1. 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

2. 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

3. 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).

5. –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

6. C. Empirical studies cestodes in beetles (1981)

8. nematode in lab mice (Scott, 1987)

10. Ticks on moose (Samuel, 1980’s)

12. nematode in Grouse in Scotland (Hudson, 1992, 1998) –Hudson, PJ et al. Science 282: 2256-2258. –affects on individuals –mortality of infected hosts

16. d. Nematodes in UK hares (Townsend et al., 2009)

17. 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