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

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

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

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)

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

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

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)

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?

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

Variety of Mutualisms Resources Protection Obligate mutualisms 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

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)

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)

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