Chapter 14. Parasitism

1. Chapter 14. Parasitism • What’s a parasite? – hard to define • Intimate contact (feed off host) • Usually do not kill host (parasitoids do) • Herbivores(?) • Parasitic Plants • Holoparasites (lack chlorophyll) – Rafflesia (biggest flower) • Hemiparasites (photosynthesize) – Mistletoe • Microparasites – reproduce inside host • Bacteria, viruses • Macroparasites – release juvenile outside • E.g. trematodes • Ectoparasites vs. endoparasites

2. “Weird” Parasites • Nest Parasites • Brownheaded Cowbird • European Cuckoo • Sexual Parasites • Gynogenetic fishes • Amazon molly • Resided/Finescale Dace hybrid

3. Parasitism Common • Possibly more parasites than anything else • 50% of insects parasitic • Potentially 4:1 parasites:free-living forms • Often complex life cycles • E.g. lancet fluke, other trematodes • Several intermediate hosts

6. Modeling Parasitism • Complex because of intermediate hosts, and infection rate • Not usually sensitive to “actual” r for parasite (this is gigantically high) • Important variables: • Rp – number of infected hosts • If Rp > 1 then parasite spreads • For microparasites • Rp = NBL • N – density of susceptible hosts • B – transmission rate of parasite • L – length of time host is infectious • Nt (host pop. size) = 1/BL (if Rp = 1) • Critical host density (upshot is disease cycles as Nt reached by recruitment)

8. Effects on natural populations • Introduced parasites – large effect • Chestnut blight, Dutch elm etc. • Natural systems • Dodder (Cuscuta) – plant parasite – may act to maintain diversity • Fuller and Blaustein – deer mice • Found infected had lower overwinter survival • Hurtrez-Bousses – microwaved blue tit nests • Found higher size at fledging and lower failure rate • Red Grouse

12. Community Effects • Brainworm – host is white-tailed deer • Not much effect • All other cervids and pronghorns susceptible • “apparent competition” – as white-tailed deer expand range, other species affected • Other examples of effects • Flour beetles, Anolis lizards

13. Biocontrol • Some success (about 16%) • E.g. myxoma and rabbits in Australia • Evolution of reduced virulence • How much of the rest deleterious uncertain • Pesticides degrade in environment • Introduced parasites remain • Switch hosts?? Cause other problems? • Some advocate shotgun approach • Some advocate “targeted” approach • I think – last-ditch effort (and maybe not even then)

15. Mutualism • Both species benefit • Plant-pollinator • Often tightly coevolved relationships • E.g. figs and fig wasps – 900 species of figs, each with its own pollinating wasp • Yucca plants and yucca moths • Perhaps each trying to “cheat”? • Reciprocal parasitism?

17. Seed Dispersal • Fruits attract dispersers • Color, smell, abundance etc. • Hypotheses for seed dispersal • Reduced competition • Colonization hypothesis • Directed dispersal hypothesis (ants) • Predator escape hypothesis

18. Variety of Mutualisms • Resources • Leaf cutting ants/fungus • Nitrogen fixing bacteria / plants • Protection • Cleaner fish and “customers” • Some are mimics (cheaters) • Ants and aphids • Ants and acacia trees (herbivory) • Obligate mutualisms • Lichens (algae and fungus) • Ruminants/bacteria • Deep sea fishes/luminescent bacteria • Corals/zooxanthellae • Endosymbiont theory

19. Modeling Mutualism • Similar to Lotka-Volterra comp. eqns. • Replace negative effect with positive • Change K to X (carrying capacity is raised) • Can become weird (unstable) or can become stable when facultative • Obligate mutualisms even more unstable (though obviously there are stable areas)

22. Indirect effects on community • Mycorrhizal fungi / plants • Reduce herbivory • Increased vigor • Increased antiherbivore defenses • Increased mycorrhizal diversity can be positive for community • Or…introduced mutualists can out-compete (endophytes in Indiana)

24. Commensalisms • Cattle egrets/cattle • Clinging seeds and hosts • Flower mites and hummingbird nostrils