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Effects on host populations
1. Introduction
• key questions
– Do affects on host individuals lead to effects on host
demography?
– Do ps regulate host populations?
• applied interest (e.g. insect biocontrol, human pathogens,
wildlife pops)
• Anderson vs. Holmes controversy
• Important considerations:
– Microparasites vs macroparasites
– Intermediate vs final hosts
– ‘normal’ vs ‘abnormal’ hosts
– Direct vs. indirect effects
– Additive vs compensatory mortality
2. Evidence
A. Anecdotal
• Smallpox, plague, distemper, rinderpest, avian cholera, ticks
and moose
• Problem of lack of controls
B. Computer Models
• Focus on patterns of abundance of host populations
– stable, cyclic, unstable
• Crofton (1970’s)
– concept of lethal limit
• Anderson and May (1980’s)
– Much more detailed mathematics
– Introduction of ’alpha’
“Parasitic species are capable of regulating the growth of host
populations, even in the complete absence of other influences
such as predation or intraspecific competition”. (May and
Anderson, 1978).
– Ps lead to stable regulation if:
• aggregated, moderate alpha, DD worm growth
– Ps lead to instability if:
• Ps-induced reduced natality, ps reproduction in host, time
lags
C. Empirical studies
• cestodes in beetles (1981)
nematode in lab mice (Scott, 1987)
Ticks on moose (Samuel, 1980’s)
• nematode in Grouse in Scotland (Hudson, 1992, 1998)
– Hudson, PJ et al. Science 282: 2256-2258.
– affects on individuals
– mortality of infected hosts
d. Nematodes in UK hares (Townsend et al., 2009)
4. Conclusions
• just because they ‘can’, doesn’t mean they ‘do’
• problem of managed or artificial model systems
• status of Anderson vs. Holmes controversy
• single factor vs. multiple factors
• indirect effects and population regulation
• interactions bw immunity, nutrition, parasitism