptible. For instance, a larval tapeworm shared by two flour beetle species increases the rate at which the dominant beetle excludes the subordinate.

Table 1 Ecological measures of parasitism

Parasite term

Definition

Abundance

Parasites per host (whether infected or not)

Aggregation

A statistical measure of the distribution of parasite abundance among hosts

Colonization

The infection of an uninfected host

Component population

All individuals of a specified life history stage at a particular place and time

Density

Parasites per sampling unit (per host, per gram tissue, etc).

Incidence

Rate at which uninfected hosts become infected

Infrapopulation

All individuals of a species of parasites in a single host

Intensity

Parasites per infected host

Prevalence

Proportion or % of hosts infected by a parasite

Suprapopulation

All developmental phases of a species at a particular place and time

Modified from Bush AO, Lafferty KD, Font JM, and Shostak AW (1997) Parasitology meets ecology: Definitions, clarifications, examples and Margolis etal. revisited. The Journal of Parasitology 83: 575-583.

Modified from Bush AO, Lafferty KD, Font JM, and Shostak AW (1997) Parasitology meets ecology: Definitions, clarifications, examples and Margolis etal. revisited. The Journal of Parasitology 83: 575-583.

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