We are interested in

• prevalence (number of infected individuals)
• intensity (average number of parasites per host)
• aggregation (distribution of parasites within hosts)

Density-dependent vs. density-independent effects

• Density-independent factors such as climate, pH, host immune system (maybe) have a constant per capita effects, regardless of the population density
• Density-dependent factors, particularly competition, have a changing (increasing) per capita effects with host population density

dN/dt = (b-d-aN)N

Density-dependent regulation can happen only occasionally (e.g. in bad years, or at particular times of year), or in only one place in a (complex) host life cycle. Thus, it can be difficult to detect density dependence. It is rarely appropriate to look at low or fluctuating population densities and conclude that density-dependent controls "are not important".

However, we can ask whether the difference between population densities in two places/times arises because of differences in density-independent or density-dependent factors. [ex: Bothriocephalus (cestodes) in Gambusia (mosquito fish): seasonal variation in numbers, but not in biomass, because size of individuals changes as they mature. Furthermore, discharges from power plants (changing water temperature and/or adding pollutants such as selenium) decreases the parasite density - possibly because the free-living stages are affected by environmental conditions. In both cases both density-dependent and density-independent factors are acting, but in the first case it is the difference in density-dependent factors that matters, while in the second case it is the difference in density-independent factors.

Density-dependent regulation affects mortality (survivorship), establishment (initial survivorship), fecundity, or size (which ultimately affects mortality/fecundity)

• Decision-dependent regulation: Affects the number within a host, and the distribution within a population, but not the number within a population (it could actually increase the prevalence/number of parasites in a population, by allowing parasites to avoid competition) Unusual: usually parasites don't have the information or ability to act on it. (Parasitoids)
• Competition:
  • exploitation competition - for host resources. Fecundity is easiest to measure; per capita egg production is what's important (or perhaps per mg, if size changes are important).
    • Hymenolepis in rats (increasing effect with time, total eggs/host decreases)
    • Polystoma in frogs; total eggs/host increases
    • Echinostoma
    • Schistosoma mansoni (in mollusks), 170/1, 400/2, 500/10; decreasing per capita
    • S. rodhaini: 500/mollusk regardless of cercarial infection
  • Interference competition - direct antagonism. Typically inferred from competitive effects at apparently low within-host densities (sea bass, nematodes in cockroaches) Probably usually caused by production of toxins (possible in Hymenolepis, but we're rarely sure).
  • Host-mediated competition: often affects establishment. Concomitant immunity to infection in schistosomes, filarial worms, e.g. Cryptocotyle in cod; plateau of metacercariae per fish over times (sometimes parasites diapause in response to immune reactions)
• Host-death dependent regulation: e.g. Trichostongylus tenuis in grouse. Heavily parasitized hosts are more likely to die (either keeling over or e.g. by being more exposed to predation because they're slower, or have to forage more to make up the costs of parasitism, or are easier to detect. Note that density-dependent host death only regulates host populations if host death is a dead end for parasites; if not (e.g. trophic transmission), it actually increases parasite population densities.